Hydrotropism: how roots search for water

Hydrotropism: how roots search for water Abstract Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions. Abscisic acid, auxin, drought, gravitropism, hydrotropism, root, water, water potential gradient Introduction Plants need to respond to a constantly changing environment and use tropisms to reposition organs for resource capture. Tropisms are directional growth movements that allow plants to respond to gravity, light, touch, water, salt, and oxygen (Gilroy and Masson, 2008; Galvan-Ampudia et al., 2013; Eysholdt-Derzsó and Sauter, 2017; Su et al., 2017). In plant roots, gravity is described as the main driver determining the direction of root growth. Gravity is perceived in the columella cells of the root cap, where displacement of statoliths leads to a lateral gradient in the shootward auxin flux. More auxin flows through the lateral root cap and epidermis on the lower side of the root, leading to differential growth in the epidermis of the elongation zone, ultimately resulting in the root tip growing downwards (Blancaflor et al., 1998; Ottenschläger et al., 2003; Swarup et al., 2005; Friml, 2010; Rahman et al., 2010). In comparison, relatively little is known about hydrotropism, the directional growth of plant roots towards a water source. Plant roots are able to perceive a water potential gradient in their surroundings and change the direction of the root tip through differential growth in the elongation zone. Water is becoming increasingly scarce on our planet, and agriculture, which uses ~70% of all freshwater globally, claims the biggest share of this limited resource (Davies and Bennett, 2015; World Water Assessment Programme, 2015). Making crop plants more resilient to drought stress has been highlighted as an important goal in a recent report on sustainable water use (World Water Assessment Programme, 2015). Current strategies to improve water use efficiency through changes in root system architecture focus on increasing the steepness of roots to exploit water resources in lower soil horizons (Henry et al., 2011; Lynch, 2013; Uga et al., 2013; Rogers and Benfey, 2015; Gao and Lynch, 2016). Hydrotropism allows roots to grow actively towards water sources which may be located in any direction. Understanding and modifying this response in plants should be considered as an additional strategy to pursue the goal of sustainable water use in agriculture. Early descriptions of hydrotropism exist (Bonnet, 1754; Knight, 1811) and, in the 19th century, Sachs, Molisch, Darwin, Wiesner, and others conducted experiments to determine which part of the root tip is necessary for the perception of the water signal (Sachs, 1872; Darwin and Darwin, 1880; Wiesner, 1881; Molisch, 1883). Growing plant roots through a sieve with moist sawdust suspended at an angle or along the outside of a clay funnel, these experiments employed moisture gradients in air to observe the bending response of roots (Fig. 1A, B). Roots were covered in a mixture of olive oil and lamp black or cauterized with silver nitrate to determine the site of perception (Darwin and Darwin, 1880). Although some of those studies came to the conclusion that the root tip is the site of perception for both gravity and water, others observed that the elongation zone is also able to perceive the water signal (for a review of early hydrotropism literature, see Hooker, 1915). Fig. 1. View largeDownload slide Historic and modern assays for hydrotropism. (A) Cross-section of the assay described by Sachs and used by the Darwins. Mesh covers the bottom of a round metal frame. Filled with moist sawdust and suspended at an angle, roots can grow through the mesh and need to bend in order to maintain contact with moisture provided by the sawdust. Redrawn from Sachs (1872). (B) Molisch’s hydrotropism assay. Roots grow through holes in the rim of a clay funnel connected to a water reservoir. Once roots reach the edge of the funnel, they have to bend in order to stay in contact with the moisture provided by the funnel surface. Redrawn from Molisch (1883). (C) Moisture in air assay. Inside a box, seedlings are mounted on a water-soaked foam or agar bloc with the root tip pointing down, and suspended in air. The water potential gradient between the moisture-containing support and the surrounding air is further increased by a dish containing a concentrated salt solution. Roots need to bend around the edge of the support in order to stay in contact with their water supply. (D) Split-agar assay. Seedlings are placed in a square Petri dish on growth medium which is in direct contact with another growth medium containing an osmolyte. Diffusion of the osmolyte establishes a water potential gradient that is able to deflect root tip growth from following the gravity vector. (E, F) Rice root bending hydrotropically in the moisture in air assay (Nakajima et al., Auxin transport and response requirements for root hydrotropism differ between plant species, Journal of Experimental Botany 2017, 68, 3441–3456, by permission of the Society of Experimental Biology). (G, H) Arabidopsis thaliana roots bending hydrotropically in the split-agar assay. The white dashed line indicates the border between the two different growth media. The arrow labelled g indicates the gravity vector in all assays. Fig. 1. View largeDownload slide Historic and modern assays for hydrotropism. (A) Cross-section of the assay described by Sachs and used by the Darwins. Mesh covers the bottom of a round metal frame. Filled with moist sawdust and suspended at an angle, roots can grow through the mesh and need to bend in order to maintain contact with moisture provided by the sawdust. Redrawn from Sachs (1872). (B) Molisch’s hydrotropism assay. Roots grow through holes in the rim of a clay funnel connected to a water reservoir. Once roots reach the edge of the funnel, they have to bend in order to stay in contact with the moisture provided by the funnel surface. Redrawn from Molisch (1883). (C) Moisture in air assay. Inside a box, seedlings are mounted on a water-soaked foam or agar bloc with the root tip pointing down, and suspended in air. The water potential gradient between the moisture-containing support and the surrounding air is further increased by a dish containing a concentrated salt solution. Roots need to bend around the edge of the support in order to stay in contact with their water supply. (D) Split-agar assay. Seedlings are placed in a square Petri dish on growth medium which is in direct contact with another growth medium containing an osmolyte. Diffusion of the osmolyte establishes a water potential gradient that is able to deflect root tip growth from following the gravity vector. (E, F) Rice root bending hydrotropically in the moisture in air assay (Nakajima et al., Auxin transport and response requirements for root hydrotropism differ between plant species, Journal of Experimental Botany 2017, 68, 3441–3456, by permission of the Society of Experimental Biology). (G, H) Arabidopsis thaliana roots bending hydrotropically in the split-agar assay. The white dashed line indicates the border between the two different growth media. The arrow labelled g indicates the gravity vector in all assays. Loomis and Ewan (1936) conducted the first assessment of how plant roots grow in soil with different water availability. Utilizing the fact that soil of a certain moisture content does not lose water to adjacent, drier soil through capillary action (Veihmeyer and Hendrickson, 1927), they created water gradients in soil to test the growth response of plant roots. Placing germinating seeds at the border between soil with either 4% or 11.8% moisture (the wilting coefficient of that soil was 7.1%), roots growing into the dry soil stopped their growth, whereas roots growing in the wet soil continued to grow and produced lateral roots (Loomis and Ewan, 1936). This is not a directional growth response in the strictest sense, but slight modifications allowed hydrotropism to be observed. When the border between wet and dry soil was set at a 45° angle and seeds were placed in wet soil some distance away from the border between wet and dry soil, several plant species, including beans (Phaseolus limensis and Phaseolus vulgaris), buckwheat (Fagopyrum esculentum), and foxtail millet (Setaria italica), had roots that did not grow along the gravity vector into dry soil but were bending to follow the line between dry and wet soil (Loomis and Ewan, 1936). In recent years, there has been renewed interest in hydrotropism (reviewed in Monshausen and Gilroy, 2009; Cassab et al., 2013; Moriwaki et al., 2013; Shkolnik and Fromm, 2016), and hydrotropic responses have been shown for pea (Pisum sativum), cucumber (Cucumis sativus), wheat (Triticum aestivum), maize (Zea mays), rice (Oryza sativa), birdsfoot trefoil (Lotus japonicus), sitka spruce (Picea sitchensis), and Arabidopsis thaliana (Table 1) (Jaffe et al., 1985; Takahashi and Scott, 1991; Coutts and Nicoll, 1993; Oyanagi et al., 1995; Mizuno et al., 2002; Takahashi et al., 2002; Nakajima et al., 2017). Although other methods to test hydrotropism exist (Tsuda et al., 2003; Eapen et al., 2015), most hydrotropism assays are currently performed using two systems (Fig. 1C–H). For the moisture in air assay (Fig. 1C, E, F), seedlings are mounted on a support, usually foam or agar blocks, in such a way that just the very root tip is suspended in air. The support acts as a source of water, and the moisture gradient to the surrounding air is increased by placing the mounted seedlings in an enclosed environment with a concentrated salt solution (Takahashi et al., 2002; Morohashi et al., 2017). Alternatively, a water potential gradient can be imposed in a split-agar-based system by adding an osmolyte, (e.g. sorbitol), to the growth medium and placing this in direct contact with the growth medium without additives (Fig. 1D, G, H). Seedlings are transferred to these plates and placed with their root tips a set distance away from the border between the two growth media (Antoni et al., 2016). In both cases, roots will experience a water potential gradient, with a wet (in contact with the foam/agar support or closer to the growth media without osmolyte) and dry (facing the air or closer to the growth media with osmolyte) side to the root. Roots showing a hydrotropic response will change the growth direction of the root tip, bending either around the supporting block or towards the medium with higher water potential. The resulting angle of deflection from vertical, gravitropic growth is then measured. These assays have been used to identify genes involved in hydrotropism and characterize cellular and molecular events of the response. This review will provide an overview of the signalling pathways and genes involved in hydrotropism, species-specific differences in the response, putative mechanisms for perception of the water gradient, describe interaction between hydrotropism and gravitropism, possible contributions of hydrotropism to drought resilience, and concludes with a series of future directions for hydrotropism research. Plant hormones and hydrotropism Auxin plays a central role in several tropisms and might be involved in hydrotropism too. However, the requirement for auxin in the hydrotropic response varies depending on the plant species examined (Table 1). In A. thaliana, the agravitropic auxin transport mutants aux1 and pin2 are not impaired in their hydrotropic response (Takahashi et al., 2002). Likewise, the auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA), 1-naphthylphthalamic acid (NPA), and 3-chloro-4-hydroxyphenylacetic acid (CHPAA) are unable to block hydrotropism; in fact, treatment with TIBA or NPA leads to an earlier increase in root tip angle in hydrotropism assays, even though final angles remain the same (Kaneyasu et al., 2007; Shkolnik et al., 2016). In addition, expression of the auxin reporters DII-Venus and DR5 remains unchanged throughout the hydrotropism response (Ponce et al., 2008b; Takahashi et al., 2009; Shkolnik et al., 2016). Even though hydrotropism in A. thaliana does not require auxin transport, a functioning response to auxin appears to be necessary for hydrotropism. Treatment with auxin response inhibitors led to contrasting results. Whereas p-chlorophenoxyisobutylacetic acid (PCIB) produced a decrease in hydrotropic response, addition of auxinole or α-(phenylethyl-2-oxo)-indole acetic acid (PEO-IAA) accelerated the response (Kaneyasu et al., 2007; Shkolnik et al., 2016). These contrasting results could be due to different modes of action and specificities of inhibitors. Auxinole and PEO-IAA have been shown to bind to the auxin receptor TRANSPORT INHIBITOR RESPONSE 1 (TIR1), whereas the mode of action of PCIB is still unclear (Oono et al., 2003; Hayashi et al., 2008, 2012). PCIB is unable to reverse the effects of exogenous IAA application on root growth, which both auxinole and PEO-IAA are able to do (Oono et al., 2003; Hayashi et al., 2008, 2012). The more specific inhibitors auxinole and PEO-IAA indicate that auxin has a negative influence on hydrotropism in A. thaliana, but this awaits independent confirmation from experiments with auxin response mutants. Table 1. Plant species-specific differences in hydrotropism Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) ND, not determined View Large Table 1. Plant species-specific differences in hydrotropism Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) ND, not determined View Large Auxin’s function in hydrotropism has been explored in four other plant species, cucumber, rice, birdsfoot trefoil, and pea. Gravitropism usually masks the hydrotropism response in both cucumber and pea, hence experiments with pea use the ageotropum mutant which is completely agravitropic, whereas experiments with cucumber seedlings are conducted either under microgravity or clinorotation, or after removal of the root tip (Jaffe et al., 1985; Morohashi et al., 2017). In cucumber, the Aux/IAA gene CsIAA1 (sometimes also referred to as CsIAA12) is differentially expressed within 30 min of exposure to a gravity or water stimulus, with increased expression occurring on the concave side of the bending root (Mizuno et al., 2002). Increased expression on the concave side of hydrotropically bending roots has also been observed for other CsIAA genes (Morohashi et al., 2017). Treatment with the auxin transport inhibitors TIBA and 9-hydroxyfluorene-9-carboxylic acid (HFCA) strongly reduces the hydrotropic response in cucumber, while PCIB and brefeldin A (BFA) have a less strong inhibitory effect (Morohashi et al., 2017). CsPIN5, which is localized in the epidermis and lateral root cap and like AtPIN2 may function in shootward transport of auxin from the root tip, is decreased on the convex side of gravitropically bending roots and on the dry side of roots exposed to a water potential gradient (Morohashi et al., 2017). Surprisingly, this differential CsPIN5 localization also takes places in hydrotropically stimulated roots that show no response because they are exposed to normal gravity (Morohashi et al., 2017). Auxin efflux transport inhibitors (HFCA, NPA, and TIBA) disrupt hydrotropism in the pea ageotropum mutant, whereas inhibitors of auxin influx (CHPAA and 1-naphthoxyacetic acid) do not seem to have a discernible effect on the response (Nakajima et al., 2017). In rice, inhibitors of auxin transport (CHPAA and TIBA), response (PCIB), and biosynthesis (kynurenine) inhibit hydrotropism, and the effect of the latter can be rescued by exogenous application of IAA (Nakajima et al., 2017). Interestingly, the hydrotropic response of birdsfoot trefoil is only inhibited by kynurenine application, which again can be rescued by IAA application, whereas CHPAA, TIBA, and PCIB do not affect hydrotropism (Nakajima et al., 2017). It seems surprising that auxin biosynthesis, but not signalling, is necessary for hydrotropism in birdsfoot trefoil. Signal transduction through ABP1, which recently has been shown not to be involved in auxin signalling (Enders et al., 2015; Gao et al., 2015), has been invoked to explain this discrepancy (Nakajima et al., 2017). An alternative explanation may be that PCIB is not specific enough to inhibit the response in L. japonicus, and that a more potent inhibitor (e.g. auxinole) could prove the necessity for auxin signalling. In summary, the involvement of auxin in hydrotropism varies widely in a plant species-specific manner. Plants usually have species-specific water requirements for successful completion of their life cycle, which might explain why gravitropism overrides hydrotropism in some species, whereas in others (e.g. rice) hydrotropism is independent of gravitropism, but still requires auxin. Understanding the role of auxin in hydrotropism will be important to understanding how gravi- and hydrotropic signals are integrated to determine the growth direction of the root tip. Abscisic acid (ABA) is involved in many processes in plant development and physiological responses, but is perhaps best known for its function in the response to drought and osmotic stress (Yamaguchi-Shinozaki and Shinozaki, 2006; Cutler et al., 2010). The core components of the ABA signalling pathway consist of cytosolic receptors of the START-domain superfamily (PYR/PYL/RCAR), clade A, type 2C protein phosphatases (PP2C), and a subclass III Snf1-related kinases (SnRK2) (Cutler et al., 2010). ABA leads to the formation of a ternary receptor–hormone–phosphatase complex that relieves the inhibition of SnRK2 kinases by PP2C phosphatases, allowing the phosphorylation of downstream targets (Fujii et al., 2009; Ma et al., 2009; Park et al., 2009). In A. thaliana, the ABA biosynthesis mutant aba1-1 has a reduced hydrotropic response, but this defect is rescued by the exogenous application of ABA (Takahashi et al., 2002). ABA signal transduction mutants also have an altered hydrotropic response, with the gain-of-function PP2C mutant abi2-1 and a hextuple receptor mutant showing a reduced response, whereas it is increased in a loss-of-function quadruple pp2c mutant (Takahashi et al., 2002; Antoni et al., 2013). The most detailed exploration of the role of ABA signalling in hydrotropism has been conducted for the SnRK2 kinases. Three family members, SnRK2.2, SnRK2.3, and SnRK2.6, are known to be involved in ABA signalling, and the snkr2.2 snrk2.3 double mutant has a strongly reduced hydrotropism (Mustilli et al., 2002; Fujii et al., 2007; Dietrich et al., 2017). Tissue-specific expression of SnRK2.2 in the double mutant background showed that expression in the cortex alone is able to rescue the response (Dietrich et al., 2017). Exogenous ABA at low concentrations promotes root elongation through increasing the length of root cells at maturity and, in the snkr2.2 snrk2.3 mutant, SnRK2.2 expression in the cortex was able to rescue this effect (Dietrich et al., 2017). A mathematical model examining the contribution of the cortex to root bending predicted that differential elongation in the cortex could be the driving force behind hydrotropic bending (Dietrich et al., 2017). This was further confirmed by blocking differential elongation in a tissue-specific manner, which only blocked hydrotropism if the cortex was affected (Dietrich et al., 2017). Together, this led to the proposal that ABA-mediated differential elongation in the cortex is the driving force behind the changes in growth direction observed in hydrotropism (Dietrich et al., 2017) (Fig. 2). With auxin transport and response in the lateral root cap and epidermis driving gravitropism (Swarup et al., 2005), the distinct role of the cortex in hydrotropism indicates that there are tissue-specific and mechanistic differences between responses to gravity and water. The position of ABA in the signalling cascade for hydrotropism is currently unclear. The rescue of the hydrotropic defect of the aba1-1 mutant by application of exogenous ABA, which is non-directional, could be taken as an indication that hydrotropic signalling does not involve an ABA gradient across the root. On the other hand, hydrotropic signalling could involve changes in ABA sensitivity on the dry and wet side of the root. It is also still unknown if the water potential gradient across the root affects the radial transport of water and signalling molecules. It seems possible that a water potential gradient could lead to changes in the direction of water flow on the dry and wet side of the root, with water flowing towards the stele on the wet side and away from the stele on the dry side of the root. This differential water flow could affect the transport direction of signalling molecules, including ABA. These different hypotheses about the mechanism of ABA in the hydrotropic response still await experimental verification. In addition, the requirement for ABA in hydrotropism of plant species other than A. thaliana still needds to be examined. Fig. 2. View largeDownload slide Hydrotropism