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.     

Sensitivity of Stomata to Abscisic Acid (An Effect of the Mesophyll)

The effects of added abscisic acid (ABA) on the stomatal behavior of Commelina communis L. were tested using three different systems. ABA was applied to isolated epidermis or to leaf pieces incubated in the light in bathing solutions perfused with CO2-free air. ABA was also fed to detached leaves in a transpiration bioassay. The apparent sensitivity of stomata to ABA was highly dependent on the method used to feed ABA. Stomata of isolated epidermis were apparently most sensitive to ABA, such that a concentration of 1 [mu]M caused almost complete stomatal closure. When pieces of whole leaves were floated on solutions of ABA of the same concentration, the stomata were almost completely open. The same concentration of ABA fed through the midrib of transpiring detached leaves caused an intermediate response. These differences in stomatal sensitivity to added ABA were found to be a function of differences in the ABA concentration in the epidermes. Comparison of the three application systems suggested that, when leaf pieces were incubated in ABA or fed with ABA through the midrib, accumulation of ABA in the epidermes was limited by the presence of the mesophyll. Even bare mesophyll incubated in ABA solution did not accumulate ABA. Accumulation of radioactivity by leaf pieces floated on [3H]ABA confirmed ABA uptake in this system. Experiments with tetcyclacis, an inhibitor of phaseic acid formation, suggested that rapid metabolism of ABA in mesophyll can have a controlling influence on ABA concentration in both the mesophyll and the epidermis. Inhibition of ABA catabolism with tetcyclacis allows ABA accumulation and increases the apparent sensitivity of stomata to applied ABA. The results are discussed in the context of an important role for ABA metabolism in the regulation of stomatal behavior.     

不同籼稻品种的矮生性与内源ABA水平及其结合蛋白的关系

试验研究了内源脱落酸及其结合蛋白与含有不同矮秆主基因的籼稻矮生性状的关系。结果表明：籼稻的矮生性也受内源ＡＢＡ及其结合蛋白协同调控。矮秆品种幼苗内源ＡＢＡ及其幼芽膜上ＡＢＡ结合蛋白水平显著高于离秆品种,且含有强矮化效应的ｓｄ－ｇ基因的新桂矮、双矮的内源ＡＢＡ及其结合蛋白水平又明显高于含ｓｄ－１半矮秆基因的广场矮。外源ＡＢＡ能显著抑制籼稻幼苗伸长,苗高下降率与ＡＢＡ结合蛋白水平有一定的关系,且矮秆品     

苹果果实组织脱落酸结合位点的亚细胞定位

在苹果（ＭａｌｕｓｐｕｍｉｌａＭｉｌ）果实细胞的可溶性组分中存在ＡＢＡ特异结合位点,这些位点能与ＡＢＡ形成稳定的３ＨＡＢＡ蛋白结合复合物,而微粒体部分未表现出３ＨＡＢＡ结合活性。可溶性组分对３ＨＡＢＡ的结合效率与结合稳定性需要活体组织完整细胞的存在。ＡＢＡ共价交联物ＡＢＡＬＹＣＨ,具有与ＡＢＡ相同的抑制红苋菜（ＡｍａｒａｎｔｈｕｓｔｒｉｃｏｌｏｒＬ．）种子萌发的生物活性,能有效地竞争抑制果实组织圆片对３ＨＡＢＡ的结合。通过ＡＢＡＬＹＣＨ对苹果果实组织圆片及果实组织游离细胞的荧光染色表明,未成熟果实的细胞外周被特异性地染色。在ＢＳＡ存在的情况下,ＡＢＡＬＹＣＨ复合物被转运进细胞,形成密集的强烈荧光团。结果表明,ＡＢＡＬＹＣＨ与３ＨＡＢＡ的竞争性结合发生在细胞质膜水平。     

The modulating effect of the perisperm-endosperm envelope on ABA-inhibition of seed germination in cucumber

