Complex regulation of ABA biosynthesis in plants.

Abscisic acid (ABA) is a plant hormone that plays important roles during many phases of the plant life cycle, including seed development and dormancy, and in plant responses to various environmental stresses. Because many of these physiological processes are correlated with endogenous ABA levels, the regulation of ABA biosynthesis is a key element facilitating the elucidation of these physiological characteristics. Recent studies on the identification of genes encoding enzymes involved in ABA biosynthesis have revealed details of the main ABA biosynthetic pathway. At the same time, the presence of gene families and their respective organ-specific expression are indicative of the complex mechanisms governing the regulation of ABA biosynthesis in response to plant organ and/or environmental conditions. There have been recent advances in the study of ABA biosynthesis and new insights into the regulation of ABA biosynthesis in relation to physiological phenomena.     

Early ABA Signaling Events in Guard Cells

The plant hormone abscisic acid (ABA) regulates a wide variety of plant physiological and developmental processes, particularly responses to environmental stress, such as drought. In response to water deficiency, plants redistribute foliar ABA and/or upregulate ABA synthesis in roots, leading to roughly a 30-fold increase in ABA concentration in the apoplast of stomatal guard cells. The elevated ABA triggers a chain of events in guard cells, causing stomatal closure and thus preventing water loss. Although the molecular nature of ABA receptor(s) remains unknown, considerable progress in the identification and characterization of its downstream signaling elements has been made by using combined physiological, biochemical, biophysical, molecular, and genetic approaches. The measurable events associated with ABA-induced stomatal closure in guard cells include, sequentially, the production of reactive oxygen species (ROS), increases in cytosolic free Ca²⁺ levels ([Ca²⁺]_i), activation of anion channels, membrane potential depolarization, cytosolic alkalinization, inhibition of K⁺ influx channels, and promotion of K⁺ efflux channels. This review provides an overview of the cellular and molecular mechanisms underlying these ABA-evoked signaling events, with particular emphasis on how ABA triggers an “electronic circuitry” involving these ionic components.     

The PP2C-SnRK2 complex: The central regulator of an abscisic acid signaling pathway

The phytohormone abscisic acid (ABA), an important bioactive compound in plants, is implicated in several essential processes such as development and the abiotic stress response. Many components have been reported to have roles in these processes. Although 2C-type protein phosphatases (PP2C) and SNF1-related protein kinases2 (SnRK2) family are known to be important signal mediators, the molecular mechanisms by which these components regulate the ABA signaling pathway have not been elucidated. Recent identification of soluble ABA receptors, PYR/PYL/RCAR, has provided a major breakthrough in understanding the signaling mechanisms of ABA and revealed the importance of PP2Cs. In addition, the physical, biochemical and physiological connections between PP2C and SnRK2 have been clearly demonstrated. Taken together, the molecular basis of the major ABA signaling pathway has been established, from perception to gene expression. In this addendum, we discuss this emerging ABA signaling pathway, which has a conventional protein phosphorylation/dephosphorylation regulatory circuit and consider its physiological and functional relevance.Key words: ABA receptor, abscisic acid, PP2C, signal transduction, SnRK2, plant hormone, phosphoarylation     

Abscisic acid receptors: past, present and future

Jin-Gui Chen (Corresponding author) Abscisic acid (ABA) is the key plant stress hormone. Consistent with the earlier studies in support of the presence of both membrane- and cytoplasm-localized ABA receptors, recent studies have identified multiple ABA receptors located in various subcellular locations. These include a chloroplast envelope-localized receptor (the H subunit of Chloroplast Mg(2+) -chelatase/ABA Receptor), two plasma membrane-localized receptors (G-protein Coupled Receptor 2 and GPCR-type G proteins), and one cytosol/nucleus-localized Pyrabactin Resistant (PYR)/PYR-Like (PYL)/Regulatory Component of ABA Receptor 1 (RCAR). Although the downstream molecular events for most of the identified ABA receptors are currently unknown, one of them, PYR/PYL/RCAR was found to directly bind and regulate the activity of a long-known central regulator of ABA signaling, the A-group protein phosphatase 2C (PP2C). Together with the Sucrose Non-fermentation Kinase Subfamily 2 (SnRK2s) protein kinases, a central signaling complex (ABA-PYR-PP2Cs-SnRK2s) that is responsible for ABA signal perception and transduction is supported by abundant genetic, physiological, biochemical and structural evidence. The identification of multiple ABA receptors has advanced our understanding of ABA signal perception and transduction while adding an extra layer of complexity.     

