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Summary: The phytohormone abscisic acid (ABA) functions in the control of plant stress responses, particularly in drought stress. A significant mechanism in attenuating and terminating ABA signals involves regulated protein turnover, with certain ABA receptors, despite their main presence in the cytosol and nucleus, subjected to vacuolar degradation via the Endosomal Sorting Complex Required for Transport (ESCRT) machinery.Collectively our findings show that discrete TOM1‐LIKE (TOL) proteins, which are functional ESCRT‐0 complex substitutes in plants, affect the trafficking for degradation of core components of the ABA signaling and transport machinery.TOL2,3,5 and 6 modulate ABA signaling where they function additively in degradation of ubiquitinated ABA receptors and transporters.TOLs colocalize with their cargo in different endocytic compartments in the root epidermis and in guard cells of stomata, where they potentially function in ABA‐controlled stomatal aperture.Although the tol2/3/5/6 quadruple mutant plant line is significantly more drought‐tolerant and has a higher ABA sensitivity than control plant lines, it has no obvious growth or development phenotype under standard conditions, making the TOL genes ideal candidates for engineering to improved plant performance. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) plays a crucial role in plant defense mechanisms under adverse environmental conditions, but its metabolism and perception in response to heavy metals are largely unknown. In Pisum sativum exposed to CdCl2, an accumulation of free ABA was detected in leaves at different developmental stages (A, youngest, unexpanded; B1, youngest, fully expanded; B2, mature; C, old), with the highest content found in A and B1 leaves. In turn, the content of ABA conjugates, which was highest in B2 and C leaves under control conditions, increased only in A leaves and decreased in leaves of later developmental stages after Cd treatment. Based on the expression of PsNCED2, PsNCED3 (9-cis-epoxycarotenoid dioxygenase), PsAO3 (aldehyde oxidase) and PsABAUGT1 (ABA-UDP-glucosyltransferase), and the activity of PsAOγ, B2 and C leaves were found to be the main sites of Cd-induced de novo synthesis of ABA from carotenoids and ABA conjugation with glucose. In turn, β-glucosidase activity and the expression of genes encoding ABA receptors (PsPYL2, PsPYL4, PsPYL8, PsPYL9) suggest that in A and B1 leaves, Cd-induced release of ABA from inactive ABA-glucosyl esters and enhanced ABA perception comes to the forefront when dealing with Cd toxicity. The distinct role of leaves at different developmental stages in defense against the harmful effects of Cd is discussed. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA), a plant hormone, is crucial for regulating various physiological and developmental processes in plants, including adaptation to biotic and abiotic stresses. Recent advancements have significantly contributed to our understanding of ABA's biosynthetic pathway, transport, signaling pathway, and metabolism. To overcome the limitations of natural ABA, scientists have developed broad-spectrum and highly active agonists of ABA receptors. However, the practical application of these receptor agonists as agrochemicals still faces several challenges. On the other hand, some ABA antagonists have also been developed to differentiate the functional differences among various receptors more accurately. This can help design ABA agonists that can selectively activate specific physiological responses, thereby eliminating the undesired physiological effects induced by ABA. This paper aims to provide a comprehensive overview of the current ABA receptor agonists and antagonists to assist in developing novel ABA functional analogs with improved efficacy and simpler chemical structures that are suitable for agricultural applications.

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Abscisic acid (ABA) is a phytohormone that plays a key role in regulating several developmental processes as well as in response to stressful conditions such as drought. Activation of the ABA signaling cascade allows the induction of an appropriate physiological response. The basic components of the ABA signaling pathway have been recognized and characterized in recent years. Pyrabactin resistance, pyrabactin resistance-like, and the regulatory component of ABA receptors (PYR/PYL/RCAR) are the major components responsible for the regulation of the ABA signaling pathway. Here, we review recent findings concerning the PYR/PYL/RCAR receptor structure, function, and interaction with other components of the ABA signaling pathway as well as the termination mechanism of ABA signals in plant cells. Since ABA is one of the basic elements related to abiotic stress, which is increasingly common in the era of climate changes, understanding the perception and transduction of the signal related to this phytohormone is of paramount importance in further increasing crop tolerance to various stress factors. [ABSTRACT FROM AUTHOR]

As changes occur in climate, abiotic stress to agricultural production is an inevitable threat to farmers’ ability to meet an increasing demand to feed people. Plants have developed a stress tolerance mechanism to reduce the effects of such environmental conditions by engaging various stress-responsive genes. Accordingly, various signal transduction networks are used to fabricate stress tolerance. Engineering of the phytohormone abscisic acid (ABA) could be a choice method for scientists to mitigate abiotic stress because of its widespread role in response to salt, drought, heat, and cold stresses including triggering stomatal regulation and leaf senescence. In addition, it plays a crucial role in seed maturation, seed dormancy, stomatal opening/closure and increases resistance against pathogens through callose depositions and regulates physiological strategies in stress signaling pathways through synchronizing of hormonal crosstalk. The transcriptional regulation can be achieved through ABA-dependent and ABA-independent signaling cascades. ABAI5 and RD29A genes are regulated in ABA-dependent and independent manners to mitigate stress tolerance. ABA regulatory components (RCARs) including pyrabactin resistance PYR/PYL genes, SnRK2 type protein kinases, transcription factors (WRKY, NAC, AREB1, bZIP, RGL2, and ABRE), reactive oxygen species, jasmonic acid and cytokinin hormones regulate ABA gene action in response to abiotic stresses.

