Uncovering C4-like photosynthesis in C3 vascular cells

Uncovering C4-like photosynthesis in C3 vascular cells Abstract In C4 plants, the vascularization of the leaf is extended to include a ring of photosynthetic bundle sheath cells, which have essential and specific functions. In contrast to the substantial knowledge of photosynthesis in C4 plants, relatively little is known about photosynthesis in C3 plant veins, which differs substantially from that in C3 mesophyll cells. In this review we highlight the specific photosynthetic machinery present in C3 vascular cells, which likely evolved prior to the divergence between C3 and C4 plants. The associated primary processes of carbon recapture, nitrogen transport, and antioxidant metabolism are discussed. This review of the basal C4 photosynthesis in C3 plants is significant in the context of promoting the potential for biotechnological development of C4-transgenic rice crops. C3 vein, C4 vein, carbon recapture, nitrogen transport, photosynthesis, plant vasculature Introduction The architecture of the vascular system of plants is determined by a range of functional demands in specific tissues (Carmeliet, 2003). In broad terms, the vascular network was considered to be simply a passive conduit (Riens et al., 1991) until the late 1960s, when the vascular anatomy of C4 photosynthetic plants was defined, although the vascular structure had been known for some time (Esau and Cheadle, 1965; Dengler et al., 1994). In C4 plants, a single ring of photosynthetically active bundle sheath cells surrounds the vascular bundle. Outside these cells is a concentric ring of specialized mesophyll cells, creating the classical Kranz anatomy (Langdale, 2011). In most C4 plants, photosynthetic reactions are partitioned between bundle sheath and mesophyll cells (Hatch 1987; Sage, 2004; Brown et al., 2005). However, in certain C4 plants, such as some C4 grasses (Ueno, 1992; Voznesenskaya et al., 2005), an inner layer of mestome sheath cells, rather than the bundle sheath, serves as the site of CO2 concentration (Sage et al., 2012). In contrast to C4 plants, there is no Kranz anatomy in C3 plants, but partial C4 photosynthesis has been detected in cells surrounding the veins of C3 plants (Hibberd and Quick, 2002). These photosynthetic, chloroplast-containing cells around the C3 plant veins were termed bundle sheath cells (Esau, 1953). The incomplete C4 photosynthesis associated with C3 veins (rather than the full C4 pathway) is an adaptation from the evolutionarily more ancient C3 photosynthesis, and has specifically evolved to function efficiently at high light intensities. The parenchymatous bundle sheath cells of C3 plants participate in a variety of metabolic processes associated with carbohydrate synthesis and storage, the import and export of nitrogen and sulfur, and the metabolism of reactive oxygen species (ROS) (Leegood, 2008). However, the molecular basis of the cross-talk between these primary functions and vascular photosynthesis has yet to be elucidated. This review focuses on recent studies concerning the importance of C4-like photosynthesis in C3 veins, including the cell-specific features of their photosynthetic machinery, carbon recapture, nitrogen transport, systemic signaling, antioxidant metabolism, and development. C4-like photosynthesis in cells surrounding the vascular system The canonical C4 photosynthetic pathway comprises four steps that occur in two morphologically and biochemically distinct cell types (Fig. 1A). The first reaction of C4 photosynthesis involves conversion of CO2 to bicarbonate, catalyzed by carbonic anhydrase. The bicarbonate is then fixed by phosphoenolpyruvate carboxylase (PEPC) to form oxaloacetate, which can be further converted to malate in a reaction catalyzed by malate dehydrogenase, or to aspartate, catalyzed by aspartate aminotransferase. The malate or aspartate then diffuses into the bundle sheath cells and is subsequently decarboxylated. The released CO2 is later refixed into a three-carbon compound by ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) in the Calvin cycle. Finally, the pyruvate generated by the decarboxylation reaction diffuses back to the mesophyll chloroplasts, where it is used in the regeneration of phosphoenolpyruvate (PEP) by pyruvate orthophosphate dikinase (PPDK) (Fig. 1A) (Langdale, 2011; Majeran et al., 2008). Fig. 1. View largeDownload slide The C4-like-photosynthetic cells surrounding the vascular system of C3 plants. (A) Transverse leaf section (Langdale, 2011) and (right) photosynthesis in NADP-malic enzyme (NADP-ME) C4 plants. (B) Transverse leaf section (Langdale, 2011) and (right) vascular C4 photosynthesis of C3 plants. (C) Malate is transported through the transpiration stream of the vascular system of C3 plants, which supplies the photosynthetic cells in twig petioles and the leaf mid-veins. Vascular cells are unlikely to receive significant amounts of CO2 via stomata. Scale bars=3 μm. Fig. 1. View largeDownload slide The C4-like-photosynthetic cells surrounding the vascular system of C3 plants. (A) Transverse leaf section (Langdale, 2011) and (right) photosynthesis in NADP-malic enzyme (NADP-ME) C4 plants. (B) Transverse leaf section (Langdale, 2011) and (right) vascular C4 photosynthesis of C3 plants. (C) Malate is transported through the transpiration stream of the vascular system of C3 plants, which supplies the photosynthetic cells in twig petioles and the leaf mid-veins. Vascular cells are unlikely to receive significant amounts of CO2 via stomata. Scale bars=3 μm. Parts of the C4 cycle, involving malate-decarboxylating and PPDK activities, were discovered in cells located around the vascular system of C3 plants (Hibberd and Quick, 2002). A radiotracer analysis showed that the carbon present in the transpiration stream can be used for photosynthesis in the vascular cells, and the presence of PEPC allowed distant heterotrophic tissues, such as roots, stems, and petioles, to produce malate from PEP. This malate was found to be transported through the veins and decarboxylated in photosynthetic cells bordering the vascular system in C3 plants. The released CO2 can then be refixed to produce carbohydrates, including sucrose and starch (Fig. 1B) (Hibberd and Quick, 2002; Berveiller and Damesin, 2008; Brown et al., 2010). The photosynthetic cells surrounding the vascular system of C3 plants are therefore considered to exhibit a more spatially separated version of the C4 photosynthetic pathway (Hibberd and Quick, 2002). This feature has been observed in the vascular tissues of phylogenetically widespread C3 plants, including twigs of Pinus silvestris (Ivanov et al., 2006), petioles of Apium graveolens and Nicotiana tabacum (Hibberd and Quick, 2002), and mid-veins of Arabidopsis thaliana (Brown et al., 2010) and Oryza sativa (Shen et al., 2016) (Fig. 1C). Recapturing respiratory carbon loss Unlike in C4 plants, where CO2 diffuses from stomata into adjacent mesophyll cells and is then fixed into malate, the malate in C3 plants is derived from the respiratory activity of distant heterotrophic tissues (Fig. 1C). C3 plants lacking chlorophyll in cells close to veins showed that the rate of net photosynthesis was most affected by high partial pressures of CO2 in the leaf, and it is possible that elevated night-time temperatures lead to increased CO2 production via higher rates of respiration (Janacek et al., 2009). In addition, high temperatures are thought to be one of the drivers for the evolution of C4 photosynthesis (Sage, 2004), so it is possible that the vascular chloroplast volume in C3 plants can expand as a consequence of high rates of night-time respiration, leading to increased intercellular CO2 concentrations at the start of the day (Janacek et al., 2009). Carbon refixation in the bark of Populus tremuloides was shown to result in a 90% reduction in the rate of respiration in woody tissues (Foote and Schaedle, 1976). Similarly, refixation in 3-year-old stem sections of Pinus sylvestris resulted in a net reduction in respiration rate of 40% (Linder and Troeng, 1980). Thus, the recycling pathways in chlorophyllous veins contribute substantially to the overall carbon budget of C3 plants, although these pathways also confound efficient efflux-based estimation of the respiration rates of woody tissues and the patterns of carbohydrate allocation (Bloemen et al., 2013). Currently, their net contribution to carbon metabolism and distribution is not well understood. Expression of specific isoforms of C4 enzymes C4 plants have been categorized into three subtypes according to their different decarboxylating enzymes: the NADP-malic enzyme (NADP-ME) type, the NAD-malic enzyme (NAD-ME) type, and the phosphoenolpyruvate carboxykinase (PEPCK) type (Yoshimura et al., 2004). However, recent studies have indicated that the three decarboxylation subtypes may have flexibility between these three pathways, and that this flexibility may potentially be expressed in response to both developmental and environmental factors (Weber and von Caemmerer, 2010; Furbank 2011). The three C4 enzymes NADP-ME, NAD-ME, and PEPCK are expressed in a vein-specific distribution pattern in C3 plants, where they are predominantly localized within the vascular tissues (Chen et al., 2004; Taylor et al., 2010; Penfield et al., 2012). In the C3 species A. thaliana, it has been shown that the 5ʹ region of each decarboxylase gene is sufficient to drive expression in the veins, while the 3ʹ region enhances expression (Fig. 2A) (Brown et al., 2010). The enzymes are encoded by multigene families, members of which show vein-specific compartmentalization (Hibberd and Covshoff, 2010). Transcripts derived from both cytosolic and chloroplast-localized NADP-ME genes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME genes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) and PPDK are responsible for the primary activity in A. thaliana mid-veins (Brown et al., 2010; Taylor et al., 2010). Translatome analysis has further indicated that the ribosomes in C3 veins can preferentially translate these proteins compared with other members of the family (Fig. 2B) (Aubry et al., 2014). Thus, these C4 genes, which are differentially expressed between veins and mesophyll cells in C3 plants, likely exhibit functional specialization in C4 plants and underwent minor adaptations from their ancestral versions in C3 plants, such as evolving vein-specific trans-factors to adapt to the existing cis-elements. Fig. 2. View largeDownload slide Schematic model of the C4 photosynthetic enzymes and machinery in C3 vascular cells. (A) The 5ʹ regions of C4 acid decarboxylase genes target them to the vasculature, and the 3ʹ regions are involved in increasing the amount of their expression (Brown et al., 2010). The C3 vasculature is therefore able to preferentially transcribe and translate these proteins (Aubry et al., 2014). (B) Cytosolic and chloroplastic NADP-ME enzymes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME enzymes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) are together responsible for the high malate-decarboxylating activity in veins (Brown et al., 2010). (C) Malate is transported into vascular photosynthetic cells from the transpiration stream of the vascular system. (D) The decarboxylation of malate can generate extra NADP(H) and CO2, resulting in a unique microenvironment (high CO2 and hypoxia) and metabolic demand (high ATP/NADPH) for the vascular chloroplasts. (E) The high internal CO2 concentrations acidify the thylakoid lumen and impair the pH-dependent high-energy-state quenching (Manetas, 2004). This in turn impedes the oxygen-evolving complex (OEC) of photosystem (PSII) and PSII-driven linear electron transport to produce NADPH (Kalachanis and Manetas, 2010). Photosystem I (PSI), cytochrome b6f (Cytb6f), and ATP synthase complexes mediate cyclic electron flow to rebalance the ATP/NADPH ratio. Fig. 2. View largeDownload slide Schematic model of the C4 photosynthetic enzymes and machinery in C3 vascular cells. (A) The 5ʹ regions of C4 acid decarboxylase genes target them to the vasculature, and the 3ʹ regions are involved in increasing the amount of their expression (Brown et al., 2010). The C3 vasculature is therefore able to preferentially transcribe and translate these proteins (Aubry et al., 2014). (B) Cytosolic and chloroplastic NADP-ME enzymes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME enzymes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) are together responsible for the high malate-decarboxylating activity in veins (Brown et al., 2010). (C) Malate is transported into vascular photosynthetic cells from the transpiration stream of the vascular system. (D) The decarboxylation of malate can generate extra NADP(H) and CO2, resulting in a unique microenvironment (high CO2 and hypoxia) and metabolic demand (high ATP/NADPH) for the vascular chloroplasts. (E) The high internal CO2 concentrations acidify the thylakoid lumen and impair the pH-dependent high-energy-state quenching (Manetas, 2004). This in turn impedes the oxygen-evolving complex (OEC) of photosystem (PSII) and PSII-driven linear electron transport to produce NADPH (Kalachanis and Manetas, 2010). Photosystem I (PSI), cytochrome b6f (Cytb6f), and ATP synthase complexes mediate cyclic