The Mg-chelatase H subunit is an abscisic acid receptor

Analysis of Arabidopsis glucose insensitive growth mutants reveals the involvement of the plastidial copper transporter PAA1 in glucose-induced intracellular signaling

Sugars play important roles in many aspects of plant growth and development, acting as both energy sources and signaling molecules. With the successful use of genetic approaches, the molecular components involved in sugar signaling have been identified and their regulatory roles in the pathways have been elucidated. Here, we describe novel mutants of Arabidopsis (Arabidopsis thaliana), named glucose insensitive growth (gig), identified by their insensitivity to high-glucose (Glc)-induced growth inhibition. The gig mutant displayed retarded growth under normal growth conditions and also showed alterations in the expression of Glc-responsive genes under high-Glc conditions. Our molecular identification reveals that GIG encodes the plastidial copper (Cu) transporter PAA1 (for P(1B)-type ATPase 1). Interestingly, double mutant analysis indicated that in high Glc, gig is epistatic to both hexokinase1 (hxk1) and aba insensitive4 (abi4), major regulators in sugar and retrograde signaling. Under high-Glc conditions, the addition of Cu had no effect on the recovery of gig/paa1 to the wild type, whereas exogenous Cu feeding could suppress its phenotype under normal growth conditions. The expression of GIG/PAA1 was also altered by mutations in the nuclear factors HXK1, ABI3, and ABI4 in high Glc. Furthermore, a transient expression assay revealed the interaction between ABI4 and the GIG/PAA1 promoter, suggesting that ABI4 actively regulates the transcription of GIG/PAA1, likely binding to the CCAC/ACGT core element of the GIG/PAA1 promoter. Our findings indicate that the plastidial Cu transporter PAA1, which is essential for plastid function and/or activity, plays an important role in bidirectional communication between the plastid and the nucleus in high Glc.

Abscisic acid substantially inhibits senescence of cucumber plants (Cucumis sativus) grown under low nitrogen conditions

Low nitrogen (N) availability such as that found in both dry land and tropical regions limits plant growth and development. The relationship between the level of abscisic acid (ABA) in a plant and its growth under low-N conditions was investigated. The level of ABA in cucumber (Cucumis sativus) plants under low-N conditions was significantly higher at 10 and 20 d after transplantation compared with that under sufficient-N conditions. Chlorophyll was preserved in the aerial parts of cucumber plants grown under low-N conditions in the presence of ABA, while there was no significant difference between control plants and ABA-applied plants under sufficient-N conditions. ABA suppressed the reduction of chlorophyll biosynthesis under low-N conditions but not under sufficient-N conditions. On the other hand, ABA decreased the expression of the chlorophyll degradation gene in older cucumber plants grown under both conditions. In addition, transcript and protein levels of a gene encoding a chlorophyll a/b binding protein were positively correlated with ABA concentration under low-N conditions. The chloroplasts in control plants were round, and the stack of thylakoid membranes was reduced compared with that of plants treated with ABA 10(-5) M. These results strongly suggest that ABA is accumulated in cucumber plants grown under low-N conditions and that accumulated ABA promotes chlorophyll biosynthesis and inhibits its degradation in those plants.

SKP1 is involved in abscisic acid signalling to regulate seed germination, stomatal opening and root growth in Arabidopsis thaliana

Abscisic acid (ABA) regulates many aspects of plant development, including seed dormancy and germination, root growth and stomatal closure. Plant SKP1 proteins are subunits of the SCF complex E3 ligases, which regulate several phytohormone signalling pathways through protein degradation. However, little is known about SKP1 proteins participating in ABA signalling. Here, we report that the overexpression of Triticum aestivum SKP1-like 1 (TSK1) in Arabidopsis thaliana (Arabidopsis) resulted in delayed seed germination and hypersensitivity to ABA. The opening of stomatal guard cells and the transcription of several ABA-responsive genes were affected in transgenic plants. In contrast, Arabidopsis skp1-like 1 (ask1)/ask1 ASK2/ask2 seedlings exhibited reduced ABA sensitivity. Furthermore, the transcription of ASK1 and ASK2 was down-regulated in abi1-1 and abi5-1 mutants compared with that in wild type. ASK1 or ASK2 overexpression could rescue or partially rescue the ABA insensitivity of abi5-1 mutants, respectively. Our work demonstrates that SKP1 is involved in ABA signalling and that SKP1-like genes may positively regulate ABA signalling by SCF-mediated protein degradation.