As changes occur in climate, abiotic stress to agricultural production is an inevitable threat to farmers' ability to meet an increasing demand to feed people. Plants have developed a stress tolerance mechanism to reduce the effects of such environmental conditions by engaging various stress-responsive genes. Accordingly, various signal transduction networks are used to fabricate stress tolerance. Engineering of the phytohormone abscisic acid (ABA) could be a choice method for scientists to mitigate abiotic stress because of its widespread role in response to salt, drought, heat, and cold stresses including triggering stomatal regulation and leaf senescence. In addition, it plays a crucial role in seed maturation, seed dormancy, stomatal opening/closure and increases resistance against pathogens through callose depositions and regulates physiological strategies in stress signaling pathways through synchronizing of hormonal crosstalk. The transcriptional regulation can be achieved through ABA-dependent and ABA-independent signaling cascades. ABAI5 and RD29A genes are regulated in ABA-dependent and independent manners to mitigate stress tolerance. ABA regulatory components (RCARs) including pyrabactin resistance PYR/PYL genes, SnRK2 type protein kinases, transcription factors (WRKY, NAC, AREB1, bZIP, RGL2, and ABRE), reactive oxygen species, jasmonic acid and cytokinin hormones regulate ABA gene action in response to abiotic stresses. [ABSTRACT FROM AUTHOR]

C-terminally encoded peptides (CEPs) are small peptides, typically post-translationally modified, and highly conserved in many species. CEPs are known to inhibit plant growth and development, but the mechanisms are not well understood. In this study, 14 CEPs were identified in Setaria italica and divided into two groups. The transcripts of most SiCEP s were more abundant in roots than in other detected tissues. SiCEP3, SiCEP4, and SiCEP5 were also highly expressed in panicles. Moreover, expression of all SiCEP s was induced by abiotic stresses and phytohormones. SiCEP3 overexpression and application of synthetic SiCEP3 both inhibited seedling growth. In the presence of abscisic acid (ABA), growth inhibition and ABA content in seedlings increased with the concentration of SiCEP3. Transcripts encoding eight ABA transporters and six ABA receptors were induced or repressed by synthetic SiCEP3, ABA, and their combination. Further analysis using loss-of-function mutants of Arabidopsis genes functioning as ABA transporters, receptors, and in the biosynthesis and degradation of ABA revealed that SiCEP3 promoted ABA import at least via NRT1.2 (NITRATE TRANSPORTER 1.2) and ABCG40 (ATP-BINDING CASSETTE G40). In addition, SiCEP3, ABA, or their combination inhibited the kinase activities of CEP receptors AtCEPR1/2. Taken together, our results indicated that the CEP-CEPR module mediates ABA signaling by regulating ABA transporters and ABA receptors in planta. [ABSTRACT FROM AUTHOR]

Uridine diphosphate‐glucosyltransferases (UGTs) maintain abscisic acid (ABA) homeostasis in Arabidopsis thaliana by converting ABA to abscisic acid‐glucose ester (ABA‐GE). UGT71C5 plays an important role in the generation of ABA‐GE. Abscisic acid receptors are crucial upstream components of the ABA signaling pathway, but how UGTs and ABA receptors function together to modulate ABA levels is unknown. Here, we demonstrated that the ABA receptors RCAR12/13 and UGT71C5 maintain ABA homeostasis in Arabidopsis following rehydration under drought stress. Biochemical analyses show that UGT71C5 directly interacted with RCAR8/12/13 in yeast cells, and the interactions between UGT71C5 and RCAR12/13 were enhanced by ABA treatment. Enzyme activity analysis showed that ABA‐GE contents were significantly elevated in the presence of RCAR12 or RCAR13, suggesting that these ABA receptors enhance the activity of UGT71C5. Determination of the content of ABA and ABA‐GE in Arabidopsis following rehydration under drought stress revealed that ABA‐GE contents were significantly higher in Arabidopsis plants overexpressing RCAR12 and RCAR13 than in non‐transformed plants and plants overexpressing RCAR11 following rehydration under drought stress. These observations suggest that RCAR12 and RCAR13 enhance the activity of UGT71C5 to glycosylate excess ABA into ABA‐GE following rehydration under drought stress, representing a rapid mechanism for regulating plant growth and development. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) is a phytohormone that plays a key role in regulating several developmental processes as well as in response to stressful conditions such as drought. Activation of the ABA signaling cascade allows the induction of an appropriate physiological response. The basic components of the ABA signaling pathway have been recognized and characterized in recent years. Pyrabactin resistance, pyrabactin resistance-like, and the regulatory component of ABA receptors (PYR/PYL/RCAR) are the major components responsible for the regulation of the ABA signaling pathway. Here, we review recent findings concerning the PYR/PYL/RCAR receptor structure, function, and interaction with other components of the ABA signaling pathway as well as the termination mechanism of ABA signals in plant cells. Since ABA is one of the basic elements related to abiotic stress, which is increasingly common in the era of climate changes, understanding the perception and transduction of the signal related to this phytohormone is of paramount importance in further increasing crop tolerance to various stress factors.

Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) is a nutritional crop, which has high rutin, and is good for health. Until now, plant genetic engineering is insufficient for Tartary buckwheat. Abscisic acid (ABA), as one of phytohormones, is involved in the regulation of plant growth and development, and responses to diverse environmental challenges. Although ABA receptors have been well characterized in Arabidopsis , it is little understood in Tartary buckwheat. In this study, we identified 12 ABA receptors, designated as FtRCAR1 through FtRCAR12 in Tartary buckwheat. FtRCARs are divided into three subfamily. Based on the similarity, we could predict that FtRCARs comprise of the monomeric (FtRCAR1, 3, 4, 5, 9, 10, 11 and 12) and the dimeric (FtRCAR2, 7 and 8) state in solution. The analysis of the transcript pattern indicated that most of FtRCARs were significantly variable among the root, stem, leaf, flower and seed, while FtRCAR4 transcript was undetectable under in all tissues. The transcript levels of FtRCARs under ABA treatment indicated that most FtRCARs transcripts were depressed, indicating a possible feedback regulation of ABA signaling. The analysis of rutin biosynthesis related-genes indicated that ABA up-graduated CHS , CHI , F3 ′ H , F3H and FLS transcript levels, while transcripts of 4CL and PAL were down-regulated. In addition, the transcription factors that mediated the rutin biosynthesis related-genes were also regulated by exogenous ABA. Thus, the identification and the characterization of FtRCARs would enable us to further understand the role of ABA signal in Tartary buckwheat. • The identification of abscisic acid receptors (FtRCARs) in Tartary Buckwheat. • The characteristics of FtRCARs are shown in the paper. • The genes in rutin biosynthesis signaling pathway were predicted to be mediated by exogenous ABA. • The transcription factors that regulate the rutin biosynthesis in Tartary Buckwheat could also be regulated by ABA. [ABSTRACT FROM AUTHOR]

To face the challenges of climate change and sustainable food production, it is essential to develop crop genome editing techniques to pinpoint key genes involved in abiotic stress signaling. The identification of those prevailing abscisic acid (ABA) receptors that mediate plant-environment interactions is quite challenging in polyploid plants because of the high number of genes in the PYR/PYL/RCAR ABA receptor family. Nicotiana benthamiana is a biotechnological crop amenable to genome editing, and given the importance of ABA signaling in coping with drought stress, we initiated the analysis of its 23-member family of ABA receptors through multiplex CRISPR/Cas9-mediated editing. We generated several high-order mutants impaired in NbPYL1-like and NbPYL8-like receptors, which showed certain insensitivity to ABA for inhibition of seedling establishment, growth, and development of shoot and lateral roots as well as reduced sensitivity to the PYL1-agonist cyanabactin (CB). However, in these high-order mutants, regulation of transpiration was not affected and was responsive to ABA treatment. This reveals a robust and redundant control of transpiration in this allotetraploid plant that probably reflects its origin from the extreme habitat of central Australia.

The observation of a much-improved fitness of wild-type plants over abscisic acid (ABA)-deficient mutants during drought has led researchers from all over to world to perform experiments aiming at a better understanding of how this hormone modulates the physiology of plants under water-limited conditions. More recently, several promising approaches manipulating ABA biosynthesis and signaling have been explored to improve water use efficiency and confer drought tolerance to major crop species. Here, we review recent progress made in the last decade on (i) ABA biosynthesis, (ii) the roles of ABA on plant-water relations and on primary and secondary metabolisms during drought, and (iii) the regulation of ABA levels and perception to improve water use efficiency and drought tolerance in crop species. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) is a ubiquitous phytohormone, plays important roles in several physiological processes, including stress adaptation, flowering, seed germination, fruit ripening, and leaf senescence etc. ABA binds with START domain proteins called Pyrabactin Resistance1 (PYR1)/PYR1-like (PYL)/Regulatory Components of ABA Receptors (RCARs) and controls the activity of PP2C phosphatase proteins and in turn the ABA-dependent signaling pathway. Fourteen ABA receptors have been identified in the model plant Arabidopsis thaliana and have shown to be involved in various biological functions. Under field conditions, exogenous application of ABA produces inadequate physiological response due to its rapid conversion into the biologically inactive metabolites. ABA shows selective binding preferences to PYL receptor subtypes and hence produces pleiotropic physiological and phenotypic effects which limit the usage of ABA in agriculture. An agrochemical meant for ameliorating the undesirable physiological effect of the plant should ideally have positive biological attributes without affecting the normal growth, development, and yield. Therefore, to overcome the limitations of ABA for its usage in various agricultural applications, several types of ABA-mimicking agents have been developed. Many compounds have been identified as having significant ABA-agonist/antagonist activity and can be employed to reverse the excessive/moderate ABA action. The present review highlights the potential usage of ABA signaling modulators for managing agronomic and postharvest traits. Besides, designing, development and versatile usage of ABA-mimicking compounds displaying ABA agonists and antagonist activities are discussed in detail. • ABA-metabolism regulation through agrochemicals is a viable strategy for managing major agronomic and postharvest traits. • ABA and its analogs show selective binding to ABA receptor subtypes imparting modulatory effect on ABA receptors • Ligand bound 3D structures of ABA receptors and computational modeling are invaluable for the synthesis of new compounds • Chemoinformatics knowledge of ABA mimetics to add value for commercial agriculture and post-harvest crop management. • Designer ABA receptor antagonist/agonist will accelerate plant breeding with optimized ABA-responsive plants. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA), one of phytohormones, is induced in response to abiotic stress to mediate plant acclimation to environmental challenge. Key players of the ABA-signaling pathway are the ABA-binding receptors (RCAR/PYR1/PYL), which perceive ABA and then inhibit PP2Cs to activate SnRK2s. Here, we report that a putative receptor-like cytoplasmic kinase (RLCK) in Arabidopsis named CARK6, which is a member of cytosolic ABA receptor kinases. We confirm that CARK6 interacts with ABA receptors, RCAR11-14 in vitro and in vivo. Induced overexpression of CARK6 in Arabidopsis enhances sensitivity to ABA by inhibition of seed germination and root elongation, and promotes the drought resistance. However, loss-of-function seedlings of cark6 are less sensitive to ABA and show reduced drought stress response with respect to water loss and stomatal aperture. In transgenic Arabidopsis complementation lines in the cark6 mutant background, stress responsivity was restored by CARK6. In conclusion, our data provide evidence that CARK6 plays a positive role in ABA signaling in Arabidopsis. • A receptor-like cytoplasmic kinase (CARK6) was identified. • CARK6 interacts with ABA receptors. • CARK6 positively regulates ABA signal transduction. [ABSTRACT FROM AUTHOR]