Abscisic acid (ABA) markedly reduced the germination of developing seeds at much lower concentrations (ABA50=0.1 mM) compared with that of mature seeds (ABA50=1.6 mM) in cucumber (Cucumis sativus L. cv. Green long). The perisperm-endosperm (PE) envelope in developing seeds showed partly differentiated lipid and callose layers, considerable ABA biosynthetic activity in endosperm cells, and appreciable permeability to applied ABA. The decrease in the sensitivity of seeds to applied ABA was coincident with the complete development of lipid and callose layers, diminished ABA biosynthetic activity in endosperm cells in imbibed mature seeds, and moderate permeability of the PE envelope to applied ABA. Decoated seeds pretreated with chloroform showed decreased germination (ABA50=0.4 mM) in response to applied ABA and increased ABA permeation through the PE envelope. ABA thus allowed to permeate into embryonic tissues substantially reduced the pregerminative activity of beta-glucanase in the radicles. The structure and biophysical/biochemical properties of the PE envelope seem to modulate the effect of ABA on the germination of developing and mature cucumber seeds.     

Control of abscisic acid catabolism and abscisic acid homeostasis is important for reproductive stage stress tolerance in cereals

Drought stress at the reproductive stage causes pollen sterility and grain loss in wheat (Triticum aestivum). Drought stress induces abscisic acid (ABA) biosynthesis genes in anthers and ABA accumulation in spikes of drought-sensitive wheat varieties. In contrast, drought-tolerant wheat accumulates lower ABA levels, which correlates with lower ABA biosynthesis and higher ABA catabolic gene expression (ABA 8'-hydroxylase). Wheat TaABA8'OH1 deletion lines accumulate higher spike ABA levels and are more drought sensitive. ABA treatment of the spike mimics the effect of drought, causing high levels of sterility. ABA treatment represses the anther cell wall invertase gene TaIVR1, and drought-tolerant lines appeared to be more sensitive to the effect of ABA. Drought-induced sterility shows similarity to cold-induced sterility in rice (Oryza sativa). In cold-stressed rice, the rate of ABA accumulation was similar in cold-sensitive and cold-tolerant lines during the first 8 h of cold treatment, but in the tolerant line, ABA catabolism reduced ABA levels between 8 and 16 h of cold treatment. The ABA biosynthesis gene encoding 9-cis-epoxycarotenoid dioxygenase in anthers is mainly expressed in parenchyma cells surrounding the vascular bundle of the anther. Transgenic rice lines expressing the wheat TaABA8'OH1 gene under the control of the OsG6B tapetum-specific promoter resulted in reduced anther ABA levels under cold conditions. The transgenic lines showed that anther sink strength (OsINV4) was maintained under cold conditions and that this correlated with improved cold stress tolerance. Our data indicate that ABA and ABA 8'-hydroxylase play an important role in controlling anther ABA homeostasis and reproductive stage abiotic stress tolerance in cereals.     

Dynamic analysis of ABA accumulation in relation to the rate of ABA catabolism in maize tissues under water deficit

The plant hormone abscisic acid (ABA) accumulates in plant tissues which experience water deficit (stress ABA). This study analysed its accumulation as a function of both synthesis and catabolism in maize tissues. By following the disappearance of the stress ABA when ABA synthesis was blocked by nordihydroguaiaretic acid (NDGA), the rate of the catabolism of stress ABA was determined. When compared with the catabolic rate of baseline (non-stress) ABA, stress ABA showed a catabolic rate >11 times higher. With such an elevated catabolic rate, it is proposed that the xanthophyll precursor pool may not be able to sustain the ABA accumulation, and such a proposition has been substantiated by further experiments where fluridone is used to limit the availability of upstream ABA precursors. When fluridone was used, stress ABA accumulation could only be sustained for a few hours, i.e. approximately 5 h for leaf and 1 h for root tissues. In detached roots, stress ABA accumulation could not be sustained even if fluridone was not used, suggesting that stress ABA accumulation in root systems requires the continuous import of ABA precursors from the shoots. Such an assumption was substantiated by the observation that defoliation or shading significantly reduced ABA accumulation in intact roots. The present study suggests that ABA catabolism is rapid enough to play an important role in the regulation of ABA accumulation.     

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.     