Functional roles of the protein phosphatase 2C, AtAIP1, in abscisic acid signaling and sugar tolerance in Arabidopsis

Biotic signaling molecules including abscisic acid (ABA) serve as an integrator of abiotic stress including high salinity and drought. Recent studies have led to the identification of an ABA signaling pathway from the ABA receptor to stomatal closure in response to abiotic stress. ABA is linked to ABA receptors and protein phosphatase 2C (PP2C) members. In this study, we reconstituted the ABA signaling pathway as a protein-protein interaction between the RCAR type receptor and AIP1, which is one of the group A PP2C member. Several ABA receptors interact with AIP1 in an ABA dependent or independent manner. aip1 null mutant plants exhibited reduced sensitivity to ABA and glucose during the seed germination and seedling stage. Taken together, these results demonstrated that AIP1 is associated with ABA-mediated cell signaling and function as positive regulators of ABA.

Thioredoxin redox regulates ATPase activity of magnesium chelatase CHLI subunit and modulates redox-mediated signaling in tetrapyrrole biosynthesis and homeostasis of reactive oxygen species in pea plants

The chloroplast thioredoxins (TRXs) function as messengers of redox signals from ferredoxin to target enzymes. In this work, we studied the regulatory impact of pea (Pisum sativum) TRX-F on the magnesium (Mg) chelatase CHLI subunit and the enzymatic activation of Mg chelatase in vitro and in vivo. In vitro, reduced TRX-F activated the ATPase activity of pea CHLI and enhanced the activity of Mg chelatase reconstituted from the three recombinant subunits CHLI, CHLD, and CHLH in combination with the regulator protein GENOMES UNCOUPLED4 (GUN4). Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that TRX-F physically interacts with CHLI but not with either of the other two subunits or GUN4. In vivo, virus-induced TRX-F gene silencing (VIGS-TRX-F) in pea plants did not result in an altered redox state of CHLI. However, simultaneous silencing of the pea TRX-F and TRX-M genes (VIGS-TRX-F/TRX-M) resulted in partially and fully oxidized CHLI in vivo. VIGS-TRX-F/TRX-M plants demonstrated a significant reduction in Mg chelatase activity and 5-aminolevulinic acid synthesizing capacity as well as reduced pigment content and lower photosynthetic capacity. These results suggest that, in vivo, TRX-M can compensate for a lack of TRX-F and that both TRXs act as important redox regulators of Mg chelatase. Furthermore, the silencing of TRX-F and TRX-M expression also affects gene expression in the tetrapyrrole biosynthesis pathway and leads to the accumulation of reactive oxygen species, which may also serve as an additional signal for the transcriptional regulation of photosynthesis-associated nuclear genes.

[Molecular mechanism for dynamic regulation of endogenous ABA signal level]

The process from stress signal perception and the trigger of ABA biosynthesis to dynamic regulation of ABA level is an important stress signaling pathway in cells. Compared to the downstream events in ABA signal transduction, the researches in this field are relatively lagged. Expression of synthase genes, such as ZEP in roots and rate-limiting enzyme genes NCED, AtRGS1 and ABA2, can be activated in response to stresses. However, the expression of genes encoding degradative enzymes, including 7'-, 8'-, 9'-hydroxylase and glucosyltransferase, negatively regulates ABA accumulation. Meanwhile, the expressions of the synthases, such as ZEP and NCED3, are induced by increasing endogenous ABA contents. Additionally, the analyses of gene expression and source-sink dynamics indicates that sustained supply from root-sourced ABA is required for the maintenance of leaf ABA dynamic pool. It is notable that miRNAs should be involved in ABA signal origin and ABA level dynamic adjustment. Further dynamic analysis of ABA metabolism revealed that endogenous ABA signal levels are synergistically controlled by the expressions of synthases and degradative enzymes.