The observation of a much-improved fitness of wild-type plants over abscisic acid (ABA)-deficient mutants during drought has led researchers from all over to world to perform experiments aiming at a better understanding of how this hormone modulates the physiology of plants under water-limited conditions. More recently, several promising approaches manipulating ABA biosynthesis and signaling have been explored to improve water use efficiency and confer drought tolerance to major crop species. Here, we review recent progress made in the last decade on (i) ABA biosynthesis, (ii) the roles of ABA on plant-water relations and on primary and secondary metabolisms during drought, and (iii) the regulation of ABA levels and perception to improve water use efficiency and drought tolerance in crop species.

Drought is one of the major abiotic stresses affecting world agriculture. Breeding drought-resistant crops is one of the most important challenges for plant biologists. PYR1/PYL/RCARs, which encode the abscisic acid (ABA) receptors, play pivotal roles in ABA signaling, but how these genes function in crop drought response remains largely unknown. Here we identified 13 PYL family members in maize (ZmPYL1-13). Changes in expression of these genes under different stresses indicated that ZmPYLs played important roles in responding to multiple abiotic stresses. Transgenic analyses of ZmPYL genes in Arabidopsis showed that overexpression of ZmPYL3, ZmPYL9, ZmPYL10, and ZmPYL13 significantly enhanced the sensitivity of transgenic plants to ABA. Additionally, transgenic lines overexpressing ZmPYL8, ZmPYL9, and ZmPYL12 were more resistant to drought. Accumulation of proline and enhanced expression of drought-related marker genes in transgenic lines further confirmed the positive roles of ZmPYL genes in plant drought resistance. Association analyses with a panel of 368 maize inbred lines identified natural variants in ZmPYL8 and ZmPYL12 that were significantly associated with maize drought resistance. Our results deepen the knowledge of the function of maize PYL genes in responses to abiotic stresses, and the natural variants identified in ZmPYL genes may serve as potential molecular markers for breeding drought-resistant maize cultivars.

PYR/PYLs function as ABA receptors and are key regulators during plant drought stress response. Previously we screened drought tolerance of Arabidopsis ABA receptors PYR/PYLs under the control of five different promoters. In this study, we characterized drought stress tolerance of AtPYL5 transgene under the control of one guard cell specific promoter, pGC1. pGC1::AtPYL5 transgenic Arabidopsis exhibited reduced transpiration rate and decreased water loss after drought treatment. Transformation of pGC1::AtPYL5 in Arabidopsis also decreased oxidative stress damage and improved photosynthesis under drought stress condition. These results indicated that pGC1::AtPYL5 construct is effective and might pave new way to develop genetically engineered plants to improve drought stress tolerance. [ABSTRACT FROM AUTHOR]

Drought is one of the major abiotic stresses affecting world agriculture. Breeding drought-resistant crops is one of the most important challenges for plant biologists. PYR1/PYL/RCARs, which encode the abscisic acid (ABA) receptors, play pivotal roles in ABA signaling, but how these genes function in crop drought response remains largely unknown. Here we identified 13 PYL family members in maize (ZmPYL1-13). Changes in expression of these genes under different stresses indicated that ZmPYLs played important roles in responding to multiple abiotic stresses. Transgenic analyses of ZmPYL genes in Arabidopsis showed that overexpression of ZmPYL3, ZmPYL9, ZmPYL10, and ZmPYL13 significantly enhanced the sensitivity of transgenic plants to ABA. Additionally, transgenic lines overexpressing ZmPYL8, ZmPYL9, and ZmPYL12 were more resistant to drought. Accumulation of proline and enhanced expression of drought-related marker genes in transgenic lines further confirmed the positive roles of ZmPYL genes in plant drought resistance. Association analyses with a panel of 368 maize inbred lines identified natural variants in ZmPYL8 and ZmPYL12 that were significantly associated with maize drought resistance. Our results deepen the knowledge of the function of maize PYL genes in responses to abiotic stresses, and the natural variants identified in ZmPYL genes may serve as potential molecular markers for breeding drought-resistant maize cultivars. [ABSTRACT FROM AUTHOR]

