Plant Physiology | DeepDyve

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2019 Plant Physiology

Stress-Induced Kinases Rapidly Relocate To Plasmodesmata Plasmodesmata are membranous pores that span the plant cell wall, creating both cytoplasmic and membrane continua between cells. By interconnecting most cells throughout the entire plant body, plasmodesmata form a symplastic network that supports and controls the movement of molecules from cell to cell, within a given tissue or organ. By regulating intercellular communication, plasmodesmata orchestrate processes related to plant growth and development as well as responses to pathogens and abiotic stresses. Plasmodesmata also act as specialized signaling hubs, capable of generating and/or relaying signaling from cell to cell through plasmodesmata-associated receptor activity. Grison et al. (pp. 142–160) show that specific plasma membrane-localized Leu-rich repeat receptor-like kinases, Qiān Shŏu (thousand hands) kinase (QSK1) and inflorescence meristem kinase2, which under optimal growth conditions are absent from plasmodesmata, rapidly relocate and cluster to the pores in response to osmotic stress. This unusual process is remarkably fast and is independent of sterol and sphingolipid membrane composition. Focusing on QSK1, previously reported to be involved in stress responses, the authors show that relocalization in response to mannitol depends on QSK1 phosphorylation. Loss-of-function mutation in QSK1 results in delayed lateral root development, and the mutant is affected in the root response to mannitol stress. Callose-mediated plasmodesmata regulation is known to regulate lateral root development. Callose levels are reduced in the qsk1 mutant background with a root phenotype resembling ectopic expression of plasmodesmata-localized β-1,3-glucanase, an enzyme that degrades callose at the pores. The results presented emphasize the dynamic nature of plasmodesmata membrane domains, which can within a few minutes of stimulation recruit plasma membrane-located receptor-like proteins that presumably trigger local mechanisms to regulate plasmodesmata aperture and, thereby, plant developmental response to environmental stresses. Cytoskeletal Targets of an Auxin Transport Inhibitor The phytohormone auxin plays critical roles in various plant developmental programs by controlling cell expansion and polarity as well as organ patterning. Auxin action relies on polar transport through different plant tissues. Auxin transport inhibitors are important tools for understanding auxin-dependent plant development. One mode of auxin transport inhibition involves changes in actin dynamics. A major role of the actin cytoskeleton in auxin transport is to regulate vesicular trafficking of auxin efflux carriers of the PIN-FORMED (PIN) family. PIN proteins show a polar localization that determines the direction of auxin flow, and PIN targeting is a highly dynamic process with constitutive cycling between the plasma membrane and endosomal compartments. The actin cytoskeleton is required for both endocytosis and recycling of PINs. Zou et al. (pp. 161–178) have characterized the action of 2,3,5-triiodobenzoic acid (TIBA) on actin dynamics. By means of live-cell imaging and quantitative analyses of actin dynamics, it was determined that TIBA increases actin filament abundance and bundling in root epidermal cells. Changes in actin organization resulted from reduced actin turnover and enhanced actin bundle formation. By surveying candidate actin-binding protein mutants with reduced TIBA sensitivity, the authors determined that Arabidopsis (Arabidopsis thaliana) villins, a class of Ca2+-dependent, actin-binding cytoskeletal proteins, contribute to TIBA action. TIBA-induced actin dynamics were altered in vln4 knockout mutants. TIBA directly binds to the C-terminal headpiece domain of VLN4 and promotes oligomerization of VLN4, leading to enhanced formation of actin bundles. These findings provide insights into the molecular mechanism by which villins contribute to the action of TIBA on the actin cytoskeleton, supporting the important role of actin dynamics in the mechanism of auxin transport. Systems Analysis of Lignin Mutants Lignin is a complex polymer deposited in plant cell walls that provides mechanical support, facilitates the transport of water and solutes through the vascular system, and aids in plant defense. Lignin waterproofs plant cells by providing a hydrophobic environment by chemical bonding with cellulose and hemicellulose. This lignin-polysaccharide matrix underlies the recalcitrance that limits chemical, enzymatic, and microbial digestion of plant cell walls, thereby hindering the conversion of lignocellulosic biomass to liquid biofuels. Genetic modification of the lignin pathway, however, can result in diverse and unpredictable impacts on plant physiology, especially the development of dwarf plants. In spite of extensive studies on individual dwarf lignin mutants in several different species, there is no clear agreement as to whether a common mechanism is responsible for these dwarf phenotypes. To address this question, Ha et al. (pp. 63–84) have analyzed transposon insertion in various single mutants of Medicago truncatula that have specific defects in the monolignol pathway as well as one double mutant. These plants exhibit growth phenotypes from essentially wild type to severely retarded. Extensive phenotypic, transcriptomic, and metabolomics analyses, including structural characterization of differentially expressed compounds, revealed diverse phenotypic consequences of the lignin pathway. Notable phenotypes among the mutants with severe growth impairment were increased trichome numbers, accumulation of a variety of triterpene saponins, and extensive but differential ectopic expression of defense response genes. The authors