Connections between sphingosine kinase and phospholipase D in the abscisic acid signaling pathway in Arabidopsis

Phosphatidic acid (PA) and phytosphingosine 1-phosphate (phyto-S1P) both are lipid messengers involved in plant response to abscisic acid (ABA). Our previous data indicate that PA binds to sphingosine kinase (SPHK) and increases its phyto-S1P-producing activity. To understand the cellular and physiological functions of the PA-SPHK interaction, we isolated Arabidopsis thaliana SPHK mutants sphk1-1 and sphk2-1 and characterized them, together with phospholipase Dα1 knock-out, pldα1, in plant response to ABA. Compared with wild-type (WT) plants, the SPHK mutants and pldα1 all displayed decreased sensitivity to ABA-promoted stomatal closure. Phyto-S1P promoted stomatal closure in sphk1-1 and sphk2-1, but not in pldα1, whereas PA promoted stomatal closure in sphk1-1, sphk2-1, and pldα1. The ABA activation of PLDα1 in leaves and protoplasts was attenuated in the SPHK mutants, and the ABA activation of SPHK was reduced in pldα1. In response to ABA, the accumulation of long-chain base phosphates was decreased in pldα1, whereas PA production was decreased in SPHK mutants, compared with WT. Collectively, these results indicate that SPHK and PLDα1 act together in ABA response and that SPHK and phyto-S1P act upstream of PLDα1 and PA in mediating the ABA response. PA is involved in the activation of SPHK, and activation of PLDα1 requires SPHK activity. The data suggest that SPHK/phyto-S1P and PLDα1A are co-dependent in amplification of response to ABA, mediating stomatal closure in Arabidopsis.

ABA signal in rice under stress conditions

Ever since its discovery, abscisic acid (ABA) has been intensively studied due to its versatile functions in plant developmental and physiological processes. Many signaling details of ABA have been well elucidated and reviewed. The identification of ABA receptors is a great breakthrough in the field of ABA study, whereas the discovery of ABA transporter has changed our concept that ABA is delivered solely by passive transport. The intensity of ABA signaling pathway is well known to be controlled by multi-regulators. Nonetheless, the interaction and coordination among ABA biosynthesis, catabolism, conjugation and transportation are seldom discussed. Here, we summarize the biological functions of ABA in response to different stresses, especially the roles of ABA in plant defense to pathogen attack, and discuss the possible relationships of these determinants in controlling the specificity and intensity of ABA signaling pathway in the rice.

Structure of the cyanobacterial Magnesium Chelatase H subunit determined by single particle reconstruction and small-angle X-ray scattering

The biosynthesis of chlorophyll, an essential cofactor for photosynthesis, requires the ATP-dependent insertion of Mg(2+) into protoporphyrin IX catalyzed by the multisubunit enzyme magnesium chelatase. This enzyme complex consists of the I subunit, an ATPase that forms a complex with the D subunit, and an H subunit that binds both the protoporphyrin substrate and the magnesium protoporphyrin product. In this study we used electron microscopy and small-angle x-ray scattering to investigate the structure of the magnesium chelatase H subunit, ChlH, from the thermophilic cyanobacterium Thermosynechococcus elongatus. Single particle reconstruction of negatively stained apo-ChlH and Chl-porphyrin proteins was used to reconstitute three-dimensional structures to a resolution of ∼30 Å. ChlH is a large, 148-kDa protein of 1326 residues, forming a cage-like assembly comprising the majority of the structure, attached to a globular N-terminal domain of ∼16 kDa by a narrow linker region. This N-terminal domain is adjacent to a 5 nm-diameter opening in the structure that allows access to a cavity. Small-angle x-ray scattering analysis of ChlH, performed on soluble, catalytically active ChlH, verifies the presence of two domains and their relative sizes. Our results provide a basis for the multiple regulatory and catalytic functions of ChlH of oxygenic photosynthetic organisms and for a chaperoning function that sequesters the enzyme-bound magnesium protoporphyrin product prior to its delivery to the next enzyme in the chlorophyll biosynthetic pathway, magnesium protoporphyrin methyltransferase.

Comparison of phytohormone signaling mechanisms

Plant hormones are crucial signaling molecules that coordinate all aspects of plant growth, development and defense. A great deal of attention has been attracted from biologists to study the molecular mechanisms for perception and signal transduction of plant hormones during the last two decades. Tremendous progress has been made in identifying receptors and key signaling components of plant hormones. The holistic picture of hormone signaling pathways is extremely complicated, this review will give a general overview of perception and signal transduction mechanisms of auxin, gibberellin, cytokinin, abscisic acid, ethylene, brassinosteroid, and jasmonate.