electron flow to rebalance the ATP/NADPH ratio. Adjusting the photosynthetic machinery and photochemistry It has been proposed that high malate-decarboxylating metabolism in the veins shaped the structure and function of the photosystems in C4 plants, as well as the associated electron flow (Kalachanis and Manetas, 2010), by favoring changes in the state of photosystem I, photosystem II, and the inter-system communication (Fig. 2C, 2D) (Kotakis et al., 2006). The specific vascular photosynthetic system of C3 plants may be related to this function. Similar to the C4 bundle sheath cells, the chlorophyllous twig cortices of Eleagnus angustifolius have a low dark-adapted photosystem II photochemical efficiency, as well as limited photosystem II-driven linear electron transport (Kotakis et al., 2006). The chronic photoinhibition of photosystem II activity that occurs in these tissues is attributed to the acidification of the protoplasm by internal CO2 concentrations, leading to an impairment of the pH-dependent high-energy-state quenching, followed by a reduction in the efficiency of heat dissipation (Manetas, 2004). The primary cause of the impairment is located on the donor side (oxygen-evolving complex) of photosystem II (Kalachanis and Manetas, 2010), which is blocked by a ‘traffic jam’ consisting of selective subunits of the photosystem II protein complex, such as PsbP, PsbQ, and PsbR (Fig. 2E) (Shen et al., 2016). However, photosystem I activity is potentially sufficient for the plant to engage in cyclic electron transport to restore the ATP/NADPH ratio (Kotakis et al., 2006) and to provide the additional ATP required for the decarboxylation of malate ascending from the roots (Yiotis and Manetas, 2010). Therefore, the C3 vascular chloroplasts can be considered to have the capacity to perform photosynthesis, albeit a form that is structurally and metabolically distinct from the photosynthesis occurring in the mesophyll. Channeling nitrogen transport The fact that C4 plants utilize available nitrogen more efficiently than C3 plants is partially due to the ‘division of labor’ between bundle sheath and mesophyll cells (Moore and Black, 1979). Similarly, a subtle partitioning of nitrogen metabolism has been observed in the vascular cells of C3 plants, where nitrogen-metabolism enzymes or their encoding genes exhibit spatial localization together with C4 enzymes. For example, the cytosolic glutamine synthetase (GS1) and chloroplast NADH-glutamate synthase (NADH-GOGAT2) genes are expressed primarily in the vascular parenchyma cells of senescing rice leaf blades (Sakurai et al., 1996; Yamaya and Kusano, 2014; Bailey and Leegood, 2016). In addition, the cytosolic and mitochondrial glutamate dehydrogenases (GDH1 and GDH3) are abundant in the vascular ribosomes of C3 plants (Aubry et al., 2014), along with high levels of asparagine synthetase (Nakano et al., 2000). The coordinated localization of C4 enzymes with these nitrogen-metabolic enzymes is essential for nitrogen transport. Malate can be converted into oxaloacetate by C4 enzymes, and sequentially reassimilated into transport amino acids (glutamine, asparagine, and glutamate) via the co-localized nitrogen-compound-metabolizing enzymes (Fig. 3) (Kamachi et al., 1992; Hayakawa et al., 1994). The conversion pathway also requires the metabolic steps of the partial tricarboxylic acid (TCA) cycle in the mitochondria to provide carbon skeletons (Taylor et al., 2010). Accordingly, mutants of A. thaliana with reduced chlorophyll levels in the veins have a reduced production of TCA cycle intermediates and amino acids (Janacek et al., 2009). The absence of individual C4 decarboxylating proteins in the mid-vein also leads to a decline in malate withdrawal from the TCA cycle, thereby regulating the flux of carbon skeletons to amino acids in a light-dependent manner (Brown et al., 2010). Fig. 3. View largeDownload slide Schematic presentation of the role of the vascular C4 photosynthetic pathway in nitrogen transport, which allows carbon skeletons from malate to flow into the pathway that produces transport amino acids. The transport processes proposed are based on a portion of the tricarboxylic acid (TCA) cycle, where both the enzyme activity (shown in red) and metabolite levels (shown in blue) increase to produce transport amino acids. Cooperation of the C4 enzymes PEPCK, PPDK, and NAD(P)-ME and the nitrogen-compound-metabolizing enzymes glutamine synthetase (GS), glutamate dehydrogenase (GDH), and chloroplast NADH-glutamate synthase (NADP-GOGAT) in the vascular photosynthetic cells is required for nitrogen transport in order to convert malate to oxaloacetate and then to the transport amino acids glutamate, glutamine, and asparagine (Lin and Wu, 2004; Taylor et al., 2010). Fig. 3. View largeDownload slide Schematic presentation of the role of the vascular C4 photosynthetic pathway in nitrogen transport, which allows carbon skeletons from malate to flow into the pathway that produces transport amino acids. The transport processes proposed are based on a portion of the tricarboxylic acid (TCA) cycle, where both the enzyme activity (shown in red) and metabolite levels (shown in blue) increase to produce transport amino acids. Cooperation of the C4 enzymes PEPCK, PPDK, and NAD(P)-ME and the nitrogen-compound-metabolizing enzymes glutamine synthetase (GS), glutamate dehydrogenase (GDH), and chloroplast NADH-glutamate synthase (NADP-GOGAT) in the vascular photosynthetic cells is required for nitrogen transport in order to convert malate to oxaloacetate and then to the transport amino acids glutamate, glutamine, and asparagine (Lin and Wu, 2004; Taylor et al., 2010). Orchestrating systemic signaling and antioxidant metabolism In the C4 plant maize (Zea mays), chloroplast metabolism in the bundle sheath and mesophyll cells differs in terms of ROS defense components, in order to cope with the different cellular environments (Friso et al., 2010). Owing to their central position, leaf veins in C3 plants also have the potential to regulate ROS signaling in various cell types and organs (Kangasjärvi et al., 2009). Karpinski et al. (1999) proposed a phenomenon, termed systemic acquired acclimation, in which light stress initiates systemic signals that spread through the veins to confer stress resistance in non-exposed parts of the plant (Fig. 4A). Fig. 4. View largeDownload slide Schematic overview of signaling networks involved in systemic acquired acclimation processes in the vascular photosynthetic cells. (A) H2O2 accumulation in an Arabidopsis leaf, revealed by diaminobenzidine staining. The top half of the leaf was treated with high light, while the bottom half was kept in the dark. Note the considerable reduction in the degree of staining from the vascular tissue to the adjacent mid-vein cells (Fryer et al., 2002). (B) The structural and functional properties of the photosynthetic machinery in the vasculature contribute to cell-specific accumulation of reactive oxygen species (ROS) and antioxidant metabolism (Kangasjärvi et al., 2009; Kinsman and Pyke, 1998). I: The decarboxylation of the imported malate consumes excess NADP+. II: In chloroplasts, O2 can be used as an alternative electron acceptor in photosystem I (PSI) to promote O2- generation and the Mehler reaction (Asada, 1999; Shen et al., 2015). The thylakoid-membrane-attached Cu/Mn superoxide dismutase (SOD) catalyzes the disproportionation of O2– into H2O2 signals. III: In the plasma membrane and cell wall, NADP-ME2 is involved in meeting the demand for reducing power for the production of H2O2 through NADPH oxidase and/or peroxidases (Voll et al., 2012). IV: In the nucleus, the H2O2 signals can synergistically up-regulate the expression of cytosolic APX2 and chloroplastic Cu/Mn SOD (Rossel et al., 2007). These unique antioxidant enzymes control the bursts of H2O2 accumulation (Kangasjärvi et al., 2009). V: H2O2 accumulation initiates unidentified systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. As a result of the systemic signals, shaded leaf tissues undergo photosynthetic adjustments as well as up-regulation of the antioxidative capacity and cross-tolerance of biotic stresses (Kangasjärvi et al., 2009). This process is mediated by the zinc finger transcription factor ZAT10 (Rossel et al., 2007). Fig. 4. View largeDownload slide Schematic overview of signaling networks involved in systemic acquired acclimation processes in the vascular photosynthetic cells. (A) H2O2 accumulation in an Arabidopsis leaf, revealed by diaminobenzidine staining. The top half of the leaf was treated with high light, while the bottom half was kept in the dark. Note the considerable reduction in the degree of staining from the vascular tissue to the adjacent mid-vein cells (Fryer et al., 2002). (B) The structural and functional properties of the photosynthetic machinery in the vasculature contribute to cell-specific accumulation of reactive oxygen species (ROS) and antioxidant metabolism (Kangasjärvi et al., 2009; Kinsman and Pyke, 1998). I: The decarboxylation of the imported malate consumes excess NADP+. II: In chloroplasts, O2 can be used as an alternative electron acceptor in photosystem I (PSI) to promote O2- generation and the Mehler reaction (Asada, 1999; Shen et al., 2015). The thylakoid-membrane-attached Cu/Mn superoxide dismutase (SOD) catalyzes the disproportionation of O2– into H2O2 signals. III: In the plasma membrane and cell wall, NADP-ME2 is involved in meeting the demand for reducing power for the production of H2O2 through NADPH oxidase and/or peroxidases (Voll et al., 2012). IV: In the nucleus, the H2O2 signals can synergistically up-regulate the expression of cytosolic APX2 and chloroplastic Cu/Mn SOD (Rossel et al., 2007). These unique antioxidant enzymes control the bursts of H2O2 accumulation (Kangasjärvi et al., 2009). V: H2O2 accumulation initiates unidentified systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. As a result of the systemic signals, shaded leaf tissues undergo photosynthetic adjustments as well as up-regulation of the antioxidative capacity and cross-tolerance of biotic stresses (Kangasjärvi et al., 2009). This process is mediated by the zinc finger transcription factor ZAT10 (Rossel et al., 2007). The structural and functional properties of the photosynthetic machinery in vascular chloroplasts potentially differ from those in the mesophyll cells, associated with light-dependent accumulation of ROS (Kinsman and Pyke, 1998), which supply the reducing equivalents to mediate H2O2 signaling (Heyno et al., 2014; Kuźniak et al., 2016). The decarboxylation of malate consumes excess NADP+ on the acceptor side of photosystem I (Fig. 4B), such that O2 can be used as an alternative electron acceptor to promote the Mehler reaction and ROS metabolism (Fig. 4B) (Asada, 1999; Shen et al., 2015). In addition, NADP-ME2, which is responsible for maximal activity of NADP-ME in the veins (Brown et al., 2010), appears to be necessary for the production of ROS. NADP-ME2 is also involved in fulfilling the demand for reducing power in order to produce superoxide and H2O2 through NADPH oxidase and/or cell wall peroxidases (Fig. 4B) (Voll et al., 2012). Doulis et al. (1997) found that antioxidants in maize leaves are partitioned between mesophyll and bundle sheath cells according to the availability of NADPH. For example, ascorbate peroxidase (APX) and superoxide dismutase (SOD) enzymes are exclusively localized in the bundle sheath. The presence of this detoxification system in C4 plants agrees with the low rate of linear electron transport and oxygen-evolving-complex activity, as well as with the associated risk of generating ROS (Friso et al., 2010). The veins of the model C3 plant A. thaliana also have a versatile antioxidant network, presumably to control the generation of ROS signals and prevent oxidative damage following bursts of ROS production (Kangasjärvi et al., 2009). The cytosolic enzyme APX2 (Fryer et al., 2003), a microRNA (miR398) targeting Cu/Zn SOD (Sunkar et al., 2006), and chloroplast glutaredoxin (Cheng et al., 2006) are photosynthetically induced in the vascular tissues of C3 plants as regulators of H2O2 pulses. Other studies have confirmed that H2O2 signals can synergistically up-regulate the expression of specific isoenzymes of APX and SOD in veins (Fig. 4B) (Rossel et al., 2007). In turn, vein-specific ROS signals and antioxidant metabolism promote unidentified systemic signals that contribute to the capacity of plants to detect and signal the severity of environmental stresses (Kangasjärvi et al., 2009). The mobile signaling components are transported by the vascular system and are sensed in distal tissues (Rossel et al., 2007; Mühlenbock et al., 2008; Pogson et al., 2008). In A. thaliana, the zinc finger transcription factor ZAT10 accumulates preferentially in distal photosynthetic vascular tissues, triggering the non-exposed parts to undergo photosynthetic adjustments, as well as up-regulating antioxidative capacity and cross-tolerance against biotic stresses (Fig. 4B) (Rossel et al., 2007; Kangasjärvi et al., 2009). The vein-specific C4-like photosynthetic machinery together with ROS signaling and antioxidant metabolism are therefore proposed to promote pre-acclimation to subsequent stresses in C3 plants. Manipulating