mechanism in Arabidopsis thaliana. A. thaliana roots exposed to a water potential gradient perceive reduced water availability through an as yet unknown mechanism in the elongation zone. Reactive oxygen species (ROS) are able to inhibit hydrotropism, but currently the stage at which the response is affected is unknown. Abscisic acid and MIZ2/GNOM are required for hydrotropism and could be involved in perception and differential growth. The role of auxin is currently unclear, but a lateral auxin gradient does not develop during hydrotropism in A. thaliana. Bending of the root tip is achieved by differential elongation of cortex cells; abscisic acid and expression of MIZ1 and SnRK2.2 in the cortex cell file are required for this. Hydrotropic bending of the root tip will trigger a gravitropic response through statolith relocalization, which provides feedback inhibition. Statoliths and differentially expanding cortex cells have been drawn for emphasis and are not to scale. Fig. 2. View largeDownload slide Hydrotropism mechanism in Arabidopsis thaliana. A. thaliana roots exposed to a water potential gradient perceive reduced water availability through an as yet unknown mechanism in the elongation zone. Reactive oxygen species (ROS) are able to inhibit hydrotropism, but currently the stage at which the response is affected is unknown. Abscisic acid and MIZ2/GNOM are required for hydrotropism and could be involved in perception and differential growth. The role of auxin is currently unclear, but a lateral auxin gradient does not develop during hydrotropism in A. thaliana. Bending of the root tip is achieved by differential elongation of cortex cells; abscisic acid and expression of MIZ1 and SnRK2.2 in the cortex cell file are required for this. Hydrotropic bending of the root tip will trigger a gravitropic response through statolith relocalization, which provides feedback inhibition. Statoliths and differentially expanding cortex cells have been drawn for emphasis and are not to scale. Key hydrotropism genes Forward genetic screens have only led to the isolation of a few hydrotropism-related genes in A. thaliana; no hydrotropic response 1 (nhr1) and altered hydrotropic response 1 (ahr1) are semi-dominant mutants affected in hydrotropism (Eapen et al., 2003; Saucedo et al., 2012). Homozygous nhr1 plants never reach the reproductive stage, and the genes affected in both mutants have not yet been cloned (Eapen et al., 2003; Saucedo et al., 2012; Salazar-Blas et al., 2017). mizu-kussei 1 (miz1), described by Kobayashi et al. (2007), is caused by a recessive mutation in At2g41660. Apart from a complete absence of hydrotropism and slightly reduced root phototropism and waving, miz1 plants grow normally and in particular show a normal gravitropism response and root tip anatomy (Kobayashi et al., 2007). Overexpression of MIZ1 leads to increased root curvature in hydrotropism assays (Miyazawa et al., 2012). Unfortunately, MIZ1 is a protein of unknown function, containing only a conserved domain of uncharacterized function (DUF617 domain). Homologues containing a DUF617 domain have been found in rice and Physcomitrella patens but not in algae, suggesting that acquisition of MIZ1 function may have taken place during the evolution of land plants (Kobayashi et al., 2007). It is still unclear at which step of the hydrotropism response MIZ1 functions, but subcellular localization of MIZ1 showed that it is a soluble protein associated with the cytosolic side of the endoplasmatic reticulum (ER) membrane (Yamazaki et al., 2012). ABA and blue light are both able to up-regulate MIZ1 expression (Moriwaki et al., 2012). MIZ1 itself appears to influence auxin accumulation, as free IAA concentrations in miz1 and MIZ1-overexpressing roots increase and decrease, respectively (Moriwaki et al., 2011). Whether this is directly linked to the role of MIZ1 in hydrotropism is unclear, and overexpression or loss of MIZ1 function do not affect PIN gene expression and localization (Moriwaki et al., 2011). A MIZ1–green fluoprescent protein (GFP) fusion under the control of its own promoter has shown that the protein is strongly expressed in cortex cells around the transition zone between the meristem and elongation zone, the lateral root cap and columella, and also, to a lesser extent, in the epidermis and stele, but MIZ1–GFP intensity and localization do not change during the hydrotropic response (Yamazaki et al., 2012; Moriwaki et al., 2013). Recently, it was shown that expression of MIZ1 in the cortex alone is able to rescue the hydrotropism response of miz1 mutants, highlighting the important role of this tissue in hydrotropism (Dietrich et al., 2017) (Fig. 2). A second mutant isolated through forward screens, miz2, is a weak GNOM allele (G951E) (Miyazawa et al., 2009b). GNOM is a GDP/GTP exchange factor for small G proteins of the ARF class (ARF-GEF) regulating intracellular vesicle trafficking, whose best characterized function is polar targeting of PIN proteins to the plasma membrane (Geldner et al., 2003). Importantly, miz2 does not affect auxin response or PIN localization (Miyazawa et al., 2009a, b ). The G951E mutation of miz2 is downstream of the Sec7 domain and affects an amino acid conserved in GNOM homologues in other plant species (Miyazawa et al., 2009b). Treatment with BFA, a known inhibitor of ARF-GEFs, phenocopies miz2. In addition, the hydrotropic response of the BFA-resistant GNM696L allele cannot be blocked by BFA, whereas the weak gnomB/E allele is ahydrotropic (Miyazawa et al., 2009b). Supporting evidence of the importance of vesicle trafficking for hydrotropism comes from a phospholipase D mutant that is slightly impaired in hydrotropism (Taniguchi et al., 2010). There appears to be no direct interaction between MIZ2 and MIZ1, as MIZ1–GFP is still correctly localized in the miz2 mutant (Moriwaki et al., 2011). Interestingly though, miz2 plants that overexpress MIZ1 show an ahydrotropic phenotype, demonstrating that MIZ2 is epistatic to MIZ1 (Miyazawa et al., 2012). Where and how do roots sense water? Which part of the root is able to sense a gradient in water availability is a question that has fascinated people since the early days of hydrotropism research. The Darwins describe experiments where covering the root tip with a mixture of olive oil and lamp black abolishes the hydrotropic response, concluding that the very root tip is necessary for the perception of gravity and water. This led them to coin the, since then much repeated, description of the root tip as the ‘brain of the root’ (Darwin and Darwin, 1880). Darwin’s contemporaries already criticized those experiments, especially with regards to the effect of the applied mixture on root growth rates and the difficulties in applying the mixture in an even manner and to a precise region of the root (Wiesner, 1881; Molisch, 1883). Similar problems affect more recent experiments. A role for the root cap in hydrotropism perception was reported for pea and maize, but root growth rates were not always recorded (Takahashi and Scott, 1991, 1993; Takahashi and Suge, 1991; Takano et al., 1995; Hirasawa et al., 1997). In addition, while surgical ablation experiments record the length of root tip removed, usually no relationship to anatomical markers along the root axis is given and it is therefore difficult to know whether just the columella or larger parts, including the meristem or perhaps even the elongation zone, were removed. Miyazawa et al. (2008) used heavy-ion microbeam irradiation and laser ablation to ablate either the columella or what is described as the elongation zone of A. thaliana roots, and reported conflicting results. While irradiation of the elongation zone led to a reduction in hydrotropic bending, the same treatment of the columella did not (Miyazawa et al., 2008). On the other hand, laser ablation of the columella did reduce the hydrotropic response (Miyazawa et al., 2008). However, root growth rates following both treatments were extremely slow, so that these results have to be considered with caution. More recently, laser ablation and microdissection were again used to determine the root tissue responsible for hydrotropism perception in A. thaliana. Root growth rates were reported for these experiments and were in the expected range. Whereas laser ablation of the columella inhibited the gravitropic response as reported by Blancaflor et al. (1998), the hydrotropic response was not perturbed (Dietrich et al., 2017). Removal of the root cap and meristem by either laser ablation or microdissection also did not inhibit hydrotropism, demonstrating that the elongation zone of the root is able to perceive and respond to the hydrotropic signal (Dietrich et al., 2017) (Fig. 2). While some contribution from the columella and root cap in hydrotropism perception cannot be totally excluded, these results place perception for hydro- and gravitropism in separate tissues. In addition, removal of the columella in rice and cucumber does not impair hydrotropism, demonstrating that in other plant species hydrotropism perception also does not depend on this tissue (Morohashi et al., 2017; Nakajima et al., 2017; Fujii et al., 2018). How could roots be able to sense a water potential gradient in the elongation zone? The difference in water potential across the root is rather small, and was calculated to reach a maximum of <10 kPa across a 100 µm wide A. thaliana root during a standard split-agar hydrotropism assay, which is <3% of the maximum absolute water potential experienced at the root midline (Dietrich et al., 2017). Mechanosensitive ion channels could potentially be triggered by changes in cell volume if a root is exposed to a water potential gradient (Hamilton et al., 2015). Pea is the only plant species where turgor measurements have been performed during hydrotropism, but differences in turgor between the wet and dry side of the root were not observed (Hirasawa et al., 1997; Miyamoto et al., 2002). The miz2 (GNG951E) phenotype strongly implies that membrane proteins play an important part in the hydrotropism response (Miyazawa et al., 2009b). Although GNOM is best known for its role in endosomal recycling of PIN proteins (Geldner et al., 2003), localization of PIN1 is unaffected in miz2 (Miyazawa et al., 2009a). Hydrotropism may rely on endosomal recycling of other proteins, trafficking from the ER to the Golgi or endocytosis, processes that also rely on GNOM (Paez Valencia et al., 2016). It is highly likely that hydrotropism is intricately linked to water uptake and transport in the root. Radial water uptake from the soil towards the xylem vessels in the vasculature follows two paths, the apoplastic route along cell walls and the cell-to-cell path that is comprised of transcellular (across membranes) and symplastic (through plasmodesmata) transport (Li et al., 2014). Root hydraulic conductivity (Lpr) is a measure for water transported through the root. Aquaporins are membrane channels that transport water and small neutral molecules, and one subfamily, the plasma membrane intrinsic proteins (PIPs), contributes significantly to Lpr (Sutka et al., 2011; Li et al., 2014). How could aquaporins and changes in Lpr contribute to hydrotropic signalling? Lpr is reduced by abiotic stress in many plant species (Aroca et al., 2012). PIP activity is regulated at many levels—transcriptionally, translationally, through gating of the channel itself by phosphorylation, protons, or divalent cations, and by cellular trafficking (Li et al., 2014)—and reduction of root hydraulic conductivity under abiotic stress could be achieved using any of these regulatory mechanisms. For salt stress, it was demonstrated that treatment with 100 mM sodium chloride reduces Lpr by ~60% within 1 h and decreases aquaporin transcript abundance (Boursiac et al., 2005). Down-regulation of aquaporin gene expression, however, takes longer than the decrease in Lpr, but other regulation mechanisms respond more rapidly to salt stress. At 45 min after the start of salt treatment, a substantial amount of a PIP2;1–GFP fusion protein had become internalized, and removal from the plasma membrane involved clathrin and membrane raft-associated pathways (Boursiac et al., 2008; Li et al., 2011). Another pathway for removal of aquaporins from the plasma membrane involves tryptophan-rich sensory protein/translocator (TSPO), which is induced by abiotic stress and was shown to interact with PIP2;7, leading to internalization and autophagic degradation of the aquaporin (Hachez et al., 2014). In addition, PIP1;2 and PIP2;1 were recently shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane and were regulated in their water transporting activity by this interaction (Bellati et al., 2016). Similar to the examples for salt and TSPO regulating PIPs at the plasma membrane, the low water potential during hydrotropism could affect the presence in the membrane of aquaporins through endosomal recycling, which would explain the requirement for MIZ2/GNOM (Fig. 3). The interaction of aquaporins with RLKs could also be affected by low water potential, possibly leading to changes in cell elongation through signalling via the RLKs in addition to regulation of PIP activity by the RLKs (Fig. 3). These hypothetical regulation mechanisms of aquaporin activity or membrane presence could lead to a change in hydraulic conductivity, with two possible outcomes. Cell or tissue growth could be affected by changes in Lpr, as was demonstrated for lateral root primordia emergence (Péret et al., 2012). Alternatively, hydraulic conductivity was shown to affect radial ABA transport along the apoplastic pathway through solvent drag (Freundl et al., 1998). This could lead to changes in ABA concentration on the dry and wet side of the root, driving differential cell elongation. Fig. 3. View largeDownload slide Potential mechanisms for perception and response to low water potential. Low water potential could affect the membrane presence or activity of plasma membrane intrinsic proteins (PIPs). This could affect cell elongation through several independent pathways: PIPs were shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane. This interaction was shown to regulate PIP activity, but could potentially also affect signalling from the RLK to change cell elongation. Changes in aquaporin activity or presence due to low water potential will also lead to a change in hydraulic conductivity, with two possible outcomes. Hydraulic conductivity could affect cell elongation directly (as demonstrated for lateral root primordia), but can also affect radial ABA transport in the root. Changes in local ABA concentration could be the driver of differential cell elongation, leading ultimately to root bending. Perception would not necessarily require sensing of a water potential gradient at opposing sides of the root, but could work through a water potential set point, below which PIP membrane presence or activity changes, initiating the signal cascade leading to cell elongation. MIZ2/GNOM is required to facilitate cycling of PIPs (and RLKs) to and from the plasma membrane in this model. Aquaporin regulation in a single layer or all tissue layers of the root may be necessary for this mechanism. Fig. 3. View largeDownload slide Potential mechanisms for perception and response to low water potential. Low water potential could affect the membrane presence or activity of plasma membrane intrinsic proteins (PIPs). This could affect cell elongation through several independent pathways: PIPs were shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane. This interaction was shown to regulate PIP activity, but could potentially also affect signalling from the RLK to change cell elongation. Changes in aquaporin activity or presence due to low water potential will also lead to a change in hydraulic conductivity, with two possible outcomes. Hydraulic conductivity could affect cell elongation directly (as demonstrated for lateral root primordia), but can also affect radial ABA transport in the root. Changes in local ABA concentration could be the driver of differential cell elongation, leading ultimately to root bending. Perception would not necessarily require sensing of a water potential gradient at opposing sides of the root, but could work through a water potential set point, below which PIP membrane presence or activity changes, initiating the signal cascade leading to cell elongation. MIZ2/GNOM is required to facilitate cycling of PIPs (and RLKs) to and from the plasma membrane in this model. Aquaporin regulation in a single layer or all tissue layers of the root may be necessary for this mechanism. These hypothetical perception mechanisms linked to aquaporins would not necessarily require sensing of the water potential gradient at opposing sides of the root, but could utilize a water potential set point, below which aquaporin membrane presence or activity changes, setting in motion the signalling cascade leading to cell elongation. However, at the moment, the identity of the hydrotropic signal perceived by the root is still unclear. Interaction between hydro- and gravitropism Several tropisms can adjust the growth direction of the root tip, and interaction and competition between the responses to different environmental cues will determine the final growth direction. To understand hydrotropism, its interaction with gravitropism is central. It has been argued that gravitropism determines the ‘default’ growth direction of the root, which is then adjusted by tropic responses to other environmental cues (Blancaflor and Masson, 2003; Rosquete and Kleine-Vehn, 2013; Krieger et al., 2016). In the interaction between hydro- and gravitropism, a clear distinction has to be drawn between plant species that depend on auxin and its transport for their hydrotropic response and those where hydrotropism is independent of development of a lateral auxin gradient. In those plant species which require auxin transport, the gravitropic response can be assumed to counteract hydrotropism, unless the water potential gradient aligns with the gravity vector. This would explain why hydrotropism in pea and cucumber can only be observed if gravitropism has been removed. Still, there are plants (e.g. rice), that rely on auxin transport for both tropisms but react to a water gradient in the presence of gravity. How can such differences be explained? It is still unclear whether plant species requiring auxin for hydrotropism develop a lateral auxin gradient during the response. If they do, species-specific differences in the interaction between gravi- and hydrotropism could be due to differences in the establishment of these auxin gradients. Another factor influencing the interaction between the two tropisms could be timing and sensitivity of each response. Presentation time, defined as the minimum exposure time needed to elicit a response, has been determined for the gravitropism response of various plant roots (Kiss and Sack, 1989; Kiss et al., 1996; Hou et al., 2003). Usually, root curvature in response to a 90° stimulus is plotted against stimulation time and the presentation time determined by regression analysis (Kiss et al., 1996). For hydrotropism, the presentation time has so far only been determined for ageotropum peas following the method described for gravitropism (Stinemetz et al., 1996). Equally, data on the strength of the water potential gradient necessary for triggering hydrotropism are scarce (Takano et al., 1995). Natural variation has been reported to exist for gravitropic presentation times (Tanimoto et al., 2008; Moulia and Fournier, 2009), and a more detailed examination of presentation times and response strength for both hydro- and gravitropism should help to understand species-specific differences in the interaction between those tropisms. In A. thaliana, hydrotropism is independent of the development of a lateral auxin gradient (Shkolnik et al., 2016). Plants treated with auxin transport and response inhibitors (Shkolnik et al., 2016) and the pgm1 mutant which lacks statoliths (Takahashi et al., 2003) show a faster hydrotropic response. Together with the observation that statolith degradation occurs in roots exposed to a water potential gradient in A. thaliana and Raphanus sativus (Takahashi et al., 2003; Ponce et al., 2008a), this has been taken as evidence to support the hypothesis that gravitropic responsiveness needs to be reduced so that hydrotropism can take place. In contrast, exposure of roots to 150 mM sodium chloride leads to agravitropic growth and degradation of statoliths, but several salt overly sensitive mutants, which display the same agravitropic growth on medium with salt, retain their statoliths (Sun et al., 2008). This indicates that statolith degradation on exposure to environmental stress may be a mere correlation and not causative for the response. In addition, the agravitropic pin2 and aux1 mutants do not have an accelerated hydrotropic response (Takahashi et al., 