Defects in RNA processing and degradation pathways often lead to developmental abnormalities, impaired hormonal signaling and altered resistance to abiotic and biotic stress. Here we report that components of the 5'-3' mRNA decay pathway, DCP5, LSM1-7 and XRN4, contribute to a proper response to a key plant hormone abscisc acid (ABA), albeit in a different manner. Plants lacking DCP5 are more sensitive to ABA during germination, whereas lsm1a lsm1b and xrn4-5 mutants are affected at the early stages of vegetative growth. In addition, we show that DCP5 and LSM1 regulate mRNA stability and act in translational repression of the main components of the early ABA signaling, PYR/PYL ABA receptors and SnRK2s protein kinases. mRNA decapping DCP and LSM1-7 complexes also appear to modulate ABA-dependent expression of stress related transcription factors from the AP2/ERF/DREB family that in turn affect the level of genes regulated by the PYL/PYR/RCAR-PP2C-SnRK2 pathway. These observations suggest that ABA signaling through PYL/PYR/RCAR receptors and SnRK2s kinases is regulated directly and indirectly by the cytoplasmic mRNA decay pathway. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) is a vital phytohormone that accumulates in response to various biotic and abiotic stresses, as well as plant growth. In Arabidopsis thaliana, there are 14 members of the ABA receptor family, which are key positive regulators involved in ABA signaling. Besides reduced drought stress tolerance, the quadruple and sextuple mutants (pyr1pyl1pyl2pyl4 (1124) and pyr1pyl1pyl2pyl4pyl5pyl8 (112458) show abnormal growth phenotypes, such as decreases in yield and height, under non-stress conditions. However, it remains unknown whether ABA receptors mediate ABA signaling to regulate plant growth and development. Here, we showed the primary metabolite profiles of 1124, 112458 and wild-type (WT) plants grown under normal conditions. The metabolic changes were significantly different between ABA receptor mutants and WT. Guanosine, for the biosynthesis of cyclic guanosine 3′,5′-monophosphate (cGMP), is an important second messenger that acts to regulate the level of ABA. In addition, other amino acids were increased in the 112458 mutant, including proline. These results, together with phenotype analysis, indicated that ABA receptors are involved in ABA signaling to modulate metabolism and plant growth under normal conditions.

Pyrabactin resistance-like (PYL) proteins were identified as receptors of the plant hormone ABA. The PYL family consists of multiple members that are differently expressed in various tissues, exhibit distinct biochemical properties and have diverse biological functions. In the present study, we explored the expression patterns of the rice (Oryza sativa L.) OsPYL family genes and determined that OsPYL8 and OsPYL9 are specifically expressed in the endosperms. Sequence analysis and deletion experiments revealed that the OsPYL8 and OsPYL9 promoters contain multiple motifs involved in endosperm-specific expression. Transgenic rice plants overexpressing OsPYL8 or OsPYL9 showed hypersensitivity toABA during seed germination, suggesting that both OsPYL8 and OsPYL9 act as positive regulators of the ABA signalling pathway in the seed. OsPYL8 and OsPYL9 interact with OsPP2C51 and OsPP2C68, whose expression is induced in the endosperms by ABA. Our results provided a foundation for future studies on OsPYL8- and OsPYL9-mediated ABA signalling in the rice endosperms. [ABSTRACT FROM AUTHOR]

Recent discovery of PYR/PYL/RCAR-type abscisic acid (ABA) receptors has become one of most significant advances in plant science in the past decade. In mammals, endosomal sorting acts as an important pathway to downregulate different types of receptors, but its role in plant hormone signaling is poorly understood. Here, we report that an ubiquitin E2-like protein, VPS23A, which is a key component of ESCRT-I, negatively regulates ABA signaling. VPS23A has epistatic relationship with PYR / PYL / RCAR -type ABA receptors and disruption of VPS23A enhanced the activity of key kinase OST1 in the ABA signaling pathway under ABA treatment. Moreover, VPS23A interacts with PYR1/PYLs and K63-linked diubiquitin, and PYL4 possesses K63-linked ubiquitinated modification in vivo . Further analysis revealed that VPS23A affects the subcellular localization of PYR1 and the stability of PYL4. Taken together, our results suggest that VPS23A affects PYR1/PYL4 via vacuole-mediated degradation, providing an advanced understanding of both the turnover of ABA receptors and ESCRTs in plant hormone signaling. [ABSTRACT FROM AUTHOR]