mesophyll development Vein development precedes the development of mesophyll cells, thereby positioning it for the subsequent regulation of mesophyll development (Kinsman and Pyke, 1998; Pyke et al., 1991). Specifically, the veins, and the photosynthesis associated with them, are proposed to provide essential primary metabolites or metabolic signals to the developing mesophyll cells. This phenomenon has resulted in the proposal of the supply (Fig. 5A) and the signal (Fig. 5C) hypotheses, both of which relate to genes expressed in tissues predominantly surrounding the veins (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). Janacek et al. (2009) reported that when chlorophyll accumulation in A. thaliana mutant veins was prevented, the leaves developed more slowly. The ppt1 mutant lacks a PEP/phosphate translocator in the inner membrane of the chloroplast envelope. Consequently, it cannot provide the plastidic shikimate pathway with PEP, which is essential for C3 mesophyll development. Despite the vein-specific expression of the gene in question, the ppt1 mutation was found to primarily affect chloroplast development in palisade mesophyll cells (Li et al., 1995; Streatfield et al., 1999; He et al., 2004). Fig. 5. View largeDownload slide In the young and expanding ‘sink’ leaves, the precocious vasculature and associated photosynthesis are proposed to provide essential primary metabolites or metabolic signals for developing mesophyll, resulting in the signaling (A) and supply (C) hypotheses (Lundquist et al., 2014). Malate is supplied to the vascular C4-like photosynthetic cells by the xylem stream. NADP-ME, NAD-ME, and pyruvate orthophosphate dikinase (PPDK) then catalyze the conversion of malate to PEP. The carbon skeleton is translocated by PEP/phosphate translocator 1 (PPT1) of the chloroplast envelope inner membrane to enter the shikimate pathway (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). (B) In the signaling hypothesis, dehydrodiconiferyl alcohol glucoside (DCG) is produced as a phenylpropanoid-derived secondary metabolite to regulate cell division and expansion, and can be transported to leaf mesophyll cells to trigger their development (Tamagnone et al., 1998; Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014). (D) In the supply hypothesis, the photosynthetic electron transporter plastoquinone (PQ), which is also derived from the shikimate pathway, is incorporated into the vascular photosystem to ensure the production of soluble sugars. The product, sucrose, can be supplied to growing mesophyll cells (Hibberd and Quick, 2002; Lundquist et al., 2014), where it modulates their photosynthetic development (Vaughn et al., 2002). In addition, glutamine, produced preferentially in vascular cells, is transported to feed the mesophyll cells (Kichey et al., 2005). TPT, triose-phosphate/phosphate translocator. Fig. 5. View largeDownload slide In the young and expanding ‘sink’ leaves, the precocious vasculature and associated photosynthesis are proposed to provide essential primary metabolites or metabolic signals for developing mesophyll, resulting in the signaling (A) and supply (C) hypotheses (Lundquist et al., 2014). Malate is supplied to the vascular C4-like photosynthetic cells by the xylem stream. NADP-ME, NAD-ME, and pyruvate orthophosphate dikinase (PPDK) then catalyze the conversion of malate to PEP. The carbon skeleton is translocated by PEP/phosphate translocator 1 (PPT1) of the chloroplast envelope inner membrane to enter the shikimate pathway (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). (B) In the signaling hypothesis, dehydrodiconiferyl alcohol glucoside (DCG) is produced as a phenylpropanoid-derived secondary metabolite to regulate cell division and expansion, and can be transported to leaf mesophyll cells to trigger their development (Tamagnone et al., 1998; Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014). (D) In the supply hypothesis, the photosynthetic electron transporter plastoquinone (PQ), which is also derived from the shikimate pathway, is incorporated into the vascular photosystem to ensure the production of soluble sugars. The product, sucrose, can be supplied to growing mesophyll cells (Hibberd and Quick, 2002; Lundquist et al., 2014), where it modulates their photosynthetic development (Vaughn et al., 2002). In addition, glutamine, produced preferentially in vascular cells, is transported to feed the mesophyll cells (Kichey et al., 2005). TPT, triose-phosphate/phosphate translocator. In the supply hypothesis (Fig. 5D), sucrose is produced by the vascular photosynthetic cells and then loaded into the phloem and translocated to growing mesophyll cells (Lundquist et al., 2014). Chlorophyll fluorescence measurements indicate that there is a reduced pool of the photosynthetic electron transporter plastoquinone, which is derived from the shikimate pathway, in the ppt1 mutant. The impairment of vascular chloroplast homeostasis may then disrupt the delivery of carbohydrate to the growing mesophyll cells (Lundquist et al., 2014). In addition, glutamine produced in vascular cells is transported to feed the mesophyll cells (Kichey et al., 2005). In the signal hypothesis (Fig. 5C), the vein-located PEP/phosphate translocator is also involved in the generation of shikimate-pathway-derived signal molecules that are transported to mesophyll cells to trigger their development. It has been proposed that dehydrodiconiferyl alcohol glucoside, a phenylpropanoid-derived secondary metabolite, acts as a molecular signal in a cytokinin-mediated pathway controlling cell division and expansion of mesophyll palisade tissue (Knappe et al., 2003; Lundquist et al., 2014; Tamagnone et al., 1998; Voll et al., 2003). Notably, photosynthesis in the vein-associated cells also plays a prominent role in providing carbon skeletons to the shikimate pathway via NADP-ME, PPDK, and NAD-ME (Hibberd and Quick, 2002; Brown et al., 2010). The presence of an endogenous PPDK in the veins circumvents the metabolic constraints of ppt1 and rescues the mutant phenotype (Voll et al., 2003). It is thought that the xylem stream can supply the vascular C4-like photosynthetic cells with malate to produce PEP. The carbon skeletons for the shikimate pathway are translocated by the PEP/phosphate translocator 1 (PPT1) into the chloroplast envelope in cells in the veins (Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014; Rosar et al., 2012). Therefore, the co-expression of C4-type enzymes in the C3 veins has been suggested to be essential for providing metabolites and signals to regulate mesophyll development. A C4-like photosynthetic machinery is used in the photosynthetic cells of C3 plant veins to compensate for the loss of respiratory carbon, coordinate nitrogen transport, regulate systemic signaling, and direct the growth of leaves. The similarities and dissimilarities in photosynthesis between C3 and C4 plants, as well as between mesophyll and vascular cells, represent an interesting topic for future studies aiming to identify the molecular regulators of spatial and temporal differentiation of vascular chloroplasts. The reticulate mutants, a subset of variegated mutants of A. thaliana with mesophyll-specific yellowish defects but normal green veins (Lundquist et al., 2014), represent a useful system in which to elucidate the processes of vein-specific development and function in C3 leaves, and to provide insight into intercellular signaling. Gene editing technology, such the CRISPR/Cas9 system, also has great potential for application in cultivated crops in order to achieve efficient targeted mutagenesis at the veinal loci (Miao et al., 2013; Nekrasov et al., 2013). In addition, immunogenic tagging can be employed to isolate individual chloroplasts from vascular cells expressing yellow fluorescent protein on the outer surface of the chloroplasts (Truernit and Hibberd, 2007). Such techniques have the potential to reveal the factors required for vein-specific compartmentalization during development, and to elucidate their function and architecture. Furthermore, research into C3 vascular photosynthesis will likely be valuable in understanding the polyphyletic evolution of C4 photosynthesis from ancestral C3 plants and in promoting the biotechnological development of C4-transgenic rice cultivars. Acknowledgements This work was supported by the Agricultural Independent Innovation Foundation of Jiangsu Province (grant no. CX173022), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the National Natural Science Foundation of China (grant no.31271621). References Asada K . 1999 . The water-water cycle in chloroplasts: scavenging of active oxygens and dissipation of excess photons . Annual Review of Plant Physiology and Plant Molecular Biology 50 , 601 – 639 . Google Scholar CrossRef Search ADS PubMed Aubry S , Smith-Unna RD , Boursnell CM , Kopriva S , Hibberd JM . 2014 . Transcript residency on ribosomes reveals a key role for the Arabidopsis thaliana bundle sheath in sulfur and glucosinolate metabolism . The Plant Journal 78 , 659 – 673 . Google Scholar CrossRef Search ADS PubMed Bailey KJ , Leegood RC . 2016 . Nitrogen recycling from the xylem in rice leaves: dependence upon metabolism and associated changes in xylem hydraulics . Journal of Experimental Botany 67 , 2901 – 2911 . Google Scholar CrossRef Search ADS PubMed Berveiller D , Damesin C . 2008 . Carbon assimilation by tree stems: potential involvement of phosphoenolpyruvate carboxylase . Trees – Structure and Function 22 , 149 – 157 . Google Scholar CrossRef Search ADS Bloemen J , McGuire MA , Aubrey DP , Teskey RO , Steppe K . 2013 . Transport of root-respired CO₂ via the transpiration stream affects aboveground carbon assimilation and CO₂ efflux in trees . New Phytologist 197 , 555 – 565 . Google Scholar CrossRef Search ADS PubMed Brown NJ , Parsley K , Hibberd JM . 2005 . The future of C4 research – maize, Flaveria or Cleome ? Trends in Plant Science 10 , 215 – 221 . Google Scholar CrossRef Search ADS PubMed Brown NJ , Palmer BG , Stanley S et al. 2010 . C4 acid decarboxylases required for C4 photosynthesis are active in the mid-vein of the C3 species Arabidopsis thaliana, and are important in sugar and amino acid metabolism . The Plant Journal 61 , 122 – 133 . Google Scholar CrossRef Search ADS PubMed Carmeliet P . 2003 . Blood vessels and nerves: common signals, pathways and diseases . Nature Reviews. Genetics 4 , 710 – 720 . Google Scholar CrossRef Search ADS PubMed Chen ZH , Walker RP , Técsi LI , Lea PJ , Leegood RC . 2004 . Phosphoenolpyruvate carboxykinase in cucumber plants is increased both by ammonium and by acidification, and is present in the phloem . Planta 219 , 48 – 58 . Google Scholar CrossRef Search ADS PubMed Cheng NH , Liu JZ , Brock A , Nelson RS , Hirschi KD . 2006 . AtGRXcp, an Arabidopsis chloroplastic glutaredoxin, is critical for protection against protein oxidative damage . The Journal of Biological Chemistry 281 , 26280 – 26288 . Google Scholar CrossRef Search ADS PubMed Dengler NG , Dengler RE , Donnelly PM , Hattersley PW . 1994 . Quantitative leaf anatomy of C3 and C4 grasses (Poaceae): bundle sheath and mesophyll surface area relationships . Annals of Botany 73 , 241 – 255 . Google Scholar CrossRef Search ADS Doulis AG , Debian N , Kingston-Smith AH , Foyer CH . 1997 . Differential localization of antioxidants in maize leaves . Plant Physiology 114 , 1031 – 1037 . Google Scholar CrossRef Search ADS PubMed Esau K . 1953 . Plant anatomy . New York : John Wiley & Sons . Esau K , Cheadle , VI . 1965 . Cytologic studies on phloem . University of California (Berkeley) Publications of Botany 36 , 253 – 344 . Foote KC , Schaedle M . 1976 . Diurnal and seasonal patterns of photosynthesis and respiration by stems of Populus tremuloides Michx . Plant Physiology 58 , 651 – 655 . Google Scholar CrossRef Search ADS PubMed Friso G , Majeran W , Huang M , Sun Q , van Wijk KJ . 2010 . Reconstruction of metabolic pathways, protein expression, and homeostasis machineries across maize bundle sheath and mesophyll chloroplasts: large-scale quantitative proteomics using the first maize genome assembly . Plant Physiology 152 , 1219 – 1250 . Google Scholar CrossRef Search ADS PubMed Fryer MJ , Ball L , Oxborough K , Karpinski S , Mullineaux PM , Baker NR . 2003 . Control of Ascorbate Peroxidase 2 expression by hydrogen peroxide and leaf water status during excess light stress reveals a functional organisation of Arabidopsis leaves . The Plant Journal 33 , 691 – 705 . Google Scholar CrossRef Search ADS PubMed Fryer MJ , Oxborough K , Mullineaux PM , Baker NR . 2002 . Imaging of photo-oxidative stress responses in leaves . Journal of Experimental Botany 53 , 1249 – 1254 . Google Scholar PubMed Furbank RT . 2011 . Evolution of the C4 photosynthetic mechanism: are there really three C4 acid decarboxylation types ? Journal of Experimental Botany 62 , 3103 – 3108 . Google Scholar CrossRef Search ADS PubMed Hatch MD . 1987 . C4 photosynthesis: a unique blend of modified biochemistry, anatomy and ultrastructure . Biochimica et Biophysica Acta 895 , 81 – 106 . Google Scholar CrossRef Search ADS Hayakawa T , Nakamura T , Hattori F , Mae T , Ojima K , Yamaya T . 