2002). Detailed analysis of the kinetics of gravitropism shows that the rate of gravitropic root bending in A. thaliana depends on the stimulation angle, with smaller stimulation angles resulting in reduced bending rates (Mullen et al., 2000). In addition, a threshold angle of 15° from the vertical has to be reached before 50% of a population of seedlings respond gravitropically (Mullen et al., 2000). Therefore, a water potential gradient can lead to a substantial change in root angle before a gravity response is triggered. Furthermore, this gravitropic response will be slow to begin with, as the stimulation angle is small. Recently, a study investigated the interaction between hydro- and gravitropism and the role of reactive oxygen species (ROS) (Krieger et al., 2016). A very interesting observation of this study was that hydro- and gravitropism lead to bending of the root tip in different regions, with gravitropic bending initiating relatively close to the root tip in the distal elongation zone whereas hydrotropic bending takes place in a more shootward region of the elongation zone (central elongation zone) (Krieger et al., 2016), which provides further confirmation that gravitropism and hydrotropism employ different tissues in their bending mechanisms. Auxin-induced ROS production is necessary for gravitropism (Joo et al., 2001), and using the fluorescent dye dihydrorhodamine-123, Krieger et al. (2016) demonstrated that 2 h after gravistimulation a transient ROS increase was visible on the concave side of the distal elongation zone of the bending root. Using the moisture in air assay, a ROS increase was observed on the concave side of the central elongation zone of hydrotropically bending roots (Krieger et al., 2016). However, when calcium chloride was replaced with distilled water in the assay (i.e. under conditions that do not induce hydrotropic bending in roots), a similar ROS increase in the same location was observed (Krieger et al., 2016). ROS distribution, however, was unchanged when hydrotropism was induced in roots using the split-agar assay, and the authors attribute the spurious ROS accumulation in the moisture in air assay to the mechanical tension the roots were under (Krieger et al., 2016). Treatment with ROS scavengers and NADPH oxidase inhibitors showed that ROS production in fact inhibited hydrotropism (Krieger et al., 2016). Ascorbate peroxidase (apx1-2) and respiratory burst oxidase homologue (rbohC) mutants showed decreased and increased hydrotropic curvature, respectively, further confirming the inhibition of hydrotropism by ROS (Krieger et al., 2016). How ROS inhibit hydrotropism is currently unknown. Interestingly though, the same study also showed that after 4 h of hydrostimulation, a 90° gravitropic stimulus was unable to elicit an increase in ROS or a lateral auxin gradient (Krieger et al., 2016). Clearly more work is still necessary to understand exactly how hydro- and gravitropism interact, but this is an exciting first glimpse showing that hydrotropism is able to influence the gravitropic response. Can hydrotropism improve drought acclimation? Drought stress is a major limiting factor in crop production, and complex plant responses exist to escape, avoid, or tolerate limited water availability (Wery, 2005; Gaur et al., 2008). Drought can lead to an increase in the root to shoot ratio of plants, usually due to shoot growth being more strongly affected by drought (Blum, 2005), and maintaining yield under drought conditions can be linked to a well-developed root system, particularly in those regions of the soil still containing water (Comas et al., 2013). Irrigation is used to prevent drought stress in crops, and agriculture uses 70% of globally available freshwater, mostly for this purpose (Du et al., 2015; World Water Assessment Programme, 2015). Climate change, however, will make water availability more unpredictable, with increased likelihoods for extreme weather events and changes in rainfall patterns (IPCC, 2014). A variety of strategies are pursued to make agricultural water use more sustainable and ‘produce more crop per drop’ (Morison et al., 2008; Du et al., 2015). How could hydrotropism, which allows roots to forage for water in soil, contribute to this? Conservation tillage, which minimizes the amount of soil disturbance, increases soil water availability through improved physical soil properties, increased organic matter, and reduced evaporation due to crop residue left on the surface (Triplett and Dick, 2008), and is now widely adopted in many rain-fed agriculture systems (Brunel et al., 2013; Peiretti and Dumanski, 2014). Currently little is known about water distribution in soils under conservation tillage, but water may be more heterogeneously distributed than under conventional tillage, which would make crops with an increased hydrotropism response more efficient. For agricultural systems using irrigation, deficit and partial root zone drying (PRD) irrigation systems have been demonstrated to increase the water use efficiency in a number of crops (Kang and Zhang, 2004). In these systems, less water than is needed to cover evapotranspiration demand is supplied, sometimes only to part of the root system (PRD). As a result, plants produce less shoot biomass and decrease stomatal conductance, whilst still producing similar or slightly reduced yields compared with fully irrigated crops. Under PRD, it is thought that the drying part of the root system produces a signal that regulates stomatal conductance, whereas the irrigated part supplies the shoot with sufficient water to produce the crop (Kang and Zhang, 2004; Sobeih et al., 2004). The applied effects of PRD on plant growth have been extensively studied, and are reviewed elsewhere (Kang and Zhang, 2004). For root growth, it was shown that PRD leads to an increase in tomato root dry weight, particularly in those parts of the root system that were rewatered after a previous drying period (Mingo et al., 2004), and an increase in the root surface area of maize (Zhenchang et al., 2016). That hydrotropism can contribute to directional root growth in soil has been demonstrated for A. thaliana grown in soil microcosms with a lateral water gradient (Iwata et al., 2013). Plants showed increased root growth in the area with higher water content (Iwata et al., 2013). This behaviour was dependent on a functioning hydrotropism response, as plants overexpressing MIZ1 had an increased tendency to grow roots in soil with high water content, whereas miz1 plants grew roots in a random fashion, unrelated to water distribution in the soil (Iwata et al., 2013). In maize, a recent study tried to link hydrotropic responsiveness to yield under PRD irrigation and drought (Eapen et al., 2017). The hydrotropic response of part of a collection of maize hybrid lines from the Drought Tolerance Maize for Africa project was analysed at 4 d after germination, and representative lines with strong and weak hydrotropic responses were tested in field trials (Eapen et al., 2017). Although one line with a strong hydrotropic response showed increased yield under PRD irrigation and drought stress, the results were more ambiguous for other lines (Eapen et al., 2017). Interestingly, however, there seemed to be a stronger correlation between root weight and grain yield in the lines with a strong hydrotropic response compared with those lines which only weakly responded to the stimulus (Eapen et al., 2017). Root biomass and root system architecture traits might have been confounding factors in this study, and highlight the need for rigorous experimental design when assessing the contribution of hydrotropism to crop performance. These new developments are an indication that crops with an improved hydrotropic response could be beneficial in agricultural systems using conservation tillage or deficit/PRD irrigation systems, contributing to improved water use efficiency. Conclusions and future directions Hydrotropism research has taken a leap forward in the last few years with a number of discoveries describing the site of perception, bending mechanism, and interaction with gravitropism. Hydrotropism has now been shown to exist in an increasing number of plant species and, interestingly, species-specific mechanistic differences in the response exist. New techniques will allow us to understand this tropism and how it contributes to water uptake and drought responses in plants. Although progress has been made in understanding hydrotropism, many more questions still remain open. Most importantly, it is still unclear what the water signal is and how it is perceived. With the recent discovery that the columella may not be necessary for hydrotropism and that the signal can be perceived by the elongation zone of A. thaliana (Dietrich et al., 2017), the pool of potential candidates for hydrotropism perception has widened and changed again. Development of sensors for calcium, ABA, pH, ROS, and other signalling molecules has improved dramatically over recent years (Nagai et al., 2004; Jones et al., 2014; Waadt et al., 2014; Krieger et al., 2016), but these sensors may still not be sensitive enough to detect changes during hydrotropism. It might be necessary to determine the signal indirectly, and a better understanding of hydrotropism response kinetics may help in this respect. The presentation time for the hydrotropic signal has been determined so far only for pea, and water potential gradients used in assays are usually chosen on the basis of returning the maximum response without affecting root growth (Stinemetz et al., 1996; Takahashi et al., 2002). A systematic evaluation of presentation times and the strength of the water potential gradient needed to trigger the response may inform the search for the elusive water signal. New developments in microfluidic devices now allow precise delivery of stimuli at high spatial and temporal resolution, and will be instrumental in determining these parameters (Meier et al., 2010; Stanley et al., 2018). Once the signal for hydrotropism has been found, it should be easier to connect genes known to be involved in hydrotropism (e.g. MIZ1, MIZ2/GNOM, and ABA genes) to the signal transduction pathway of the hydrotropic response. Alternatively, a reverse approach could be used, starting from these known components to search for interaction partners that are specific to hydrotropism. In A. thaliana, the cortex tissue has been shown to play an important role in hydrotropism, with evidence that differential elongation in this tissue drives the bending response (Dietrich et al., 2017). Other plant species in which hydrotropism has been observed have a cortex consisting of multiple cell layers, and it will be interesting to see whether hydrotropic bending in those species uses the same mechanistic principle to drive the bending response. The interaction of hydrotropism with other tropisms, gravitropism in particular, is another area of great interest. For those plant species that require auxin transport for hydrotropism, it will be important to determine whether a lateral auxin gradient develops during the response and how such a gradient is affected by gravitropism. Mathematical modelling has provided new insights into gravitropism (Swarup et al., 2005; Band et al., 2012) and has been used to investigate the bending response in hydrotropism (Dietrich et al., 2017). Development of new models that combine hydro- and gravitropic responses will be an important part of understanding how these tropisms interact and direct root tip growth angles. Until now, all hydrotropism experiments have been performed on primary roots of plants. Wiesner and Molisch already observed that lateral roots grow more easily in the direction of water than primary roots (Wiesner, 1881; Molisch, 1883). Hydrotropism research needs to extend its scope and investigate the response of lateral roots. Lateral roots, which have a different gravitropic set point angle and are therefore less responsive to gravity than primary roots (Roychoudhry et al., 2013, 2017), are in theory more responsive to water potential gradients. Lateral roots make up the majority of any plant root system and, although hydrotropism assays for lateral roots will be technically more difficult, these should give us a better appreciation of whether hydrotropism is able to increase water uptake. Ultimately, hydrotropic responses will have to be assessed in soil. Methods now exist that allow the visualization of roots and water in soil and to compute water fluxes into the root (Daly et al., 2015, 2018). Development of a hydrotropic assay in soil will be a necessity to understand the true contribution of this tropism to water uptake and drought acclimation in plants. Acknowledgements The author would like to thank Malcolm J. Bennett and Darren M. Wells for discussion of the manuscript. This work was supported by the Leverhulme Trust [grant no. RPG-2016-409]. References Antoni R , Dietrich D , Bennett MJ , Rodriguez PL . 2016 . Hydrotropism: analysis of the root response to a moisture gradient . Methods in Molecular Biology 1398 , 3 – 9 . Google Scholar CrossRef Search ADS Antoni R , Gonzalez-Guzman M , Rodriguez L et al. 2013 . PYRABACTIN RESISTANCE1-LIKE8 plays an important role for the regulation of abscisic acid signaling in root . Plant Physiology 161 , 931 – 941 . Google Scholar CrossRef Search ADS Aroca R , Porcel R , Ruiz-Lozano JM . 2012 . Regulation of root water uptake under abiotic stress conditions . Journal of Experimental Botany 63 , 43 – 57 . Google Scholar CrossRef Search ADS Band LR , Wells DM , Larrieu A et al. 2012 . Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism . Proceedings of the National Academy of Sciences, USA 109 , 4668 – 4673 . Google Scholar CrossRef Search ADS Bellati J , Champeyroux C , Hem S , Rofidal V , Krouk G , Maurel C , Santoni V . 2016 . Novel aquaporin regulatory mechanisms revealed by interactomics . Molecular and Cellular Proteomics 15 , 3473 – 3487 . Google Scholar CrossRef Search ADS Blancaflor EB , Fasano JM , Gilroy S . 1998 . Mapping the functional roles of cap cells in the response of Arabidopsis primary roots to gravity . Plant Physiology 116 , 213 – 222 . Google Scholar CrossRef Search ADS Blancaflor EB , Masson PH . 2003 . Plant gravitropism. Unraveling the ups and downs of a complex process . Plant Physiology 133 , 1677 – 1690 . Google Scholar CrossRef Search ADS Blum A . 2005 . Drought resistance, water-use efficiency, and yield potential—are they compatible, dissonant, or mutually exclusive ? Australian Journal of Agricultural Research 56 , 1159 – 1168 . Google Scholar CrossRef Search ADS Bonnet C . 1754 . Recherches sur l’usage des feuilles dans les plantes et sur quelques autres sujets relatifs a l’histoire de la vegetation . Goettingen, Leiden : Elie Luzac . Google Scholar CrossRef Search ADS Boursiac Y , Boudet J , Postaire O , Luu DT , Tournaire-Roux C , Maurel C . 2008 . Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization . The Plant Journal 56 , 207 – 218 . Google Scholar CrossRef Search ADS Boursiac Y , Chen S , Luu DT , Sorieul M , van den Dries N , Maurel C . 2005 . Early effects of salinity on water transport in Arabidopsis roots. Molecular and cellular features of aquaporin expression . Plant Physiology 139 , 790 – 805 . Google Scholar CrossRef Search ADS Brunel N , Seguel O , Acevedo E . 2013 . Conservation tillage and water availability for wheat in the dryland of central Chile . Journal of Soil Science and Plant Nutrition 13 , 622 – 637 . Cassab GI , Eapen D , Campos ME . 2013 . Root hydrotropism: an update . American Journal of Botany 100 , 14 – 24 . Google Scholar CrossRef Search ADS Comas LH , Becker SR , Cruz VM , Byrne PF , Dierig DA . 2013 . Root traits contributing to plant productivity under drought . Frontiers in Plant Science 4 , 442 . Google Scholar CrossRef Search ADS Coutts MP , Nicoll BC . 1993 . Orientation of the lateral roots of trees: II. Hydrotropic and gravitropic responses of lateral roots of Sitka spruce grown in air at different humidities . New Phytologist 124 , 277 – 281 . Google Scholar CrossRef Search ADS Cutler SR , Rodriguez PL , Finkelstein RR , Abrams SR . 2010 . Abscisic acid: emergence of a core signaling network . Annual Review of Plant Biology 61 , 651 – 679 . Google Scholar CrossRef Search ADS Daly KR , Mooney SJ , Bennett MJ , Crout NM , Roose T , Tracy SR . 2015 . Assessing the influence of the rhizosphere on soil hydraulic properties using X-ray computed tomography and numerical modelling . Journal of Experimental Botany 66 , 2305 – 2314 . Google Scholar CrossRef Search ADS Daly KR , Tracy SR , Crout NMJ , Mairhofer S , Pridmore TP , Mooney SJ , Roose T . 2018 . Quantification of root water uptake in soil using X-ray computed tomography and image-based modelling . Plant, Cell and Environment 41 , 121 – 133 . Google Scholar CrossRef Search ADS Darwin C , Darwin F . 1880 . The power of movement in plants . London : John Murray . Google Scholar CrossRef Search ADS Davies WJ , Bennett MJ . 2015 . Achieving more crop per drop . Nature Plants 1 , 15118 . Google Scholar CrossRef Search ADS Dietrich D , Pang L , Kobayashi A et al. 2017 . Root hydrotropism is controlled via a cortex-specific growth mechanism . Nature Plants 3 , 17057 . Google Scholar CrossRef Search ADS Du T , Kang S , Zhang J , Davies WJ . 2015 . Deficit irrigation and sustainable water-resource strategies in agriculture for China’s food security . Journal of Experimental Botany 66 , 2253 – 2269 . Google Scholar CrossRef Search ADS Eapen D , Barroso ML , Campos ME , Ponce G , Corkidi G , Dubrovsky JG , Cassab GI . 2003 . A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis . Plant Physiology 131 , 536 – 546 . Google Scholar CrossRef Search ADS Eapen D , Martínez JJ , Cassab GI . 2015 . Assays for root hydrotropism and response to water stress . In: Blancaflor EB , ed. Plant gravitropism: methods and protocols . New York : Springer New York , 133 – 142 . Google Scholar CrossRef Search ADS Eapen D , Martínez-Guadarrama J , Hernández-Bruno O , Flores L , Nieto-Sotelo J , Cassab GI . 2017 . Synergy between root hydrotropic response and root biomass in maize (Zea mays L.) enhances drought avoidance . Plant Science 265 , 87 – 99 . Google Scholar CrossRef Search ADS Enders TA , Oh S , Yang Z , Montgomery BL , Strader LC . 2015 . Genome sequencing of Arabidopsis abp1-5 reveals second-site mutations that may affect phenotypes . The Plant Cell 27 , 1820 – 1826 . Google Scholar CrossRef Search ADS Eysholdt-Derzsó E , Sauter M . 2017 . Root bending is antagonistically affected by hypoxia and ERF-mediated transcription via auxin signaling . Plant Physiology 175 , 412 – 423 . Google Scholar CrossRef Search ADS Freundl E , Steudle E , Hartung W . 1998 . Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem . Planta 207 , 8 – 19 . Google Scholar CrossRef Search ADS Friml J . 2010 . Subcellular trafficking of PIN auxin efflux carriers in auxin transport . European Journal of Cell Biology 89 , 231 – 235 . Google Scholar CrossRef Search ADS Fujii H , Chinnusamy V , Rodrigues A , Rubio S , Antoni R , Park SY , Cutler SR , Sheen J , Rodriguez PL , Zhu JK . 2009 . In vitro reconstitution of an abscisic acid signalling pathway . Nature 462 , 660 – 664 . Google Scholar CrossRef Search ADS Fujii N , Miyabayashi S , Sugita T et al. 2018 . Root-tip-mediated inhibition of hydrotropism is accompanied with the suppression of asymmetric expression of auxin-inducible genes in response to moisture gradients in cucumber roots . PLoS One 13 , e0189827 . Google Scholar CrossRef Search ADS Fujii H , Verslues PE , Zhu JK . 2007 . Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis . The Plant Cell 19 , 485 – 494 . Google Scholar CrossRef Search ADS Galvan-Ampudia CS , Julkowska MM , Darwish E , Gandullo J , Korver RA , Brunoud G , Haring MA , Munnik T , Vernoux T , Testerink C . 2013 . Halotropism is a response of plant roots to avoid a saline environment . Current Biology 23 , 2044 – 2050 . Google Scholar CrossRef Search ADS Gao Y , Lynch JP . 2016 . Reduced crown root number improves water acquisition under water deficit stress in maize (Zea mays L.) . Journal of Experimental Botany 67 , 4545 – 4557 . Google Scholar CrossRef Search ADS Gao Y , Zhang Y , Zhang D , Dai X , Estelle M , Zhao Y . 2015 . Auxin binding protein 1 (ABP1) is not required for either auxin signaling or Arabidopsis development . Proceedings of the National Academy of Sciences, USA 112 , 2275 – 2280 . Google Scholar CrossRef Search ADS Gaur PM , Krishnamurthy L , Kashiwagi J . 2008 . Improving drought-avoidance root traits in chickpea (Cicer arietinum L.)—current status of research at ICRISAT . Plant Production Science 11 , 3 – 11 . Google Scholar CrossRef Search ADS Geldner N , Anders N , Wolters H , Keicher J , Kornberger W , Muller P , Delbarre A , Ueda T , Nakano A , Jürgens G . 