Light-harvesting chlorophyll a/b-binding proteins are required for stomatal response to abscisic acid in Arabidopsis

The light-harvesting chlorophyll a/b binding proteins (LHCB) are perhaps the most abundant membrane proteins in nature. It is reported here that the down-regulation or disruption of any member of the LHCB family, LHCB1, LHCB2, LHCB3, LHCB4, LHCB5, or LHCB6, reduces responsiveness of stomatal movement to ABA, and therefore results in a decrease in plant tolerance to drought stress in Arabidopsis thaliana. By contrast, over-expression of a LHCB member, LHCB6, enhances stomatal sensitivity to ABA. In addition, the reactive oxygen species (ROS) homeostasis and a set of ABA-responsive genes are altered in the lhcb mutants. These data demonstrate that LHCBs play a positive role in guard cell signalling in response to ABA and suggest that they may be involved in ABA signalling partly by modulating ROS homeostasis.

Characterization of potential ABA receptors in Vitis vinifera

Molecular control mechanisms for abiotic stress tolerance are based on the activation and regulation of specific stress-related genes. The phytohormone abscisic acid (ABA) is a key endogenous messenger in a plant's response to such stresses. A novel ABA binding mechanism which plays a key role in plant cell signaling cascades has recently been uncovered. In the absence of ABA, a type 2C protein phosphatase (PP2C) interacts and inhibits the kinase SnRK2. Binding of ABA to the PYR/PYLs receptors enables interaction between the ABA receptor and the PP2C protein, and abrogates the SnRK2 inactivation. The active SnRK2 is then free to activate the ABA-responsive element Binding Factors which target ABA-dependent gene expression. We used the grape as a model to study the ABA perception mechanism in fruit trees. The grape ABA signaling cascade consists of at least seven ABA receptors and six PP2Cs. We used a yeast two-hybrid system to examine physical interaction in vitro between the grape ABA receptors and their interacting partners, and found that twenty-two receptor-PP2C interactions can occur. Moreover, quantifying these affinities by the use of the LacZ reporter enables us to show that VvPP2C4 and VvPP2C9 are the major binding partners of the ABA receptor. We also tested in vivo the root and leaf gene expression of the various ABA receptors and PP2Cs in the presence of exogenic ABA and under different abiotic stresses such as high salt concentration, cold and drought, and found that many of these genes are regulated by such abiotic environmental factors. Our results indicate organ specificity in the ABA receptor genes and stress specificity in the VvPP2Cs. We suggest that VvPP2C4 is the major PP2C involved in ABA perception in leaves and roots, and VvRCAR6 and VvRCAR5 respectively, are the major receptors involved in ABA perception in these organs. Identification, characterization and manipulation of the central players in the ABA signaling cascades in fruit trees is likely to prove essential for improving their performance in the future.

Molecular phylogenetic and expression analysis of the complete WRKY transcription factor family in maize

The WRKY transcription factors function in plant growth and development, and response to the biotic and abiotic stresses. Although many studies have focused on the functional identification of the WRKY transcription factors, much less is known about molecular phylogenetic and global expression analysis of the complete WRKY family in maize. In this study, we identified 136 WRKY proteins coded by 119 genes in the B73 inbred line from the complete genome and named them in an orderly manner. Then, a comprehensive phylogenetic analysis of five species was performed to explore the origin and evolutionary patterns of these WRKY genes, and the result showed that gene duplication is the major driving force for the origin of new groups and subgroups and functional divergence during evolution. Chromosomal location analysis of maize WRKY genes indicated that 20 gene clusters are distributed unevenly in the genome. Microarray-based expression analysis has revealed that 131 WRKY transcripts encoded by 116 genes may participate in the regulation of maize growth and development. Among them, 102 transcripts are stably expressed with a coefficient of variation (CV) value of <15%. The remaining 29 transcripts produced by 25 WRKY genes with the CV value of >15% are further analysed to discover new organ- or tissue-specific genes. In addition, microarray analyses of transcriptional responses to drought stress and fungal infection showed that maize WRKY proteins are involved in stress responses. All these results contribute to a deep probing into the roles of WRKY transcription factors in maize growth and development and stress tolerance.

Possible Role of Nutritional Priming for Early Salt and Drought Stress Responses in Medicago truncatula

Most legume species establish a symbiotic association with soil bacteria. The plant accommodates the differentiated rhizobia in specialized organs, the root nodules. In this environment, the microsymbiont reduces atmospheric nitrogen (N) making it available for plant metabolism. Symbiotic N-fixation is driven by the respiration of the host photosynthates and thus constitutes an additional carbon sink for the plant. Molecular phenotypes of symbiotic and non-symbiotic Medicago truncatula are identified. The implication of nodule symbiosis on plant abiotic stress response mechanisms is not well understood. In this study, we exposed nodulated and non-symbiotic N-fertilized plants to salt and drought conditions. We assessed the stress effects with proteomic and metabolomic methods and found a nutritionally regulated phenotypic plasticity pivotal for a differential stress adjustment strategy.