1994 . Cellular localization of NADH-dependent glutamate-synthase protein in vascular bundles of unexpanded leaf blades and young grains of rice plants . Planta 193 , 455 – 460 . Google Scholar CrossRef Search ADS He Y , Tang RH , Hao Y et al. 2004 . Nitric oxide represses the Arabidopsis floral transition . Science 305 , 1968 – 1971 . Google Scholar CrossRef Search ADS PubMed Heyno E , Innocenti G , Lemaire SD , Issakidis-Bourguet E , Krieger-Liszkay A . 2014 . Putative role of the malate valve enzyme NADP-malate dehydrogenase in H2O2 signalling in Arabidopsis . Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 369 , 20130228 . Google Scholar CrossRef Search ADS PubMed Hibberd JM , Covshoff S . 2010 . The regulation of gene expression required for C4 photosynthesis . Annual Review of Plant Biology 61 , 181 – 207 . Google Scholar CrossRef Search ADS PubMed Hibberd JM , Quick WP . 2002 . Characteristics of C4 photosynthesis in stems and petioles of C3 flowering plants . Nature 415 , 451 – 454 . Google Scholar CrossRef Search ADS PubMed Ivanov AG , Krol M , Sveshnikov D , Malmberg G , Gardeström P , Hurry V , Oquist G , Huner NP . 2006 . Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine . Planta 223 , 1165 – 1177 . Google Scholar CrossRef Search ADS PubMed Janacek SH , Trenkamp S , Palmer B et al. 2009 . Photosynthesis in cells around veins of the C3 plant Arabidopsis thaliana is important for both the shikimate pathway and leaf senescence as well as contributing to plant fitness . The Plant Journal 59 , 329 – 343 . Google Scholar CrossRef Search ADS PubMed Kalachanis D , Manetas Y . 2010 . Analysis of fast chlorophyll fluorescence rise (O-K-J-I-P) curves in green fruits indicates electron flow limitations at the donor side of PSII and the acceptor sides of both photosystems . Physiologia Plantarum 139 , 313 – 323 . Google Scholar PubMed Kamachi K , Yamaya T , Hayakawa T , Mae T , Ojima K . 1992 . Vascular bundle-specific localization of cytosolic glutamine synthetase in rice leaves . Plant Physiology 99 , 1481 – 1486 . Google Scholar CrossRef Search ADS PubMed Kangasjärvi S , Nurmi M , Tikkanen M , Aro EM . 2009 . Cell-specific mechanisms and systemic signalling as emerging themes in light acclimation of C3 plants . Plant cell & Environment 32 , 1230 – 1240 . Google Scholar CrossRef Search ADS Karpinski S , Reynolds H , Karpinska B , Wingsle G , Creissen G , Mullineaux P . 1999 . Systemic signaling and acclimation in response to excess excitation energy in Arabidopsis . Science 284 , 654 – 657 . Google Scholar CrossRef Search ADS PubMed Kichey T , Le Gouis J , Sangwan B , Hirel B , Dubois F . 2005 . Changes in the cellular and subcellular localization of glutamine synthetase and glutamate dehydrogenase during flag leaf senescence in wheat (Triticum aestivum L.) . Plant & Cell Physiology 46 , 964 – 974 . Google Scholar CrossRef Search ADS PubMed Kinsman EA , Pyke KA . 1998 . Bundle sheath cells and cell-specific plastid development in Arabidopsis leaves . Development 125 , 1815 – 1822 . Google Scholar PubMed Knappe S , Löttgert T , Schneider A , Voll L , Flügge UI , Fischer K . 2003 . Characterization of two functional phosphoenolpyruvate/phosphate translocator (PPT) genes in Arabidopsis – AtPPT1 may be involved in the provision of signals for correct mesophyll development . The Plant Journal 36 , 411 – 420 . Google Scholar CrossRef Search ADS PubMed Kotakis CH , Petropoulou Y , Stamatakis K , Yiotis CH , Manetas Y . 2006 . Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity . Planta 225 , 245 – 253 . Google Scholar CrossRef Search ADS PubMed Kuźniak E , Kornas A , Kaźmierczak A , Rozpądek P , Nosek M , Kocurek M , Zellnig G , Müller M , Miszalski Z . 2016 . Photosynthesis-related characteristics of the midrib and the interveinal lamina in leaves of the C3-CAM intermediate plant Mesembryanthemum crystallinum . Annals of Botany 117 , 1141 – 1151 . Google Scholar CrossRef Search ADS PubMed Langdale JA . 2011 . C4 cycles: past, present, and future research on C4 photosynthesis . The Plant Cell 23 , 3879 – 3892 . Google Scholar CrossRef Search ADS PubMed Leegood RC . 2008 . Roles of the bundle sheath cells in leaves of C3 plants . Journal of Experimental Botany 59 , 1663 – 1673 . Google Scholar CrossRef Search ADS PubMed Li H , Culligan K , Dixon RA , Chory J . 1995 . CUE1: a mesophyll cell-specific positive regulator of light-controlled gene expression in Arabidopsis . The Plant Cell 7 , 1599 – 1610 . Google Scholar CrossRef Search ADS PubMed Lin JF , Wu SH . 2004 . Molecular events in senescing Arabidopsis leaves . The Plant Journal 39 , 612 – 628 . Google Scholar CrossRef Search ADS PubMed Linder S , Troeng E . 1980 . Photosynthesis and transpiration of 20-year-old Scots pine . Ecological Bulletins 32 , 153 – 163 . Lundquist PK , Rosar C , Bräutigam A , Weber AP . 2014 . Plastid signals and the bundle sheath: mesophyll development in reticulate mutants . Molecular Plant 7 , 14 – 29 . Google Scholar CrossRef Search ADS PubMed Majeran W , Zybailov B , Ytterberg AJ , Dunsmore J , Sun Q , van Wijk KJ . 2008 . Consequences of C4 differentiation for chloroplast membrane proteomes in maize mesophyll and bundle sheath cells . Molecular & Cellular Proteomics 7 , 1609 – 1638 . Google Scholar CrossRef Search ADS Manetas Y . 2004 . Probing corticular photosynthesis through in vivo chlorophyll fluorescence measurements: evidence that high internal CO2 levels suppress electron flow and increase the risk of photoinhibition . Physiologia Plantarum 120 , 509 – 517 . Google Scholar CrossRef Search ADS PubMed Miao J , Guo D , Zhang J , Huang Q , Qin G , Zhang X , Wan J , Gu H , Qu LJ . 2013 . Targeted mutagenesis in rice using CRISPR-Cas system . Cell Research 23 , 1233 – 1236 . Google Scholar CrossRef Search ADS PubMed Moore R , Black CC . 1979 . Nitrogen assimilation pathways in leaf mesophyll and bundle sheath cells of C4 photosynthesis plants formulated from comparative studies with Digitaria sanguinalis (L.) Scop . Plant Physiology 64 , 309 – 313 . Google Scholar CrossRef Search ADS PubMed Mühlenbock P , Szechynska-Hebda M , Plaszczyca M , Baudo M , Mateo A , Mullineaux PM , Parker JE , Karpinska B , Karpinski S . 2008 . Chloroplast signaling and LESION SIMULATING DISEASE1 regulate crosstalk between light acclimation and immunity in Arabidopsis . The Plant Cell 20 , 2339 – 2356 . Google Scholar CrossRef Search ADS PubMed Nakano K , Suzuki T , Hayakawa T , Yamaya T . 2000 . Organ and cellular localization of asparagine synthetase in rice plants . Plant & Cell Physiology 41 , 874 – 880 . Google Scholar CrossRef Search ADS PubMed Nekrasov V , Staskawicz B , Weigel D , Jones JD , Kamoun S . 2013 . Targeted mutagenesis in the model plant Nicotiana benthamiana using Cas9 RNA-guided endonuclease . Nature Biotechnology 31 , 691 – 693 . Google Scholar CrossRef Search ADS PubMed Penfield S , Clements S , Bailey KJ , Gilday AD , Leegood RC , Gray JE , Graham IA . 2012 . Expression and manipulation of PHOSPHOENOLPYRUVATE CARBOXYKINASE 1 identifies a role for malate metabolism in stomatal closure . The Plant Journal 69 , 679 – 688 . Google Scholar CrossRef Search ADS PubMed Pogson BJ , Woo NS , Förster B , Small ID . 2008 . Plastid signalling to the nucleus and beyond . Trends in Plant Science 13 , 602 – 609 . Google Scholar CrossRef Search ADS PubMed Pyke KA , Marrison JL , Leech AM . 1991 . Temporal and spatial development of the cells of the expanding first leaf of Arabidopsis thaliana (L.) Heynh . Journal of Experimental Botany 42 , 1407 – 1416 . Google Scholar CrossRef Search ADS Riens B , Lohaus G , Heineke D , Heldt HW . 1991 . Amino acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the phloem sap of spinach leaves . Plant Physiology 97 , 227 – 233 . Google Scholar CrossRef Search ADS PubMed Rosar C , Kanonenberg K , Nanda AM , Mielewczik M , Bräutigam A , Novák O , Strnad M , Walter A , Weber AP . 2012 . The leaf reticulate mutant dov1 is impaired in the first step of purine metabolism . Molecular Plant 5 , 1227 – 1241 . Google Scholar CrossRef Search ADS PubMed Rossel JB , Wilson PB , Hussain D et al. 2007 . Systemic and intracellular responses to photooxidative stress in Arabidopsis . The Plant Cell 19 , 4091 – 4110 . Google Scholar CrossRef Search ADS PubMed Sage RF . 2004 . The evolution of C4 photosynthesis . New Phytologist 161 , 341 – 370 . Google Scholar CrossRef Search ADS Sage RF , Sage TL , Kocacinar F . 2012 . Photorespiration and the evolution of C4 photosynthesis . Annual Review of Plant Biology 63 , 19 – 47 . Google Scholar CrossRef Search ADS PubMed Sakurai N , Hayakawa T , Nakamura T , Yamaya T . 1996 . Changes in the cellular localization of cytosolic glutamine synthetase protein in vascular bundles of rice leaves at various stages of development . Planta 200 , 306 – 311 . Google Scholar CrossRef Search ADS Shen WJ , Chen GX , Xu JG , Zhen XH , Ma J , Zhang XJ , Lv CG , Gao ZP . 2015 . High light acclimation of Oryza sativa L. leaves involves specific photosynthetic-sourced changes of NADPH/NADP⁺ in the midvein . Protoplasma 252 , 77 – 87 . Google Scholar CrossRef Search ADS PubMed Shen WJ , Ye LH , Ma J et al. 2016 . The existence of C4-bundle-sheath-like photosynthesis in the mid-vein of C3 rice . Rice 9 , 1 – 20 . Google Scholar CrossRef Search ADS PubMed Streatfield SJ , Weber A , Kinsman EA et al. 1999 . The phosphoe- nolpyruvate/phosphate translocator is required for phenolic metabolism, palisade cell development, and plastid-dependent nuclear gene expression . The Plant Cell 11 , 1609 – 1622 . Google Scholar CrossRef Search ADS PubMed Sunkar R , Kapoor A , Zhu JK . 2006 . Posttranscriptional induction of two Cu/Zn superoxide dismutase genes in Arabidopsis is mediated by downregulation of miR398 and important for oxidative stress tolerance . The Plant Cell 18 , 2051 – 2065 . Google Scholar CrossRef Search ADS PubMed Tamagnone L , Merida A , Stacey N et al. 1998 . Inhibition of phenolic acid metabolism results in precocious cell death and altered cell morphology in leaves of transgenic tobacco plants . The Plant Cell 10 , 1801 – 1816 . Google Scholar CrossRef Search ADS PubMed Taylor L , Nunes-Nesi A , Parsley K , Leiss A , Leach G , Coates S , Wingler A , Fernie AR , Hibberd JM . 2010 . Cytosolic pyruvate,orthophosphate dikinase functions in nitrogen remobilization during leaf senescence and limits individual seed growth and nitrogen content . The Plant Journal 62 , 641 – 652 . Google Scholar CrossRef Search ADS PubMed Truernit E , Hibberd JM . 2007 . Immunogenic tagging of chloroplasts allows their isolation from defined cell types . The Plant Journal 50 , 926 – 932 . Google Scholar CrossRef Search ADS PubMed Ueno O . 1992 . Immunogold localization of photosynthetic enzymes in leaves of Aristida latifolia, a unique C4 grass with a double chlorenchymatous bundle sheath . Physiologia Plantarum 85 , 189 – 196 . Google Scholar CrossRef Search ADS Vaughn MW , Harrington GN , Bush DR . 2002 . Sucrose-mediated transcriptional regulation of sucrose symporter activity in the phloem . Proceedings of the National Academy of Sciences, USA 99 , 10876 – 10880 . Google Scholar CrossRef Search ADS Voll L , Häusler RE , Hecker R , Weber A , Weissenböck G , Fiene G , Waffenschmidt S , Flügge UI . 2003 . The phenotype of the Arabidopsis cue1 mutant is not simply caused by a general restriction of the shikimate pathway . The Plant Journal 36 , 301 – 317 . Google Scholar CrossRef Search ADS PubMed Voll LM , Zell MB , Engelsdorf T , Saur A , Wheeler MG , Drincovich MF , Weber AP , Maurino VG . 2012 . Loss of cytosolic NADP-malic enzyme 2 in Arabidopsis thaliana is associated with enhanced susceptibility to Colletotrichum higginsianum . New Phytologist 195 , 189 – 202 . Google Scholar CrossRef Search ADS PubMed Voznesenskaya EV , Chuong SDX , Koteyeva NK , Edwards GE , Franceschi VR . 2005 . Functional compartmentation of C4 photosynthesis in the triple-layered chlorenchyma of Aristida (Poaceae) . Functional Plant Biology 32 , 67 – 77 . Google Scholar CrossRef Search ADS Weber AP , von Caemmerer S . 2010 . Plastid transport and metabolism of C3 and C4 plants – comparative analysis and possible biotechnological exploitation . Current Opinion in Plant Biology 13 , 257 – 265 . Google Scholar CrossRef Search ADS PubMed Yamaya T , Kusano M . 2014 . Evidence supporting distinct functions of three cytosolic glutamine synthetases and two NADH-glutamate synthases in rice . Journal of Experimental Botany 65 , 5519 – 5525 . Google Scholar CrossRef Search ADS PubMed Yiotis C , Manetas Y . 2010 . Sinks for photosynthetic electron flow in green petioles and pedicels of Zantedeschia aethiopica: evidence for innately high photorespiration and cyclic electron flow rates . Planta 232 , 523 – 531 . Google Scholar CrossRef Search ADS PubMed Yoshimura Y , Kubota F , Ueno O . 2004 . Structural and biochemical bases of photorespiration in C4 plants: quantification of organelles and glycine decarboxylase . Planta 220 , 307 – 317 . Google Scholar CrossRef Search ADS PubMed © 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

Uncovering C4-like photosynthesis in C3 vascular cells

Journal of Experimental Botany , Volume Advance Article (15) – Apr 19, 2018

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Oxford University Press
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© 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
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0022-0957