2003 . The Arabidopsis GNOM ARF-GEF mediates endosomal recycling, auxin transport, and auxin-dependent plant growth . Cell 112 , 219 – 230 . Google Scholar CrossRef Search ADS Gilroy S , Masson PH , eds. 2008 . Plant tropisms . Oxford : Blackwell Publishing Ltd . Hachez C , Veljanovski V , Reinhardt H , Guillaumot D , Vanhee C , Chaumont F , Batoko H . 2014 . The Arabidopsis abiotic stress-induced TSPO-related protein reduces cell-surface expression of the aquaporin PIP2;7 through protein–protein interactions and autophagic degradation . The Plant Cell 26 , 4974 – 4990 . Google Scholar CrossRef Search ADS Hamilton ES , Schlegel AM , Haswell ES . 2015 . United in diversity: mechanosensitive ion channels in plants . Annual Review of Plant Biology 66 , 113 – 137 . Google Scholar CrossRef Search ADS Hayashi K , Neve J , Hirose M , Kuboki A , Shimada Y , Kepinski S , Nozaki H . 2012 . Rational design of an auxin antagonist of the SCF(TIR1) auxin receptor complex . ACS Chemical Biology 7 , 590 – 598 . Google Scholar CrossRef Search ADS Hayashi K-I , Tan X , Zheng N , Hatate T , Kimura Y , Kepinski S , Nozaki H . 2008 . Small-molecule agonists and antagonists of F-box protein–substrate interactions in auxin perception and signaling . Proceedings of the National Academy of Sciences, USA 105 , 5632 – 5637 . Google Scholar CrossRef Search ADS Henry A , Gowda VRP , Torres RO , McNally KL , Serraj R . 2011 . Variation in root system architecture and drought response in rice (Oryza sativa): phenotyping of the OryzaSNP panel in rainfed lowland fields . Field Crops Research 120 , 205 – 214 . Google Scholar CrossRef Search ADS Hirasawa T , Takahashi H , Suge H , Ishihara K . 1997 . Water potential, turgor and cell wall properties in elongating tissues of the hydrotropically bending roots of pea (Pisum sativum L) . Plant, Cell and Environment 20 , 381 – 386 . Google Scholar CrossRef Search ADS Hooker HD . 1915 . Hydrotropism in roots of Lupinus albus . Annals of Botany 29 , 265 – 283 . Google Scholar CrossRef Search ADS Hou G , Mohamalawari DR , Blancaflor EB . 2003 . Enhanced gravitropism of roots with a disrupted cap actin cytoskeleton . Plant Physiology 131 , 1360 – 1373 . Google Scholar CrossRef Search ADS IPCC . 2014 . Climate change 2014: synthesis report. Contribution of working groups I, II and II to the fifth assessment report of the intergovernmental panel on climate change . Core Writing Team , Pachauri RK , Meyer LA , eds. Geneva, Switzerland : IPCC . Iwata S , Miyazawa Y , Fujii N , Takahashi H . 2013 . MIZ1-regulated hydrotropism functions in the growth and survival of Arabidopsis thaliana under natural conditions . Annals of Botany 112 , 103 – 114 . Google Scholar CrossRef Search ADS Jaffe MJ , Takahashi H , Biro RL . 1985 . A pea mutant for the study of hydrotropism in roots . Science 230 , 445 – 447 . Google Scholar CrossRef Search ADS Jones AM , Danielson JA , Manojkumar SN , Lanquar V , Grossmann G , Frommer WB . 2014 . Abscisic acid dynamics in roots detected with genetically encoded FRET sensors . eLife 3 , e01741 . Joo JH , Bae YS , Lee JS . 2001 . Role of auxin-induced reactive oxygen species in root gravitropism . Plant Physiology 126 , 1055 – 1060 . Google Scholar CrossRef Search ADS Kaneyasu T , Kobayashi A , Nakayama M , Fujii N , Takahashi H , Miyazawa Y . 2007 . Auxin response, but not its polar transport, plays a role in hydrotropism of Arabidopsis roots . Journal of Experimental Botany 58 , 1143 – 1150 . Google Scholar CrossRef Search ADS Kang S , Zhang J . 2004 . Controlled alternate partial root-zone irrigation: its physiological consequences and impact on water use efficiency . Journal of Experimental Botany 55 , 2437 – 2446 . Google Scholar CrossRef Search ADS Kiss JZ , Sack FD . 1989 . Reduced gravitropic sensitivity in roots of a starch-deficient mutant of Nicotiana sylvestris . Planta 180 , 123 – 130 . Google Scholar CrossRef Search ADS Kiss JZ , Wright JB , Caspar T . 1996 . Gravitropism in roots of intermediate-starch mutants of Arabidopsis . Physiologia Plantarum 97 , 237 – 244 . Google Scholar CrossRef Search ADS Knight T . 1811 . On the causes which influence the direction of the growth of roots . Philosophical Transactions of the Royal Society of London 101 , 209 – 219 . Google Scholar CrossRef Search ADS Kobayashi A , Takahashi A , Kakimoto Y , Miyazawa Y , Fujii N , Higashitani A , Takahashi H . 2007 . A gene essential for hydrotropism in roots . Proceedings of the National Academy of Sciences, USA 104 , 4724 – 4729 . Google Scholar CrossRef Search ADS Krieger G , Shkolnik D , Miller G , Fromm H . 2016 . Reactive oxygen species tune root tropic responses . Plant Physiology 172 , 1209 – 1220 . Li G , Santoni V , Maurel C . 2014 . Plant aquaporins: roles in plant physiology . Biochimica et Biophysica Acta 1840 , 1574 – 1582 . Google Scholar CrossRef Search ADS Li X , Wang X , Yang Y , Li R , He Q , Fang X , Luu DT , Maurel C , Lin J . 2011 . Single-molecule analysis of PIP2;1 dynamics and partitioning reveals multiple modes of Arabidopsis plasma membrane aquaporin regulation . The Plant Cell 23 , 3780 – 3797 . Google Scholar CrossRef Search ADS Loomis WE , Ewan LM . 1936 . Hydrotropic responses of roots in soil . Botanical Gazette 97 , 728 – 743 . Google Scholar CrossRef Search ADS Lynch JP . 2013 . Steep, cheap and deep: an ideotype to optimize water and N acquisition by maize root systems . Annals of Botany 112 , 347 – 357 . Google Scholar CrossRef Search ADS Ma Y , Szostkiewicz I , Korte A , Moes D , Yang Y , Christmann A , Grill E . 2009 . Regulators of PP2C phosphatase activity function as abscisic acid sensors . Science 324 , 1064 – 1068 . Meier M , Lucchetta EM , Ismagilov RF . 2010 . Chemical stimulation of the Arabidopsis thaliana root using multi-laminar flow on a microfluidic chip . Lab on a Chip 10 , 2147 – 2153 . Google Scholar CrossRef Search ADS Mingo DM , Theobald JC , Bacon MA , Davies WJ , Dodd IC . 2004 . Biomass allocation in tomato (Lycopersicon esculentum) plants grown under partial rootzone drying: enhancement of root growth . Functional Plant Biology 31 , 971 – 978 . Google Scholar CrossRef Search ADS Miyamoto N , Ookawa T , Takahashi H , Hirasawa T . 2002 . Water uptake and hydraulic properties of elongating cells in hydrotropically bending roots of Pisum sativum L . Plant and Cell Physiology 43 , 393 – 401 . Google Scholar CrossRef Search ADS Miyazawa Y , Ito Y , Moriwaki T , Kobayashi A , Fujii N , Takahashi H . 2009a. A molecular mechanism unique to hydrotropism in roots . Plant Science 177 , 297 – 301 . Google Scholar CrossRef Search ADS Miyazawa Y , Moriwaki T , Uchida M , Kobayashi A , Fujii N , Takahashi H . 2012 . Overexpression of MIZU-KUSSEI1 enhances the root hydrotropic response by retaining cell viability under hydrostimulated conditions in Arabidopsis thaliana . Plant and Cell Physiology 53 , 1926 – 1933 . Google Scholar CrossRef Search ADS Miyazawa Y , Sakashita T , Funayama T et al. 2008 . Effects of locally targeted heavy-ion and laser microbeam on root hydrotropism in Arabidopsis thaliana . Journal of Radiation Research 49 , 373 – 379 . Google Scholar CrossRef Search ADS Miyazawa Y , Takahashi A , Kobayashi A , Kaneyasu T , Fujii N , Takahashi H . 2009b. GNOM-mediated vesicular trafficking plays an essential role in hydrotropism of Arabidopsis roots . Plant Physiology 149 , 835 – 840 . Google Scholar CrossRef Search ADS Mizuno H , Kobayashi A , Fujii N , Yamashita M , Takahashi H . 2002 . Hydrotropic response and expression pattern of auxin-inducible gene, CS-IAA1, in the primary roots of clinorotated cucumber seedlings . Plant and Cell Physiology 43 , 793 – 801 . Google Scholar CrossRef Search ADS Molisch H . 1883 . Untersuchungen ueber den Hydrotropismus. Sitzungsberichte k. k . Akademie Wien 88 , 897 – 943 . Monshausen GB , Gilroy S . 2009 . The exploring root–root growth responses to local environmental conditions . Current Opinion in Plant Biology 12 , 766 – 772 . Google Scholar CrossRef Search ADS Morison JI , Baker NR , Mullineaux PM , Davies WJ . 2008 . Improving water use in crop production . Philosophical Transactions of the Royal Society B: Biological Sciences 363 , 639 – 658 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Fujii N , Takahashi H . 2012 . Light and abscisic acid signalling are integrated by MIZ1 gene expression and regulate hydrotropic response in roots of Arabidopsis thaliana . Plant, Cell and Environment 35 , 1359 – 1368 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Kobayashi A , Takahashi H . 2013 . Molecular mechanisms of hydrotropism in seedling roots of Arabidopsis thaliana (Brassicaceae) . American Journal of Botany 100 , 25 – 34 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Kobayashi A , Uchida M , Watanabe C , Fujii N , Takahashi H . 2011 . Hormonal regulation of lateral root development in Arabidopsis modulated by MIZ1 and requirement of GNOM activity for MIZ1 function . Plant Physiology 157 , 1209 – 1220 . Google Scholar CrossRef Search ADS Morohashi K , Okamoto M , Yamazaki C et al. 2017 . Gravitropism interferes with hydrotropism via counteracting auxin dynamics in cucumber roots: clinorotation and spaceflight experiments . New Phytologist 215 , 1476 – 1489 . Google Scholar CrossRef Search ADS Moulia B , Fournier M . 2009 . The power and control of gravitropic movements in plants: a biomechanical and systems biology view . Journal of Experimental Botany 60 , 461 – 486 . Google Scholar CrossRef Search ADS Mullen JL , Wolverton C , Ishikawa H , Evans ML . 2000 . Kinetics of constant gravitropic stimulus responses in Arabidopsis roots using a feedback system . Plant Physiology 123 , 665 – 670 . Google Scholar CrossRef Search ADS Mustilli AC , Merlot S , Vavasseur A , Fenzi F , Giraudat J . 2002 . Arabidopsis OST1 protein kinase mediates the regulation of stomatal aperture by abscisic acid and acts upstream of reactive oxygen species production . The Plant Cell 14 , 3089 – 3099 . Google Scholar CrossRef Search ADS Nagai T , Yamada S , Tominaga T , Ichikawa M , Miyawaki A . 2004 . Expanded dynamic range of fluorescent indicators for Ca2+ by circularly permuted yellow fluorescent proteins . Proceedings of the National Academy of Sciences, USA 101 , 10554 – 10559 . Google Scholar CrossRef Search ADS Nakajima Y , Nara Y , Kobayashi A , Sugita T , Miyazawa Y , Fujii N , Takahashi H . 2017 . Auxin transport and response requirements for root hydrotropism differ between plant species . Journal of Experimental Botany 68 , 3441 – 3456 . Google Scholar CrossRef Search ADS Oono Y , Ooura C , Rahman A , Aspuria ET , Hayashi K , Tanaka A , Uchimiya H . 2003 . p-Chlorophenoxyisobutyric acid impairs auxin response in Arabidopsis root . Plant Physiology 133 , 1135 – 1147 . Google Scholar CrossRef Search ADS Ottenschläger I , Wolff P , Wolverton C , Bhalerao RP , Sandberg G , Ishikawa H , Evans M , Palme K . 2003 . Gravity-regulated differential auxin transport from columella to lateral root cap cells . Proceedings of the National Academy of Sciences, USA 100 , 2987 – 2991 . Google Scholar CrossRef Search ADS Oyanagi A , Takahashi H , Suge H . 1995 . Interactions between hydrotropism and gravitropism in the primary seminal roots of Triticum aestivum L . Annals of Botany 75 , 229 – 235 . Google Scholar CrossRef Search ADS Paez Valencia J , Goodman K , Otegui MS . 2016 . Endocytosis and endosomal trafficking in plants . Annual Review of Plant Biology 67 , 309 – 335 . Google Scholar CrossRef Search ADS Park SY , Fung P , Nishimura N et al. 2009 . Abscisic acid inhibits type 2C protein phosphatases via the PYR/PYL family of START proteins . Science 324 , 1068 – 1071 . Peiretti R , Dumanski J . 2014 . The transformation of agriculture in Argentina through soil conservation . International Soil and Water Conservation Research 2 , 14 – 20 . Google Scholar CrossRef Search ADS Péret B , Li G , Zhao J et al. 2012 . Auxin regulates aquaporin function to facilitate lateral root emergence . Nature Cell Biology 14 , 991 – 998 . Google Scholar CrossRef Search ADS Ponce G , Rasgado FA , Cassab GI . 2008a. Roles of amyloplasts and water deficit in root tropisms . Plant, Cell and Environment 31 , 205 – 217 . Google Scholar CrossRef Search ADS Ponce G , Rasgado F , Cassab GI . 2008b. How amyloplasts, water deficit and root tropisms interact ? Plant Signaling and Behavior 3 , 460 – 462 . Google Scholar CrossRef Search ADS Rahman A , Takahashi M , Shibasaki K , Wu S , Inaba T , Tsurumi S , Baskin TI . 2010 . Gravitropism of Arabidopsis thaliana roots requires the polarization of PIN2 toward the root tip in meristematic cortical cells . The Plant Cell 22 , 1762 – 1776 . Google Scholar CrossRef Search ADS Rogers ED , Benfey PN . 2015 . Regulation of plant root system architecture: implications for crop advancement . Current Opinion in Biotechnology 32 , 93 – 98 . Google Scholar CrossRef Search ADS Rosquete MR , Kleine-Vehn J . 2013 . Halotropism: turning down the salty date . Current Biology 23 , R927 – R929 . Google Scholar CrossRef Search ADS Roychoudhry S , Del Bianco M , Kieffer M , Kepinski S . 2013 . Auxin controls gravitropic setpoint angle in higher plant lateral branches . Current Biology 23 , 1497 – 1504 . Google Scholar CrossRef Search ADS Roychoudhry S , Kieffer M , Del Bianco M , Liao CY , Weijers D , Kepinski S . 2017 . The developmental and environmental regulation of gravitropic setpoint angle in Arabidopsis and bean . Scientific Reports 7 , 42664 . Google Scholar CrossRef Search ADS Sachs J . 1872 . Ablenkung der Wurzel von ihrer normalen Wachsthumsrichtung durch feuchte Koerper . In: Sachs J , ed. Arbeiten des Botanischen Institus in Wuerzburg . Leipzig : Wilhelm Engelmann , 209 – 222 . Salazar-Blas A , Noriega-Calixto L , Campos ME , Eapen D , Cruz-Vázquez T , Castillo-Olamendi L , Sepulveda-Jiménez G , Porta H , Dubrovsky JG , Cassab GI . 2017 . Robust root growth in altered hydrotropic response1 (ahr1) mutant of Arabidopsis is maintained by high rate of cell production at low water potential gradient . Journal of Plant Physiology 208 , 102 – 114 . Google Scholar CrossRef Search ADS Saucedo M , Ponce G , Campos ME , Eapen D , García E , Luján R , Sánchez Y , Cassab GI . 2012 . An altered hydrotropic response (ahr1) mutant of Arabidopsis recovers root hydrotropism with cytokinin . Journal of Experimental Botany 63 , 3587 – 3601 . Google Scholar CrossRef Search ADS Shkolnik D , Fromm H . 2016 . The Cholodny–Went theory does not explain hydrotropism . Plant Science 252 , 400 – 403 . Google Scholar CrossRef Search ADS Shkolnik D , Krieger G , Nuriel R , Fromm H . 2016 . Hydrotropism: root bending does not require auxin redistribution . Molecular Plant 9 , 757 – 759 . Google Scholar CrossRef Search ADS Sobeih WY , Dodd IC , Bacon MA , Grierson D , Davies WJ . 2004 . Long-distance signals regulating stomatal conductance and leaf growth in tomato (Lycopersicon esculentum) plants subjected to partial root-zone drying . Journal of Experimental Botany 55 , 2353 – 2363 . Google Scholar CrossRef Search ADS Stanley CE , Shrivastava J , Brugman R , Heinzelmann E , van Swaay D , Grossmann G . 2018 . Dual-flow-RootChip reveals local adaptations of roots towards environmental asymmetry at the physiological and genetic levels . New Phytologist 217 , 1357 – 1369 . Google Scholar CrossRef Search ADS Stinemetz C , Takahashi H , Suge H . 1996 . Characterization of hydrotropism: the timing of perception and signal movement from the root cap in the agravitropic pea mutant ageotropum . Plant and Cell Physiology 37 , 800 – 805 . Google Scholar CrossRef Search ADS Su SH , Gibbs NM , Jancewicz AL , Masson PH . 2017 . Molecular mechanisms of root gravitropism . Current Biology 27 , R964 – R972 . Google Scholar CrossRef Search ADS Sun F , Zhang W , Hu H , Li B , Wang Y , Zhao Y , Li K , Liu M , Li X . 2008 . Salt modulates gravity signaling pathway to regulate growth direction of primary roots in Arabidopsis . Plant Physiology 146 , 178 – 188 . Google Scholar CrossRef Search ADS Sutka M , Li G , Boudet J , Boursiac Y , Doumas P , Maurel C . 2011 . Natural variation of root hydraulics in Arabidopsis grown in normal and salt-stressed conditions . Plant Physiology 155 , 1264 – 1276 . Google Scholar CrossRef Search ADS Swarup R , Kramer EM , Perry P , Knox K , Leyser HM , Haseloff J , Beemster GT , Bhalerao R , Bennett MJ . 2005 . Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal . Nature Cell Biology 7 , 1057 – 1065 . Google Scholar CrossRef Search ADS Takahashi N , Goto N , Okada K , Takahashi H . 2002 . Hydrotropism in abscisic acid, wavy, and gravitropic mutants of Arabidopsis thaliana . Planta 216 , 203 – 211 . Google Scholar CrossRef Search ADS Takahashi H , Miyazawa Y , Fujii N . 2009 . Hormonal interactions during root tropic growth: hydrotropism versus gravitropism . Plant Molecular Biology 69 , 489 – 502 . Google Scholar CrossRef Search ADS Takahashi H , Scott TK . 1991 . Hydrotropism and its interaction with gravitropism in maize roots . Plant Physiology 96 , 558 – 564 . Google Scholar CrossRef Search ADS Takahashi H , Scott TK . 1993 . Intensity of hydrostimulation for the induction of root hydrotropism and its sensing by the root cap . Plant, Cell and Environment 16 , 99 – 103 . Google Scholar CrossRef Search ADS Takahashi H , Suge H . 1991 . Root hydrotropism of an agravitropic pea mutant, ageotropum . Physiologia Plantarum 82 , 24 – 31 . Google Scholar CrossRef Search ADS Takahashi N , Yamazaki Y , Kobayashi A , Higashitani A , Takahashi H . 2003 . Hydrotropism interacts with gravitropism by degrading amyloplasts in seedling roots of Arabidopsis and radish . Plant Physiology 132 , 805 – 810 . Google Scholar CrossRef Search ADS Takano M , Takahashi H , Hirasawa T , Suge H . 1995 . Hydrotropism in roots: sensing of a gradient in water potential by the root cap . Planta 197 , 410 – 413 . Google Scholar CrossRef Search ADS Taniguchi YY , Taniguchi M , Tsuge T , Oka A , Aoyama T . 2010 . Involvement of Arabidopsis thaliana phospholipase Dzeta2 in root hydrotropism through the suppression of root gravitropism . Planta 231 , 491 – 497 . Google Scholar CrossRef Search ADS Tanimoto M , Tremblay R , Colasanti J . 2008 . Altered gravitropic response, amyloplast sedimentation and circumnutation in the Arabidopsis shoot gravitropism 5 mutant are associated with reduced starch levels . Plant Molecular Biology 67 , 57 – 69 . Google Scholar CrossRef Search ADS Triplett GB , Dick WA . 2008 . No-tillage crop production: a revolution in agriculture ! Agronomy Journal 100 , 153 – 165 . Google Scholar CrossRef Search ADS Tsuda S , Miyamoto N , Takahashi H , Ishihara K , Hirasawa T . 2003 . Roots of Pisum sativum L. exhibit hydrotropism in response to a water potential gradient in vermiculite . Annals of Botany 92 , 767 – 770 . Google Scholar CrossRef Search ADS Uga Y , Sugimoto K , Ogawa S et al. 2013 . Control of root system architecture by DEEPER ROOTING 1 increases rice yield under drought conditions . Nature Genetics 45 , 1097 – 1102 . Google Scholar CrossRef Search ADS Veihmeyer FJ , Hendrickson AH . 1927 . Soil-moisture conditions in relation to plant growth . Plant Physiology 2 , 71 – 82 . Google Scholar CrossRef Search ADS Waadt R , Hitomi K , Nishimura N , Hitomi C , Adams SR , Getzoff ED , Schroeder JI . 2014 . FRET-based reporters for the direct visualization of abscisic acid concentration changes and distribution in Arabidopsis . eLife 3 , e01739 . Google Scholar CrossRef Search ADS Wery J . 2005 . Differential effects of soil water deficit on the basic plant functions and their significance to analyse crop responses to water deficit in indeterminate plants . Australian Journal of Agricultural Research 56 , 1201 – 1209 . Google Scholar CrossRef Search ADS Wiesner J . 1881 . Das Bewegungsvermoegen der Pflanzen . Wien : Alfred Hoelder . World Water Assessment Programme (WWAP) . 2015 . The United Nations World Water Development Report 2015: water for a sustainable world . Paris : UNESCO . Yamaguchi-Shinozaki K , Shinozaki K . 2006 . Transcriptional regulatory networks in cellular responses and tolerance to dehydration and cold stresses . Annual Review of Plant Biology 57 , 781 – 803 . Google Scholar CrossRef Search ADS Yamazaki T , Miyazawa Y , Kobayashi A , Moriwaki T , Fujii N , Takahashi H . 2012 . MIZ1, an essential protein for root hydrotropism, is associated with the cytoplasmic face of the endoplasmic reticulum membrane in Arabidopsis root cells . FEBS Letters 586 , 398 – 402 . Google Scholar CrossRef Search ADS Zhenchang W , Xiaofei Y , Liang F , Jianbin Z . 2016 . Partial rootzone drying irrigation increase root surface area, root hydraulic conductivity and water use efficiency in maize . International Journal of Environmental Monitoring and Analysis 4 , 146 – 153 . Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Experimental Botany Oxford University Press

Hydrotropism: how roots search for water

Loading next page...
 
/lp/ou_press/hydrotropism-how-roots-search-for-water-5RrJzjFaSI
Publisher
Oxford University Press
Copyright
© The Author(s) 2018. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com
ISSN
0022-0957
eISSN
1460-2431
D.O.I.