Protein phosphatases (PPs) counteract kinases in reversible phosphorylation events during numerous signal transduction pathways in eukaryotes. Type 2C PPs (PP2Cs) represent the major group of PPs in plants, and recent discovery of novel abscisic acid (ABA) receptors (ABARs) has placed the PP2Cs at the center stage of the major signaling pathway regulating plant responses to stresses and plant development. Several studies have provided deep insight into vital roles of the PP2Cs in various plant processes. Global analyses of thePP2Cgene family in model plants have contributed to our understanding of their genomic diversity and conservation, across plant species. In this review, we discuss the genomic and structural accounts ofPP2Csin plants. Recent advancements in their interaction paradigm with ABARs and sucrose nonfermenting related kinases 2 (SnRK2s) in ABA signaling are also highlighted. In addition, expression analyses and important roles ofPP2Cs in the regulation of biotic and abiotic stress responses, potassium (K+) deficiency signaling, plant immunity and development are elaborated. Knowledge of functional roles of specific PP2Cs could be exploited for the genetic manipulation of crop plants. Genetic engineering usingPP2Cgenes could provide great impetus in the agricultural biotechnology sector in terms of imparting desired traits, including a higher degree of stress tolerance and productivity without a yield penalty. [ABSTRACT FROM PUBLISHER]

Methyl jasmonate (MeJA) induces stomatal closure. It has been shown that stomata of many ABA-insensitive mutants are also insensitive to MeJA, and a low amount of ABA is a prerequisite for the MeJA response. However, the molecular mechanisms of the interaction between ABA and MeJA signaling remain to be elucidated. Here we studied the interplay of signaling of the two hormones in guard cells using the quadruple ABA receptor mutant pyr1 pyl1 pyl2 pyl4 and ABA-activated protein kinase mutants ost1-2 and srk2e. In the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, nitric oxide (NO) production, cytosolic alkalization and plasma membrane Ca2+-permeable current (ICa) activation were not impaired. At the same time, the inactivation of the inward-rectifying K+ current was impaired. In contrast to the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, NO production and cytosolic alkalization were impaired in ost1-2 and srk2e as well as in aba2-2, the ABA-deficient mutant. The activation of ICa was also impaired in srk2e. Collectively, these results indicated that OST1 was essential for MeJA-induced stomatal closure, while PYR1, PYL1, PYL2 and PYL4 ABA receptors were not sufficient factors. MeJA did not appear to activate OST1 kinase activity. This implies that OST1 mediates MeJA signaling through an undetectable level of activity or a nonenzymatic action. MeJA induced the expression of an ABA synthesis gene, NCED3, and increased ABA contents only modestly. Taken together with previous reports, this study suggests that MeJA signaling in guard cells is primed by ABA and is not brought about through the pathway mediated by PYR1, PYL1 PYL2 and PYL4. [ABSTRACT FROM AUTHOR]

• A total of seven LchiPYL genes were comprehensively identified and characterized in the Liriodendron chinense genome. • LchiPYL genes were highly conserved in plant species. • The expression patterns of seven LchiPYL genes in buds responding to ABA, photoperiod, and chilling stress were established. Abscisic acid (ABA) is a signaling phytohormone in plants to improve stress adaption under harsh environments. ABA is sensed by the pyrabactin resistance1/PYR1-like/regulatory components of the ABA receptor (PYR1/PYL/RCAR) (named PYLs) and is the core regulator of ABA stress signaling. ABA receptor PYL gene family have been well studied in rubber tree, apple, Theobroma cacao, Ricinus communis , and Vitis vinifera. We first revealed the genome-wide comprehensive analysis to classify the PYL genes in the L. chinense. This study highlighted the seven LchiPYL genes in the L. chinense genome. The detailed investigations about gene structure variations, chromosomal distributions, phylogenetic tree, 3D structure, motif analysis, cis-regulatory elements, subcellular location, and expression profiles in buds and stress responses were carried out in this article. Phylogenetic tree analysis exploited that the seven LchiPYL genes were divided into four groups (Group I–IV) from L. chinense, and three were highly linked with other species. By analyzing the cis-elements in the promoters, we identified five hormones-, six stress-, three growth and biological process, and two metabolic-related responsive elements. The expression analysis showed that all seven genes were up-and down-regulated against response to ABA, photoperiod, chilling, and chilling + photoperiod treatments. Our findings opened up new future research directions and provided insight into the PYL family genes in Liriodendron chinense. [ABSTRACT FROM AUTHOR]

Summary An integral part of global environment change is an increase in the atmospheric concentration of CO 2 ([CO 2 ]) [ 1 ]. Increased [CO 2 ] reduces leaf stomatal apertures and density of stomata that plays out as reductions in evapotranspiration [ 2–4 ]. Surprisingly, given the importance of transpiration to the control of terrestrial water fluxes [ 5 ] and plant nutrient acquisition [ 6 ], we know comparatively little about the molecular components involved in the intracellular signaling pathways by which [CO 2 ] controls stomatal development and function [ 7 ]. Here, we report that elevated [CO 2 ]-induced closure and reductions in stomatal density require the generation of reactive oxygen species (ROS), thereby adding a new common element to these signaling pathways. We also show that the PYR/RCAR family of ABA receptors [ 8, 9 ] and ABA itself are required in both responses. Using genetic approaches, we show that ABA in guard cells or their precursors is sufficient to mediate the [CO 2 ]-induced stomatal density response. Taken together, our results suggest that stomatal responses to increased [CO 2 ] operate through the intermediacy of ABA. In the case of [CO 2 ]-induced reductions in stomatal aperture, this occurs by accessing the guard cell ABA signaling pathway. In both [CO 2 ]-mediated responses, our data are consistent with a mechanism in which ABA increases the sensitivity of the system to [CO 2 ] but could also be explained by requirement for a CO 2 -induced increase in ABA biosynthesis specifically in the guard cell lineage. Furthermore, the dependency of stomatal [CO 2 ] signaling on ABA suggests that the ABA pathway is, in evolutionary terms, likely to be ancestral. [ABSTRACT FROM AUTHOR]