propose that reallocation of resources into defense pathways is linked to the severity of the final growth phenotype in monolignol pathway mutants of M. truncatula, although it remains unclear whether this is a cause or an effect of the growth impairment. A Circadian Clock Protein Regulates Fitness under Water Limitation The circadian clock of plants coordinates many molecular, physiological, and metabolic processes to optimize the plant's health and survival in an ever-changing environment. The core circadian clock component TIMING OF CAB EXPRESSION1 (TOC1) integrates environmental stress responses in plants through its effects on abscisic acid signaling and growth suppression. The interaction of TOC1 with PHYTOCHROME B under the high far-red to red light conditions that characterize the end of day, when TOC1 is expressed, suggests a connection between circadian control of light responses and sensitivity to abscisic acid. Using micrografting and experimentally controlled drought scenarios in both field and glasshouse experiments, Valim et al. (pp. 305–318) studied the desert annual, Nicotiana attenuata, to explore the consequences of silencing TOC1 on fitness in the face of drought. TOC1-silenced plants displayed altered physiological responses to drought, including decreased leaf water loss and increased water-use efficiency. However, the authors found that TOC1-silenced plants incurred severe fitness disadvantages under synchronized drought stress and a controlled watering regime. Despite marked decreases in leaf water loss, TOC1-deficient lines failed to maintain fitness in response to drought stress as measured by total seed capsule production. Restoring TOC1 transcript levels in shoots via micrografting was sufficient to restore wild-type drought responses under field conditions. Microarray data identified a coexpression module in leaves strongly linking red and far-red light signaling to drought responses in a TOC1-dependent manner, but experiments with phytochrome-deficient lines revealed that the effects of TOC1 deficiency under drought cannot be attributed to changes in red/far-red light perception alone. These results reveal a role for TOC1 in integrating developmental as well as physiological responses for maintaining fitness in the face of drought. How Carrots Get Their Colors Carrot (Daucus carota ssp. sativus) are classified into two groups: the carotene group (variety sativus) and the anthocyanin group (variety atrorubens). Carotene group members, also known as nonpurple carrots, accumulate massive amounts of carotenoids in their roots. Anthocyanin group members, also known as purple carrots, accumulate high levels of anthocyanins in their roots. Anthocyanins are water-soluble flavonoid compounds and confer red, blue, and purple pigmentation to plants. In addition to their important roles in plants, they are also beneficial human nutrients. Nonpurple carrots are considered to have arisen from purple carrots that acquired mutations. In addition, purple carrot root pigmentation extensively varies across different carrot genotypes, ranging from the purple peridermal carrot type (purple periderm but nonpurple phloem and xylem) to the solid purple carrot type (purple periderm, phloem, and xylem). A quantitative trait locus associated with anthocyanin pigmentation in purple carrot roots has been identified on chromosome 3 and includes two candidate genes, DcMYB6 and DcMYB7. Xu et al. (pp. 195–207) have characterized the functions of DcMYB6 and DcMYB7 in carrots. Overexpression of DcMYB7, but not DcMYB6, in the orange carrot ‘Kurodagosun’ led to anthocyanin accumulation in roots. Knockout of DcMYB7 in the solid purple (purple periderm, phloem, and xylem) carrot ‘Deep Purple’ resulted in carrots with yellow roots. Evidence is presented that supports the hypothesis that another genetic factor suppresses DcMYB7 expression in the phloem and xylem of purple peridermal carrot root tissues. These results improve our understanding of the molecular mechanism behind the origin of the nonpurple root phenotype during carrot domestication and are also valuable for breeding programs aimed at modifying anthocyanin biosynthesis in carrot and other plant species. Regulation of Pavement Cell Morphogenesis Simple plant cell morphologies, such as cylindrical shoot cells, are determined by the extensibility pattern of the primary cell wall, which is thought to be largely dominated by cellulose microfibrils, but the mechanism leading to more complex shapes, such as the brick-shaped or jigsaw-like patterns in the epidermal pavement cells of many eudicot leaves, is much less well understood. Details about the manner in which cell wall polymers at the periclinal wall regulate the morphogenetic process in epidermal pavement cells and mechanistic information about the initial steps leading to the characteristic undulations in the cell borders are elusive. Altatouri et al. (pp. 127–141) have used genetics and imaging methods to study the impact of the spatiotemporal dynamics of cellulose and homogalacturonan pectin distribution during lobe formation in the epidermal pavement cells of Arabidopsis cotyledons. Their data suggest that lobe initiation involves a modulation of cell wall stiffness through local enrichment in demethylated pectin, whereas a subsequent increase in lobe amplitude is mediated by the stress-induced deposition of aligned cellulose microfibrils. The results of this study provide evidence for the initiation mechanism of cell shaping and the roles of cellulose microfibrils and demethyl-esterified pectin during the developmental process of the undulations of pavement cells. Author notes www.plantphysiol.org/cgi/doi/10.1104/pp.19.00986 © 2019 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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On the Inside

Minorsky, Peter V.