WRKY transcription factors: key components in abscisic acid signalling

WRKY transcription factors (TFs) are key regulators of many plant processes, including the responses to biotic and abiotic stresses, senescence, seed dormancy and seed germination. For over 15 years, limited evidence has been available suggesting that WRKY TFs may play roles in regulating plant responses to the phytohormone abscisic acid (ABA), notably some WRKY TFs are ABA-inducible repressors of seed germination. However, the roles of WRKY TFs in other aspects of ABA signalling, and the mechanisms involved, have remained unclear. Recent significant progress in ABA research has now placed specific WRKY TFs firmly in ABA-responsive signalling pathways, where they act at multiple levels. In Arabidopsis, WRKY TFs appear to act downstream of at least two ABA receptors: the cytoplasmic PYR/PYL/RCAR-protein phosphatase 2C-ABA complex and the chloroplast envelope-located ABAR-ABA complex. In vivo and in vitro promoter-binding studies show that the target genes for WRKY TFs that are involved in ABA signalling include well-known ABA-responsive genes such as ABF2, ABF4, ABI4, ABI5, MYB2, DREB1a, DREB2a and RAB18. Additional well-characterized stress-inducible genes such as RD29A and COR47 are also found in signalling pathways downstream of WRKY TFs. These new insights also reveal that some WRKY TFs are positive regulators of ABA-mediated stomatal closure and hence drought responses. Conversely, many WRKY TFs are negative regulators of seed germination, and controlling seed germination appears a common function of a subset of WRKY TFs in flowering plants. Taken together, these new data demonstrate that WRKY TFs are key nodes in ABA-responsive signalling networks.

Structural insights into PYR/PYL/RCAR ABA receptors and PP2Cs

Abscisic acid (ABA) plays an essential function in plant physiology since it is required for biotic and abiotic stress responses as well as control of plant growth and development. A new family of soluble ABA receptors, named PYR/PYL/RCAR, has emerged as ABA sensors able to inhibit the activity of specific protein phosphatases type-2C (PP2Cs) in an ABA-dependent manner. The structural and functional mechanism by which ABA is perceived by these receptors and consequently leads to inhibition of the PP2Cs has been recently elucidated. The module PYR/PYL/RCAR-ABA-PP2C offers an elegant and unprecedented mechanism to control phosphorylation signaling cascades in a ligand-dependent manner. The knowledge of their three-dimensional structures paves the way to the design of ABA agonists able to modulate the plant stress response.

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.

Suppression of 9-cis-epoxycarotenoid dioxygenase, which encodes a key enzyme in abscisic acid biosynthesis, alters fruit texture in transgenic tomato

Cell wall catabolism during fruit ripening is under complex control and is key for fruit quality and shelf life. To examine the role of abscisic acid (ABA) in tomato (Solanum lycopersicum) fruit ripening, we suppressed SlNCED1, which encodes 9-cis-epoxycarotenoid dioxygenase (NCED), a key enzyme in the biosynthesis of ABA. To suppress SlNCED1 specifically in tomato fruits, and thus avoid the pleiotropic phenotypes associated with ABA deficiency, we used an RNA interference construct driven by the fruit-specific E8 promoter. ABA accumulation and SlNCED1 transcript levels in the transgenic fruit were down-regulated to between 20% and 50% of the levels measured in the control fruit. This significant reduction in NCED activity led to a down-regulation in the transcription of genes encoding major cell wall catabolic enzymes, specifically polygalacturonase (SlPG), pectin methyl esterase (SlPME), β-galactosidase precursor mRNA (SlTBG), xyloglucan endotransglycosylase (SlXET), endo-1,4-β-cellulose (SlCels), and expansin (SlExp). This resulted in an increased accumulation of pectin during ripening. In turn, this led to a significant extension of the shelf life to 15 to 29 d compared with a shelf life of only 7 d for the control fruit and an enhancement of fruit firmness at the mature stage by 30% to 45%. In conclusion, ABA affects cell wall catabolism during tomato fruit ripening via down-regulation of the expression of major catabolic genes (SlPG, SlPME, SlTBG, SlXET, SlCels, and SlExp).