10.1093/jxb/ery034
Publisher site
See Article on Publisher Site

Abstract

Abstract Fresh water is an increasingly scarce resource for agriculture. Plant roots mediate water uptake from the soil and have developed a number of adaptive traits such as hydrotropism to aid water foraging. Hydrotropism modifies root growth to respond to a water potential gradient in soil and grow towards areas with a higher moisture content. Abscisic acid (ABA) and a small number of genes, including those encoding ABA signal transducers, MIZ2/GNOM, and the hydrotropism-specific MIZ1, are known to be necessary for the response in Arabidopsis thaliana, whereas the role of auxin in hydrotropism appears to vary depending on the plant species. This review will describe recent progress characterizing the hormonal regulation of hydrotropism. Recent advances in identifying the sites of hydrotropic perception and response, together with its interaction with gravitropism, will also be discussed. Finally, I will describe putative mechanisms for perception of the water potential gradient and a potential role for hydrotropism in acclimatizing plants to drought conditions. Abscisic acid, auxin, drought, gravitropism, hydrotropism, root, water, water potential gradient Introduction Plants need to respond to a constantly changing environment and use tropisms to reposition organs for resource capture. Tropisms are directional growth movements that allow plants to respond to gravity, light, touch, water, salt, and oxygen (Gilroy and Masson, 2008; Galvan-Ampudia et al., 2013; Eysholdt-Derzsó and Sauter, 2017; Su et al., 2017). In plant roots, gravity is described as the main driver determining the direction of root growth. Gravity is perceived in the columella cells of the root cap, where displacement of statoliths leads to a lateral gradient in the shootward auxin flux. More auxin flows through the lateral root cap and epidermis on the lower side of the root, leading to differential growth in the epidermis of the elongation zone, ultimately resulting in the root tip growing downwards (Blancaflor et al., 1998; Ottenschläger et al., 2003; Swarup et al., 2005; Friml, 2010; Rahman et al., 2010). In comparison, relatively little is known about hydrotropism, the directional growth of plant roots towards a water source. Plant roots are able to perceive a water potential gradient in their surroundings and change the direction of the root tip through differential growth in the elongation zone. Water is becoming increasingly scarce on our planet, and agriculture, which uses ~70% of all freshwater globally, claims the biggest share of this limited resource (Davies and Bennett, 2015; World Water Assessment Programme, 2015). Making crop plants more resilient to drought stress has been highlighted as an important goal in a recent report on sustainable water use (World Water Assessment Programme, 2015). Current strategies to improve water use efficiency through changes in root system architecture focus on increasing the steepness of roots to exploit water resources in lower soil horizons (Henry et al., 2011; Lynch, 2013; Uga et al., 2013; Rogers and Benfey, 2015; Gao and Lynch, 2016). Hydrotropism allows roots to grow actively towards water sources which may be located in any direction. Understanding and modifying this response in plants should be considered as an additional strategy to pursue the goal of sustainable water use in agriculture. Early descriptions of hydrotropism exist (Bonnet, 1754; Knight, 1811) and, in the 19th century, Sachs, Molisch, Darwin, Wiesner, and others conducted experiments to determine which part of the root tip is necessary for the perception of the water signal (Sachs, 1872; Darwin and Darwin, 1880; Wiesner, 1881; Molisch, 1883). Growing plant roots through a sieve with moist sawdust suspended at an angle or along the outside of a clay funnel, these experiments employed moisture gradients in air to observe the bending response of roots (Fig. 1A, B). Roots were covered in a mixture of olive oil and lamp black or cauterized with silver nitrate to determine the site of perception (Darwin and Darwin, 1880). Although some of those studies came to the conclusion that the root tip is the site of perception for both gravity and water, others observed that the elongation zone is also able to perceive the water signal (for a review of early hydrotropism literature, see Hooker, 1915). Fig. 1. View largeDownload slide Historic and modern assays for hydrotropism. (A) Cross-section of the assay described by Sachs and used by the Darwins. Mesh covers the bottom of a round metal frame. Filled with moist sawdust and suspended at an angle, roots can grow through the mesh and need to bend in order to maintain contact with moisture provided by the sawdust. Redrawn from Sachs (1872). (B) Molisch’s hydrotropism assay. Roots grow through holes in the rim of a clay funnel connected to a water reservoir. Once roots reach the edge of the funnel, they have to bend in order to stay in contact with the moisture provided by the funnel surface. Redrawn from Molisch (1883). (C) Moisture in air assay. Inside a box, seedlings are mounted on a water-soaked foam or agar bloc with the root tip pointing down, and suspended in air. The water potential gradient between the moisture-containing support and the surrounding air is further increased by a dish containing a concentrated salt solution. Roots need to bend around the edge of the support in order to stay in contact with their water supply. (D) Split-agar assay. Seedlings are placed in a square Petri dish on growth medium which is in direct contact with another growth medium containing an osmolyte. Diffusion of the osmolyte establishes a water potential gradient that is able to deflect root tip growth from following the gravity vector. (E, F) Rice root bending hydrotropically in the moisture in air assay (Nakajima et al., Auxin transport and response requirements for root hydrotropism differ between plant species, Journal of Experimental Botany 2017, 68, 3441–3456, by permission of the Society of Experimental Biology). (G, H) Arabidopsis thaliana roots bending hydrotropically in the split-agar assay. The white dashed line indicates the border between the two different growth media. The arrow labelled g indicates the gravity vector in all assays. Fig. 1. View largeDownload slide Historic and modern assays for hydrotropism. (A) Cross-section of the assay described by Sachs and used by the Darwins. Mesh covers the bottom of a round metal frame. Filled with moist sawdust and suspended at an angle, roots can grow through the mesh and need to bend in order to maintain contact with moisture provided by the sawdust. Redrawn from Sachs (1872). (B) Molisch’s hydrotropism assay. Roots grow through holes in the rim of a clay funnel connected to a water reservoir. Once roots reach the edge of the funnel, they have to bend in order to stay in contact with the moisture provided by the funnel surface. Redrawn from Molisch (1883). (C) Moisture in air assay. Inside a box, seedlings are mounted on a water-soaked foam or agar bloc with the root tip pointing down, and suspended in air. The water potential gradient between the moisture-containing support and the surrounding air is further increased by a dish containing a concentrated salt solution. Roots need to bend around the edge of the support in order to stay in contact with their water supply. (D) Split-agar assay. Seedlings are placed in a square Petri dish on growth medium which is in direct contact with another growth medium containing an osmolyte. Diffusion of the osmolyte establishes a water potential gradient that is able to deflect root tip growth from following the gravity vector. (E, F) Rice root bending hydrotropically in the moisture in air assay (Nakajima et al., Auxin transport and response requirements for root hydrotropism differ between plant species, Journal of Experimental Botany 2017, 68, 3441–3456, by permission of the Society of Experimental Biology). (G, H) Arabidopsis thaliana roots bending hydrotropically in the split-agar assay. The white dashed line indicates the border between the two different growth media. The arrow labelled g indicates the gravity vector in all assays. Loomis and Ewan (1936) conducted the first assessment of how plant roots grow in soil with different water availability. Utilizing the fact that soil of a certain moisture content does not lose water to adjacent, drier soil through capillary action (Veihmeyer and Hendrickson, 1927), they created water gradients in soil to test the growth response of plant roots. Placing germinating seeds at the border between soil with either 4% or 11.8% moisture (the wilting coefficient of that soil was 7.1%), roots growing into the dry soil stopped their growth, whereas roots growing in the wet soil continued to grow and produced lateral roots (Loomis and Ewan, 1936). This is not a directional growth response in the strictest sense, but slight modifications allowed hydrotropism to be observed. When the border between wet and dry soil was set at a 45° angle and seeds were placed in wet soil some distance away from the border between wet and dry soil, several plant species, including beans (Phaseolus limensis and Phaseolus vulgaris), buckwheat (Fagopyrum esculentum), and foxtail millet (Setaria italica), had roots that did not grow along the gravity vector into dry soil but were bending to follow the line between dry and wet soil (Loomis and Ewan, 1936). In recent years, there has been renewed interest in hydrotropism (reviewed in Monshausen and Gilroy, 2009; Cassab et al., 2013; Moriwaki et al., 2013; Shkolnik and Fromm, 2016), and hydrotropic responses have been shown for pea (Pisum sativum), cucumber (Cucumis sativus), wheat (Triticum aestivum), maize (Zea mays), rice (Oryza sativa), birdsfoot trefoil (Lotus japonicus), sitka spruce (Picea sitchensis), and Arabidopsis thaliana (Table 1) (Jaffe et al., 1985; Takahashi and Scott, 1991; Coutts and Nicoll, 1993; Oyanagi et al., 1995; Mizuno et al., 2002; Takahashi et al., 2002; Nakajima et al., 2017). Although other methods to test hydrotropism exist (Tsuda et al., 2003; Eapen et al., 2015), most hydrotropism assays are currently performed using two systems (Fig. 1C–H). For the moisture in air assay (Fig. 1C, E, F), seedlings are mounted on a support, usually foam or agar blocks, in such a way that just the very root tip is suspended in air. The support acts as a source of water, and the moisture gradient to the surrounding air is increased by placing the mounted seedlings in an enclosed environment with a concentrated salt solution (Takahashi et al., 2002; Morohashi et al., 2017). Alternatively, a water potential gradient can be imposed in a split-agar-based system by adding an osmolyte, (e.g. sorbitol), to the growth medium and placing this in direct contact with the growth medium without additives (Fig. 1D, G, H). Seedlings are transferred to these plates and placed with their root tips a set distance away from the border between the two growth media (Antoni et al., 2016). In both cases, roots will experience a water potential gradient, with a wet (in contact with the foam/agar support or closer to the growth media without osmolyte) and dry (facing the air or closer to the growth media with osmolyte) side to the root. Roots showing a hydrotropic response will change the growth direction of the root tip, bending either around the supporting block or towards the medium with higher water potential. The resulting angle of deflection from vertical, gravitropic growth is then measured. These assays have been used to identify genes involved in hydrotropism and characterize cellular and molecular events of the response. This review will provide an overview of the signalling pathways and genes involved in hydrotropism, species-specific differences in the response, putative mechanisms for perception of the water gradient, describe interaction between hydrotropism and gravitropism, possible contributions of hydrotropism to drought resilience, and concludes with a series of future directions for hydrotropism research. Plant hormones and hydrotropism Auxin plays a central role in several tropisms and might be involved in hydrotropism too. However, the requirement for auxin in the hydrotropic response varies depending on the plant species examined (Table 1). In A. thaliana, the agravitropic auxin transport mutants aux1 and pin2 are not impaired in their hydrotropic response (Takahashi et al., 2002). Likewise, the auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA), 1-naphthylphthalamic acid (NPA), and 3-chloro-4-hydroxyphenylacetic acid (CHPAA) are unable to block hydrotropism; in fact, treatment with TIBA or NPA leads to an earlier increase in root tip angle in hydrotropism assays, even though final angles remain the same (Kaneyasu et al., 2007; Shkolnik et al., 2016). In addition, expression of the auxin reporters DII-Venus and DR5 remains unchanged throughout the hydrotropism response (Ponce et al., 2008b; Takahashi et al., 2009; Shkolnik et al., 2016). Even though hydrotropism in A. thaliana does not require auxin transport, a functioning response to auxin appears to be necessary for hydrotropism. Treatment with auxin response inhibitors led to contrasting results. Whereas p-chlorophenoxyisobutylacetic acid (PCIB) produced a decrease in hydrotropic response, addition of auxinole or α-(phenylethyl-2-oxo)-indole acetic acid (PEO-IAA) accelerated the response (Kaneyasu et al., 2007; Shkolnik et al., 2016). These contrasting results could be due to different modes of action and specificities of inhibitors. Auxinole and PEO-IAA have been shown to bind to the auxin receptor TRANSPORT INHIBITOR RESPONSE 1 (TIR1), whereas the mode of action of PCIB is still unclear (Oono et al., 2003; Hayashi et al., 2008, 2012). PCIB is unable to reverse the effects of exogenous IAA application on root growth, which both auxinole and PEO-IAA are able to do (Oono et al., 2003; Hayashi et al., 2008, 2012). The more specific inhibitors auxinole and PEO-IAA indicate that auxin has a negative influence on hydrotropism in A. thaliana, but this awaits independent confirmation from experiments with auxin response mutants. Table 1. Plant species-specific differences in hydrotropism Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) ND, not determined View Large Table 1. Plant species-specific differences in hydrotropism Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) Plant species Gravitropism masks hydrotropism Root cap needed for hydrotropism Auxin transport inhibitor blocks hydrotropism Auxin response inhibitor blocks hydrotropism Auxin biosynthesis inhibitor blocks hydrotropism Hydrotropism genes References Pea Yes ND Yes/no Only at 100 µM PCIB ND ND Jaffe et al. (1985); Nakajima et al. (2017) Cucumber Yes No Yes Yes (PCIB) ND IAA1, PIN5 Mizuno et al. (2002); Morohashi et al. (2017) A. thaliana No No No Yes (PCIB), no (auxinole, PEO-IAA) ND MIZ1, MIZ2 PYR/PYL, PP2CA, SnRK2.2, ABA1, PLDζ2 Takahashi et al. (2002); Kaneyasu et al. (2007); Kobayashi et al. (2007); Miyazawa et al. (2009b); Taniguchi et al. (2010); Antoni et al. (2013); Shkolnik et al. (2016); Dietrich et al. (2017) Rice No No Yes Yes (PCIB) Yes ND Nakajima et al. (2017) Lotus japonicus No ND No No (PCIB) Yes ND Nakajima et al. (2017) ND, not determined View Large Auxin’s function in hydrotropism has been explored in four other plant species, cucumber, rice, birdsfoot trefoil, and pea. Gravitropism usually masks the hydrotropism response in both cucumber and pea, hence experiments with pea use the ageotropum mutant which is completely agravitropic, whereas experiments with cucumber seedlings are conducted either under microgravity or clinorotation, or after removal of the root tip (Jaffe et al., 1985; Morohashi et al., 2017). In cucumber, the Aux/IAA gene CsIAA1 (sometimes also referred to as CsIAA12) is differentially expressed within 30 min of exposure to a gravity or water stimulus, with increased expression occurring on the concave side of the bending root (Mizuno et al., 2002). Increased expression on the concave side of hydrotropically bending roots has also been observed for other CsIAA genes (Morohashi et al., 2017). Treatment with the auxin transport inhibitors TIBA and 9-hydroxyfluorene-9-carboxylic acid (HFCA) strongly reduces the hydrotropic response in cucumber, while PCIB and brefeldin A (BFA) have a less strong inhibitory effect (Morohashi et al., 2017). CsPIN5, which is localized in the epidermis and lateral root cap and like AtPIN2 may function in shootward transport of auxin from the root tip, is decreased on the convex side of gravitropically bending roots and on the dry side of roots exposed to a water potential gradient (Morohashi et al., 2017). Surprisingly, this differential CsPIN5 localization also takes places in hydrotropically stimulated roots that show no response because they are exposed to normal gravity (Morohashi et al., 2017). Auxin efflux transport inhibitors (HFCA, NPA, and TIBA) disrupt hydrotropism in the pea ageotropum mutant, whereas inhibitors of auxin influx (CHPAA and 1-naphthoxyacetic acid) do not seem to have a discernible effect on the response (Nakajima et al., 2017). In rice, inhibitors of auxin transport (CHPAA and TIBA), response (PCIB), and biosynthesis (kynurenine) inhibit hydrotropism, and the effect of the latter can be rescued by exogenous application of IAA (Nakajima et al., 2017). Interestingly, the hydrotropic response of birdsfoot trefoil is only inhibited by kynurenine application, which again can be rescued by IAA application, whereas CHPAA, TIBA, and PCIB do not affect hydrotropism (Nakajima et al., 2017). It seems surprising that auxin biosynthesis, but not signalling, is necessary for hydrotropism in birdsfoot trefoil. Signal transduction through ABP1, which recently has been shown not to be involved in auxin signalling (Enders et al., 2015; Gao et al., 2015), has been invoked to explain this discrepancy (Nakajima et al., 2017). An alternative explanation may be that PCIB is not specific enough to inhibit the response in L. japonicus, and that a more potent inhibitor (e.g. auxinole) could prove the necessity for auxin signalling. In summary, the involvement of auxin in hydrotropism varies widely in a plant species-specific manner. Plants usually have species-specific water requirements for successful completion of their life cycle, which might explain why gravitropism overrides hydrotropism in some species, whereas in others (e.g. rice) hydrotropism is independent of gravitropism, but still requires auxin. Understanding the role of auxin in hydrotropism will be important to understanding how gravi- and hydrotropic signals are integrated to determine the growth direction of the root tip. Abscisic acid (ABA) is involved in many processes in plant development and physiological responses, but is perhaps best known for its function in the response to drought and osmotic stress (Yamaguchi-Shinozaki and Shinozaki, 2006; Cutler et al., 2010). The core components of the ABA signalling pathway consist of cytosolic receptors of the START-domain superfamily (PYR/PYL/RCAR), clade A, type 2C protein phosphatases (PP2C), and a subclass III Snf1-related kinases (SnRK2) (Cutler et al., 2010). ABA leads to the formation of a ternary receptor–hormone–phosphatase complex that relieves the inhibition of SnRK2 kinases by PP2C phosphatases, allowing the phosphorylation of downstream targets (Fujii et al., 2009; Ma et al., 2009; Park et al., 2009). In A. thaliana, the ABA biosynthesis mutant aba1-1 has a reduced hydrotropic response, but this defect is rescued by the exogenous application of ABA (Takahashi et al., 2002). ABA signal transduction mutants also have an altered hydrotropic response, with the gain-of-function PP2C mutant abi2-1 and a hextuple receptor mutant showing a reduced response, whereas it is increased in a loss-of-function quadruple pp2c mutant (Takahashi et al., 2002; Antoni et al., 2013). The most detailed exploration of the role of ABA signalling in hydrotropism has been conducted for the SnRK2 kinases. Three family members, SnRK2.2, SnRK2.3, and SnRK2.6, are known to be involved in ABA signalling, and the snkr2.2 snrk2.3 double mutant has a strongly reduced hydrotropism (Mustilli et al., 2002; Fujii et al., 2007; Dietrich et al., 2017). Tissue-specific expression of SnRK2.2 in the double mutant background showed that expression in the cortex alone is able to rescue the response (Dietrich et al., 2017). Exogenous ABA at low concentrations promotes root elongation through increasing the length of root cells at maturity and, in the snkr2.2 snrk2.3 mutant, SnRK2.2 expression in the cortex was able to rescue this effect (Dietrich et al., 2017). A mathematical model examining the contribution of the cortex to root bending predicted that differential elongation in the cortex could be the driving force behind hydrotropic bending (Dietrich et al., 2017). This was further confirmed by blocking differential elongation in a tissue-specific manner, which only blocked hydrotropism if the cortex was affected (Dietrich et al., 2017). Together, this led to the proposal that ABA-mediated differential elongation in the cortex is the driving force behind the changes in growth direction observed in hydrotropism (Dietrich et al., 2017) (Fig. 2). With auxin transport and response in the lateral root cap and epidermis driving gravitropism (Swarup et al., 2005), the distinct role of the