2019 Plant Physiology

doi: 10.1104/pp.19.00986pmid: 33814622

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Enhancing Wheat Rubisco Activase Thermostability by Mutagenesis of Conserved Residues from Heat-Adapted Species

Annunziata, Maria Grazia

2019 Plant Physiology

doi: 10.1104/pp.19.00794pmid: 31467136

Rubisco is the central enzyme of photosynthetic carbon assimilation in the Calvin-Benson-Bassham (CBB) cycle. It catalyzes the carboxylation of one molecule of ribulose-1,5-bisphosphate (RuBP) producing two molecules of 3-phosphoglycerate, from which sugars, starch, amino acids, and fatty acids are synthesized (Bhat et al., 2017). The catalytically active form of Rubisco is tightly regulated. Its activation (carbamylation) occurs spontaneously in the presence of CO2 and Mg2+ and allows the correct position of RuBP for efficient electrophilic attack by the second CO2 molecule that will be fixed in the CBB cycle (Andersson, 2008). Instead, Rubisco activity is impaired by several sugar phosphates, even by excess RuBP (Sulpice et al., 2007). The release of sugar phosphates from Rubisco is facilitated by its catalytic chaperone Rubisco activase (Rca), allowing spontaneous re‐carbamylation (Portis, 2003). Rca belongs to the AAA+ protein superfamily and uses ATP to remodel Rubisco and release the sugar phosphate inhibitors (Mueller-Cajar, 2017). Although Rca enhances Rubisco activity, it is heat labile; therefore, during heat stress it limits the photosynthetic machinery. In light of the negative impact of global warming on crop productivity, improving the thermostability of Rca is considered one of the most promising approaches to enhance heat tolerance of photosynthesis. In this issue of Plant Physiology, Scafaro et al. (2019) report the relative expression and protein abundance of the three wheat Rca isoforms—TaRca1-β, TaRca2-α, and TaRca2-β—and characterize their in vitro thermal stability. Spring and winter wheat cultivars at different developmental stages were exposed to high temperatures (36°C and 38°C, respectively). Heat induced the expression of the TaRca1-β gene in both cultivars, suggesting that the wheat heat stress response is mediated by the corresponding protein. To test this hypothesis, the three TaRca isoforms were extracted and isolated from leaves of the two cultivars and used for an Rca in vitro activity assay. Furthermore, the temperature responsible for the loss of enzyme functionality and structural integrity was determined by differential scanning fluorimetry and enzymatic rate of preheated protein. These different approaches confirmed that the TaRca1-β isoform is indeed a heat stable variant of Rca for wheat, whereas the TaRca2-β isoform is the most heat sensitive. In order to improve TaRca2-β thermal stability, Scafaro and coworkers generated different mutations based on protein sequence alignments with thermostable wheat and rice isoforms and with protein sequences of cold and warm adapted species. They discovered three to 11 mutated amino acids able to confer thermostability to the enzyme, and each mutation gave an additive affect to the thermal stability of the enzyme. In conclusion, Scafaro et al. (2019) found that inducing the expression of TaRca1-β—the most thermally stable Rca isoform in wheat—is a mechanism that wheat has evolved in response to heat stress. Moreover, thermal stability of the TaRca2-β isoform was increased in the mutational variants created. Their discoveries suggest that engineering Rca and/or manipulating its regulation could be possible strategies to improve Rubisco carboxylation efficiency in crop plants. Also promising could be coengineering of Rubisco and Rca, mimicking the process that probably occurred during natural evolution. However, additional work is needed, especially in planta studies, to test the hypothesis that thermostable Rca can improve photosynthesis under elevated temperature. LITERATURE CITED Andersson I ( 2008 ) Catalysis and regulation in Rubisco . J Exp Bot 59 : 1555 – 1568 Google Scholar Crossref Search ADS PubMed WorldCat Bhat JY , Thieulin-Pardo G, Hartl FU, Hayer-Hartl M ( 2017 ) Rubisco activases: AAA+ chaperones adapted to enzyme repair . Front Mol Biosci 4 : 20 Google Scholar Crossref Search ADS PubMed WorldCat Mueller-Cajar O ( 2017 ) The diverse AAA+ machines that repair inhibited rubisco active sites . Front Mol Biosci 4 : 31 Google Scholar Crossref Search ADS PubMed WorldCat Portis AR , Jr . ( 2003 ) Rubisco activase - Rubisco's catalytic chaperone . Photosynth Res 75 : 11 – 27 Google Scholar Crossref Search ADS PubMed WorldCat Scafaro AP , Bautsoens N, den Boer B, Van Rie J, Gallé A ( 2019 ) Improving the thermostability of Rubisco activase from wheat . Plant Physiol 181 : 43 – 54 Google Scholar Crossref Search ADS PubMed WorldCat Sulpice R , Tschoep H, VON Korff M, Büssis D, Usadel B, Höhne M, Witucka-Wall H, Altmann T, Stitt M, Gibon Y ( 2007 ) Description and applications of a rapid and sensitive non-radioactive microplate-based assay for maximum and initial activity of d-ribulose-1,5-bisphosphate carboxylase/oxygenase . Plant Cell Environ 30 : 1163 – 1175 Google Scholar Crossref Search ADS PubMed WorldCat Author notes 2 Senior author. www.plantphysiol.org/cgi/doi/10.1104/pp.19.00794 © 2019 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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Liguleless1, a Conserved Gene Regulating Leaf Angle and a Target for Yield Improvement in Wheat