Stomatal closure is induced rather by prevailing xylem abscisic acid than by accumulated amount of xylem-derived abscisic acid

We have studied the stomatal response in relation to the xylem-derived abscisic acid (ABA) accumulation in sunflower leaves. When ABA was introduced into detached leaves of the sunflower through xylem flux, stomatal conductance was regulated, water flux was changed as a result and at the same time the xylem-derived ABA was metabolised in the leaves. We computed the xylem-derived ABA accumulation in the leaves as a function of time by taking into account the variation of ABA flux into the leaves (the product of water flux and ABA concentration) and a continuing ABA metabolism. We found that ABA accumulation was rapid during an initial lag phase, much slowed down during the decreasing phase of stomatal conductance, but still substantial when stomatal conductance reached a new stable state. The results show a poor link between the kinetics of ABA-induced stomatal closure and the xylem-derived ABA accumulation. Xylem-derived ABA was metabolised rapidly in the leaves. Tetcyclacis, as an inhibitor, substantially inhibited this process. Two hours after ABA was fed into a leaf, about 70% of the fed ABA was metabolised, but when tetcyclacis was added into the feeding solution, less than 30% of ABA was metabolised, even after 24 h of incubation. The inhibition of ABA metabolism by tetcyclacis did not lead to more stomatal closure, which was still concentration-dependent. Since the accumulation of xylem-derived ABA was enhanced substantially by the presence of tetcyclacis, these results strongly indicate that stomata mainly respond to the prevailing ABA concentration in the xylem stream, rather than to the accumulated amount of xylem-derived ABA in the leaves.     

Radial transport of water and abscisic acid (ABA) in roots of Zea mays under conditions of nutrient deficiency

Radial water (J(V)) and abscisic acid (ABA) flows (J(ABA)) through maize root seedlings have been investigated under different conditions of nutrient deficiency. Whereas J(V) was reduced under nitrogen deficiency, potassium deficiency stimulated J(V). A substantial increase of J(ABA) was observed in roots kept under potassium deficiency. The observed changes of J(V) might have resulted from changed barrier properties of the endodermis. Nitrogen and potassium deficiency also caused an accumulation of endogenous ABA in root tissues. Under all conditions studied, except under K(+)-deficiency, external ABA (100 nM) caused an increase of J(V). The data of this study were used to analyse the relations between internal and endogenous root ABA, J(V), and J(ABA). The internal ABA of root tissues was positively correlated with J(V) and was highly significant (P <0.001 for internal and P=0.03 for endogenous root ABA) within the range 2-300 pmol g(-1) FW. It was also highly positively correlated to the radial ABA flows. There was also a highly positive correlation between J(V) and J(ABA). The data of this study indicate, for the first time, the relations between internal ABA, water, and ABA flows. Independent of treatment with external ABA, an ABA transport by solvent drag across the endodermis is confirmed.     

ABA信号转运调节的基因表达与源库动力学分析

通过对拟南芥NCED3、AA03及SDR1蛋白亚细胞定位分析及根系和叶片ABA池的动态库变化研究,结果表明气孔运动的有效ABA信号来自于保卫细胞之外,SDR与ABA前体加工和运输有关。胁迫处理后根系合成酶基因转录水平显著高于叶片,但叶片ABA水平是根系的10倍以上,离体叶片和附体叶片ABA含量测定表明,叶片ABA池的形成主要决定于根源ABA的输入。氟啶酮药剂阻断和遮荫实验说明根系ABA池受叶源类胡萝素前体供应影响。叶片ABA水平受根源ABA和叶源类胡萝素前体库双向转运调节,维管束组织系统可能协同和整合了这一复杂调节机制。该结论为逆境ABA信号转递机制研究和操纵内源ABA含量增强植物抗逆性的应用提供相关资料。     

Is the ABA concentration in the sap collected by pressurizing leaves relevant for analysing drought effects on stomata? Evidence from ABA-fed leaves of transgenic plants with modified capacities to synthesize ABA.