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10.1093/jxb/ery155
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Abstract

Abstract In C4 plants, the vascularization of the leaf is extended to include a ring of photosynthetic bundle sheath cells, which have essential and specific functions. In contrast to the substantial knowledge of photosynthesis in C4 plants, relatively little is known about photosynthesis in C3 plant veins, which differs substantially from that in C3 mesophyll cells. In this review we highlight the specific photosynthetic machinery present in C3 vascular cells, which likely evolved prior to the divergence between C3 and C4 plants. The associated primary processes of carbon recapture, nitrogen transport, and antioxidant metabolism are discussed. This review of the basal C4 photosynthesis in C3 plants is significant in the context of promoting the potential for biotechnological development of C4-transgenic rice crops. C3 vein, C4 vein, carbon recapture, nitrogen transport, photosynthesis, plant vasculature Introduction The architecture of the vascular system of plants is determined by a range of functional demands in specific tissues (Carmeliet, 2003). In broad terms, the vascular network was considered to be simply a passive conduit (Riens et al., 1991) until the late 1960s, when the vascular anatomy of C4 photosynthetic plants was defined, although the vascular structure had been known for some time (Esau and Cheadle, 1965; Dengler et al., 1994). In C4 plants, a single ring of photosynthetically active bundle sheath cells surrounds the vascular bundle. Outside these cells is a concentric ring of specialized mesophyll cells, creating the classical Kranz anatomy (Langdale, 2011). In most C4 plants, photosynthetic reactions are partitioned between bundle sheath and mesophyll cells (Hatch 1987; Sage, 2004; Brown et al., 2005). However, in certain C4 plants, such as some C4 grasses (Ueno, 1992; Voznesenskaya et al., 2005), an inner layer of mestome sheath cells, rather than the bundle sheath, serves as the site of CO2 concentration (Sage et al., 2012). In contrast to C4 plants, there is no Kranz anatomy in C3 plants, but partial C4 photosynthesis has been detected in cells surrounding the veins of C3 plants (Hibberd and Quick, 2002). These photosynthetic, chloroplast-containing cells around the C3 plant veins were termed bundle sheath cells (Esau, 1953). The incomplete C4 photosynthesis associated with C3 veins (rather than the full C4 pathway) is an adaptation from the evolutionarily more ancient C3 photosynthesis, and has specifically evolved to function efficiently at high light intensities. The parenchymatous bundle sheath cells of C3 plants participate in a variety of metabolic processes associated with carbohydrate synthesis and storage, the import and export of nitrogen and sulfur, and the metabolism of reactive oxygen species (ROS) (Leegood, 2008). However, the molecular basis of the cross-talk between these primary functions and vascular photosynthesis has yet to be elucidated. This review focuses on recent studies concerning the importance of C4-like photosynthesis in C3 veins, including the cell-specific features of their photosynthetic machinery, carbon recapture, nitrogen transport, systemic signaling, antioxidant metabolism, and development. C4-like photosynthesis in cells surrounding the vascular system The canonical C4 photosynthetic pathway comprises four steps that occur in two morphologically and biochemically distinct cell types (Fig. 1A). The first reaction of C4 photosynthesis involves conversion of CO2 to bicarbonate, catalyzed by carbonic anhydrase. The bicarbonate is then fixed by phosphoenolpyruvate carboxylase (PEPC) to form oxaloacetate, which can be further converted to malate in a reaction catalyzed by malate dehydrogenase, or to aspartate, catalyzed by aspartate aminotransferase. The malate or aspartate then diffuses into the bundle sheath cells and is subsequently decarboxylated. The released CO2 is later refixed into a three-carbon compound by ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) in the Calvin cycle. Finally, the pyruvate generated by the decarboxylation reaction diffuses back to the mesophyll chloroplasts, where it is used in the regeneration of phosphoenolpyruvate (PEP) by pyruvate orthophosphate dikinase (PPDK) (Fig. 1A) (Langdale, 2011; Majeran et al., 2008). Fig. 1. View largeDownload slide The C4-like-photosynthetic cells surrounding the vascular system of C3 plants. (A) Transverse leaf section (Langdale, 2011) and (right) photosynthesis in NADP-malic enzyme (NADP-ME) C4 plants. (B) Transverse leaf section (Langdale, 2011) and (right) vascular C4 photosynthesis of C3 plants. (C) Malate is transported through the transpiration stream of the vascular system of C3 plants, which supplies the photosynthetic cells in twig petioles and the leaf mid-veins. Vascular cells are unlikely to receive significant amounts of CO2 via stomata. Scale bars=3 μm. Fig. 1. View largeDownload slide The C4-like-photosynthetic cells surrounding the vascular system of C3 plants. (A) Transverse leaf section (Langdale, 2011) and (right) photosynthesis in NADP-malic enzyme (NADP-ME) C4 plants. (B) Transverse leaf section (Langdale, 2011) and (right) vascular C4 photosynthesis of C3 plants. (C) Malate is transported through the transpiration stream of the vascular system of C3 plants, which supplies the photosynthetic cells in twig petioles and the leaf mid-veins. Vascular cells are unlikely to receive significant amounts of CO2 via stomata. Scale bars=3 μm. Parts of the C4 cycle, involving malate-decarboxylating and PPDK activities, were discovered in cells located around the vascular system of C3 plants (Hibberd and Quick, 2002). A radiotracer analysis showed that the carbon present in the transpiration stream can be used for photosynthesis in the vascular cells, and the presence of PEPC allowed distant heterotrophic tissues, such as roots, stems, and petioles, to produce malate from PEP. This malate was found to be transported through the veins and decarboxylated in photosynthetic cells bordering the vascular system in C3 plants. The released CO2 can then be refixed to produce carbohydrates, including sucrose and starch (Fig. 1B) (Hibberd and Quick, 2002; Berveiller and Damesin, 2008; Brown et al., 2010). The photosynthetic cells surrounding the vascular system of C3 plants are therefore considered to exhibit a more spatially separated version of the C4 photosynthetic pathway (Hibberd and Quick, 2002). This feature has been observed in the vascular tissues of phylogenetically widespread C3 plants, including twigs of Pinus silvestris (Ivanov et al., 2006), petioles of Apium graveolens and Nicotiana tabacum (Hibberd and Quick, 2002), and mid-veins of Arabidopsis thaliana (Brown et al., 2010) and Oryza sativa (Shen et al., 2016) (Fig. 1C). Recapturing respiratory carbon loss Unlike in C4 plants, where CO2 diffuses from stomata into adjacent mesophyll cells and is then fixed into malate, the malate in C3 plants is derived from the respiratory activity of distant heterotrophic tissues (Fig. 1C). C3 plants lacking chlorophyll in cells close to veins showed that the rate of net photosynthesis was most affected by high partial pressures of CO2 in the leaf, and it is possible that elevated night-time temperatures lead to increased CO2 production via higher rates of respiration (Janacek et al., 2009). In addition, high temperatures are thought to be one of the drivers for the evolution of C4 photosynthesis (Sage, 2004), so it is possible that the vascular chloroplast volume in C3 plants can expand as a consequence of high rates of night-time respiration, leading to increased intercellular CO2 concentrations at the start of the day (Janacek et al., 2009). Carbon refixation in the bark of Populus tremuloides was shown to result in a 90% reduction in the rate of respiration in woody tissues (Foote and Schaedle, 1976). Similarly, refixation in 3-year-old stem sections of Pinus sylvestris resulted in a net reduction in respiration rate of 40% (Linder and Troeng, 1980). Thus, the recycling pathways in chlorophyllous veins contribute substantially to the overall carbon budget of C3 plants, although these pathways also confound efficient efflux-based estimation of the respiration rates of woody tissues and the patterns of carbohydrate allocation (Bloemen et al., 2013). Currently, their net contribution to carbon metabolism and distribution is not well understood. Expression of specific isoforms of C4 enzymes C4 plants have been categorized into three subtypes according to their different decarboxylating enzymes: the NADP-malic enzyme (NADP-ME) type, the NAD-malic enzyme (NAD-ME) type, and the phosphoenolpyruvate carboxykinase (PEPCK) type (Yoshimura et al., 2004). However, recent studies have indicated that the three decarboxylation subtypes may have flexibility between these three pathways, and that this flexibility may potentially be expressed in response to both developmental and environmental factors (Weber and von Caemmerer, 2010; Furbank 2011). The three C4 enzymes NADP-ME, NAD-ME, and PEPCK are expressed in a vein-specific distribution pattern in C3 plants, where they are predominantly localized within the vascular tissues (Chen et al., 2004; Taylor et al., 2010; Penfield et al., 2012). In the C3 species A. thaliana, it has been shown that the 5ʹ region of each decarboxylase gene is sufficient to drive expression in the veins, while the 3ʹ region enhances expression (Fig. 2A) (Brown et al., 2010). The enzymes are encoded by multigene families, members of which show vein-specific compartmentalization (Hibberd and Covshoff, 2010). Transcripts derived from both cytosolic and chloroplast-localized NADP-ME genes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME genes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) and PPDK are responsible for the primary activity in A. thaliana mid-veins (Brown et al., 2010; Taylor et al., 2010). Translatome analysis has further indicated that the ribosomes in C3 veins can preferentially translate these proteins compared with other members of the family (Fig. 2B) (Aubry et al., 2014). Thus, these C4 genes, which are differentially expressed between veins and mesophyll cells in C3 plants, likely exhibit functional specialization in C4 plants and underwent minor adaptations from their ancestral versions in C3 plants, such as evolving vein-specific trans-factors to adapt to the existing cis-elements. Fig. 2. View largeDownload slide Schematic model of the C4 photosynthetic enzymes and machinery in C3 vascular cells. (A) The 5ʹ regions of C4 acid decarboxylase genes target them to the vasculature, and the 3ʹ regions are involved in increasing the amount of their expression (Brown et al., 2010). The C3 vasculature is therefore able to preferentially transcribe and translate these proteins (Aubry et al., 2014). (B) Cytosolic and chloroplastic NADP-ME enzymes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME enzymes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) are together responsible for the high malate-decarboxylating activity in veins (Brown et al., 2010). (C) Malate is transported into vascular photosynthetic cells from the transpiration stream of the vascular system. (D) The decarboxylation of malate can generate extra NADP(H) and CO2, resulting in a unique microenvironment (high CO2 and hypoxia) and metabolic demand (high ATP/NADPH) for the vascular chloroplasts. (E) The high internal CO2 concentrations acidify the thylakoid lumen and impair the pH-dependent high-energy-state quenching (Manetas, 2004). This in turn impedes the oxygen-evolving complex (OEC) of photosystem (PSII) and PSII-driven linear electron transport to produce NADPH (Kalachanis and Manetas, 2010). Photosystem I (PSI), cytochrome b6f (Cytb6f), and ATP synthase complexes mediate cyclic electron flow to rebalance the ATP/NADPH ratio. Fig. 2. View largeDownload slide Schematic model of the C4 photosynthetic enzymes and machinery in C3 vascular cells. (A) The 5ʹ regions of C4 acid decarboxylase genes target them to the vasculature, and the 3ʹ regions are involved in increasing the amount of their expression (Brown et al., 2010). The C3 vasculature is therefore able to preferentially transcribe and translate these proteins (Aubry et al., 2014). (B) Cytosolic and chloroplastic NADP-ME enzymes (NADP-ME2 and NADP-ME4), mitochondrial NAD-ME enzymes (NAD-ME1 and NAD-ME2), and cytosolic PEPCK (PEPCK1) are together responsible for the high malate-decarboxylating activity in veins (Brown et al., 2010). (C) Malate is transported into vascular photosynthetic cells from the transpiration stream of the vascular system. (D) The decarboxylation of malate can generate extra NADP(H) and CO2, resulting in a unique microenvironment (high CO2 and hypoxia) and metabolic demand (high ATP/NADPH) for the vascular chloroplasts. (E) The high internal CO2 concentrations acidify the thylakoid lumen and impair the pH-dependent high-energy-state quenching (Manetas, 2004). This in turn impedes the oxygen-evolving complex (OEC) of photosystem (PSII) and PSII-driven linear electron transport to produce NADPH (Kalachanis and Manetas, 2010). Photosystem I (PSI), cytochrome b6f (Cytb6f), and ATP