Yu, Yunqing

2019 Plant Physiology

doi: 10.1104/pp.19.00872pmid: 31467137

A major challenge in modern agriculture is to increase crop yield with diminishing agricultural land to feed the fast-growing global population. The erect leaf phenotype is a highly desirable trait that allows for higher planting density and maximizes light interception capacity and thus increases photosynthetic efficiency and crop yield (Donald, 1968). In cereal crops, which all belong to the grass family (Poaceae), a leaf is composed of a sheath, blade, and the junction between the two, called the “laminar joint.” The laminar joint includes the auricle (an appendage or expanded region at the base of the blade surrounding the sheath) and the ligule (which is the membranous outgrowth of the sheath; Fig. 1). “Leaf angle” is defined as the angle between the leaf blade midrib and the stem (Fig. 1A), and is influenced by leaf development, particularly at the laminar joint. Classic liguleless (lg) mutants, such as lg1, lg2, lg3, and lg4 in maize (Zea mays) and Oslg1 in rice (Oryza sativa), are deficient in the formation of the ligule and auricle, resulting in smaller leaf angles (Mantilla-Perez and Salas Fernandez, 2017). In this issue of Plant Physiology, Liu et al. (2019) show that the LG1 homolog in wheat (Triticum aestivum), TaSPL8, plays a conserved role in regulating leaf angle. They further demonstrate that TaSPL8 functions through regulating brassinosteroid biosynthesis and auxin signaling. Figure 1. Open in new tabDownload slide Photographs of wheat leaves showing the lamina joint, ligule, and auricle. A, T. aestivum leaves. The lamina joint is indicated in white rectangles. Leaf angle is indicated with black lines. B and C, Enlargement of leaf regions showing the various parts. B, Leaf sheath wrapped around the stem. C, Sheath dissected from the stem. Scale bars = 1 cm. Figure and images by Yunqing Yu. Figure 1. Open in new tabDownload slide Photographs of wheat leaves showing the lamina joint, ligule, and auricle. A, T. aestivum leaves. The lamina joint is indicated in white rectangles. Leaf angle is indicated with black lines. B and C, Enlargement of leaf regions showing the various parts. B, Leaf sheath wrapped around the stem. C, Sheath dissected from the stem. Scale bars = 1 cm. Figure and images by Yunqing Yu. Liu et al. (2019) reported a compact plant architecture wheat mutant with smaller laminar joint and vertical leaves identified from a fast neutron radiation mutagenesis screen. Map-based cloning combined with bulked segregant RNA sequencing narrowed down the candidates to 83 genes, among which TaSPL8, encoding a SQUAMOSA promoter-binding protein-like (SPL) transcription factor and homolog of LG1, was identified as the causal gene. Because wheat is a hexaploid, the authors generated clustered regularly interspaced short palindromic repeats (CRISPR) mutants to knock out all three TaSPL8 homeologs. The homeologs have additive effects on the phenotype, and the triple mutants completely lack the lamina joint. In addition, the contribution of the three homeologs differs, with TaSPL8 from the D subgenome showing the largest effect (Liu et al., 2019). Overall, the phenotypes of Taspl8, particularly the higher order mutants, resemble the defects in the ligule and auricle development seen in maize and rice lg1 mutants (Becraft et al., 1990; Lee et al., 2007), suggesting that the functions of LG1/SPL8 are conserved across different grass species. ZmLG1 is specifically expressed at the preligule band, a linear layer of small cells at the boundary between the preblade and presheath. ZmLG1 activates the expression of auxin transporter ZmPIN1a, which induces auxin accumulation at the boundary and promotes ligule formation (Johnston et al., 2014). Leaf angle is regulated by brassinosteroid signaling, and exogenous 24-epibrassinolide promotes cell division and elongation of the adaxial cells at the laminar joint, resulting in increased leaf angles (Luo