Most studies on the role of ABA in the stomatal response of the whole plant to drought rely on a good estimate of ABA concentration in xylem sap. In this report, varying volumes of sap (V(sap)) were collected by pressurizing leaves cut from several lines of N. plumbaginifolia with modified capacities to synthesize ABA. Leaves were fed with solutions of known ABA concentration ([ABA](solution) from 0-500 micromol m(-3)) for 2-3 h before sap collection. ABA concentration in extruded sap ([ABA](sap)) was compared with [ABA](solution). In low-volume extracts (less than 0.35 mm(3) cm(-2) leaf area) collected from leaves of well-watered plants, [ABA](sap) was close to [ABA](solution). For all lines, [ABA](sap) decreased with increasing V(sap). The same dilution effect was observed for leaves pressurized just after sampling on droughted plants, suggesting, as for detached leaves fed with ABA, that [ABA](sap) in low-volume extracts approximated well with the concentration of ABA entering leaves still attached on droughted plants. However, ABA-fed leaves sampled from droughted plants yielded higher [ABA](sap) than ABA-fed leaves sampled from well-watered plants. [ABA](sap) was also increased, although very slightly, when leaves were preincubated in highly enriched ABA solution. This indicates that some leaf ABA contributed to the ABA concentration returned in the extruded sap. Consistently, [ABA](sap) in medium-volume extracts (0.35-0.65 mm(3) cm(-2) leaf area) was lower for leaves sampled on under-producing lines than on the wild type. Despite these distortions between [ABA](solution) and [ABA](sap) in medium-volume extracts, stomatal conductance of ABA-fed leaves closely correlated with [ABA](sap) with a similar relationship in all cases, whilst relationships with [ABA](solution) were more scattered.     

Abscisic acid biosynthesis during corn embryo development

In this study we examined the biosynthesis of abscisic acid (ABA) by developing corn (Zea mays L.) embryos. Three comparisons were made: ABA biosynthesis in embryos isolated from kernels grown in vitro with those grown in the field; the developmental profile of ABA content with that of biosynthesis; and ABA biosynthesis in corn embryos lacking carotenoid precursors with ABA biosynthesis in normal embryos. Embryos were harvested at various times during seed development and divided into two groups. Endogenous levels of ABA were measured in one group of embryos and ABA biosynthetic capacity was measured in the other group. The ABA biosynthetic capacity was measured with and without tetcyclacis (an inhibitor of ABA degradation) in embryos from both field-grown and in-vitro-grown corn kernels. Reduced-carotenoid (either fluridone-treated or genetically viviparous) embryos were also included in the study. Corn kernels developing under field and in-vitro conditions differed from each other in their responses to tetcyclacis and in their profiles of ABA biosynthesis during development. Therefore, in-vitro kernel culture may not be an appropriate substitute for field conditions for studies of embryo development. The developmental profiles of endogenous ABA content differed from those of ABA biosynthesis in isolated embryos of both in-vitro-and field-grown kernels. This indicated that ABA levels in the developing embryos were determined by import from the maternal tissues available to the embryos rather than by in-situ biosynthesis. In embryos with reduced levels of carotenoids, either fluridone-treated or genetically viviparous embryos, ABA biosynthesis was low or nonexistent. This result is expected for the presence of an indirect pathway of ABA biosynthesis and in the absence of ABA precursors.Abbreviations ABA abscisic acid - DAP days after pollination     

Decoding ABA and osmostress signalling in plants from an evolutionary point of view

The plant hormone abscisic acid (ABA) is fundamental for land plant adaptation to water-limited conditions. Osmostress, such as drought, induces ABA accumulation in angiosperms, triggering physiological responses such as stomata closure. The core components of angiosperm ABA signalling are soluble ABA receptors, group A protein phosphatase type 2C and SNF1-related protein kinase2 (SnRK2). ABA also has various functions in non-angiosperms, however, suggesting that its role in adaptation to land may not have been angiosperm-specific. Indeed, among land plants, the core ABA signalling components are evolutionarily conserved, implying their presence in a common ancestor. Results of ongoing functional genomics studies of ABA signalling components in bryophytes and algae have expanded our understanding of the evolutionary role of ABA signalling, with genome sequencing uncovering the ABA core module even in algae. In this review, we describe recent discoveries involving the ABA core module in non-angiosperms, tracing the footprints of how ABA evolved as a phytohormone. We also cover the latest findings on Raf-like kinases as upstream regulators of the core ABA module component SnRK2. Finally, we discuss the origin of ABA signalling from an evolutionary perspective.     