cortex in hydrotropism indicates that there are tissue-specific and mechanistic differences between responses to gravity and water. The position of ABA in the signalling cascade for hydrotropism is currently unclear. The rescue of the hydrotropic defect of the aba1-1 mutant by application of exogenous ABA, which is non-directional, could be taken as an indication that hydrotropic signalling does not involve an ABA gradient across the root. On the other hand, hydrotropic signalling could involve changes in ABA sensitivity on the dry and wet side of the root. It is also still unknown if the water potential gradient across the root affects the radial transport of water and signalling molecules. It seems possible that a water potential gradient could lead to changes in the direction of water flow on the dry and wet side of the root, with water flowing towards the stele on the wet side and away from the stele on the dry side of the root. This differential water flow could affect the transport direction of signalling molecules, including ABA. These different hypotheses about the mechanism of ABA in the hydrotropic response still await experimental verification. In addition, the requirement for ABA in hydrotropism of plant species other than A. thaliana still needds to be examined. Fig. 2. View largeDownload slide Hydrotropism mechanism in Arabidopsis thaliana. A. thaliana roots exposed to a water potential gradient perceive reduced water availability through an as yet unknown mechanism in the elongation zone. Reactive oxygen species (ROS) are able to inhibit hydrotropism, but currently the stage at which the response is affected is unknown. Abscisic acid and MIZ2/GNOM are required for hydrotropism and could be involved in perception and differential growth. The role of auxin is currently unclear, but a lateral auxin gradient does not develop during hydrotropism in A. thaliana. Bending of the root tip is achieved by differential elongation of cortex cells; abscisic acid and expression of MIZ1 and SnRK2.2 in the cortex cell file are required for this. Hydrotropic bending of the root tip will trigger a gravitropic response through statolith relocalization, which provides feedback inhibition. Statoliths and differentially expanding cortex cells have been drawn for emphasis and are not to scale. Fig. 2. View largeDownload slide Hydrotropism mechanism in Arabidopsis thaliana. A. thaliana roots exposed to a water potential gradient perceive reduced water availability through an as yet unknown mechanism in the elongation zone. Reactive oxygen species (ROS) are able to inhibit hydrotropism, but currently the stage at which the response is affected is unknown. Abscisic acid and MIZ2/GNOM are required for hydrotropism and could be involved in perception and differential growth. The role of auxin is currently unclear, but a lateral auxin gradient does not develop during hydrotropism in A. thaliana. Bending of the root tip is achieved by differential elongation of cortex cells; abscisic acid and expression of MIZ1 and SnRK2.2 in the cortex cell file are required for this. Hydrotropic bending of the root tip will trigger a gravitropic response through statolith relocalization, which provides feedback inhibition. Statoliths and differentially expanding cortex cells have been drawn for emphasis and are not to scale. Key hydrotropism genes Forward genetic screens have only led to the isolation of a few hydrotropism-related genes in A. thaliana; no hydrotropic response 1 (nhr1) and altered hydrotropic response 1 (ahr1) are semi-dominant mutants affected in hydrotropism (Eapen et al., 2003; Saucedo et al., 2012). Homozygous nhr1 plants never reach the reproductive stage, and the genes affected in both mutants have not yet been cloned (Eapen et al., 2003; Saucedo et al., 2012; Salazar-Blas et al., 2017). mizu-kussei 1 (miz1), described by Kobayashi et al. (2007), is caused by a recessive mutation in At2g41660. Apart from a complete absence of hydrotropism and slightly reduced root phototropism and waving, miz1 plants grow normally and in particular show a normal gravitropism response and root tip anatomy (Kobayashi et al., 2007). Overexpression of MIZ1 leads to increased root curvature in hydrotropism assays (Miyazawa et al., 2012). Unfortunately, MIZ1 is a protein of unknown function, containing only a conserved domain of uncharacterized function (DUF617 domain). Homologues containing a DUF617 domain have been found in rice and Physcomitrella patens but not in algae, suggesting that acquisition of MIZ1 function may have taken place during the evolution of land plants (Kobayashi et al., 2007). It is still unclear at which step of the hydrotropism response MIZ1 functions, but subcellular localization of MIZ1 showed that it is a soluble protein associated with the cytosolic side of the endoplasmatic reticulum (ER) membrane (Yamazaki et al., 2012). ABA and blue light are both able to up-regulate MIZ1 expression (Moriwaki et al., 2012). MIZ1 itself appears to influence auxin accumulation, as free IAA concentrations in miz1 and MIZ1-overexpressing roots increase and decrease, respectively (Moriwaki et al., 2011). Whether this is directly linked to the role of MIZ1 in hydrotropism is unclear, and overexpression or loss of MIZ1 function do not affect PIN gene expression and localization (Moriwaki et al., 2011). A MIZ1–green fluoprescent protein (GFP) fusion under the control of its own promoter has shown that the protein is strongly expressed in cortex cells around the transition zone between the meristem and elongation zone, the lateral root cap and columella, and also, to a lesser extent, in the epidermis and stele, but MIZ1–GFP intensity and localization do not change during the hydrotropic response (Yamazaki et al., 2012; Moriwaki et al., 2013). Recently, it was shown that expression of MIZ1 in the cortex alone is able to rescue the hydrotropism response of miz1 mutants, highlighting the important role of this tissue in hydrotropism (Dietrich et al., 2017) (Fig. 2). A second mutant isolated through forward screens, miz2, is a weak GNOM allele (G951E) (Miyazawa et al., 2009b). GNOM is a GDP/GTP exchange factor for small G proteins of the ARF class (ARF-GEF) regulating intracellular vesicle trafficking, whose best characterized function is polar targeting of PIN proteins to the plasma membrane (Geldner et al., 2003). Importantly, miz2 does not affect auxin response or PIN localization (Miyazawa et al., 2009a, b ). The G951E mutation of miz2 is downstream of the Sec7 domain and affects an amino acid conserved in GNOM homologues in other plant species (Miyazawa et al., 2009b). Treatment with BFA, a known inhibitor of ARF-GEFs, phenocopies miz2. In addition, the hydrotropic response of the BFA-resistant GNM696L allele cannot be blocked by BFA, whereas the weak gnomB/E allele is ahydrotropic (Miyazawa et al., 2009b). Supporting evidence of the importance of vesicle trafficking for hydrotropism comes from a phospholipase D mutant that is slightly impaired in hydrotropism (Taniguchi et al., 2010). There appears to be no direct interaction between MIZ2 and MIZ1, as MIZ1–GFP is still correctly localized in the miz2 mutant (Moriwaki et al., 2011). Interestingly though, miz2 plants that overexpress MIZ1 show an ahydrotropic phenotype, demonstrating that MIZ2 is epistatic to MIZ1 (Miyazawa et al., 2012). Where and how do roots sense water? Which part of the root is able to sense a gradient in water availability is a question that has fascinated people since the early days of hydrotropism research. The Darwins describe experiments where covering the root tip with a mixture of olive oil and lamp black abolishes the hydrotropic response, concluding that the very root tip is necessary for the perception of gravity and water. This led them to coin the, since then much repeated, description of the root tip as the ‘brain of the root’ (Darwin and Darwin, 1880). Darwin’s contemporaries already criticized those experiments, especially with regards to the effect of the applied mixture on root growth rates and the difficulties in applying the mixture in an even manner and to a precise region of the root (Wiesner, 1881; Molisch, 1883). Similar problems affect more recent experiments. A role for the root cap in hydrotropism perception was reported for pea and maize, but root growth rates were not always recorded (Takahashi and Scott, 1991, 1993; Takahashi and Suge, 1991; Takano et al., 1995; Hirasawa et al., 1997). In addition, while surgical ablation experiments record the length of root tip removed, usually no relationship to anatomical markers along the root axis is given and it is therefore difficult to know whether just the columella or larger parts, including the meristem or perhaps even the elongation zone, were removed. Miyazawa et al. (2008) used heavy-ion microbeam irradiation and laser ablation to ablate either the columella or what is described as the elongation zone of A. thaliana roots, and reported conflicting results. While irradiation of the elongation zone led to a reduction in hydrotropic bending, the same treatment of the columella did not (Miyazawa et al., 2008). On the other hand, laser ablation of the columella did reduce the hydrotropic response (Miyazawa et al., 2008). However, root growth rates following both treatments were extremely slow, so that these results have to be considered with caution. More recently, laser ablation and microdissection were again used to determine the root tissue responsible for hydrotropism perception in A. thaliana. Root growth rates were reported for these experiments and were in the expected range. Whereas laser ablation of the columella inhibited the gravitropic response as reported by Blancaflor et al. (1998), the hydrotropic response was not perturbed (Dietrich et al., 2017). Removal of the root cap and meristem by either laser ablation or microdissection also did not inhibit hydrotropism, demonstrating that the elongation zone of the root is able to perceive and respond to the hydrotropic signal (Dietrich et al., 2017) (Fig. 2). While some contribution from the columella and root cap in hydrotropism perception cannot be totally excluded, these results place perception for hydro- and gravitropism in separate tissues. In addition, removal of the columella in rice and cucumber does not impair hydrotropism, demonstrating that in other plant species hydrotropism perception also does not depend on this tissue (Morohashi et al., 2017; Nakajima et al., 2017; Fujii et al., 2018). How could roots be able to sense a water potential gradient in the elongation zone? The difference in water potential across the root is rather small, and was calculated to reach a maximum of <10 kPa across a 100 µm wide A. thaliana root during a standard split-agar hydrotropism assay, which is <3% of the maximum absolute water potential experienced at the root midline (Dietrich et al., 2017). Mechanosensitive ion channels could potentially be triggered by changes in cell volume if a root is exposed to a water potential gradient (Hamilton et al., 2015). Pea is the only plant species where turgor measurements have been performed during hydrotropism, but differences in turgor between the wet and dry side of the root were not observed (Hirasawa et al., 1997; Miyamoto et al., 2002). The miz2 (GNG951E) phenotype strongly implies that membrane proteins play an important part in the hydrotropism response (Miyazawa et al., 2009b). Although GNOM is best known for its role in endosomal recycling of PIN proteins (Geldner et al., 2003), localization of PIN1 is unaffected in miz2 (Miyazawa et al., 2009a). Hydrotropism may rely on endosomal recycling of other proteins, trafficking from the ER to the Golgi or endocytosis, processes that also rely on GNOM (Paez Valencia et al., 2016). It is highly likely that hydrotropism is intricately linked to water uptake and transport in the root. Radial water uptake from the soil towards the xylem vessels in the vasculature follows two paths, the apoplastic route along cell walls and the cell-to-cell path that is comprised of transcellular (across membranes) and symplastic (through plasmodesmata) transport (Li et al., 2014). Root hydraulic conductivity (Lpr) is a measure for water transported through the root. Aquaporins are membrane channels that transport water and small neutral molecules, and one subfamily, the plasma membrane intrinsic proteins (PIPs), contributes significantly to Lpr (Sutka et al., 2011; Li et al., 2014). How could aquaporins and changes in Lpr contribute to hydrotropic signalling? Lpr is reduced by abiotic stress in many plant species (Aroca et al., 2012). PIP activity is regulated at many levels—transcriptionally, translationally, through gating of the channel itself by phosphorylation, protons, or divalent cations, and by cellular trafficking (Li et al., 2014)—and reduction of root hydraulic conductivity under abiotic stress could be achieved using any of these regulatory mechanisms. For salt stress, it was demonstrated that treatment with 100 mM sodium chloride reduces Lpr by ~60% within 1 h and decreases aquaporin transcript abundance (Boursiac et al., 2005). Down-regulation of aquaporin gene expression, however, takes longer than the decrease in Lpr, but other regulation mechanisms respond more rapidly to salt stress. At 45 min after the start of salt treatment, a substantial amount of a PIP2;1–GFP fusion protein had become internalized, and removal from the plasma membrane involved clathrin and membrane raft-associated pathways (Boursiac et al., 2008; Li et al., 2011). Another pathway for removal of aquaporins from the plasma membrane involves tryptophan-rich sensory protein/translocator (TSPO), which is induced by abiotic stress and was shown to interact with PIP2;7, leading to internalization and autophagic degradation of the aquaporin (Hachez et al., 2014). In addition, PIP1;2 and PIP2;1 were recently shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane and were regulated in their water transporting activity by this interaction (Bellati et al., 2016). Similar to the examples for salt and TSPO regulating PIPs at the plasma membrane, the low water potential during hydrotropism could affect the presence in the membrane of aquaporins through endosomal recycling, which would explain the requirement for MIZ2/GNOM (Fig. 3). The interaction of aquaporins with RLKs could also be affected by low water potential, possibly leading to changes in cell elongation through signalling via the RLKs in addition to regulation of PIP activity by the RLKs (Fig. 3). These hypothetical regulation mechanisms of aquaporin activity or membrane presence could lead to a change in hydraulic conductivity, with two possible outcomes. Cell or tissue growth could be affected by changes in Lpr, as was demonstrated for lateral root primordia emergence (Péret et al., 2012). Alternatively, hydraulic conductivity was shown to affect radial ABA transport along the apoplastic pathway through solvent drag (Freundl et al., 1998). This could lead to changes in ABA concentration on the dry and wet side of the root, driving differential cell elongation. Fig. 3. View largeDownload slide Potential mechanisms for perception and response to low water potential. Low water potential could affect the membrane presence or activity of plasma membrane intrinsic proteins (PIPs). This could affect cell elongation through several independent pathways: PIPs were shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane. This interaction was shown to regulate PIP activity, but could potentially also affect signalling from the RLK to change cell elongation. Changes in aquaporin activity or presence due to low water potential will also lead to a change in hydraulic conductivity, with two possible outcomes. Hydraulic conductivity could affect cell elongation directly (as demonstrated for lateral root primordia), but can also affect radial ABA transport in the root. Changes in local ABA concentration could be the driver of differential cell elongation, leading ultimately to root bending. Perception would not necessarily require sensing of a water potential gradient at opposing sides of the root, but could work through a water potential set point, below which PIP membrane presence or activity changes, initiating the signal cascade leading to cell elongation. MIZ2/GNOM is required to facilitate cycling of PIPs (and RLKs) to and from the plasma membrane in this model. Aquaporin regulation in a single layer or all tissue layers of the root may be necessary for this mechanism. Fig. 3. View largeDownload slide Potential mechanisms for perception and response to low water potential. Low water potential could affect the membrane presence or activity of plasma membrane intrinsic proteins (PIPs). This could affect cell elongation through several independent pathways: PIPs were shown to interact directly with receptor-like kinases (RLKs) in the plasma membrane. This interaction was shown to regulate PIP activity, but could potentially also affect signalling from the RLK to change cell elongation. Changes in aquaporin activity or presence due to low water potential will also lead to a change in hydraulic conductivity, with two possible outcomes. Hydraulic conductivity could affect cell elongation directly (as demonstrated for lateral root primordia), but can also affect radial ABA transport in the root. Changes in local ABA concentration could be the driver of differential cell elongation, leading ultimately to root bending. Perception would not necessarily require sensing of a water potential gradient at opposing sides of the root, but could work through a water potential set point, below which PIP membrane presence or activity changes, initiating the signal cascade leading to cell elongation. MIZ2/GNOM is required to facilitate cycling of PIPs (and RLKs) to and from the plasma membrane in this model. Aquaporin regulation in a single layer or all tissue layers of the root may be necessary for this mechanism. These hypothetical perception mechanisms linked to aquaporins would not necessarily require sensing of the water potential gradient at opposing sides of the root, but could utilize a water potential set point, below which aquaporin membrane presence or activity changes, setting in motion the signalling cascade leading to cell elongation. However, at the moment, the identity of the hydrotropic signal perceived by the root is still unclear. Interaction between hydro- and gravitropism Several tropisms can adjust the growth direction of the root tip, and interaction and competition between the responses to different environmental cues will determine the final growth direction. To understand hydrotropism, its interaction with gravitropism is central. It has been argued that gravitropism determines the ‘default’ growth direction of the root, which is then adjusted by tropic responses to other environmental cues (Blancaflor and Masson, 2003; Rosquete and Kleine-Vehn, 2013; Krieger et al., 2016). In the interaction between hydro- and gravitropism, a clear distinction has to be drawn between plant species that depend on auxin and its transport for their hydrotropic response and those where hydrotropism is independent of development of a lateral auxin gradient. In those plant species which require auxin transport, the gravitropic response can be assumed to counteract hydrotropism, unless the water potential gradient aligns with the gravity vector. This would explain why hydrotropism in pea and cucumber can only be observed if gravitropism has been removed. Still, there are plants (e.g. rice), that rely on auxin transport for both tropisms but react to a water gradient in the presence of gravity. How can such differences be explained? It is still unclear whether plant species requiring auxin for hydrotropism develop a lateral auxin gradient during the response. If they do, species-specific differences in the interaction between gravi- and hydrotropism could be due to differences in the establishment of these auxin gradients. Another factor influencing the interaction between the two tropisms could be timing and sensitivity of each response. Presentation time, defined as the minimum exposure time needed to elicit a response, has been determined for the gravitropism response of various plant roots (Kiss and Sack, 1989; Kiss et al., 1996; Hou et al., 2003). Usually, root curvature in response to a 90° stimulus is plotted against stimulation time and the presentation time determined by regression analysis (Kiss et al., 1996). For hydrotropism, the presentation time has so far only been determined for ageotropum peas following the method described for gravitropism (Stinemetz et al., 1996). Equally, data on the strength of the water potential gradient necessary for triggering hydrotropism are scarce (Takano et al., 1995). Natural variation has been reported to exist for gravitropic presentation times (Tanimoto et al., 2008; Moulia and Fournier, 2009), and a more detailed examination of presentation times and response strength for both hydro- and gravitropism should help to understand species-specific differences in the interaction between those tropisms. In A. thaliana, hydrotropism is independent of the development of a lateral auxin gradient (Shkolnik et al., 2016). Plants treated with auxin transport and response inhibitors (Shkolnik et al., 2016) and the pgm1 mutant which lacks statoliths (Takahashi et al., 2003) show a faster hydrotropic response. Together with the