et al., 2016; Mantilla-Perez and Salas Fernandez, 2017). However, the link between LG1 and brassinosteroid signaling and direct targets of LG1 is largely unknown. Liu et al. (2019) screened for TaSPL8 targets in the downregulated genes in the Taspl8 triple mutant compared to the wild type, using the conserved SPL binding motif GTAC (Birkenbihl et al., 2005). Two candidates were identified: an AUXIN RESPONSE FACTOR6 and a brassinosteroid biosynthetic gene D2. Direct binding of TaSPL8 to their promoters containing the GTAC motif was further validated by an electrophoretic mobility shift assay, chromatin immunoprecipitation followed by quantitative PCR in wheat, and a transient luciferase reporter assay in tobacco (Nicotiana tabacum) leaves. These results provide insight into the molecular function of LG1 in boundary establishment and laminar joint development. The optimal leaf angle for maximal yield is unknown. A maize leaf blade has an average of leaf angle of 35° from the stem. Zmlg2, with a 14° leaf angle, has a significant increase in grain yield, particularly at higher planting density, whereas Zmlg1, with a more extreme leaf angle of 2°, does not have improved yield—suggesting that intermediate leaf angles may be more favorable than completely vertical leaves in crop breeding (Lambert and Johnson, 1978; Mantilla-Perez and Salas Fernandez, 2017). Although there is no difference in grain number per spike and grain weight between the wild type and Taspl8 triple mutants, spike number (i.e. tiller number) is significantly higher in the triple mutants than in the wild type, particularly at higher planting density, which may enhance yield. Future studies of grain yield in mutants with intermediate leaf angles may further benefit wheat breeding. LITERATURE CITED Becraft PW , Bongard-Pierce DK, Sylvester AW, Poethig RS, Freeling M ( 1990 ) The liguleless-1 gene acts tissue specifically in maize leaf development . Dev Biol 141 : 220 – 232 Google Scholar Crossref Search ADS PubMed WorldCat Birkenbihl RP , Jach G, Saedler H, Huijser P ( 2005 ) Functional dissection of the plant-specific SBP-domain: Overlap of the DNA-binding and nuclear localization domains . J Mol Biol 352 : 585 – 596 Google Scholar Crossref Search ADS PubMed WorldCat Donald CM ( 1968 ) The breeding of crop ideotypes . Euphytica 17 : 385 – 403 Google Scholar Crossref Search ADS WorldCat Johnston R , Wang M, Sun Q, Sylvester AW, Hake S, Scanlon MJ ( 2014 ) Transcriptomic analyses indicate that maize ligule development recapitulates gene expression patterns that occur during lateral organ initiation . Plant Cell 26 : 4718 – 4732 Google Scholar Crossref Search ADS PubMed WorldCat Lambert R , Johnson R ( 1978 ) Leaf angle, tassel morphology, and the performance of maize hybrids 1 . Crop Sci 18 : 499 – 502 Google Scholar Crossref Search ADS WorldCat Lee J , Park J-J, Kim SL, Yim J, An G ( 2007 ) Mutations in the rice liguleless gene result in a complete loss of the auricle, ligule, and laminar joint . Plant Mol Biol 65 : 487 – 499 Google Scholar Crossref Search ADS PubMed WorldCat Liu K , Cao J, Yu K, Liu X, Gao Y, Chen Q, Zhang W, Peng H, Du J, Xin M ( 2019 ) Wheat TaSPL8 modulates leaf angle through auxin and brassinosteroid signaling . Plant Physiol 181 : 179 – 194 Google Scholar Crossref Search ADS PubMed WorldCat Luo X , Zheng J, Huang R, Huang Y, Wang H, Jiang L, Fang X ( 2016 ) Phytohormones signaling and crosstalk regulating leaf angle in rice . Plant Cell Rep 35 : 2423 – 2433 Google Scholar Crossref Search ADS PubMed WorldCat Mantilla-Perez MB , Salas Fernandez MG ( 2017 ) Differential manipulation of leaf angle throughout the canopy: Current status and prospects . J Exp Bot 68 : 5699 – 5717 Google Scholar Crossref Search ADS PubMed WorldCat Author notes 2 Senior author. www.plantphysiol.org/cgi/doi/10.1104/pp.19.00872 © 2019 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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NPR1 Has Everything under Control