Regulatory mechanism of abscisic acid signaling

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

Identification of an important site for function of the type 2C protein phosphatase ABI2 in abscisic acid signalling in Arabidopsis

It is known that the clade A protein phosphatase 2Cs (PP2Cs), including ABI1 and ABI2 and other PP2C members, are key players that function directly downstream of the PYR/PYL/RCAR abscisic acid (ABA) receptors. Here, identification of a crucial site for function of ABI2 protein phosphatase in ABA signalling is reported. It was observed that a calcium-dependent protein kinase (CDPK) phosphorylation site-like motif (CPL) in the ABI2 molecule is required for the interactions of ABI2 with the two members of the ABA receptors PYL5 and PYL9 and with a downstream protein kinase SnRK2.6, and for the catalytic activity of ABI2 in vitro, as well as for the response of ABI2 to the ABA receptors PYL5/PYL9 in relation to the ABA receptor-induced inhibition of the ABI2 phosphatase activity. Further, genetic evidence was provided to demonstrate that this CPL is required for the function of ABI2 to mediate ABA signalling. These data reveal that this CPL is an important site necessary for both the phosphatase activity of ABI2 and the functional interaction between ABI2 and PYL5/9 ABA receptors, providing new information to understand primary events of ABA signal transduction.

Transcriptional regulation of SlPYL, SlPP2C, and SlSnRK2 gene families encoding ABA signal core components during tomato fruit development and drought stress

In order to characterize the potential transcriptional regulation of core components of abscisic acid (ABA) signal transduction in tomato fruit development and drought stress, eight SlPYL (ABA receptor), seven SlPP2C (type 2C protein phosphatase), and eight SlSnRK2 (subfamily 2 of SNF1-related kinases) full-length cDNA sequences were isolated from the tomato nucleotide database of NCBI GenBank. All SlPYL, SlPP2C, and SlSnRK2 genes obtained are homologous to Arabidopsis AtPYL, AtPP2C, and AtSnRK2 genes, respectively. Based on phylogenetic analysis, SlPYLs and SlSnRK2s were clustered into three subfamilies/subclasses, and all SlPP2Cs belonged to PP2C group A. Within the SlPYL gene family, SlPYL1, SlPYL2, SlPYL3, and SlPYL6 were the major genes involved in the regulation of fruit development. Among them, SlPYL1 and SlPYL2 were expressed at high levels throughout the process of fruit development and ripening; SlPYL3 was strongly expressed at the immature green (IM) and mature green (MG) stages, while SlPYL6 was expressed strongly at the IM and red ripe (RR) stages. Within the SlPP2C gene family, the expression of SlPP2C, SlPP2C3, and SlPP2C4 increased after the MG stage; SlPP2C1 and SlPP2C5 peaked at the B3 stage, while SlPP2C2 and SlPP2C6 changed little during fruit development. Within the SlSnRK2 gene family, the expression of SlSnRK2.2, SlSnRK2.3, SlSnRK2.4, and SlSnRK2C was higher than that of other members during fruit development. Additionally, most SlPYL genes were down-regulated, while most SlPP2C and SlSnRK2 genes were up-regulated by dehydration in tomato leaf.

AtPUB19, a U-box E3 ubiquitin ligase, negatively regulates abscisic acid and drought responses in Arabidopsis thaliana

Ubiquitination is an important protein post-translational modification, which is involved in various cellular processes in higher plants, and U-box E3 ligases play important roles in diverse functions in eukaryotes. Here, we describe the functions of Arabidopsis thaliana PUB19 (AtPUB19), which we demonstrated in an in vitro assay to encode a U-box type E3 ubiquitin ligase. AtPUB19 was up-regulated by drought, salt, cold, and abscisic acid (ABA). Down-regulation of AtPUB19 led to hypersensitivity to ABA, enhanced ABA-induced stomatal closing, and enhanced drought tolerance, while AtPUB19 overexpression resulted in the reverse phenotypes. Molecular analysis showed that the expression levels of a number of ABA and stress marker genes were altered in both AtPUB19 overexpressing and atpub19-1 mutant plants. In summary, our data show that AtPUB19 negatively regulates ABA and drought responses in A. thaliana.