We provide the first evidence for a mechanism of growth inhibition by ABA during germination and seedling establishment based on inhibition of PM H+-ATPase and altered pH, K+, and anion homeostasis.The stress hormone abscisic acid (ABA) induces expression of defence genes in many organs, modulates ion homeostasis and metabolism in guard cells, and inhibits germination and seedling growth. Concerning the latter effect, several mutants of Arabidopsis thaliana with improved capability for H+ efflux (wat1-1D, overexpression of AKT1 and ost2-1D) are less sensitive to inhibition by ABA than the wild type. This suggested that ABA could inhibit H+ efflux (H+-ATPase) and induce cytosolic acidification as a mechanism of growth inhibition. Measurements to test this hypothesis could not be done in germinating seeds and we used roots as the most convenient system. ABA inhibited the root plasma-membrane H+-ATPase measured in vitro (ATP hydrolysis by isolated vesicles) and in vivo (H+ efflux from seedling roots). This inhibition involved the core ABA signalling elements: PYR/PYL/RCAR ABA receptors, ABA-inhibited protein phosphatases (HAB1), and ABA-activated protein kinases (SnRK2.2 and SnRK2.3). Electrophysiological measurements in root epidermal cells indicated that ABA, acting through the PYR/PYL/RCAR receptors, induced membrane hyperpolarization (due to K+ efflux through the GORK channel) and cytosolic acidification. This acidification was not observed in the wat1-1D mutant. The mechanism of inhibition of the H+-ATPase by ABA and its effects on cytosolic pH and membrane potential in roots were different from those in guard cells. ABA did not affect the in vivo phosphorylation level of the known activating site (penultimate threonine) of H+-ATPase in roots, and SnRK2.2 phosphorylated in vitro the C-terminal regulatory domain of H+-ATPase while the guard-cell kinase SnRK2.6/OST1 did not. [ABSTRACT FROM AUTHOR]

Abscisic acid (ABA) is a phytohormone that plays important roles in the regulation of seed dormancy and adaptation to abiotic stresses. Previous work identified OsPYL/RCARs as functional ABA receptors regulating ABA-dependent gene expression in Oryza sativa. OsPYL/RCARs thus are considered to be good candidate genes for improvement of abiotic stress tolerance in crops. This work demonstrates that the cytosolic ABA receptor OsPYL/RCAR5 in O. sativa functions as a positive regulator of abiotic stress-responsive gene expression. The constitutive expression of OsPYL/RCAR5 in rice driven by the Zea mays ubiquitin promoter induced the expression of many stress-responsive genes even under normal growth conditions and resulted in improved drought and salt stress tolerance in rice. However, it slightly reduced plant height under paddy field conditions and severely reduced total seed yield. This suggests that, although exogenous expression of OsPYL/RCAR5 is able to improve abiotic stress tolerance in rice, fine regulation of its expression will be required to avoid deleterious effects on agricultural traits. [ABSTRACT FROM PUBLISHER]

To face the challenges of climate change and sustainable food production, it is essential to develop crop genome editing techniques to pinpoint key genes involved in abiotic stress signaling. The identification of those prevailing abscisic acid (ABA) receptors that mediate plant-environment interactions is quite challenging in polyploid plants because of the high number of genes in the PYR/PYL/RCAR ABA receptor family. Nicotiana benthamiana is a biotechnological crop amenable to genome editing, and given the importance of ABA signaling in coping with drought stress, we initiated the analysis of its 23-member family of ABA receptors through multiplex CRISPR/Cas9-mediated editing. We generated several high-order mutants impaired in NbPYL1-like and NbPYL8-like receptors, which showed certain insensitivity to ABA for inhibition of seedling establishment, growth, and development of shoot and lateral roots as well as reduced sensitivity to the PYL1-agonist cyanabactin (CB). However, in these high-order mutants, regulation of transpiration was not affected and was responsive to ABA treatment. This reveals a robust and redundant control of transpiration in this allotetraploid plant that probably reflects its origin from the extreme habitat of central Australia. [ABSTRACT FROM AUTHOR]

Plants perceiving drought activate multiple responses to improve survival, including large-scale alterations in gene expression. This article reports on the roles in the drought response of two Arabidopsis thaliana homeodomain-leucine zipper class I genes; ATHB7 and ATHB12, both strongly induced by water-deficit and abscisic acid (ABA). ABA-mediated transcriptional regulation of both genes is shown to depend on the activity of protein phosphatases type 2C (PP2C). ATHB7 and ATHB12 are, thus, targets of the ABA signalling mechanism defined by the PP2Cs and the PYR/PYL family of ABA receptors, with which the PP2C proteins interact. Our results from chromatin immunoprecipitation and gene expression analyses demonstrate that ATHB7 and ATHB12 act as positive transcriptional regulators of PP2C genes, and thereby as negative regulators of abscisic acid signalling. In support of this notion, our results also show that ATHB7 and ATHB12 act to repress the transcription of genes encoding the ABA receptors PYL5 and PYL8 in response to an ABA stimulus. In summary, we demonstrate that ATHB7 and ATHB12 have essential functions in the primary response to drought, as mediators of a negative feedback effect on ABA signalling in the plant response to water deficit. [ABSTRACT FROM AUTHOR]