Somatostatins and their receptors in fish

Somatostatin (SRIF) is a multigene family of peptides. SRIF-14 is conserved with identical primary structure in species across the vertebrates. The presence of multiple SRIF genes has been demonstrated in a number of fish species. Notably, three distinct SRIF genes have been identified in goldfish. One of these genes, which encodes [Pro(2)]SRIF-14, has also been identified in sturgeon and African lungfish, and is closely associated with the amphibian [Pro(2),Met(13)]SRIF-14 gene and mammalian cortistatin gene. The main neuroendocrine role of SRIF-14 peptide that has been determined in fish is the inhibition of pituitary growth hormone secretion. The functions of SRIF-14 variant or larger forms of SRIF peptide and the regulation of SRIF gene expression remain to be explored. Type one and two SRIF receptors have been identified from goldfish and type three SRIF receptor from an electric fish. Fish SRIF receptors display considerable homology to mammalian counterparts in terms of primary structure and negative coupling to adenylate cyclase. The identification of the multiple gene family of SRIF peptides and multiple types of SRIF receptors in fish opens a new avenue for the study of physiological roles of SRIF, and the molecular and cellular mechanisms of SRIF actions in fish.     

Turning on gibberellin and abscisic acid signaling

The phytohormones gibberellin (GA) and abscisic acid (ABA) play essential and often antagonistic roles in regulating plant growth, development and stress responses. The long-awaited identification of receptors for both GA and ABA has shed light upon the initial events that surround the perception of these two phytohormones. The discovery of these receptors also challenges conventional views of plant hormone signaling and raises intriguing questions regarding the nature of GA and ABA perception and the initiation of their signaling pathways. Moreover, recent advances in understanding GA and ABA signaling point to the existence of multiple, non-linear cell- and compartment-specific pathways that regulate genomic and non-genomic responses to these phytohormones.     

Signaling mechanisms in abscisic acid-mediated stomatal closure

The plant hormone abscisic acid (ABA) plays a central role in the regulation of stomatal movements under water-deficit conditions. The identification of ABA receptors and the ABA signaling core consisting of PYR/PYL/RCAR ABA receptors, PP2C protein phosphatases and SnRK2 protein kinases has led to studies that have greatly advanced our knowledge of the molecular mechanisms mediating ABA-induced stomatal closure in the past decade. This review focuses on recent progress in illuminating the regulatory mechanisms of ABA signal transduction, and the physiological importance of basal ABA signaling in stomatal regulation by CO₂ and, as hypothesized here, vapor-pressure deficit. Furthermore, advances in understanding the interactions of ABA and other stomatal signaling pathways are reviewed here. We also review recent studies investigating the use of ABA signaling mechanisms for the manipulation of stomatal conductance and the enhancement of drought tolerance and water-use efficiency of plants.     

植物激素脱落酸受体的研究进展

脱落酸(abscisic acid, ABA)广泛参与植物生长发育的调控和对多种环境胁迫的适应性反应。有关ABA受体的研究已经在检测受体位置、纯化ABA特异性的结合蛋白和克隆ABA受体基因方面做出了许多重要的工作。最近相继发现一种RNA结合蛋白FCA和一种编码Mg离子螯合酶（Mg-chelatase）H亚基的CHLH作为两种不同的ABA受体分别调控植物的开花时间和介导种子萌发、幼苗生长及叶片的气孔运动。本文从实验策略的角度重点分析总结了研究脱落酸受体相对有效的途径与方法, 同时就有关的研究结果给予了评论和展望。     

植物激素脱落酸受体的研究进展

脱落酸（abscisic acid,ABA）广泛参与植物生长发育的调控和对多种环境胁迫的适应性反应。有关ABA受体的研究已经在检测受体位置、纯化ABA特异性的结合蛋白和克隆ABA受体基因方面做出了许多重要的工作。最近相继发现一种RNA结合蛋白FCA和一种编码Mg离子螯合酶（Mg-chelatase）H亚基的CHLH作为两种不同的ABA受体分别调控植物的开花时间和介导种子萌发、幼苗生长及叶片的气孔运动。本文从实验策略的角度重点分析总结了研究脱落酸受体相对有效的途径与方法,同时就有关的研究结果给予了评论和展望。     

A thermodynamic switch modulates abscisic acid receptor sensitivity

Abscisic acid (ABA) is a key hormone regulating plant growth, development and the response to biotic and abiotic stress. ABA binding to pyrabactin resistance (PYR)/PYR1-like (PYL)/Regulatory Component of Abscisic acid Receptor (RCAR) intracellular receptors promotes the formation of stable complexes with certain protein phosphatases type 2C (PP2Cs), leading to the activation of ABA signalling. The PYR/PYL/RCAR family contains 14 genes in Arabidopsis and is currently the largest plant hormone receptor family known; however, it is unclear what functional differentiation exists among receptors. Here, we identify two distinct classes of receptors, dimeric and monomeric, with different intrinsic affinities for ABA and whose differential properties are determined by the oligomeric state of their apo forms. Moreover, we find a residue in PYR1, H60, that is variable between family members and plays a key role in determining oligomeric state. In silico modelling of the ABA activation pathway reveals that monomeric receptors have a competitive advantage for binding to ABA and PP2Cs. This work illustrates how receptor oligomerization can modulate hormonal responses and more generally, the sensitivity of a ligand-dependent signalling system.     