Mhamdi, Amna

2019 Plant Physiology

doi: 10.1104/pp.19.00890pmid: 31467138

NONEXPRESSER OF PR GENES1 (NPR1) controls plant immunity and is key to salicylic acid (SA)-dependent signaling pathways. First identified via a genetic screen, NPR1 is named after the npr1 mutants that failed to activate PATHOGENESIS-RELATED (PR) gene expression and were, therefore, defective in resistance to virulent bacteria in local and systemic tissues (Cao et al., 1994; Shah et al., 1997). Although the role of Arabidopsis (Arabidopsis thaliana) NPR1 has been studied for decades, it is only relatively recently that NPR1 emerged as a bona fide receptor for SA (Fu et al., 2012; Wu et al., 2012; Ding et al., 2018). In the absence of SA, NPR1 is cytosolic and exists as oligomers held together by intermolecular disulfide bridges. When plants are challenged with pathogens, SA accumulation and associated redox changes trigger the reduction of NPR1 and its translocation to the nucleus. The reduced NPR1 interacts with transcription factors and enables the expression of target genes (Després et al., 2003; Mou et al., 2003; Tada et al., 2008; Lindermayr et al., 2010). Yet, despite its importance in immunity, there remain several gaps in our understanding of how NPR1 expression is regulated. In this issue of Plant Physiology, Chen and colleagues shed light on this question and reveal a novel mechanism by which NPR1 controls its own expression and that of its target genes (Chen et al., 2019). The authors first demonstrate that a functional NPR1 protein promotes NPR1 gene expression by binding to its promoter. NPR1 interacts with WRKY18 transcription factors and with CYCLIN-DEPENDENT KINASE8 (CDK8) to regulate its own transcription. In this model, SA promotes an interaction between NPR1 and both partners. In addition, Chen et al. (2019) investigated the roles of CDK8 and the associated mediator subunits 12 and 13 in recruiting RNA polymerase II to the promoter of NPR1 and its PR1 target. Analysis of loss-of-function mutants revealed that CDK8 positively regulates NPR1 and PR1 gene expression and is required to fully induce SA responses, although this CDK8 function is independent from its kinase activity (Fig. 1). Figure 1. Open in new tabDownload slide Express yourself: how NPR1 regulates its own and target gene expression. SA (red spheres) and redox changes associated with pathogen challenge trigger the monomerization of NPR1 and its translocation to the nucleus. NPR1 binds to SA and interacts with CDK8 and the associated MED12 and MED13 as well as with WRKY18, thus facilitating the recruitment of RNA pol II to the NPR1 promoter and promoting its own expression. Similarly, NPR1 associates with CDK8, TGA5, and TGA7, which bind to the PR1 promoter, recruiting RNA polymerase II and promoting PR1 gene expression. Question marks highlight knowledge gaps related to the binding of SA to NPR1 and the gradient of free and bound SA between the nucleus and the cytosol. Abbreviations: CDK8, CYCLIN-DEPENDENT KINASE8; MED,12/13, mediator subunit 12/13; NPR1, NON-EXPRESSOR OF PAT2HOGENESIS-RELATED GENES1; PR1, PATHOGENESIS-RELATED1; SA, salicylic acid; TGA; TGACG cis-element-binding protein; TRX, thioredoxins; WRKY, WRKY transcription factors. Adapted from Chen et al. (2019). Figure 1. Open in new tabDownload slide Express yourself: how NPR1 regulates its own and target gene expression. SA (red spheres) and redox changes associated with pathogen challenge trigger the monomerization of NPR1 and its translocation to the nucleus. NPR1 binds to SA and interacts with CDK8 and the associated MED12 and MED13 as well as with WRKY18, thus facilitating the recruitment of RNA pol II to the NPR1 promoter and promoting its own expression. Similarly, NPR1 associates with CDK8, TGA5, and TGA7, which bind to the PR1 promoter, recruiting RNA polymerase II and promoting PR1 gene expression. Question marks highlight knowledge gaps related to the binding of SA to NPR1 and the gradient of free and bound SA between the nucleus and the cytosol. Abbreviations: CDK8, CYCLIN-DEPENDENT KINASE8; MED,12/13, mediator subunit 12/13; NPR1, NON-EXPRESSOR OF PAT2HOGENESIS-RELATED GENES1; PR1, PATHOGENESIS-RELATED1; SA, salicylic acid; TGA; TGACG cis-element-binding protein; TRX, thioredoxins; WRKY, WRKY transcription factors. Adapted from Chen et al. (2019). Interestingly, Chen et al. (2019) also throw new light onto how NPR1 regulates PR1 expression. NPR1 forms a complex with TGACG cis-element-binding protein (TGA) transcription factors TGA5, TGA7, and CDK8, which forms a bridge between the