EDL3 is an F-box protein involved in the regulation of abscisic acid signalling in Arabidopsis thaliana

The EID1-like protein 3 (EDL3) shows high similarity to EID1 (Empfindlicher im dunkelroten Licht 1), an F-box protein that functions as a negative regulator in the signalling cascade downstream of the phytochrome A photoreceptor in Arabidopsis thaliana. Analyses revealed a strong and rapid induction of EDL3 gene expression under osmotic stress, high salinity, and upon abscisic acid (ABA) application. Therefore, it was speculated that EDL3 is involved in the regulation of responses controlled by this plant hormone, which not only regulates many aspects of plant development but also integrates responses towards temperature, drought, osmotic, and salt stresses. Physiological data obtained with over-expresser lines and a conditional knock-down mutant demonstrated that EDL3 functions as a positive regulator in ABA-dependent signalling cascades that control seed germination, root growth, greening of etiolated seedlings, and transition to flowering. Results further demonstrate that EDL3 regulates anthocyanin accumulation under drought stress. The observed effects on physiological responses fit to tissue-specific expression patterns obtained with EDL3-promoter:GUS lines. Bimolecular Fluorescence Complementation assays and yeast two-hybrid analyses showed that EDL3 carries a functional F-box domain. Thus, the protein is presumed to act as a component of a ubiquitin ligase complex that specifically directs negatively acting factors in ABA signalling to degradation via the proteasome.

The Arabidopsis Ca(2+) -dependent protein kinase CPK12 negatively regulates abscisic acid signaling in seed germination and post-germination growth

• Ca(2+) -dependent protein kinase (CDPK) is believed to be involved in abscisic acid (ABA) signaling, and several members of the Arabidopsis CDPK superfamily have been identified as positive ABA signaling regulators, but it remains unknown if CDPK negatively regulates ABA signaling. • Here, we investigated the function of an Arabidopsis (Arabidopsis thaliana) CDPK, CPK12, in ABA signaling pathway. • We generated Arabidopsis CPK12-RNAi lines, and observed that downregulation of CPK12 resulted in ABA hypersensitivity in seed germination and post-germination growth, and altered expression of a set of ABA-responsive genes. Expression assay showed that CPK12 was ubiquitously expressed and localized to both cytosol and nucleus. Biochemical assays showed that CPK12 interacted with, phosphorylated and stimulated a type 2C protein phosphatase ABI2, and phosphorylated two ABA-responsive transcription factors (ABF1 and ABF4) in vitro. • Our findings show that the Arabidopsis CPK12 is a negative ABA-signaling regulator in seed germination and post-germination growth, suggesting that different members of the CDPK family may constitute a regulation loop by functioning positively and negatively in ABA signal transduction.

News on ABA transport, protein degradation, and ABFs/WRKYs in ABA signaling

The recent identification of abscisic acid (ABA) transporters provides an important insight into the delivery of ABA from the vascular system and its uptake by target cells. A putative connection with PYR/PYL receptors is envisaged, linking ABA uptake and intracellular perception by a fast and efficient mechanism. Downstream signaling of the core pathway involves regulation of ABA-responsive element binding factors (ABFs/AREBs) through phosphorylation, ubiquitination, and sumoylation in the case of ABI5. Several E3 ligases appear to regulate ABA signaling either positively or negatively, although relatively few targets are known yet. ABFs/AREBs are themselves subjected to transcriptional regulation, and some transcription factors (TFs) harboring the WRKY domain (WRKYs) appear to regulate their expression through W-box sequences present in the promoters of ABFs/AREBs.

FaPYR1 is involved in strawberry fruit ripening

Although the plant hormone abscisic acid (ABA) has been suggested to play a role in the ripening of non-climatic fruit, direct genetic/molecular evidence is lacking. In the present study, a strawberry gene homologous to the Arabidopsis ABA receptor gene PYR1, named FaPYR1, was isolated and characterized. The 627 bp cDNA includes an intact open reading frame that encodes a deduced protein of 208 amino acids, in which putative conserved domains were detected by homology analysis. Using tobacco rattle virus-induced gene silencing (VIGS), the FaPYR1 gene was silenced in strawberry fruit. Down-regulation of the FaPYR1 gene not only significantly delayed fruit ripening, but also markedly altered ABA content, ABA sensitivity, and a set of ABA-responsive gene transcripts, including ABI1 and SnRK2. Furthermore, the loss of red colouring in FaPYR1 RNAi (RNA interference) fruits could not be rescued by exogenously applied ABA, which could promote the ripening of wild-type fruits. Collectively, these results demonstrate that the putative ABA receptor FaPYR1 acts as a positive regulator in strawberry fruit ripening. It was also revealed that the application of the VIGS technique in strawberry fruit could be used as a novel tool for studying strawberry fruit development.