observation that statolith degradation occurs in roots exposed to a water potential gradient in A. thaliana and Raphanus sativus (Takahashi et al., 2003; Ponce et al., 2008a), this has been taken as evidence to support the hypothesis that gravitropic responsiveness needs to be reduced so that hydrotropism can take place. In contrast, exposure of roots to 150 mM sodium chloride leads to agravitropic growth and degradation of statoliths, but several salt overly sensitive mutants, which display the same agravitropic growth on medium with salt, retain their statoliths (Sun et al., 2008). This indicates that statolith degradation on exposure to environmental stress may be a mere correlation and not causative for the response. In addition, the agravitropic pin2 and aux1 mutants do not have an accelerated hydrotropic response (Takahashi et al., 2002). Detailed analysis of the kinetics of gravitropism shows that the rate of gravitropic root bending in A. thaliana depends on the stimulation angle, with smaller stimulation angles resulting in reduced bending rates (Mullen et al., 2000). In addition, a threshold angle of 15° from the vertical has to be reached before 50% of a population of seedlings respond gravitropically (Mullen et al., 2000). Therefore, a water potential gradient can lead to a substantial change in root angle before a gravity response is triggered. Furthermore, this gravitropic response will be slow to begin with, as the stimulation angle is small. Recently, a study investigated the interaction between hydro- and gravitropism and the role of reactive oxygen species (ROS) (Krieger et al., 2016). A very interesting observation of this study was that hydro- and gravitropism lead to bending of the root tip in different regions, with gravitropic bending initiating relatively close to the root tip in the distal elongation zone whereas hydrotropic bending takes place in a more shootward region of the elongation zone (central elongation zone) (Krieger et al., 2016), which provides further confirmation that gravitropism and hydrotropism employ different tissues in their bending mechanisms. Auxin-induced ROS production is necessary for gravitropism (Joo et al., 2001), and using the fluorescent dye dihydrorhodamine-123, Krieger et al. (2016) demonstrated that 2 h after gravistimulation a transient ROS increase was visible on the concave side of the distal elongation zone of the bending root. Using the moisture in air assay, a ROS increase was observed on the concave side of the central elongation zone of hydrotropically bending roots (Krieger et al., 2016). However, when calcium chloride was replaced with distilled water in the assay (i.e. under conditions that do not induce hydrotropic bending in roots), a similar ROS increase in the same location was observed (Krieger et al., 2016). ROS distribution, however, was unchanged when hydrotropism was induced in roots using the split-agar assay, and the authors attribute the spurious ROS accumulation in the moisture in air assay to the mechanical tension the roots were under (Krieger et al., 2016). Treatment with ROS scavengers and NADPH oxidase inhibitors showed that ROS production in fact inhibited hydrotropism (Krieger et al., 2016). Ascorbate peroxidase (apx1-2) and respiratory burst oxidase homologue (rbohC) mutants showed decreased and increased hydrotropic curvature, respectively, further confirming the inhibition of hydrotropism by ROS (Krieger et al., 2016). How ROS inhibit hydrotropism is currently unknown. Interestingly though, the same study also showed that after 4 h of hydrostimulation, a 90° gravitropic stimulus was unable to elicit an increase in ROS or a lateral auxin gradient (Krieger et al., 2016). Clearly more work is still necessary to understand exactly how hydro- and gravitropism interact, but this is an exciting first glimpse showing that hydrotropism is able to influence the gravitropic response. Can hydrotropism improve drought acclimation? Drought stress is a major limiting factor in crop production, and complex plant responses exist to escape, avoid, or tolerate limited water availability (Wery, 2005; Gaur et al., 2008). Drought can lead to an increase in the root to shoot ratio of plants, usually due to shoot growth being more strongly affected by drought (Blum, 2005), and maintaining yield under drought conditions can be linked to a well-developed root system, particularly in those regions of the soil still containing water (Comas et al., 2013). Irrigation is used to prevent drought stress in crops, and agriculture uses 70% of globally available freshwater, mostly for this purpose (Du et al., 2015; World Water Assessment Programme, 2015). Climate change, however, will make water availability more unpredictable, with increased likelihoods for extreme weather events and changes in rainfall patterns (IPCC, 2014). A variety of strategies are pursued to make agricultural water use more sustainable and ‘produce more crop per drop’ (Morison et al., 2008; Du et al., 2015). How could hydrotropism, which allows roots to forage for water in soil, contribute to this? Conservation tillage, which minimizes the amount of soil disturbance, increases soil water availability through improved physical soil properties, increased organic matter, and reduced evaporation due to crop residue left on the surface (Triplett and Dick, 2008), and is now widely adopted in many rain-fed agriculture systems (Brunel et al., 2013; Peiretti and Dumanski, 2014). Currently little is known about water distribution in soils under conservation tillage, but water may be more heterogeneously distributed than under conventional tillage, which would make crops with an increased hydrotropism response more efficient. For agricultural systems using irrigation, deficit and partial root zone drying (PRD) irrigation systems have been demonstrated to increase the water use efficiency in a number of crops (Kang and Zhang, 2004). In these systems, less water than is needed to cover evapotranspiration demand is supplied, sometimes only to part of the root system (PRD). As a result, plants produce less shoot biomass and decrease stomatal conductance, whilst still producing similar or slightly reduced yields compared with fully irrigated crops. Under PRD, it is thought that the drying part of the root system produces a signal that regulates stomatal conductance, whereas the irrigated part supplies the shoot with sufficient water to produce the crop (Kang and Zhang, 2004; Sobeih et al., 2004). The applied effects of PRD on plant growth have been extensively studied, and are reviewed elsewhere (Kang and Zhang, 2004). For root growth, it was shown that PRD leads to an increase in tomato root dry weight, particularly in those parts of the root system that were rewatered after a previous drying period (Mingo et al., 2004), and an increase in the root surface area of maize (Zhenchang et al., 2016). That hydrotropism can contribute to directional root growth in soil has been demonstrated for A. thaliana grown in soil microcosms with a lateral water gradient (Iwata et al., 2013). Plants showed increased root growth in the area with higher water content (Iwata et al., 2013). This behaviour was dependent on a functioning hydrotropism response, as plants overexpressing MIZ1 had an increased tendency to grow roots in soil with high water content, whereas miz1 plants grew roots in a random fashion, unrelated to water distribution in the soil (Iwata et al., 2013). In maize, a recent study tried to link hydrotropic responsiveness to yield under PRD irrigation and drought (Eapen et al., 2017). The hydrotropic response of part of a collection of maize hybrid lines from the Drought Tolerance Maize for Africa project was analysed at 4 d after germination, and representative lines with strong and weak hydrotropic responses were tested in field trials (Eapen et al., 2017). Although one line with a strong hydrotropic response showed increased yield under PRD irrigation and drought stress, the results were more ambiguous for other lines (Eapen et al., 2017). Interestingly, however, there seemed to be a stronger correlation between root weight and grain yield in the lines with a strong hydrotropic response compared with those lines which only weakly responded to the stimulus (Eapen et al., 2017). Root biomass and root system architecture traits might have been confounding factors in this study, and highlight the need for rigorous experimental design when assessing the contribution of hydrotropism to crop performance. These new developments are an indication that crops with an improved hydrotropic response could be beneficial in agricultural systems using conservation tillage or deficit/PRD irrigation systems, contributing to improved water use efficiency. Conclusions and future directions Hydrotropism research has taken a leap forward in the last few years with a number of discoveries describing the site of perception, bending mechanism, and interaction with gravitropism. Hydrotropism has now been shown to exist in an increasing number of plant species and, interestingly, species-specific mechanistic differences in the response exist. New techniques will allow us to understand this tropism and how it contributes to water uptake and drought responses in plants. Although progress has been made in understanding hydrotropism, many more questions still remain open. Most importantly, it is still unclear what the water signal is and how it is perceived. With the recent discovery that the columella may not be necessary for hydrotropism and that the signal can be perceived by the elongation zone of A. thaliana (Dietrich et al., 2017), the pool of potential candidates for hydrotropism perception has widened and changed again. Development of sensors for calcium, ABA, pH, ROS, and other signalling molecules has improved dramatically over recent years (Nagai et al., 2004; Jones et al., 2014; Waadt et al., 2014; Krieger et al., 2016), but these sensors may still not be sensitive enough to detect changes during hydrotropism. It might be necessary to determine the signal indirectly, and a better understanding of hydrotropism response kinetics may help in this respect. The presentation time for the hydrotropic signal has been determined so far only for pea, and water potential gradients used in assays are usually chosen on the basis of returning the maximum response without affecting root growth (Stinemetz et al., 1996; Takahashi et al., 2002). A systematic evaluation of presentation times and the strength of the water potential gradient needed to trigger the response may inform the search for the elusive water signal. New developments in microfluidic devices now allow precise delivery of stimuli at high spatial and temporal resolution, and will be instrumental in determining these parameters (Meier et al., 2010; Stanley et al., 2018). Once the signal for hydrotropism has been found, it should be easier to connect genes known to be involved in hydrotropism (e.g. MIZ1, MIZ2/GNOM, and ABA genes) to the signal transduction pathway of the hydrotropic response. Alternatively, a reverse approach could be used, starting from these known components to search for interaction partners that are specific to hydrotropism. In A. thaliana, the cortex tissue has been shown to play an important role in hydrotropism, with evidence that differential elongation in this tissue drives the bending response (Dietrich et al., 2017). Other plant species in which hydrotropism has been observed have a cortex consisting of multiple cell layers, and it will be interesting to see whether hydrotropic bending in those species uses the same mechanistic principle to drive the bending response. The interaction of hydrotropism with other tropisms, gravitropism in particular, is another area of great interest. For those plant species that require auxin transport for hydrotropism, it will be important to determine whether a lateral auxin gradient develops during the response and how such a gradient is affected by gravitropism. Mathematical modelling has provided new insights into gravitropism (Swarup et al., 2005; Band et al., 2012) and has been used to investigate the bending response in hydrotropism (Dietrich et al., 2017). Development of new models that combine hydro- and gravitropic responses will be an important part of understanding how these tropisms interact and direct root tip growth angles. Until now, all hydrotropism experiments have been performed on primary roots of plants. Wiesner and Molisch already observed that lateral roots grow more easily in the direction of water than primary roots (Wiesner, 1881; Molisch, 1883). Hydrotropism research needs to extend its scope and investigate the response of lateral roots. Lateral roots, which have a different gravitropic set point angle and are therefore less responsive to gravity than primary roots (Roychoudhry et al., 2013, 2017), are in theory more responsive to water potential gradients. Lateral roots make up the majority of any plant root system and, although hydrotropism assays for lateral roots will be technically more difficult, these should give us a better appreciation of whether hydrotropism is able to increase water uptake. Ultimately, hydrotropic responses will have to be assessed in soil. Methods now exist that allow the visualization of roots and water in soil and to compute water fluxes into the root (Daly et al., 2015, 2018). Development of a hydrotropic assay in soil will be a necessity to understand the true contribution of this tropism to water uptake and drought acclimation in plants. Acknowledgements The author would like to thank Malcolm J. Bennett and Darren M. Wells for discussion of the manuscript. This work was supported by the Leverhulme Trust [grant no. RPG-2016-409]. References Antoni R , Dietrich D , Bennett MJ , Rodriguez PL . 2016 . Hydrotropism: analysis of the root response to a moisture gradient . Methods in Molecular Biology 1398 , 3 – 9 . Google Scholar CrossRef Search ADS Antoni R , Gonzalez-Guzman M , Rodriguez L et al. 2013 . PYRABACTIN RESISTANCE1-LIKE8 plays an important role for the regulation of abscisic acid signaling in root . Plant Physiology 161 , 931 – 941 . Google Scholar CrossRef Search ADS Aroca R , Porcel R , Ruiz-Lozano JM . 2012 . Regulation of root water uptake under abiotic stress conditions . Journal of Experimental Botany 63 , 43 – 57 . Google Scholar CrossRef Search ADS Band LR , Wells DM , Larrieu A et al. 2012 . Root gravitropism is regulated by a transient lateral auxin gradient controlled by a tipping-point mechanism . Proceedings of the National Academy of Sciences, USA 109 , 4668 – 4673 . Google Scholar CrossRef Search ADS Bellati J , Champeyroux C , Hem S , Rofidal V , Krouk G , Maurel C , Santoni V . 2016 . Novel aquaporin regulatory mechanisms revealed by interactomics . Molecular and Cellular Proteomics 15 , 3473 – 3487 . Google Scholar CrossRef Search ADS Blancaflor EB , Fasano JM , Gilroy S . 1998 . Mapping the functional roles of cap cells in the response of Arabidopsis primary roots to gravity . Plant Physiology 116 , 213 – 222 . Google Scholar CrossRef Search ADS Blancaflor EB , Masson PH . 2003 . Plant gravitropism. Unraveling the ups and downs of a complex process . Plant Physiology 133 , 1677 – 1690 . Google Scholar CrossRef Search ADS Blum A . 2005 . Drought resistance, water-use efficiency, and yield potential—are they compatible, dissonant, or mutually exclusive ? Australian Journal of Agricultural Research 56 , 1159 – 1168 . Google Scholar CrossRef Search ADS Bonnet C . 1754 . Recherches sur l’usage des feuilles dans les plantes et sur quelques autres sujets relatifs a l’histoire de la vegetation . Goettingen, Leiden : Elie Luzac . Google Scholar CrossRef Search ADS Boursiac Y , Boudet J , Postaire O , Luu DT , Tournaire-Roux C , Maurel C . 2008 . Stimulus-induced downregulation of root water transport involves reactive oxygen species-activated cell signalling and plasma membrane intrinsic protein internalization . The Plant Journal 56 , 207 – 218 . Google Scholar CrossRef Search ADS Boursiac Y , Chen S , Luu DT , Sorieul M , van den Dries N , Maurel C . 2005 . Early effects of salinity on water transport in Arabidopsis roots. Molecular and cellular features of aquaporin expression . Plant Physiology 139 , 790 – 805 . Google Scholar CrossRef Search ADS Brunel N , Seguel O , Acevedo E . 2013 . Conservation tillage and water availability for wheat in the dryland of central Chile . Journal of Soil Science and Plant Nutrition 13 , 622 – 637 . Cassab GI , Eapen D , Campos ME . 2013 . Root hydrotropism: an update . American Journal of Botany 100 , 14 – 24 . Google Scholar CrossRef Search ADS Comas LH , Becker SR , Cruz VM , Byrne PF , Dierig DA . 2013 . Root traits contributing to plant productivity under drought . Frontiers in Plant Science 4 , 442 . Google Scholar CrossRef Search ADS Coutts MP , Nicoll BC . 1993 . Orientation of the lateral roots of trees: II. Hydrotropic and gravitropic responses of lateral roots of Sitka spruce grown in air at different humidities . New Phytologist 124 , 277 – 281 . Google Scholar CrossRef Search ADS Cutler SR , Rodriguez PL , Finkelstein RR , Abrams SR . 2010 . Abscisic acid: emergence of a core signaling network . Annual Review of Plant Biology 61 , 651 – 679 . Google Scholar CrossRef Search ADS Daly KR , Mooney SJ , Bennett MJ , Crout NM , Roose T , Tracy SR . 2015 . Assessing the influence of the rhizosphere on soil hydraulic properties using X-ray computed tomography and numerical modelling . Journal of Experimental Botany 66 , 2305 – 2314 . Google Scholar CrossRef Search ADS Daly KR , Tracy SR , Crout NMJ , Mairhofer S , Pridmore TP , Mooney SJ , Roose T . 2018 . Quantification of root water uptake in soil using X-ray computed tomography and image-based modelling . Plant, Cell and Environment 41 , 121 – 133 . Google Scholar CrossRef Search ADS Darwin C , Darwin F . 1880 . The power of movement in plants . London : John Murray . Google Scholar CrossRef Search ADS Davies WJ , Bennett MJ . 2015 . Achieving more crop per drop . Nature Plants 1 , 15118 . Google Scholar CrossRef Search ADS Dietrich D , Pang L , Kobayashi A et al. 2017 . Root hydrotropism is controlled via a cortex-specific growth mechanism . Nature Plants 3 , 17057 . Google Scholar CrossRef Search ADS Du T , Kang S , Zhang J , Davies WJ . 2015 . Deficit irrigation and sustainable water-resource strategies in agriculture for China’s food security . Journal of Experimental Botany 66 , 2253 – 2269 . Google Scholar CrossRef Search ADS Eapen D , Barroso ML , Campos ME , Ponce G , Corkidi G , Dubrovsky JG , Cassab GI . 2003 . A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis . Plant Physiology 131 , 536 – 546 . Google Scholar CrossRef Search ADS Eapen D , Martínez JJ , Cassab GI . 2015 . Assays for root hydrotropism and response to water stress . In: Blancaflor EB , ed. Plant gravitropism: methods and protocols . New York : Springer New York , 133 – 142 . Google Scholar CrossRef Search ADS Eapen D , Martínez-Guadarrama J , Hernández-Bruno O , Flores L , Nieto-Sotelo J , Cassab GI . 2017 . Synergy between root hydrotropic response and root biomass in maize (Zea mays L.) enhances drought avoidance . Plant Science 265 , 87 – 99 . Google Scholar CrossRef Search ADS Enders TA , Oh S , Yang Z , Montgomery BL , Strader LC . 2015 . Genome sequencing of Arabidopsis abp1-5 reveals second-site mutations that may affect phenotypes . The Plant Cell 27 , 1820 – 1826 . Google Scholar CrossRef Search ADS Eysholdt-Derzsó E , Sauter M . 2017 . Root bending is antagonistically affected by hypoxia and ERF-mediated transcription via auxin signaling . Plant Physiology 175 , 412 – 423 . Google Scholar CrossRef Search ADS Freundl E , Steudle E , Hartung W . 1998 . Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem . Planta 207 , 8 – 19 . Google Scholar CrossRef Search ADS Friml J . 2010 . Subcellular trafficking of PIN auxin efflux carriers in auxin transport . European Journal of Cell Biology 89 , 231 – 235 . Google Scholar CrossRef Search ADS Fujii H , Chinnusamy V , Rodrigues A , Rubio S , Antoni R , Park SY , Cutler SR , Sheen J , Rodriguez PL , Zhu JK . 2009 . In vitro reconstitution of an abscisic acid signalling pathway . Nature 462 , 660 – 664 . Google Scholar CrossRef Search ADS Fujii N , Miyabayashi S , Sugita T et al. 2018 . Root-tip-mediated inhibition of hydrotropism is accompanied with the suppression of asymmetric expression of auxin-inducible genes in response to moisture gradients in cucumber roots . PLoS One 13 , e0189827 . Google Scholar CrossRef Search ADS Fujii H , Verslues PE , Zhu JK . 2007 . Identification of two protein kinases required for abscisic acid regulation of seed germination, root growth, and gene expression in Arabidopsis . The Plant Cell 19 , 485 – 494 . Google Scholar CrossRef Search ADS Galvan-Ampudia CS , Julkowska MM , Darwish E , Gandullo J , Korver RA , Brunoud G , Haring MA , Munnik T , Vernoux T , Testerink C . 2013 . Halotropism is a response of plant roots to avoid a saline environment . Current Biology 23 , 2044 – 2050 . Google Scholar CrossRef Search ADS Gao Y , Lynch JP . 2016 . Reduced crown root number improves water acquisition under water deficit stress in maize (Zea mays L.) . Journal of Experimental Botany 67 , 4545 – 4557 . Google Scholar CrossRef Search ADS Gao Y , Zhang Y , Zhang D , Dai X , Estelle M , Zhao Y . 