RNA polymerase II and the PR1 promoter and consequently facilitates PR1 gene transcription. Collectively, these findings enhance our understanding of the mechanisms by which NPR1, a master regulator of immune response, regulates the expression of itself and target genes (Fig. 1). So, do we now fully understand how increased SA availability activates NPR1-dependent and -independent gene transcription? Probably only partly. This exciting study raises many questions. How does SA facilitate the binding of NPR1 to the respective interacting proteins? How does NPR1 compete with other potential SA receptors (e.g. NPR3 and NPR4) to promote its own expression? Is direct binding of SA to NPR1 required for controlling its expression, and does it occur only at NPR1 or is it necessary for recruiting CDK8 and transcription factors to NPR1? Answers will presumably be found by studying the affinity of NPR1 for SA and the different protein domains or redox-sensitive residues that might determine the conformational changes dictated by SA. The future development of in vivo SA detection techniques should also improve our knowledge on how SA gradients between the cytosol and nucleus are maintained and how direct binding of SA to transcription regulators facilitates myriad functions related to hormone-mediated immune responses. Redox regulation occurs at different nodes to facilitate the establishment of plant immunity (Després et al., 2003; Mou et al., 2003; Lindermayr et al., 2010; Wu et al., 2012), and many posttranscriptional and posttranslational modifications shape NPR1-mediated signaling (Spoel et al., 2009; Saleh et al., 2015; Ding et al., 2018). Nevertheless, it remains unclear how these different modifications are integrated to tune the function of the fascinating NPR1. Another intriguing question is to what extent pathogen effectors might target the NPR1 signaling module to hijack plant immunity. LITERATURE CITED Cao H , Bowling SA, Gordon AS, Dong X ( 1994 ) Characterization of an Arabidopsis mutant that is nonresponsive to inducers of systemic acquired resistance . Plant Cell 6 : 1583 – 1592 Google Scholar Crossref Search ADS PubMed WorldCat Chen J , Mohan R, Zhang Y, Li M, Chen H, Palmer IA, Chang M, Qi G, Spoel SH, Mengiste T, et al. ( 2019 ) NPR1 promotes its own and target gene expression in plant defense by recruiting CDK8 . Plant Physiol 181 : 289 – 304 Google Scholar Crossref Search ADS PubMed WorldCat Després C , Chubak C, Rochon A, Clark R, Bethune T, Desveaux D, Fobert PR ( 2003 ) The Arabidopsis NPR1 disease resistance protein is a novel cofactor that confers redox regulation of DNA binding activity to the basic domain/leucine zipper transcription factor TGA1 . Plant Cell 15 : 2181 – 2191 Google Scholar Crossref Search ADS PubMed WorldCat Ding Y , Sun T, Ao K, Peng Y, Zhang Y, Li X, Zhang Y ( 2018 ) Opposite roles of salicylic acid receptors NPR1 and NPR3/NPR4 in transcriptional regulation of plant immunity . Cell 173 : 1454 – 1467.e15 Google Scholar Crossref Search ADS PubMed WorldCat Fu ZQ , Yan S, Saleh A, Wang W, Ruble J, Oka N, Mohan R, Spoel SH, Tada Y, Zheng N, et al. ( 2012 ) NPR3 and NPR4 are receptors for the immune signal salicylic acid in plants . Nature 486 : 228 – 232 Google Scholar Crossref Search ADS PubMed WorldCat Lindermayr C , Sell S, Müller B, Leister D, Durner J ( 2010 ) Redox regulation of the NPR1-TGA1 system of Arabidopsis thaliana by nitric oxide . Plant Cell 22 : 2894 – 2907 Google Scholar Crossref Search ADS PubMed WorldCat Mou Z , Fan W, Dong X ( 2003 ) Inducers of plant systemic acquired resistance regulate NPR1 function through redox changes . Cell 113 : 935 – 944 Google Scholar Crossref Search ADS PubMed WorldCat Saleh A , Withers J, Mohan R, Marqués J, Gu Y, Yan S, Zavaliev R, Nomoto M, Tada Y, Dong X ( 2015 ) Posttranslational modifications of the master transcriptional regulator NPR1 enable dynamic but tight control of plant immune responses . Cell Host Microbe 18 : 169 – 182 Google Scholar Crossref Search ADS PubMed WorldCat Shah J , Tsui F, Klessig DF ( 1997 ) Characterization of a salicylic acid-insensitive mutant (sai1) of Arabidopsis thaliana, identified in a selective screen utilizing the SA-inducible expression of the tms2 gene . Mol Plant Microbe Interact 10 : 69 – 78 Google Scholar Crossref Search ADS PubMed WorldCat Spoel SH , Mou Z, Tada Y, Spivey NW, Genschik P, Dong X ( 2009 ) Proteasome-mediated turnover of the transcription coactivator NPR1 plays dual roles in regulating plant immunity . 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journal article