Abscisic acid activates ATPase in developing apple fruit especially in fruit phloem cells

Abscisic acid (ABA) improves the sink strength by promoting the phloem unloading and regulating the assimilate metabolism in the economic sink organs of crops, although its mechanism remains unknown. The present experiment, using the techniques of the in vivo injection of ABA into the intact apple fruit attached to a growing apple tree and the in vivo incubation of the fruit tissue in the ABA‐contained medium, showed that ABA strongly activated the fruit ATPase especially P‐ATPase, of which the activity was doubled by ABA treatment. This ATPase activation was shown to be in vivo tissue‐dependent. The ABA‐induced P‐ATPase activation was fruit developmental stage‐, ABA dose‐, medium pH‐ and incubation time‐dependent. Physiological active (+)ABA was shown more effective to stimulate P‐ATPase activity than (+/–)ABA, and two ABA analogues (–)ABA and trans‐ABA, had no effect on P‐ATPase activation, indicating that only physiologically active cis(+)ABA can induce the enzyme activation, and so the ABA‐induced effects are stereospecific. The protein synthesis inhibitor cycloheximide was shown to have no effect on P‐ATPase activation by ABA, suggesting that synthesis of new proteins was not involved in the enzyme activation. The cytochemical assay revealed that P‐ATPase was activated by ABA in both the phloem and its surrounding flesh parenchyma cells, and that the most strongly P‐ATPase activation was observed in the plasma membrane of sieve element/companion cell complex. These data suggest that the improvement of phloem unloading by ABA previously reported in this fruit as in other crop sink organs may be attributed, at least partly, to the ABA‐induced ATPase activation especially in phloem cells.     

Ectopic expression of ABSCISIC ACID 2/GLUCOSE INSENSITIVE 1 in Arabidopsis promotes seed dormancy and stress tolerance

Abscisic acid (ABA) is an important phytohormone that plays a critical role in seed development, dormancy, and stress tolerance. 9-cis-Epoxycarotenoid dioxygenase is the key enzyme controlling ABA biosynthesis and stress tolerance. In this study, we investigated the effect of ectopic expression of another ABA biosynthesis gene, ABA2 (or GLUCOSE INSENSITIVE 1 [GIN1]) encoding a short-chain dehydrogenase/reductase in Arabidopsis (Arabidopsis thaliana). We show that ABA2-overexpressing transgenic plants with elevated ABA levels exhibited seed germination delay and more tolerance to salinity than wild type when grown on agar plates and/or in soil. However, the germination delay was abolished in transgenic plants showing ABA levels over 2-fold higher than that of wild type grown on 250 mm NaCl. The data suggest that there are distinct mechanisms underlying ABA-mediated inhibition of seed germination under diverse stress. The ABA-deficient mutant aba2, with a shorter primary root, can be restored to normal root growth by exogenous application of ABA, whereas transgenic plants overexpressing ABA2 showed normal root growth. The data reflect that the basal levels of ABA are essential for maintaining normal primary root elongation. Furthermore, analysis of ABA2 promoter activity with ABA2::beta-glucuronidase transgenic plants revealed that the promoter activity was enhanced by multiple prolonged stresses, such as drought, salinity, cold, and flooding, but not by short-term stress treatments. Coincidently, prolonged drought stress treatment led to the up-regulation of ABA biosynthetic and sugar-related genes. Thus, the data support ABA2 as a late expression gene that might have a fine-tuning function in mediating ABA biosynthesis through primary metabolic changes in response to stress.     