synthase complexes mediate cyclic electron flow to rebalance the ATP/NADPH ratio. Adjusting the photosynthetic machinery and photochemistry It has been proposed that high malate-decarboxylating metabolism in the veins shaped the structure and function of the photosystems in C4 plants, as well as the associated electron flow (Kalachanis and Manetas, 2010), by favoring changes in the state of photosystem I, photosystem II, and the inter-system communication (Fig. 2C, 2D) (Kotakis et al., 2006). The specific vascular photosynthetic system of C3 plants may be related to this function. Similar to the C4 bundle sheath cells, the chlorophyllous twig cortices of Eleagnus angustifolius have a low dark-adapted photosystem II photochemical efficiency, as well as limited photosystem II-driven linear electron transport (Kotakis et al., 2006). The chronic photoinhibition of photosystem II activity that occurs in these tissues is attributed to the acidification of the protoplasm by internal CO2 concentrations, leading to an impairment of the pH-dependent high-energy-state quenching, followed by a reduction in the efficiency of heat dissipation (Manetas, 2004). The primary cause of the impairment is located on the donor side (oxygen-evolving complex) of photosystem II (Kalachanis and Manetas, 2010), which is blocked by a ‘traffic jam’ consisting of selective subunits of the photosystem II protein complex, such as PsbP, PsbQ, and PsbR (Fig. 2E) (Shen et al., 2016). However, photosystem I activity is potentially sufficient for the plant to engage in cyclic electron transport to restore the ATP/NADPH ratio (Kotakis et al., 2006) and to provide the additional ATP required for the decarboxylation of malate ascending from the roots (Yiotis and Manetas, 2010). Therefore, the C3 vascular chloroplasts can be considered to have the capacity to perform photosynthesis, albeit a form that is structurally and metabolically distinct from the photosynthesis occurring in the mesophyll. Channeling nitrogen transport The fact that C4 plants utilize available nitrogen more efficiently than C3 plants is partially due to the ‘division of labor’ between bundle sheath and mesophyll cells (Moore and Black, 1979). Similarly, a subtle partitioning of nitrogen metabolism has been observed in the vascular cells of C3 plants, where nitrogen-metabolism enzymes or their encoding genes exhibit spatial localization together with C4 enzymes. For example, the cytosolic glutamine synthetase (GS1) and chloroplast NADH-glutamate synthase (NADH-GOGAT2) genes are expressed primarily in the vascular parenchyma cells of senescing rice leaf blades (Sakurai et al., 1996; Yamaya and Kusano, 2014; Bailey and Leegood, 2016). In addition, the cytosolic and mitochondrial glutamate dehydrogenases (GDH1 and GDH3) are abundant in the vascular ribosomes of C3 plants (Aubry et al., 2014), along with high levels of asparagine synthetase (Nakano et al., 2000). The coordinated localization of C4 enzymes with these nitrogen-metabolic enzymes is essential for nitrogen transport. Malate can be converted into oxaloacetate by C4 enzymes, and sequentially reassimilated into transport amino acids (glutamine, asparagine, and glutamate) via the co-localized nitrogen-compound-metabolizing enzymes (Fig. 3) (Kamachi et al., 1992; Hayakawa et al., 1994). The conversion pathway also requires the metabolic steps of the partial tricarboxylic acid (TCA) cycle in the mitochondria to provide carbon skeletons (Taylor et al., 2010). Accordingly, mutants of A. thaliana with reduced chlorophyll levels in the veins have a reduced production of TCA cycle intermediates and amino acids (Janacek et al., 2009). The absence of individual C4 decarboxylating proteins in the mid-vein also leads to a decline in malate withdrawal from the TCA cycle, thereby regulating the flux of carbon skeletons to amino acids in a light-dependent manner (Brown et al., 2010). Fig. 3. View largeDownload slide Schematic presentation of the role of the vascular C4 photosynthetic pathway in nitrogen transport, which allows carbon skeletons from malate to flow into the pathway that produces transport amino acids. The transport processes proposed are based on a portion of the tricarboxylic acid (TCA) cycle, where both the enzyme activity (shown in red) and metabolite levels (shown in blue) increase to produce transport amino acids. Cooperation of the C4 enzymes PEPCK, PPDK, and NAD(P)-ME and the nitrogen-compound-metabolizing enzymes glutamine synthetase (GS), glutamate dehydrogenase (GDH), and chloroplast NADH-glutamate synthase (NADP-GOGAT) in the vascular photosynthetic cells is required for nitrogen transport in order to convert malate to oxaloacetate and then to the transport amino acids glutamate, glutamine, and asparagine (Lin and Wu, 2004; Taylor et al., 2010). Fig. 3. View largeDownload slide Schematic presentation of the role of the vascular C4 photosynthetic pathway in nitrogen transport, which allows carbon skeletons from malate to flow into the pathway that produces transport amino acids. The transport processes proposed are based on a portion of the tricarboxylic acid (TCA) cycle, where both the enzyme activity (shown in red) and metabolite levels (shown in blue) increase to produce transport amino acids. Cooperation of the C4 enzymes PEPCK, PPDK, and NAD(P)-ME and the nitrogen-compound-metabolizing enzymes glutamine synthetase (GS), glutamate dehydrogenase (GDH), and chloroplast NADH-glutamate synthase (NADP-GOGAT) in the vascular photosynthetic cells is required for nitrogen transport in order to convert malate to oxaloacetate and then to the transport amino acids glutamate, glutamine, and asparagine (Lin and Wu, 2004; Taylor et al., 2010). Orchestrating systemic signaling and antioxidant metabolism In the C4 plant maize (Zea mays), chloroplast metabolism in the bundle sheath and mesophyll cells differs in terms of ROS defense components, in order to cope with the different cellular environments (Friso et al., 2010). Owing to their central position, leaf veins in C3 plants also have the potential to regulate ROS signaling in various cell types and organs (Kangasjärvi et al., 2009). Karpinski et al. (1999) proposed a phenomenon, termed systemic acquired acclimation, in which light stress initiates systemic signals that spread through the veins to confer stress resistance in non-exposed parts of the plant (Fig. 4A). Fig. 4. View largeDownload slide Schematic overview of signaling networks involved in systemic acquired acclimation processes in the vascular photosynthetic cells. (A) H2O2 accumulation in an Arabidopsis leaf, revealed by diaminobenzidine staining. The top half of the leaf was treated with high light, while the bottom half was kept in the dark. Note the considerable reduction in the degree of staining from the vascular tissue to the adjacent mid-vein cells (Fryer et al., 2002). (B) The structural and functional properties of the photosynthetic machinery in the vasculature contribute to cell-specific accumulation of reactive oxygen species (ROS) and antioxidant metabolism (Kangasjärvi et al., 2009; Kinsman and Pyke, 1998). I: The decarboxylation of the imported malate consumes excess NADP+. II: In chloroplasts, O2 can be used as an alternative electron acceptor in photosystem I (PSI) to promote O2- generation and the Mehler reaction (Asada, 1999; Shen et al., 2015). The thylakoid-membrane-attached Cu/Mn superoxide dismutase (SOD) catalyzes the disproportionation of O2– into H2O2 signals. III: In the plasma membrane and cell wall, NADP-ME2 is involved in meeting the demand for reducing power for the production of H2O2 through NADPH oxidase and/or peroxidases (Voll et al., 2012). IV: In the nucleus, the H2O2 signals can synergistically up-regulate the expression of cytosolic APX2 and chloroplastic Cu/Mn SOD (Rossel et al., 2007). These unique antioxidant enzymes control the bursts of H2O2 accumulation (Kangasjärvi et al., 2009). V: H2O2 accumulation initiates unidentified systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. As a result of the systemic signals, shaded leaf tissues undergo photosynthetic adjustments as well as up-regulation of the antioxidative capacity and cross-tolerance of biotic stresses (Kangasjärvi et al., 2009). This process is mediated by the zinc finger transcription factor ZAT10 (Rossel et al., 2007). Fig. 4. View largeDownload slide Schematic overview of signaling networks involved in systemic acquired acclimation processes in the vascular photosynthetic cells. (A) H2O2 accumulation in an Arabidopsis leaf, revealed by diaminobenzidine staining. The top half of the leaf was treated with high light, while the bottom half was kept in the dark. Note the considerable reduction in the degree of staining from the vascular tissue to the adjacent mid-vein cells (Fryer et al., 2002). (B) The structural and functional properties of the photosynthetic machinery in the vasculature contribute to cell-specific accumulation of reactive oxygen species (ROS) and antioxidant metabolism (Kangasjärvi et al., 2009; Kinsman and Pyke, 1998). I: The decarboxylation of the imported malate consumes excess NADP+. II: In chloroplasts, O2 can be used as an alternative electron acceptor in photosystem I (PSI) to promote O2- generation and the Mehler reaction (Asada, 1999; Shen et al., 2015). The thylakoid-membrane-attached Cu/Mn superoxide dismutase (SOD) catalyzes the disproportionation of O2– into H2O2 signals. III: In the plasma membrane and cell wall, NADP-ME2 is involved in meeting the demand for reducing power for the production of H2O2 through NADPH oxidase and/or peroxidases (Voll et al., 2012). IV: In the nucleus, the H2O2 signals can synergistically up-regulate the expression of cytosolic APX2 and chloroplastic Cu/Mn SOD (Rossel et al., 2007). These unique antioxidant enzymes control the bursts of H2O2 accumulation (Kangasjärvi et al., 2009). V: H2O2 accumulation initiates unidentified systemic signals that spread through the vasculature to confer stress resistance in non-exposed parts of the plant. As a result of the systemic signals, shaded leaf tissues undergo photosynthetic adjustments as well as up-regulation of the antioxidative capacity and cross-tolerance of biotic stresses (Kangasjärvi et al., 2009). This process is mediated by the zinc finger transcription factor ZAT10 (Rossel et al., 2007). The structural and functional properties of the photosynthetic machinery in vascular chloroplasts potentially differ from those in the mesophyll cells, associated with light-dependent accumulation of ROS (Kinsman and Pyke, 1998), which supply the reducing equivalents to mediate H2O2 signaling (Heyno et al., 2014; Kuźniak et al., 2016). The decarboxylation of malate consumes excess NADP+ on the acceptor side of photosystem I (Fig. 4B), such that O2 can be used as an alternative electron acceptor to promote the Mehler reaction and ROS metabolism (Fig. 4B) (Asada, 1999; Shen et al., 2015). In addition, NADP-ME2, which is responsible for maximal activity of NADP-ME in the veins (Brown et al., 2010), appears to be necessary for the production of ROS. NADP-ME2 is also involved in fulfilling the demand for reducing power in order to produce superoxide and H2O2 through NADPH oxidase and/or cell wall peroxidases (Fig. 4B) (Voll et al., 2012). Doulis et al. (1997) found that antioxidants in maize leaves are partitioned between mesophyll and bundle sheath cells according to the availability of NADPH. For example, ascorbate peroxidase (APX) and superoxide dismutase (SOD) enzymes are exclusively localized in the bundle sheath. The presence of this detoxification system in C4 plants agrees with the low rate of linear electron transport and oxygen-evolving-complex activity, as well as with the associated risk of generating ROS (Friso et al., 2010). The veins of the model C3 plant A. thaliana also have a versatile antioxidant network, presumably to control the generation of ROS signals and prevent oxidative damage following bursts of ROS production (Kangasjärvi et al., 2009). The cytosolic enzyme APX2 (Fryer et al., 2003), a microRNA (miR398) targeting Cu/Zn SOD (Sunkar et al., 2006), and chloroplast glutaredoxin (Cheng et al., 2006) are photosynthetically induced in the vascular tissues of C3 plants as regulators of H2O2 pulses. Other studies have confirmed that H2O2 signals can synergistically up-regulate the expression of specific isoenzymes of APX and SOD in veins (Fig. 4B) (Rossel et al., 2007). In turn, vein-specific ROS signals and antioxidant metabolism promote unidentified systemic signals that contribute to the capacity of plants to detect and signal the severity of environmental stresses (Kangasjärvi et al., 2009). The mobile signaling components are transported by the vascular system and are sensed in distal tissues (Rossel et al., 2007; Mühlenbock et al., 2008; Pogson et al., 2008). In A. thaliana, the zinc finger transcription factor ZAT10 accumulates preferentially in distal photosynthetic vascular tissues, triggering the non-exposed parts to undergo photosynthetic adjustments, as well as up-regulating antioxidative capacity and cross-tolerance against biotic stresses (Fig. 4B) (Rossel et al., 2007; Kangasjärvi et al., 2009). The vein-specific C4-like photosynthetic machinery together with ROS signaling and antioxidant metabolism are therefore