LitStream Collection

Buffering Lipid Synthesis by Conditional Inhibition

Yeats, Trevor H.

2019 Plant Physiology

doi: 10.1104/pp.19.00865pmid: 31467139

Lipid synthesis is a ubiquitous but costly branch of primary metabolism in plants. All cells must make fatty acids as precursors for cellular membranes and signaling molecules, and some cells make large quantities of fatty acid-derived products, such as oils (triacylglycerol) or cuticles (wax and cutin). Accordingly, plants have evolved robust mechanisms for regulating lipid biosynthesis. These include the conserved transcriptional activator WRINKLED1 (WRI1) and posttranslational regulation of lipid biosynthetic enzymes. In this issue of Plant Physiology, Liu et al. (2019) reveal how WRI1 induces expression of inhibitors of fatty acid synthesis while also promoting expression of associated biosynthetic genes. This counterintuitive coupling of transcriptional activation to posttranslational inhibition illustrates how regulatory networks can evolve to refine control of metabolic processes. In plants, fatty acid synthesis depends on two large plastid-localized enzyme complexes: acetyl-CoA carboxylase (ACCase) and fatty acid synthase. ACCase produces the primary substrate of fatty acid synthase, malonyl-CoA, so its activity is rate limiting for the production of fatty acids. In most plants, with the notable exception of the grasses, plastid-localized ACCase consists of four distinct subunits: biotin carboxyl carrier protein (BCCP), biotin carboxylase, and α- and β-subunits of carboxyltransferase. The synthesis of malonyl-CoA occurs in two steps: first, the biotin attached to BCCP is carboxylated by the action of biotin carboxylase, and then the carboxyl moiety is transferred to acetyl-CoA by the action of carboxyltransferase, yielding malonyl-CoA. Biotin attachment domain-containing proteins (BADCs) are homologs of BCCP that lack a conserved biotin-anchoring Lys, and thus do not participate directly in ACCase catalysis. Rather, BADCs have been shown to inhibit ACCase by competing for occupancy of the BCCP site of ACCase (Salie et al., 2016; Keereetaweep et al., 2018). BADC-mediated inhibition of ACCase is important for regulation of ACCase in the context of seed oil synthesis, since BADC mutants accumulate up to 30% more triacylglycerol (Salie et al., 2016; Keereetaweep et al., 2018). BADC proteins are also likely involved in feedback regulation of ACCase, as irreversible inhibition of ACCase in response to excess fatty acids is mediated by BADCs (Keereetaweep et al., 2018). Now, Liu et al. (2019) have revealed a seemingly paradoxical twist in the regulation of ACCase. WRI1, a transcriptional regulator of lipid synthesis, binds to the promoters of BADC-encoding genes and activates their transcription, while similarly promoting expression of the functional BCCP subunit of ACCase. Why do plants keep a foot on the gas pedal and the brake at the same time? The authors suggest that this may be important for moderating excess induction of lipid biosynthesis. Consistent with this idea, badc1badc2 double mutants phenocopy wri1 mutants with regard to impaired root growth and auxin homeostasis, highlighting the need for tight control of lipid synthesis during development. The regulation of lipid synthesis is of considerable practical interest, since oilseed crops are a predominant source of triacylglycerols for food and industrial applications. This new understanding of how WRI1 and BADCs are involved in regulating ACCase activity sets the stage for some novel approaches to increasing oil content in crops. Specifically, gene editing technologies, such as CRISPR, enable targeted promoter engineering that could be used to tune or eliminate the binding of WRI1 to BADC promoters during seed filling. This might afford a direct increase in lipid synthesis, or alleviate regulatory bottlenecks that have limited the effectiveness of other strategies for lipid metabolic engineering (Bates, 2016). Of course, it would be wise to anticipate the discovery of even more wrinkles in the regulatory network of lipid synthesis. LITERATURE CITED Bates PD ( 2016 ) Understanding the control of acyl flux through the lipid metabolic network of plant oil biosynthesis . Biochim Biophys Acta 1861 ( 9 Pt B ): 1214 – 1225 Google Scholar Crossref Search ADS PubMed WorldCat Keereetaweep J , Liu H, Zhai Z, Shanklin J ( 2018 ) Biotin attachment domain-containing proteins irreversibly inhibit acetyl CoA carboxylase . Plant Physiol 177 : 208 – 215 Google Scholar Crossref Search ADS PubMed WorldCat Liu H , Zhai Z, Kuczynski K, Keereetaweep J, Schwender J, Shanklin J ( 2019 ) WRINKLED1 regulates BIOTIN ATTACHMENT DOMAIN-CONTAINING proteins that inhibit fatty acid synthesis . Plant Physiol 181 : 55 – 62 Google Scholar Crossref Search ADS PubMed WorldCat Salie MJ , Zhang N, Lancikova V, Xu D, Thelen JJ ( 2016 ) A family of negative regulators targets the committed step of de novo fatty acid biosynthesis . Plant Cell 28 : 2312 – 2325 Google Scholar Crossref Search ADS PubMed WorldCat Author notes 2 Senior author. www.plantphysiol.org/cgi/doi/10.1104/pp.19.00865 © 2019 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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