2015 . Auxin binding protein 1 (ABP1) is not required for either auxin signaling or Arabidopsis development . Proceedings of the National Academy of Sciences, USA 112 , 2275 – 2280 . Google Scholar CrossRef Search ADS Gaur PM , Krishnamurthy L , Kashiwagi J . 2008 . Improving drought-avoidance root traits in chickpea (Cicer arietinum L.)—current status of research at ICRISAT . Plant Production Science 11 , 3 – 11 . Google Scholar CrossRef Search ADS Geldner N , Anders N , Wolters H , Keicher J , Kornberger W , Muller P , Delbarre A , Ueda T , Nakano A , Jürgens G . 2003 . The Arabidopsis GNOM ARF-GEF mediates endosomal recycling, auxin transport, and auxin-dependent plant growth . Cell 112 , 219 – 230 . Google Scholar CrossRef Search ADS Gilroy S , Masson PH , eds. 2008 . Plant tropisms . Oxford : Blackwell Publishing Ltd . Hachez C , Veljanovski V , Reinhardt H , Guillaumot D , Vanhee C , Chaumont F , Batoko H . 2014 . The Arabidopsis abiotic stress-induced TSPO-related protein reduces cell-surface expression of the aquaporin PIP2;7 through protein–protein interactions and autophagic degradation . The Plant Cell 26 , 4974 – 4990 . Google Scholar CrossRef Search ADS Hamilton ES , Schlegel AM , Haswell ES . 2015 . United in diversity: mechanosensitive ion channels in plants . Annual Review of Plant Biology 66 , 113 – 137 . Google Scholar CrossRef Search ADS Hayashi K , Neve J , Hirose M , Kuboki A , Shimada Y , Kepinski S , Nozaki H . 2012 . Rational design of an auxin antagonist of the SCF(TIR1) auxin receptor complex . ACS Chemical Biology 7 , 590 – 598 . Google Scholar CrossRef Search ADS Hayashi K-I , Tan X , Zheng N , Hatate T , Kimura Y , Kepinski S , Nozaki H . 2008 . Small-molecule agonists and antagonists of F-box protein–substrate interactions in auxin perception and signaling . Proceedings of the National Academy of Sciences, USA 105 , 5632 – 5637 . Google Scholar CrossRef Search ADS Henry A , Gowda VRP , Torres RO , McNally KL , Serraj R . 2011 . Variation in root system architecture and drought response in rice (Oryza sativa): phenotyping of the OryzaSNP panel in rainfed lowland fields . Field Crops Research 120 , 205 – 214 . Google Scholar CrossRef Search ADS Hirasawa T , Takahashi H , Suge H , Ishihara K . 1997 . Water potential, turgor and cell wall properties in elongating tissues of the hydrotropically bending roots of pea (Pisum sativum L) . Plant, Cell and Environment 20 , 381 – 386 . Google Scholar CrossRef Search ADS Hooker HD . 1915 . Hydrotropism in roots of Lupinus albus . Annals of Botany 29 , 265 – 283 . Google Scholar CrossRef Search ADS Hou G , Mohamalawari DR , Blancaflor EB . 2003 . Enhanced gravitropism of roots with a disrupted cap actin cytoskeleton . Plant Physiology 131 , 1360 – 1373 . Google Scholar CrossRef Search ADS IPCC . 2014 . Climate change 2014: synthesis report. Contribution of working groups I, II and II to the fifth assessment report of the intergovernmental panel on climate change . Core Writing Team , Pachauri RK , Meyer LA , eds. Geneva, Switzerland : IPCC . Iwata S , Miyazawa Y , Fujii N , Takahashi H . 2013 . MIZ1-regulated hydrotropism functions in the growth and survival of Arabidopsis thaliana under natural conditions . Annals of Botany 112 , 103 – 114 . Google Scholar CrossRef Search ADS Jaffe MJ , Takahashi H , Biro RL . 1985 . A pea mutant for the study of hydrotropism in roots . Science 230 , 445 – 447 . Google Scholar CrossRef Search ADS Jones AM , Danielson JA , Manojkumar SN , Lanquar V , Grossmann G , Frommer WB . 2014 . Abscisic acid dynamics in roots detected with genetically encoded FRET sensors . eLife 3 , e01741 . Joo JH , Bae YS , Lee JS . 2001 . Role of auxin-induced reactive oxygen species in root gravitropism . Plant Physiology 126 , 1055 – 1060 . Google Scholar CrossRef Search ADS Kaneyasu T , Kobayashi A , Nakayama M , Fujii N , Takahashi H , Miyazawa Y . 2007 . Auxin response, but not its polar transport, plays a role in hydrotropism of Arabidopsis roots . Journal of Experimental Botany 58 , 1143 – 1150 . Google Scholar CrossRef Search ADS Kang S , Zhang J . 2004 . Controlled alternate partial root-zone irrigation: its physiological consequences and impact on water use efficiency . Journal of Experimental Botany 55 , 2437 – 2446 . Google Scholar CrossRef Search ADS Kiss JZ , Sack FD . 1989 . Reduced gravitropic sensitivity in roots of a starch-deficient mutant of Nicotiana sylvestris . Planta 180 , 123 – 130 . Google Scholar CrossRef Search ADS Kiss JZ , Wright JB , Caspar T . 1996 . Gravitropism in roots of intermediate-starch mutants of Arabidopsis . Physiologia Plantarum 97 , 237 – 244 . Google Scholar CrossRef Search ADS Knight T . 1811 . On the causes which influence the direction of the growth of roots . Philosophical Transactions of the Royal Society of London 101 , 209 – 219 . Google Scholar CrossRef Search ADS Kobayashi A , Takahashi A , Kakimoto Y , Miyazawa Y , Fujii N , Higashitani A , Takahashi H . 2007 . A gene essential for hydrotropism in roots . Proceedings of the National Academy of Sciences, USA 104 , 4724 – 4729 . Google Scholar CrossRef Search ADS Krieger G , Shkolnik D , Miller G , Fromm H . 2016 . Reactive oxygen species tune root tropic responses . Plant Physiology 172 , 1209 – 1220 . Li G , Santoni V , Maurel C . 2014 . Plant aquaporins: roles in plant physiology . Biochimica et Biophysica Acta 1840 , 1574 – 1582 . Google Scholar CrossRef Search ADS Li X , Wang X , Yang Y , Li R , He Q , Fang X , Luu DT , Maurel C , Lin J . 2011 . Single-molecule analysis of PIP2;1 dynamics and partitioning reveals multiple modes of Arabidopsis plasma membrane aquaporin regulation . The Plant Cell 23 , 3780 – 3797 . Google Scholar CrossRef Search ADS Loomis WE , Ewan LM . 1936 . Hydrotropic responses of roots in soil . Botanical Gazette 97 , 728 – 743 . Google Scholar CrossRef Search ADS Lynch JP . 2013 . Steep, cheap and deep: an ideotype to optimize water and N acquisition by maize root systems . Annals of Botany 112 , 347 – 357 . Google Scholar CrossRef Search ADS Ma Y , Szostkiewicz I , Korte A , Moes D , Yang Y , Christmann A , Grill E . 2009 . Regulators of PP2C phosphatase activity function as abscisic acid sensors . Science 324 , 1064 – 1068 . Meier M , Lucchetta EM , Ismagilov RF . 2010 . Chemical stimulation of the Arabidopsis thaliana root using multi-laminar flow on a microfluidic chip . Lab on a Chip 10 , 2147 – 2153 . Google Scholar CrossRef Search ADS Mingo DM , Theobald JC , Bacon MA , Davies WJ , Dodd IC . 2004 . Biomass allocation in tomato (Lycopersicon esculentum) plants grown under partial rootzone drying: enhancement of root growth . Functional Plant Biology 31 , 971 – 978 . Google Scholar CrossRef Search ADS Miyamoto N , Ookawa T , Takahashi H , Hirasawa T . 2002 . Water uptake and hydraulic properties of elongating cells in hydrotropically bending roots of Pisum sativum L . Plant and Cell Physiology 43 , 393 – 401 . Google Scholar CrossRef Search ADS Miyazawa Y , Ito Y , Moriwaki T , Kobayashi A , Fujii N , Takahashi H . 2009a. A molecular mechanism unique to hydrotropism in roots . Plant Science 177 , 297 – 301 . Google Scholar CrossRef Search ADS Miyazawa Y , Moriwaki T , Uchida M , Kobayashi A , Fujii N , Takahashi H . 2012 . Overexpression of MIZU-KUSSEI1 enhances the root hydrotropic response by retaining cell viability under hydrostimulated conditions in Arabidopsis thaliana . Plant and Cell Physiology 53 , 1926 – 1933 . Google Scholar CrossRef Search ADS Miyazawa Y , Sakashita T , Funayama T et al. 2008 . Effects of locally targeted heavy-ion and laser microbeam on root hydrotropism in Arabidopsis thaliana . Journal of Radiation Research 49 , 373 – 379 . Google Scholar CrossRef Search ADS Miyazawa Y , Takahashi A , Kobayashi A , Kaneyasu T , Fujii N , Takahashi H . 2009b. GNOM-mediated vesicular trafficking plays an essential role in hydrotropism of Arabidopsis roots . Plant Physiology 149 , 835 – 840 . Google Scholar CrossRef Search ADS Mizuno H , Kobayashi A , Fujii N , Yamashita M , Takahashi H . 2002 . Hydrotropic response and expression pattern of auxin-inducible gene, CS-IAA1, in the primary roots of clinorotated cucumber seedlings . Plant and Cell Physiology 43 , 793 – 801 . Google Scholar CrossRef Search ADS Molisch H . 1883 . Untersuchungen ueber den Hydrotropismus. Sitzungsberichte k. k . Akademie Wien 88 , 897 – 943 . Monshausen GB , Gilroy S . 2009 . The exploring root–root growth responses to local environmental conditions . Current Opinion in Plant Biology 12 , 766 – 772 . Google Scholar CrossRef Search ADS Morison JI , Baker NR , Mullineaux PM , Davies WJ . 2008 . Improving water use in crop production . Philosophical Transactions of the Royal Society B: Biological Sciences 363 , 639 – 658 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Fujii N , Takahashi H . 2012 . Light and abscisic acid signalling are integrated by MIZ1 gene expression and regulate hydrotropic response in roots of Arabidopsis thaliana . Plant, Cell and Environment 35 , 1359 – 1368 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Kobayashi A , Takahashi H . 2013 . Molecular mechanisms of hydrotropism in seedling roots of Arabidopsis thaliana (Brassicaceae) . American Journal of Botany 100 , 25 – 34 . Google Scholar CrossRef Search ADS Moriwaki T , Miyazawa Y , Kobayashi A , Uchida M , Watanabe C , Fujii N , Takahashi H . 2011 . Hormonal regulation of lateral root development in Arabidopsis modulated by MIZ1 and requirement of GNOM activity for MIZ1 function . Plant Physiology 157 , 1209 – 1220 . Google Scholar CrossRef Search ADS Morohashi K , Okamoto M , Yamazaki C et al. 2017 . Gravitropism interferes with hydrotropism via counteracting auxin dynamics in cucumber roots: clinorotation and spaceflight experiments . New Phytologist 215 , 1476 – 1489 . Google Scholar CrossRef Search ADS Moulia B , Fournier M . 2009 . The power and control of gravitropic movements in plants: a biomechanical and systems biology view . Journal of Experimental Botany 60 , 461 – 486 . Google Scholar CrossRef Search ADS Mullen JL , Wolverton C , Ishikawa H , Evans ML . 2000 . Kinetics of constant gravitropic stimulus responses in Arabidopsis roots using a feedback system . Plant Physiology 123 , 665 – 670 . Google Scholar CrossRef Search ADS Mustilli AC , Merlot S , Vavasseur A , Fenzi F , Giraudat J . 2002 . Arabidopsis OST1 protein kinase mediates the regulation of stomatal aperture by abscisic acid and acts upstream of reactive oxygen species production . The Plant Cell 14 , 3089 – 3099 . Google Scholar CrossRef Search ADS Nagai T , Yamada S , Tominaga T , Ichikawa M , Miyawaki A . 2004 . Expanded dynamic range of fluorescent indicators for Ca2+ by circularly permuted yellow fluorescent proteins . Proceedings of the National Academy of Sciences, USA 101 , 10554 – 10559 . Google Scholar CrossRef Search ADS Nakajima Y , Nara Y , Kobayashi A , Sugita T , Miyazawa Y , Fujii N , Takahashi H . 2017 . Auxin transport and response requirements for root hydrotropism differ between plant species . Journal of Experimental Botany 68 , 3441 – 3456 . Google Scholar CrossRef Search ADS Oono Y , Ooura C , Rahman A , Aspuria ET , Hayashi K , Tanaka A , Uchimiya H . 2003 . p-Chlorophenoxyisobutyric acid impairs auxin response in Arabidopsis root . Plant Physiology 133 , 1135 – 1147 . Google Scholar CrossRef Search ADS Ottenschläger I , Wolff P , Wolverton C , Bhalerao RP , Sandberg G , Ishikawa H , Evans M , Palme K . 2003 . Gravity-regulated differential auxin transport from columella to lateral root cap cells . Proceedings of the National Academy of Sciences, USA 100 , 2987 – 2991 . Google Scholar CrossRef Search ADS Oyanagi A , Takahashi H , Suge H . 1995 . Interactions between hydrotropism and gravitropism in the primary seminal roots of Triticum aestivum L . Annals of Botany 75 , 229 – 235 . Google Scholar CrossRef Search ADS Paez Valencia J , Goodman K , Otegui MS . 2016 . Endocytosis and endosomal trafficking in plants . Annual Review of Plant Biology 67 , 309 – 335 . Google Scholar CrossRef Search ADS Park SY , Fung P , Nishimura N et al. 2009 . Abscisic acid inhibits type 2C protein phosphatases via the PYR/PYL family of START proteins . Science 324 , 1068 – 1071 . Peiretti R , Dumanski J . 2014 . The transformation of agriculture in Argentina through soil conservation . International Soil and Water Conservation Research 2 , 14 – 20 . Google Scholar CrossRef Search ADS Péret B , Li G , Zhao J et al. 2012 . Auxin regulates aquaporin function to facilitate lateral root emergence . Nature Cell Biology 14 , 991 – 998 . Google Scholar CrossRef Search ADS Ponce G , Rasgado FA , Cassab GI . 2008a. Roles of amyloplasts and water deficit in root tropisms . Plant, Cell and Environment 31 , 205 – 217 . Google Scholar CrossRef Search ADS Ponce G , Rasgado F , Cassab GI . 2008b. How amyloplasts, water deficit and root tropisms interact ? Plant Signaling and Behavior 3 , 460 – 462 . Google Scholar CrossRef Search ADS Rahman A , Takahashi M , Shibasaki K , Wu S , Inaba T , Tsurumi S , Baskin TI . 2010 . Gravitropism of Arabidopsis thaliana roots requires the polarization of PIN2 toward the root tip in meristematic cortical cells . The Plant Cell 22 , 1762 – 1776 . Google Scholar CrossRef Search ADS Rogers ED , Benfey PN . 2015 . Regulation of plant root system architecture: implications for crop advancement . Current Opinion in Biotechnology 32 , 93 – 98 . Google Scholar CrossRef Search ADS Rosquete MR , Kleine-Vehn J . 2013 . Halotropism: turning down the salty date . Current Biology 23 , R927 – R929 . Google Scholar CrossRef Search ADS Roychoudhry S , Del Bianco M , Kieffer M , Kepinski S . 2013 . Auxin controls gravitropic setpoint angle in higher plant lateral branches . Current Biology 23 , 1497 – 1504 . Google Scholar CrossRef Search ADS Roychoudhry S , Kieffer M , Del Bianco M , Liao CY , Weijers D , Kepinski S . 2017 . The developmental and environmental regulation of gravitropic setpoint angle in Arabidopsis and bean . Scientific Reports 7 , 42664 . Google Scholar CrossRef Search ADS Sachs J . 1872 . Ablenkung der Wurzel von ihrer normalen Wachsthumsrichtung durch feuchte Koerper . In: Sachs J , ed. Arbeiten des Botanischen Institus in Wuerzburg . Leipzig : Wilhelm Engelmann , 209 – 222 . Salazar-Blas A , Noriega-Calixto L , Campos ME , Eapen D , Cruz-Vázquez T , Castillo-Olamendi L , Sepulveda-Jiménez G , Porta H , Dubrovsky JG , Cassab GI . 2017 . Robust root growth in altered hydrotropic response1 (ahr1) mutant of Arabidopsis is maintained by high rate of cell production at low water potential gradient . Journal of Plant Physiology 208 , 102 – 114 . Google Scholar CrossRef Search ADS Saucedo M , Ponce G , Campos ME , Eapen D , García E , Luján R , Sánchez Y , Cassab GI . 2012 . An altered hydrotropic response (ahr1) mutant of Arabidopsis recovers root hydrotropism with cytokinin . Journal of Experimental Botany 63 , 3587 – 3601 . Google Scholar CrossRef Search ADS Shkolnik D , Fromm H . 2016 . The Cholodny–Went theory does not explain hydrotropism . Plant Science 252 , 400 – 403 . Google Scholar CrossRef Search ADS Shkolnik D , Krieger G , Nuriel R , Fromm H . 2016 . Hydrotropism: root bending does not require auxin redistribution . Molecular Plant 9 , 757 – 759 . Google Scholar CrossRef Search ADS Sobeih WY , Dodd IC , Bacon MA , Grierson D , Davies WJ . 2004 . Long-distance signals regulating stomatal conductance and leaf growth in tomato (Lycopersicon esculentum) plants subjected to partial root-zone drying . Journal of Experimental Botany 55 , 2353 – 2363 . Google Scholar CrossRef Search ADS Stanley CE , Shrivastava J , Brugman R , Heinzelmann E , van Swaay D , Grossmann G . 2018 . Dual-flow-RootChip reveals local adaptations of roots towards environmental asymmetry at the physiological and genetic levels . New Phytologist 217 , 1357 – 1369 . Google Scholar CrossRef Search ADS Stinemetz C , Takahashi H , Suge H . 1996 . Characterization of hydrotropism: the timing of perception and signal movement from the root cap in the agravitropic pea mutant ageotropum . Plant and Cell Physiology 37 , 800 – 805 . Google Scholar CrossRef Search ADS Su SH , Gibbs NM , Jancewicz AL , Masson PH . 2017 . Molecular mechanisms of root gravitropism . Current Biology 27 , R964 – R972 . Google Scholar CrossRef Search ADS Sun F , Zhang W , Hu H , Li B , Wang Y , Zhao Y , Li K , Liu M , Li X . 2008 . Salt modulates gravity signaling pathway to regulate growth direction of primary roots in Arabidopsis . Plant Physiology 146 , 178 – 188 . Google Scholar CrossRef Search ADS Sutka M , Li G , Boudet J , Boursiac Y , Doumas P , Maurel C . 2011 . Natural variation of root hydraulics in Arabidopsis grown in normal and salt-stressed conditions . Plant Physiology 155 , 1264 – 1276 . Google Scholar CrossRef Search ADS Swarup R , Kramer EM , Perry P , Knox K , Leyser HM , Haseloff J , Beemster GT , Bhalerao R , Bennett MJ . 2005 . Root gravitropism requires lateral root cap and epidermal cells for transport and response to a mobile auxin signal . Nature Cell Biology 7 , 1057 – 1065 . Google Scholar CrossRef Search ADS Takahashi N , Goto N , Okada K , Takahashi H . 2002 . Hydrotropism in abscisic acid, wavy, and gravitropic mutants of Arabidopsis thaliana . Planta 216 , 203 – 211 . Google Scholar CrossRef Search ADS Takahashi H , Miyazawa Y , Fujii N . 2009 . Hormonal interactions during root tropic growth: hydrotropism versus gravitropism . Plant Molecular Biology 69 , 489 – 502 . Google Scholar CrossRef Search ADS Takahashi H , Scott TK . 1991 . Hydrotropism and its interaction with gravitropism in maize roots . Plant Physiology 96 , 558 – 564 . Google Scholar CrossRef Search ADS Takahashi H , Scott TK . 1993 . Intensity of hydrostimulation for the induction of root hydrotropism and its sensing by the root cap . Plant, Cell and Environment 16 , 99 – 103 . Google Scholar CrossRef Search ADS Takahashi H , Suge H . 1991 . Root hydrotropism of an agravitropic pea mutant, ageotropum . Physiologia Plantarum 82 , 24 – 31 . Google Scholar CrossRef Search ADS Takahashi N , Yamazaki Y , Kobayashi A , Higashitani A , Takahashi H . 2003 . Hydrotropism interacts with gravitropism by degrading amyloplasts in seedling roots of Arabidopsis and radish . Plant Physiology 132 , 805 – 810 . Google Scholar CrossRef Search ADS Takano M , Takahashi H , Hirasawa T , Suge H . 1995 . Hydrotropism in roots: sensing of a gradient in water potential by the root cap . Planta 197 , 410 – 413 . Google Scholar CrossRef Search ADS Taniguchi YY , Taniguchi M , Tsuge T , Oka A , Aoyama T . 2010 . Involvement of Arabidopsis thaliana phospholipase Dzeta2 in root hydrotropism through the suppression of root gravitropism . Planta 231 , 491 – 497 . Google Scholar CrossRef Search ADS Tanimoto M , Tremblay R , Colasanti J . 2008 . Altered gravitropic response, amyloplast sedimentation and circumnutation in the Arabidopsis shoot gravitropism 5 mutant are associated with reduced starch levels . Plant Molecular Biology 67 , 57 – 69 . Google Scholar CrossRef Search ADS Triplett GB , Dick WA . 2008 . No-tillage crop production: a revolution in agriculture ! Agronomy Journal 100 , 153 – 165 . Google Scholar CrossRef Search ADS Tsuda S , Miyamoto N , Takahashi H , Ishihara K , Hirasawa T . 2003 . Roots of Pisum sativum L. exhibit hydrotropism in response to a water potential gradient in vermiculite . Annals of Botany 92 , 767 – 770 . Google Scholar CrossRef Search ADS Uga Y , Sugimoto K , Ogawa S et al. 2013 . Control of root system architecture by DEEPER ROOTING 1 increases rice yield under drought conditions . Nature Genetics 45 , 1097 – 1102 . Google Scholar CrossRef Search ADS Veihmeyer FJ , Hendrickson AH . 1927 . Soil-moisture conditions in relation to plant growth . Plant Physiology 2 , 71 – 82 . Google Scholar CrossRef Search ADS Waadt R , Hitomi K , Nishimura N , Hitomi C , Adams SR , Getzoff ED , Schroeder JI . 2014 . FRET-based reporters for the direct visualization of abscisic acid concentration changes and distribution in Arabidopsis . eLife 3 , e01739 . Google Scholar CrossRef Search ADS Wery J . 2005 . Differential effects of soil water deficit on the basic plant functions and their significance to analyse crop responses to water deficit in indeterminate plants . Australian Journal of Agricultural Research 56 , 1201 – 1209 . Google Scholar CrossRef Search ADS Wiesner J . 1881 . Das Bewegungsvermoegen der Pflanzen . Wien : Alfred Hoelder . World Water Assessment Programme (WWAP) . 2015 . The United Nations World Water Development Report 2015: water for a sustainable world . Paris : UNESCO . Yamaguchi-Shinozaki K , Shinozaki K . 2006 . Transcriptional regulatory networks in cellular responses and tolerance to dehydration and cold stresses . Annual Review of Plant Biology 57 , 781 – 803 . Google Scholar CrossRef Search ADS Yamazaki T , Miyazawa Y , Kobayashi A , Moriwaki T , Fujii N , Takahashi H . 2012 . MIZ1, an essential protein for root hydrotropism, is associated with the cytoplasmic face of the endoplasmic reticulum membrane in Arabidopsis root cells . FEBS Letters 586 , 398 – 402 . Google Scholar CrossRef Search ADS Zhenchang W , Xiaofei Y , Liang F , Jianbin Z . 2016 . Partial rootzone drying irrigation increase root surface area, root hydraulic conductivity and water use efficiency in maize . International Journal of Environmental Monitoring and Analysis 4 , 146 – 153 . Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Journal

Journal of Experimental BotanyOxford University Press

Published: Feb 26, 2018

There are no references for this article.

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off