ABA signal transduction at the crossroad of biotic and abiotic stress responses

Abscisic acid (ABA) regulates key processes relevant to seed germination, plant development, and biotic and abiotic stress responses. Abiotic stress conditions such as drought induce ABA biosynthesis initiating the signalling pathways that lead to a number of molecular and cellular responses, among which the best known are the expression of stress-related genes and stomatal closure. Stomatal closure also serves as a mechanism for pathogen defence, thereby acting as a platform for crosstalk between biotic and abiotic stress responses involving ABA action. Significant advances in our understanding of ABA signal transduction have been made with combination of approaches including genetics, biochemistry, electrophysiology and chemical genetics. Molecular components associated with the ABA signalling have been identified, and their relationship in the complex network of interactions is being dissected. We focused on the recent progress in ABA signal transduction, especially those studies related to identification of ABA receptors and downstream components that lead ABA signal to cellular response. In particular, we will describe a pathway model that starts with ABA binding to the PYR/PYL/RCAR family of receptors, followed by inactivation of 2C-type protein phosphatases and activation of SnRK2-type kinases, and eventually lead to activation of ion channels in guard cells and stomatal closure.     

Action of natural abscisic acid precursors and catabolites on abscisic acid receptor complexes

The phytohormone abscisic acid (ABA) regulates stress responses and controls numerous aspects of plant growth and development. Biosynthetic precursors and catabolites of ABA have been shown to trigger ABA responses in physiological assays, but it is not clear whether these are intrinsically active or whether they are converted into ABA in planta. In this study, we analyzed the effect of ABA precursors, conjugates, and catabolites on hormone signaling in Arabidopsis (Arabidopsis thaliana). The compounds were also tested in vitro for their ability to regulate the phosphatase moiety of ABA receptor complexes consisting of the protein phosphatase 2C ABI2 and the coreceptors RCAR1/PYL9, RCAR3/PYL8, and RCAR11/PYR1. Using mutants defective in ABA biosynthesis, we show that the physiological activity associated with ABA precursors derives predominantly from their bioconversion to ABA. The ABA glucose ester conjugate, which is the most widespread storage form of ABA, showed weak ABA-like activity in germination assays and in triggering ABA signaling in protoplasts. The ABA conjugate and precursors showed negligible activity as a regulatory ligand of the ABI2/RCAR receptor complexes. The majority of ABA catabolites were inactive in our assays. To analyze the chemically unstable 8'- and 9'-hydroxylated ABA catabolites, we used stable tetralone derivatives of these compounds, which did trigger selective ABA responses. ABA synthetic analogs exhibited differential activity as regulatory ligands of different ABA receptor complexes in vitro. The data show that ABA precursors, catabolites, and conjugates have limited intrinsic bioactivity and that both natural and synthetic ABA-related compounds can be used to probe the structural requirements of ABA ligand-receptor interactions.     

Chemical biology of abscisic acid

Chemical biology is a discipline that utilizes chemicals to elucidate biological mechanisms and physiological functions. Various abscisic acid (ABA) derivatives have revealed the structural requirement for the perception by ABA receptors while biotin or caged derivatives of ABA have disclosed the localization of several ABA-binding proteins. Recently, selective ABA agonist has been used to identify ABA receptors. Furthermore, ABA biosynthesis and catabolic inhibitors have contributed to the identification of new ABA functions in plant growth and development. The physiological function of ABA in non-plant organisms has gradually been revealed. In this review, we discuss the development of small bioactive chemicals and their significance in ABA research.     

Identification and characterization of interactions between abscisic acid and mitochondrial adenine nucleotide translocators

ABA (abscisic acid) is a plant hormone involved in important processes including development and stress responses. Recent reports have identified a number of plant ABA receptors and transporters, highlighting novel mechanisms of ABA action. In the present paper we describe application of a chemical proteomics approach leading to the identification of mitochondrial ANTs (adenine nucleotide translocators) as ABA-interacting proteins. Initial in vitro studies confirmed inhibition of ANT-dependent ATP translocation by ABA. Further analysis demonstrated ANT-dependent uptake of ABA into both recombinant Arabidopsis thaliana ANT2-containing proteoliposomes and native isolated spinach mitochondria; the latter with a Km of 3.5?μM and a Vmax of 2.5 nmol/min per g of protein. ATP was found to inhibit ANT-dependent ABA translocation. Specificity profiles highlight the possibility of mechanistic differences in translocation of ABA and ATP. Finally, ABA was shown to stimulate ATPase activity in spinach mitochondrial extracts. ABA concentrations in plant cells are estimated to reach the low micromolar range during stress responses, supporting potential physiological relevance of these in vitro findings. Overall, the present in vitro work suggests the possibility of as yet uncharacterized mechanisms of ABA action in planta related to inhibition of mitochondrial ATP translocation and functional localization of ABA in the mitochondrial matrix.     