proposed to promote pre-acclimation to subsequent stresses in C3 plants. Manipulating mesophyll development Vein development precedes the development of mesophyll cells, thereby positioning it for the subsequent regulation of mesophyll development (Kinsman and Pyke, 1998; Pyke et al., 1991). Specifically, the veins, and the photosynthesis associated with them, are proposed to provide essential primary metabolites or metabolic signals to the developing mesophyll cells. This phenomenon has resulted in the proposal of the supply (Fig. 5A) and the signal (Fig. 5C) hypotheses, both of which relate to genes expressed in tissues predominantly surrounding the veins (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). Janacek et al. (2009) reported that when chlorophyll accumulation in A. thaliana mutant veins was prevented, the leaves developed more slowly. The ppt1 mutant lacks a PEP/phosphate translocator in the inner membrane of the chloroplast envelope. Consequently, it cannot provide the plastidic shikimate pathway with PEP, which is essential for C3 mesophyll development. Despite the vein-specific expression of the gene in question, the ppt1 mutation was found to primarily affect chloroplast development in palisade mesophyll cells (Li et al., 1995; Streatfield et al., 1999; He et al., 2004). Fig. 5. View largeDownload slide In the young and expanding ‘sink’ leaves, the precocious vasculature and associated photosynthesis are proposed to provide essential primary metabolites or metabolic signals for developing mesophyll, resulting in the signaling (A) and supply (C) hypotheses (Lundquist et al., 2014). Malate is supplied to the vascular C4-like photosynthetic cells by the xylem stream. NADP-ME, NAD-ME, and pyruvate orthophosphate dikinase (PPDK) then catalyze the conversion of malate to PEP. The carbon skeleton is translocated by PEP/phosphate translocator 1 (PPT1) of the chloroplast envelope inner membrane to enter the shikimate pathway (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). (B) In the signaling hypothesis, dehydrodiconiferyl alcohol glucoside (DCG) is produced as a phenylpropanoid-derived secondary metabolite to regulate cell division and expansion, and can be transported to leaf mesophyll cells to trigger their development (Tamagnone et al., 1998; Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014). (D) In the supply hypothesis, the photosynthetic electron transporter plastoquinone (PQ), which is also derived from the shikimate pathway, is incorporated into the vascular photosystem to ensure the production of soluble sugars. The product, sucrose, can be supplied to growing mesophyll cells (Hibberd and Quick, 2002; Lundquist et al., 2014), where it modulates their photosynthetic development (Vaughn et al., 2002). In addition, glutamine, produced preferentially in vascular cells, is transported to feed the mesophyll cells (Kichey et al., 2005). TPT, triose-phosphate/phosphate translocator. Fig. 5. View largeDownload slide In the young and expanding ‘sink’ leaves, the precocious vasculature and associated photosynthesis are proposed to provide essential primary metabolites or metabolic signals for developing mesophyll, resulting in the signaling (A) and supply (C) hypotheses (Lundquist et al., 2014). Malate is supplied to the vascular C4-like photosynthetic cells by the xylem stream. NADP-ME, NAD-ME, and pyruvate orthophosphate dikinase (PPDK) then catalyze the conversion of malate to PEP. The carbon skeleton is translocated by PEP/phosphate translocator 1 (PPT1) of the chloroplast envelope inner membrane to enter the shikimate pathway (Knappe et al., 2003; Voll et al., 2003; Rosar et al., 2012; Lundquist et al., 2014). (B) In the signaling hypothesis, dehydrodiconiferyl alcohol glucoside (DCG) is produced as a phenylpropanoid-derived secondary metabolite to regulate cell division and expansion, and can be transported to leaf mesophyll cells to trigger their development (Tamagnone et al., 1998; Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014). (D) In the supply hypothesis, the photosynthetic electron transporter plastoquinone (PQ), which is also derived from the shikimate pathway, is incorporated into the vascular photosystem to ensure the production of soluble sugars. The product, sucrose, can be supplied to growing mesophyll cells (Hibberd and Quick, 2002; Lundquist et al., 2014), where it modulates their photosynthetic development (Vaughn et al., 2002). In addition, glutamine, produced preferentially in vascular cells, is transported to feed the mesophyll cells (Kichey et al., 2005). TPT, triose-phosphate/phosphate translocator. In the supply hypothesis (Fig. 5D), sucrose is produced by the vascular photosynthetic cells and then loaded into the phloem and translocated to growing mesophyll cells (Lundquist et al., 2014). Chlorophyll fluorescence measurements indicate that there is a reduced pool of the photosynthetic electron transporter plastoquinone, which is derived from the shikimate pathway, in the ppt1 mutant. The impairment of vascular chloroplast homeostasis may then disrupt the delivery of carbohydrate to the growing mesophyll cells (Lundquist et al., 2014). In addition, glutamine produced in vascular cells is transported to feed the mesophyll cells (Kichey et al., 2005). In the signal hypothesis (Fig. 5C), the vein-located PEP/phosphate translocator is also involved in the generation of shikimate-pathway-derived signal molecules that are transported to mesophyll cells to trigger their development. It has been proposed that dehydrodiconiferyl alcohol glucoside, a phenylpropanoid-derived secondary metabolite, acts as a molecular signal in a cytokinin-mediated pathway controlling cell division and expansion of mesophyll palisade tissue (Knappe et al., 2003; Lundquist et al., 2014; Tamagnone et al., 1998; Voll et al., 2003). Notably, photosynthesis in the vein-associated cells also plays a prominent role in providing carbon skeletons to the shikimate pathway via NADP-ME, PPDK, and NAD-ME (Hibberd and Quick, 2002; Brown et al., 2010). The presence of an endogenous PPDK in the veins circumvents the metabolic constraints of ppt1 and rescues the mutant phenotype (Voll et al., 2003). It is thought that the xylem stream can supply the vascular C4-like photosynthetic cells with malate to produce PEP. The carbon skeletons for the shikimate pathway are translocated by the PEP/phosphate translocator 1 (PPT1) into the chloroplast envelope in cells in the veins (Knappe et al., 2003; Voll et al., 2003; Lundquist et al., 2014; Rosar et al., 2012). Therefore, the co-expression of C4-type enzymes in the C3 veins has been suggested to be essential for providing metabolites and signals to regulate mesophyll development. A C4-like photosynthetic machinery is used in the photosynthetic cells of C3 plant veins to compensate for the loss of respiratory carbon, coordinate nitrogen transport, regulate systemic signaling, and direct the growth of leaves. The similarities and dissimilarities in photosynthesis between C3 and C4 plants, as well as between mesophyll and vascular cells, represent an interesting topic for future studies aiming to identify the molecular regulators of spatial and temporal differentiation of vascular chloroplasts. The reticulate mutants, a subset of variegated mutants of A. thaliana with mesophyll-specific yellowish defects but normal green veins (Lundquist et al., 2014), represent a useful system in which to elucidate the processes of vein-specific development and function in C3 leaves, and to provide insight into intercellular signaling. Gene editing technology, such the CRISPR/Cas9 system, also has great potential for application in cultivated crops in order to achieve efficient targeted mutagenesis at the veinal loci (Miao et al., 2013; Nekrasov et al., 2013). In addition, immunogenic tagging can be employed to isolate individual chloroplasts from vascular cells expressing yellow fluorescent protein on the outer surface of the chloroplasts (Truernit and Hibberd, 2007). Such techniques have the potential to reveal the factors required for vein-specific compartmentalization during development, and to elucidate their function and architecture. Furthermore, research into C3 vascular photosynthesis will likely be valuable in understanding the polyphyletic evolution of C4 photosynthesis from ancestral C3 plants and in promoting the biotechnological development of C4-transgenic rice cultivars. Acknowledgements This work was supported by the Agricultural Independent Innovation Foundation of Jiangsu Province (grant no. CX173022), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the National Natural Science Foundation of China (grant no.31271621). References Asada K . 1999 . The water-water cycle in chloroplasts: scavenging of active oxygens and dissipation of excess photons . Annual Review of Plant Physiology and Plant Molecular Biology 50 , 601 – 639 . Google Scholar CrossRef Search ADS PubMed Aubry S , Smith-Unna RD , Boursnell CM , Kopriva S , Hibberd JM . 2014 . Transcript residency on ribosomes reveals a key role for the Arabidopsis thaliana bundle sheath in sulfur and glucosinolate metabolism . The Plant Journal 78 , 659 – 673 . Google Scholar CrossRef Search ADS PubMed Bailey KJ , Leegood RC . 2016 . Nitrogen recycling from the xylem in rice leaves: dependence upon metabolism and associated changes in xylem hydraulics . Journal of Experimental Botany 67 , 2901 – 2911 . Google Scholar CrossRef Search ADS PubMed Berveiller D , Damesin C . 2008 . Carbon assimilation by tree stems: potential involvement of phosphoenolpyruvate carboxylase . Trees – Structure and Function 22 , 149 – 157 . Google Scholar CrossRef Search ADS Bloemen J , McGuire MA , Aubrey DP , Teskey RO , Steppe K . 2013 . Transport of root-respired CO₂ via the transpiration stream affects aboveground carbon assimilation and CO₂ efflux in trees . New Phytologist 197 , 555 – 565 . Google Scholar CrossRef Search ADS PubMed Brown NJ , Parsley K , Hibberd JM . 2005 . The future of C4 research – maize, Flaveria or Cleome ? Trends in Plant Science 10 , 215 – 221 . Google Scholar CrossRef Search ADS PubMed Brown NJ , Palmer BG , Stanley S et al. 2010 . C4 acid decarboxylases required for C4 photosynthesis are active in the mid-vein of the C3 species Arabidopsis thaliana, and are important in sugar and amino acid metabolism . The Plant Journal 61 , 122 – 133 . Google Scholar CrossRef Search ADS PubMed Carmeliet P . 2003 . Blood vessels and nerves: common signals, pathways and diseases . Nature Reviews. Genetics 4 , 710 – 720 . Google Scholar CrossRef Search ADS PubMed Chen ZH , Walker RP , Técsi LI , Lea PJ , Leegood RC . 2004 . Phosphoenolpyruvate carboxykinase in cucumber plants is increased both by ammonium and by acidification, and is present in the phloem . Planta 219 , 48 – 58 . Google Scholar CrossRef Search ADS PubMed Cheng NH , Liu JZ , Brock A , Nelson RS , Hirschi KD . 2006 . AtGRXcp, an Arabidopsis chloroplastic glutaredoxin, is critical for protection against protein oxidative damage . The Journal of Biological Chemistry 281 , 26280 – 26288 . Google Scholar CrossRef Search ADS PubMed Dengler NG , Dengler RE , Donnelly PM , Hattersley PW . 1994 . Quantitative leaf anatomy of C3 and C4 grasses (Poaceae): bundle sheath and mesophyll surface area relationships . Annals of Botany 73 , 241 – 255 . Google Scholar CrossRef Search ADS Doulis AG , Debian N , Kingston-Smith AH , Foyer CH . 1997 . Differential localization of antioxidants in maize leaves . Plant Physiology 114 , 1031 – 1037 . Google Scholar CrossRef Search ADS PubMed Esau K . 1953 . Plant anatomy . New York : John Wiley & Sons . Esau K , Cheadle , VI . 1965 . Cytologic studies on phloem . University of California (Berkeley) Publications of Botany 36 , 253 – 344 . Foote KC , Schaedle M . 1976 . Diurnal and seasonal patterns of photosynthesis and respiration by stems of Populus tremuloides Michx . Plant Physiology 58 , 651 – 655 . Google Scholar CrossRef Search ADS PubMed Friso G , Majeran W , Huang M , Sun Q , van Wijk KJ . 2010 . Reconstruction of metabolic pathways, protein expression, and homeostasis machineries across maize bundle sheath and mesophyll chloroplasts: large-scale quantitative proteomics using the first maize genome assembly . Plant Physiology 152 , 1219 – 1250 . Google Scholar CrossRef Search ADS PubMed Fryer MJ , Ball L , Oxborough K , Karpinski S , Mullineaux PM , Baker NR . 2003 . Control of Ascorbate Peroxidase 2 expression by hydrogen peroxide and leaf water status during excess light stress reveals a functional organisation of Arabidopsis leaves . The Plant Journal 33 , 691 – 705 . Google Scholar CrossRef Search ADS PubMed Fryer MJ , Oxborough K , Mullineaux PM , Baker NR . 2002 . Imaging of photo-oxidative stress responses in leaves . Journal of Experimental Botany 53 , 1249 – 1254 . Google Scholar PubMed Furbank RT . 2011 . Evolution of the C4 photosynthetic mechanism: are there really three C4 acid decarboxylation types ? Journal of Experimental Botany 62 , 3103 – 3108 . Google Scholar CrossRef Search ADS PubMed Hatch MD . 1987 . C4 photosynthesis: a unique blend of modified biochemistry, anatomy and ultrastructure . Biochimica et Biophysica Acta 895 , 81 – 106 . Google Scholar CrossRef Search ADS Hayakawa T , Nakamura T , Hattori F , Mae T , Ojima K , Yamaya T . 