Nowadays, fundamentally important data toward the molecular mechanisms of phytohormone action, starting from hormonal signal perception and up to changes in hormone-regulated gene expression, became available. Signaling mechanisms for plant cell responses to ABA remained the least known. Although a substantial progress was achieved in our understanding of the role of protein kinases and protein phosphatases functioning downstream receptors, identification of ABA receptors has induced very hot debates. This review summarizes information obtained during the last decade and concerned the molecular mechanisms of perception and signal transduction of ABA signal in plants. [ABSTRACT FROM AUTHOR]

Enhancing water use efficiency (WUE) of crops in irrigated agriculture and drought tolerance in rain-fed agriculture is the major goal for sustaining and enhancing agricultural productivity in the future. The phytohormone abscisic acid (ABA) signaling pathway is a major target for the agronomic management of WUE and genetic improvement of drought tolerance in crops. The START domain proteins PYRABACTIN RESISTANCE1 (PYR1)/PYR1-like (PYL)/Regulatory Components of ABA Receptors (RCARs) of the model plant Arabidopsis thaliana have been characterized as bona fide ABA receptors (ABARs). ABA signaling pathway can be activated or repressed by using specific agonist and antagonist against ABAR and therefore, can be used to control ABA-mediated physiological changes in plants. In the present work, we have reported the 3 D structure models of three ABARs (OsPYL1-3) from drought-tolerant Indica rice N22 ( Oryza sativa L. sp. Indica cv N22) in apo- and ligand-bound conformations developed using comparative modeling techniques. Subsequently, these models were used in docking study to investigate the binding mode of known ABAR agonists and antagonists. Further, molecular dynamics studies on the selected systems verified the residues involved in protein-ligand interactions. The study identified the important ligand-binding features for the future development of specific agonists/antagonists to modulate the ABA activity in O. sativa and provides in silico models for designing and virtual screening to identify potent ABA receptor ligands.Communicated by Ramaswamy H. Sarma.

Abscisic acid (ABA) is a vital phytohormone that accumulates in response to various biotic and abiotic stresses, as well as plant growth. In Arabidopsis thaliana, there are 14 members of the ABA receptor family, which are key positive regulators involved in ABA signaling. Besides reduced drought stress tolerance, the quadruple and sextuple mutants (pyr1pyl1pyl2pyl4 (1124) and pyr1pyl1pyl2pyl4pyl5pyl8 (112458) show abnormal growth phenotypes, such as decreases in yield and height, under non-stress conditions. However, it remains unknown whether ABA receptors mediate ABA signaling to regulate plant growth and development. Here, we showed the primary metabolite profiles of 1124, 112458 and wild-type (WT) plants grown under normal conditions. The metabolic changes were significantly different between ABA receptor mutants and WT. Guanosine, for the biosynthesis of cyclic guanosine 3′,5′-monophosphate (cGMP), is an important second messenger that acts to regulate the level of ABA. In addition, other amino acids were increased in the 112458 mutant, including proline. These results, together with phenotype analysis, indicated that ABA receptors are involved in ABA signaling to modulate metabolism and plant growth under normal conditions. [ABSTRACT FROM AUTHOR]

Methyl jasmonate (MeJA) induces stomatal closure. It has been shown that stomata of many ABA-insensitive mutants are also insensitive to MeJA, and a low amount of ABA is a prerequisite for the MeJA response. However, the molecular mechanisms of the interaction between ABA and MeJA signaling remain to be elucidated. Here we studied the interplay of signaling of the two hormones in guard cells using the quadruple ABA receptor mutant pyr1 pyl1 pyl2 pyl4 and ABA-activated protein kinase mutants ost1-2 and srk2e. In the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, nitric oxide (NO) production, cytosolic alkalization and plasma membrane Ca(2+)-permeable current (ICa) activation were not impaired. At the same time, the inactivation of the inward-rectifying K(+) current was impaired. In contrast to the quadruple mutant, MeJA-induced stomatal closure, H2O2 production, NO production and cytosolic alkalization were impaired in ost1-2 and srk2e as well as in aba2-2, the ABA-deficient mutant. The activation of ICa was also impaired in srk2e. Collectively, these results indicated that OST1 was essential for MeJA-induced stomatal closure, while PYR1, PYL1, PYL2 and PYL4 ABA receptors were not sufficient factors. MeJA did not appear to activate OST1 kinase activity. This implies that OST1 mediates MeJA signaling through an undetectable level of activity or a non-enzymatic action. MeJA induced the expression of an ABA synthesis gene, NCED3, and increased ABA contents only modestly. Taken together with previous reports, this study suggests that MeJA signaling in guard cells is primed by ABA and is not brought about through the pathway mediated by PYR1, PYL1 PYL2 and PYL4., (© The Author 2016. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)