Changes of free and conjugated abscisic acid and phaseic acid in xylem sap of drought-stressed sunflower plants

Abscisic acid (ABA), conjugated abscisic acid, phaseic acid (PA), and conjugated phaseic acid were determined by enzyme-linked immunosorbent assay (ELISA) and gas chromatography (GC) in xylem sap of well-watered and drought-stressed sunflower plants. Conjugated ABA and conjugated PA were determined indirectly after chemical or enzymatic hydrolysis. Conjugated ABA was found to be the predominant ABA metabolite in xylem sap. In xylem sap from well-watered plants at least five, and in sap from drought-stressed plants at least six alkaline hydrolysable ABA conjugates were found. One of them corresponds chromatographically (HPLC) with abscisic acid glucose ester (ABAGE). Under drought conditions the concentrations of ABA, alkaline hydrolysable ABA conjugates, -glucosidase hydrolysable ABA conjugates, PA, and conjugated PA increased. After rewatering the drought-stressed plants, the ABA and the conjugated ABA content decreased. The possible function of the ABA conjugates in the xylem sap as a source of free ABA is discussed.     

Uptake, Movement, Activity, and Persistence of an Abscisic Acid Analog (8′ Acetylene ABA Methyl Ester) in Marigold and Tomato

The abscisic acid (ABA) analog 8′ acetylene ABA methyl ester (PBI 429) was evaluated for its potential to alter the growth and moisture use of bedding plants during nursery production. Treating seedlings with the ABA analog as a root-dip slowed moisture use and growth of tomato seedlings under greenhouse conditions. In marigolds, comparable ABA analog treatments had no effect on growth and limited effects on plant moisture use. To determine whether these differences in response to treatment with the ABA analog were associated with differences in absorption of the analog and/or its persistence, the ABA analog was applied either as a foliar spray or root-dip, and the resulting concentrations of the ABA analog were monitored over a 10-day interval in both the roots and the leaves. In both crops, the ABA analog was detected in both leaf and root tissues irrespective of the mode of application, suggesting systemic movement of the analog. Tissue concentrations of the ABA analog were consistently lower in the foliar treatment than in the root-dip. The uptake and the retention of the ABA analog over time was similar in leaves of the two test crops, but less of the ABA analog was absorbed and retained in the roots of marigold plants than in the tomatoes. This suggests that the observed differences in responses of these two plant species to application of ABA analogs may be related to differences in retention or accumulation of ABA in the roots rather than to differences in the total amount of ABA analog absorbed or its movement and retention in the plant system. Levels of endogenous ABA were not significantly altered by application of the ABA analog.     

ABA signal transduction from ABA receptors to ion channels

The plant hormone abscisic acid (ABA) is involved in regulating a number of major processes such as seed dormancy, seedling development, and biotic and abiotic stress responses. The function and effect of ABA on pathogens are still unclear, but the roles of ABA in seed germination and abiotic stress responses have been well characterized. Abiotic stresses elevate ABA levels and activate ABA signaling; thus, inducing a variety of responses, including the expression of stress-related genes and stomatal closure. The past decade has witnessed many significant advances in our understanding of ABA signal transduction due to application of a combination of approaches including genetics, biochemistry, electrophysiology, and chemical genetics. A number of proteins associated with the ABA signal transduction pathway such as PYR/PYL/RCAR family of START proteins, have been identified. These ABA receptors bind to ABA and positively regulate ABA signaling via inactivation of PP2C phosphatase activity, which inhibits SnRK2-type kinases by direct interaction and dephosphorylation. Additionally, SnRK2-type kinases and PP2Cs interact with one another and with other components of ABA signaling and function as positive and negative ABA regulators, respectively. In this review, we focus on ABA function to abiotic stresses and highlight each component in relation to ABA and its interactions.     

Abscisic acid receptors: multiple signal-perception sites

BACKGROUND AND AIMS: The phytohormone abscisic acid (ABA) plays a vital role in various aspects of plant growth and development and in adaptation of plants to various environmental stresses. Cell response to ABA is initiated by ABA perception with a cell receptor. Recently, three distinct ABA receptors have been identified, opening a door to uncover the initial events of ABA signal transduction. The aim of this Botanical Briefing is to present a perspective of the ABA receptors identified. SCOPE: This Briefing offers an introduction to the three ABA receptors identified and an analysis of the complexity and multiplicity of ABA receptors, and provides some viewpoints on future research.