1994 . Cellular localization of NADH-dependent glutamate-synthase protein in vascular bundles of unexpanded leaf blades and young grains of rice plants . Planta 193 , 455 – 460 . Google Scholar CrossRef Search ADS He Y , Tang RH , Hao Y et al. 2004 . Nitric oxide represses the Arabidopsis floral transition . Science 305 , 1968 – 1971 . Google Scholar CrossRef Search ADS PubMed Heyno E , Innocenti G , Lemaire SD , Issakidis-Bourguet E , Krieger-Liszkay A . 2014 . Putative role of the malate valve enzyme NADP-malate dehydrogenase in H2O2 signalling in Arabidopsis . Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 369 , 20130228 . Google Scholar CrossRef Search ADS PubMed Hibberd JM , Covshoff S . 2010 . The regulation of gene expression required for C4 photosynthesis . Annual Review of Plant Biology 61 , 181 – 207 . Google Scholar CrossRef Search ADS PubMed Hibberd JM , Quick WP . 2002 . Characteristics of C4 photosynthesis in stems and petioles of C3 flowering plants . Nature 415 , 451 – 454 . Google Scholar CrossRef Search ADS PubMed Ivanov AG , Krol M , Sveshnikov D , Malmberg G , Gardeström P , Hurry V , Oquist G , Huner NP . 2006 . Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine . Planta 223 , 1165 – 1177 . Google Scholar CrossRef Search ADS PubMed Janacek SH , Trenkamp S , Palmer B et al. 2009 . Photosynthesis in cells around veins of the C3 plant Arabidopsis thaliana is important for both the shikimate pathway and leaf senescence as well as contributing to plant fitness . The Plant Journal 59 , 329 – 343 . Google Scholar CrossRef Search ADS PubMed Kalachanis D , Manetas Y . 2010 . Analysis of fast chlorophyll fluorescence rise (O-K-J-I-P) curves in green fruits indicates electron flow limitations at the donor side of PSII and the acceptor sides of both photosystems . Physiologia Plantarum 139 , 313 – 323 . Google Scholar PubMed Kamachi K , Yamaya T , Hayakawa T , Mae T , Ojima K . 1992 . Vascular bundle-specific localization of cytosolic glutamine synthetase in rice leaves . Plant Physiology 99 , 1481 – 1486 . Google Scholar CrossRef Search ADS PubMed Kangasjärvi S , Nurmi M , Tikkanen M , Aro EM . 2009 . Cell-specific mechanisms and systemic signalling as emerging themes in light acclimation of C3 plants . Plant cell & Environment 32 , 1230 – 1240 . Google Scholar CrossRef Search ADS Karpinski S , Reynolds H , Karpinska B , Wingsle G , Creissen G , Mullineaux P . 1999 . Systemic signaling and acclimation in response to excess excitation energy in Arabidopsis . Science 284 , 654 – 657 . Google Scholar CrossRef Search ADS PubMed Kichey T , Le Gouis J , Sangwan B , Hirel B , Dubois F . 2005 . Changes in the cellular and subcellular localization of glutamine synthetase and glutamate dehydrogenase during flag leaf senescence in wheat (Triticum aestivum L.) . Plant & Cell Physiology 46 , 964 – 974 . Google Scholar CrossRef Search ADS PubMed Kinsman EA , Pyke KA . 1998 . Bundle sheath cells and cell-specific plastid development in Arabidopsis leaves . Development 125 , 1815 – 1822 . Google Scholar PubMed Knappe S , Löttgert T , Schneider A , Voll L , Flügge UI , Fischer K . 2003 . Characterization of two functional phosphoenolpyruvate/phosphate translocator (PPT) genes in Arabidopsis – AtPPT1 may be involved in the provision of signals for correct mesophyll development . The Plant Journal 36 , 411 – 420 . Google Scholar CrossRef Search ADS PubMed Kotakis CH , Petropoulou Y , Stamatakis K , Yiotis CH , Manetas Y . 2006 . Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity . Planta 225 , 245 – 253 . Google Scholar CrossRef Search ADS PubMed Kuźniak E , Kornas A , Kaźmierczak A , Rozpądek P , Nosek M , Kocurek M , Zellnig G , Müller M , Miszalski Z . 2016 . Photosynthesis-related characteristics of the midrib and the interveinal lamina in leaves of the C3-CAM intermediate plant Mesembryanthemum crystallinum . Annals of Botany 117 , 1141 – 1151 . Google Scholar CrossRef Search ADS PubMed Langdale JA . 2011 . C4 cycles: past, present, and future research on C4 photosynthesis . The Plant Cell 23 , 3879 – 3892 . Google Scholar CrossRef Search ADS PubMed Leegood RC . 2008 . Roles of the bundle sheath cells in leaves of C3 plants . Journal of Experimental Botany 59 , 1663 – 1673 . Google Scholar CrossRef Search ADS PubMed Li H , Culligan K , Dixon RA , Chory J . 1995 . CUE1: a mesophyll cell-specific positive regulator of light-controlled gene expression in Arabidopsis . The Plant Cell 7 , 1599 – 1610 . Google Scholar CrossRef Search ADS PubMed Lin JF , Wu SH . 2004 . Molecular events in senescing Arabidopsis leaves . The Plant Journal 39 , 612 – 628 . Google Scholar CrossRef Search ADS PubMed Linder S , Troeng E . 1980 . Photosynthesis and transpiration of 20-year-old Scots pine . Ecological Bulletins 32 , 153 – 163 . Lundquist PK , Rosar C , Bräutigam A , Weber AP . 2014 . Plastid signals and the bundle sheath: mesophyll development in reticulate mutants . Molecular Plant 7 , 14 – 29 . Google Scholar CrossRef Search ADS PubMed Majeran W , Zybailov B , Ytterberg AJ , Dunsmore J , Sun Q , van Wijk KJ . 2008 . Consequences of C4 differentiation for chloroplast membrane proteomes in maize mesophyll and bundle sheath cells . Molecular & Cellular Proteomics 7 , 1609 – 1638 . Google Scholar CrossRef Search ADS Manetas Y . 2004 . Probing corticular photosynthesis through in vivo chlorophyll fluorescence measurements: evidence that high internal CO2 levels suppress electron flow and increase the risk of photoinhibition . Physiologia Plantarum 120 , 509 – 517 . Google Scholar CrossRef Search ADS PubMed Miao J , Guo D , Zhang J , Huang Q , Qin G , Zhang X , Wan J , Gu H , Qu LJ . 2013 . Targeted mutagenesis in rice using CRISPR-Cas system . Cell Research 23 , 1233 – 1236 . Google Scholar CrossRef Search ADS PubMed Moore R , Black CC . 1979 . Nitrogen assimilation pathways in leaf mesophyll and bundle sheath cells of C4 photosynthesis plants formulated from comparative studies with Digitaria sanguinalis (L.) Scop . Plant Physiology 64 , 309 – 313 . Google Scholar CrossRef Search ADS PubMed Mühlenbock P , Szechynska-Hebda M , Plaszczyca M , Baudo M , Mateo A , Mullineaux PM , Parker JE , Karpinska B , Karpinski S . 2008 . Chloroplast signaling and LESION SIMULATING DISEASE1 regulate crosstalk between light acclimation and immunity in Arabidopsis . The Plant Cell 20 , 2339 – 2356 . Google Scholar CrossRef Search ADS PubMed Nakano K , Suzuki T , Hayakawa T , Yamaya T . 2000 . Organ and cellular localization of asparagine synthetase in rice plants . Plant & Cell Physiology 41 , 874 – 880 . Google Scholar CrossRef Search ADS PubMed Nekrasov V , Staskawicz B , Weigel D , Jones JD , Kamoun S . 2013 . Targeted mutagenesis in the model plant Nicotiana benthamiana using Cas9 RNA-guided endonuclease . Nature Biotechnology 31 , 691 – 693 . Google Scholar CrossRef Search ADS PubMed Penfield S , Clements S , Bailey KJ , Gilday AD , Leegood RC , Gray JE , Graham IA . 2012 . Expression and manipulation of PHOSPHOENOLPYRUVATE CARBOXYKINASE 1 identifies a role for malate metabolism in stomatal closure . The Plant Journal 69 , 679 – 688 . Google Scholar CrossRef Search ADS PubMed Pogson BJ , Woo NS , Förster B , Small ID . 2008 . Plastid signalling to the nucleus and beyond . Trends in Plant Science 13 , 602 – 609 . Google Scholar CrossRef Search ADS PubMed Pyke KA , Marrison JL , Leech AM . 1991 . Temporal and spatial development of the cells of the expanding first leaf of Arabidopsis thaliana (L.) Heynh . Journal of Experimental Botany 42 , 1407 – 1416 . Google Scholar CrossRef Search ADS Riens B , Lohaus G , Heineke D , Heldt HW . 1991 . Amino acid and sucrose content determined in the cytosolic, chloroplastic, and vacuolar compartments and in the phloem sap of spinach leaves . Plant Physiology 97 , 227 – 233 . Google Scholar CrossRef Search ADS PubMed Rosar C , Kanonenberg K , Nanda AM , Mielewczik M , Bräutigam A , Novák O , Strnad M , Walter A , Weber AP . 2012 . The leaf reticulate mutant dov1 is impaired in the first step of purine metabolism . Molecular Plant 5 , 1227 – 1241 . Google Scholar CrossRef Search ADS PubMed Rossel JB , Wilson PB , Hussain D et al. 2007 . Systemic and intracellular responses to photooxidative stress in Arabidopsis . The Plant Cell 19 , 4091 – 4110 . Google Scholar CrossRef Search ADS PubMed Sage RF . 2004 . The evolution of C4 photosynthesis . New Phytologist 161 , 341 – 370 . Google Scholar CrossRef Search ADS Sage RF , Sage TL , Kocacinar F . 2012 . Photorespiration and the evolution of C4 photosynthesis . Annual Review of Plant Biology 63 , 19 – 47 . Google Scholar CrossRef Search ADS PubMed Sakurai N , Hayakawa T , Nakamura T , Yamaya T . 1996 . Changes in the cellular localization of cytosolic glutamine synthetase protein in vascular bundles of rice leaves at various stages of development . Planta 200 , 306 – 311 . Google Scholar CrossRef Search ADS Shen WJ , Chen GX , Xu JG , Zhen XH , Ma J , Zhang XJ , Lv CG , Gao ZP . 2015 . High light acclimation of Oryza sativa L. leaves involves specific photosynthetic-sourced changes of NADPH/NADP⁺ in the midvein . Protoplasma 252 , 77 – 87 . Google Scholar CrossRef Search ADS PubMed Shen WJ , Ye LH , Ma J et al. 2016 . The existence of C4-bundle-sheath-like photosynthesis in the mid-vein of C3 rice . Rice 9 , 1 – 20 . Google Scholar CrossRef Search ADS PubMed Streatfield SJ , Weber A , Kinsman EA et al. 1999 . The phosphoe- nolpyruvate/phosphate translocator is required for phenolic metabolism, palisade cell development, and plastid-dependent nuclear gene expression . The Plant Cell 11 , 1609 – 1622 . Google Scholar CrossRef Search ADS PubMed Sunkar R , Kapoor A , Zhu JK . 2006 . Posttranscriptional induction of two Cu/Zn superoxide dismutase genes in Arabidopsis is mediated by downregulation of miR398 and important for oxidative stress tolerance . The Plant Cell 18 , 2051 – 2065 . Google Scholar CrossRef Search ADS PubMed Tamagnone L , Merida A , Stacey N et al. 1998 . Inhibition of phenolic acid metabolism results in precocious cell death and altered cell morphology in leaves of transgenic tobacco plants . The Plant Cell 10 , 1801 – 1816 . Google Scholar CrossRef Search ADS PubMed Taylor L , Nunes-Nesi A , Parsley K , Leiss A , Leach G , Coates S , Wingler A , Fernie AR , Hibberd JM . 2010 . Cytosolic pyruvate,orthophosphate dikinase functions in nitrogen remobilization during leaf senescence and limits individual seed growth and nitrogen content . The Plant Journal 62 , 641 – 652 . Google Scholar CrossRef Search ADS PubMed Truernit E , Hibberd JM . 2007 . Immunogenic tagging of chloroplasts allows their isolation from defined cell types . The Plant Journal 50 , 926 – 932 . Google Scholar CrossRef Search ADS PubMed Ueno O . 1992 . Immunogold localization of photosynthetic enzymes in leaves of Aristida latifolia, a unique C4 grass with a double chlorenchymatous bundle sheath . Physiologia Plantarum 85 , 189 – 196 . Google Scholar CrossRef Search ADS Vaughn MW , Harrington GN , Bush DR . 2002 . Sucrose-mediated transcriptional regulation of sucrose symporter activity in the phloem . Proceedings of the National Academy of Sciences, USA 99 , 10876 – 10880 . Google Scholar CrossRef Search ADS Voll L , Häusler RE , Hecker R , Weber A , Weissenböck G , Fiene G , Waffenschmidt S , Flügge UI . 2003 . The phenotype of the Arabidopsis cue1 mutant is not simply caused by a general restriction of the shikimate pathway . The Plant Journal 36 , 301 – 317 . Google Scholar CrossRef Search ADS PubMed Voll LM , Zell MB , Engelsdorf T , Saur A , Wheeler MG , Drincovich MF , Weber AP , Maurino VG . 2012 . Loss of cytosolic NADP-malic enzyme 2 in Arabidopsis thaliana is associated with enhanced susceptibility to Colletotrichum higginsianum . New Phytologist 195 , 189 – 202 . Google Scholar CrossRef Search ADS PubMed Voznesenskaya EV , Chuong SDX , Koteyeva NK , Edwards GE , Franceschi VR . 2005 . Functional compartmentation of C4 photosynthesis in the triple-layered chlorenchyma of Aristida (Poaceae) . Functional Plant Biology 32 , 67 – 77 . Google Scholar CrossRef Search ADS Weber AP , von Caemmerer S . 2010 . Plastid transport and metabolism of C3 and C4 plants – comparative analysis and possible biotechnological exploitation . Current Opinion in Plant Biology 13 , 257 – 265 . Google Scholar CrossRef Search ADS PubMed Yamaya T , Kusano M . 2014 . Evidence supporting distinct functions of three cytosolic glutamine synthetases and two NADH-glutamate synthases in rice . Journal of Experimental Botany 65 , 5519 – 5525 . Google Scholar CrossRef Search ADS PubMed Yiotis C , Manetas Y . 2010 . Sinks for photosynthetic electron flow in green petioles and pedicels of Zantedeschia aethiopica: evidence for innately high photorespiration and cyclic electron flow rates . Planta 232 , 523 – 531 . Google Scholar CrossRef Search ADS PubMed Yoshimura Y , Kubota F , Ueno O . 2004 . Structural and biochemical bases of photorespiration in C4 plants: quantification of organelles and glycine decarboxylase . Planta 220 , 307 – 317 . Google Scholar CrossRef Search ADS PubMed © 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)

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Journal of Experimental BotanyOxford University Press

Published: Apr 19, 2018

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