Perception and transduction of abscisic acid signals: keys to the function of the versatile plant hormone ABA

During the past decade, much progress has been made toward understanding the mechanisms underlying plant hormone activity, from perception to nuclear events. However, the signaling mechanisms for abscisic acid (ABA) have remained largely obscure. Recent breakthroughs identifying FCA, which is an RNA-binding protein, the Mg-chelatase H subunit, and a G protein-coupled receptor as receptors for ABA provide a major leap forward in understanding the initial steps of ABA signaling mechanisms. Recent studies have also revealed the molecular mechanisms of second messenger production, protein modifications such as phosphorylation, and regulatory mechanisms of gene expression in the ABA response. Therefore, the connections between these events are also beginning to be determined. Here, we review recent progress and discuss the overall scheme of the ABA response mechanisms.     

Interaction between abscisic acid receptor PYL3 and protein phosphatase type 2C in response to ABA signaling in maize

Abscisic acid (ABA) is a ubiquitous hormone that regulates plant growth, development and responses to environmental stresses. In recent researches, pyrabactin resistance 1-like protein (PYL) and protein phosphatase type 2C (PP2C) were identified as the direct receptor and the second component of ABA signaling pathway, respectively. However, a lot of PYL and PP2C members were found in Arabidopsis and several other plants. Some of them were found not to be involved in ABA signaling. Because of the complex diversity of the genome, few documents have been available on the molecular details of the ABA signal perception system in maize. In the present study, we conducted bioinformatics analysis to find out the candidates (ZmPYL3 and ZmPP2C16) of the PYL and PP2C members most probably involved in ABA signaling in maize, cloned their encoding genes (ZmPYL3 and ZmPP2C16), verified the interaction between these two proteins in response to exogenous ABA induction by yeast two-hybrid assay and bimolecular fluorescence complementation, and investigated the expression patterns of these two genes under the induction of exogenous ABA by real-time fluorescence quantitative PCR. The results indicated that the ZmPYL3 and ZmPP2C16 proteins interacted in vitro and in vivo in response to the induction of exogenous ABA. The downregulated expression of the ZmPYL3 gene and the upregulated expression of the ZmPP2C16 gene are responsive to the induction of exogenous ABA. The ZmPYL3 and ZmPP2C16 proteins are the most probable members of the receptors and the second components of ABA signaling pathway, respectively.     

The putative glutamate receptor 1.1 (AtGLR1.1) in Arabidopsis thaliana regulates abscisic acid biosynthesis and signaling to control development and water loss

The involvement of the putative glutamate receptor 1.1 (AtGLR1.1) gene in the regulation of abscisic acid (ABA) biosynthesis and signaling was investigated in Arabidopsis. Seeds from AtGLR1.1-deficient (antiAtGLR1.1) lines had increased sensitivity to exogenous ABA with regard to the effect of the hormone on the inhibition of seed germination and root growth. Seed germination, which was inhibited by an animal ionotropic glutamate receptor antagonist, 6,7-dinitroquinoxaline-2,3-[1H,4H]-dione, was restored by co-incubation with an inhibitor of ABA biosynthesis, fluridone. These results confirm that germination in antiAtGLR1.1 lines was inhibited by increased ABA. When antiAtGLR1.1 and WT seeds were co-incubated in fluridone and exogenous ABA, the antiAtGLR1.1 seeds were more sensitive to ABA. In addition, the antiAtGLR1.1 lines exhibited altered expression of ABA biosynthetic (ABA) and signaling (ABI) genes, when compared with WT. Combining the physiological and molecular results suggest that ABA biosynthesis and signaling in antiAtGLR1.1 lines are altered. ABA levels in leaves of antiAtGLR1.1 lines are higher than those in WT. In addition, the antiAtGLR1.1 lines had reduced stomatal apertures, and exhibited enhanced drought tolerance due to deceased water loss compared with WT lines. The results from these experiments imply that ABA biosynthesis and signaling can be regulated through AtGLR1.1 to trigger pre- and post-germination arrest and changes in whole plant responses to water stress. Combined with our earlier results, these findings suggest that AtGLR1.1 integrates and regulates the different aspects of C, N and water balance that are required for normal plant growth and development.