A chloroplast envelope-bound PHD transcription factor mediates chloroplast signals to the nucleus

Chloroplast development, maintenance and function depend on the coordinated expression of chloroplast and nuclear genes. The retrograde chloroplast signals are essential in coordinating nuclear gene expression. Although the sources of signals in chloroplasts have been identified and the associated transcription factors in the nucleus extensively studied, the molecular mechanism that relays chloroplast signals to the nucleus remains a mystery. Here we show that PTM, a chloroplast envelope-bound plant homeodomain (PHD) transcription factor with transmembrane domains, functions in multiple retrograde signal pathways. The proteolytic cleavage of PTM occurs in response to retrograde signals and amino-terminal PTM accumulates in the nucleus, where it activates ABI4 transcription in a PHD-dependent manner associated with histone modifications. These results provide a molecular basis for the critical function of PTM in retrograde chloroplast signaling and shed new light on the mechanism whereby chloroplast signals are transmitted to the nucleus through the cytosol.

Achievements and challenges in understanding plant abiotic stress responses and tolerance

Intensive research over the last decade has gradually unraveled the mechanisms that underlie how plants react to environmental adversity. Genes involved in many of the essential steps of the stress response have been identified and characterized. In particular, the recent discovery of ABA receptors, progress in understanding the transcriptional and post-transcriptional regulation of stress-responsive gene expression, and studies on hormone interactions under stress have facilitated addressing the molecular basis of how plant cells respond to abiotic stress. Here, we summarize recent research progress on these issues, especially focusing on progress related to the essential and classically important signaling pathways and genes. Despite this wealth of achievements, many challenges remain not only for the further elucidation of stress response mechanisms but also for evaluation of the natural genetic variations and associating them with specific gene functions. Finally, the proper application of this knowledge to benefit humans and agriculture is another important issue that lies ahead. Collaborative wisdom and efforts are needed to confront these challenges.

Abscisic acid plays an important role in the regulation of strawberry fruit ripening

The plant hormone abscisic acid (ABA) has been suggested to play a role in fruit development, but supporting genetic evidence has been lacking. Here, we report that ABA promotes strawberry (Fragaria ananassa) fruit ripening. Using a newly established Tobacco rattle virus-induced gene silencing technique in strawberry fruit, the expression of a 9-cis-epoxycarotenoid dioxygenase gene (FaNCED1), which is key to ABA biosynthesis, was down-regulated, resulting in a significant decrease in ABA levels and uncolored fruits. Interestingly, a similar uncolored phenotype was observed in the transgenic RNA interference (RNAi) fruits, in which the expression of a putative ABA receptor gene encoding the magnesium chelatase H subunit (FaCHLH/ABAR) was down-regulated by virus-induced gene silencing. More importantly, the uncolored phenotype of the FaNCED1-down-regulated RNAi fruits could be rescued by exogenous ABA, but the ABA treatment could not reverse the uncolored phenotype of the FaCHLH/ABAR-down-regulated RNAi fruits. We observed that down-regulation of the FaCHLH/ABAR gene in the RNAi fruit altered both ABA levels and sugar content as well as a set of ABA- and/or sugar-responsive genes. Additionally, we showed that exogenous sugars, particularly sucrose, can significantly promote ripening while stimulating ABA accumulation. These data provide evidence that ABA is a signal molecule that promotes strawberry ripening and that the putative ABA receptor, FaCHLH/ABAR, is a positive regulator of ripening in response to ABA.

Control of stomatal aperture: a renaissance of the old guard

Stomata, functionally specialized small pores on the surfaces of leaves, regulate the flow of gases in and out of plants. The pore is opened by an increase in osmotic pressure in the guard cells, resulting in the uptake of water. The subsequent increase in cell volume inflates the guard cell and culminates with the opening of the pore. Although guard cells can be regarded as one of the most thoroughly investigated cell types, our knowledge of the signaling pathways which regulate guard cell function remains fragmented. Recent research in guard cells has led to several new hypotheses, however, it is still a matter of debate as to whether guard cells function autonomously or are subject to regulation by their neighboring mesophyll cells.This review synthesizes what is known about the mechanisms and genes critical for modulating stomatal movement. Recent progress on the regulation of guard cell function is reviewed here including the involvement of environmental signals such as light, the concentration of atmospheric CO2 and endogenous plant hormones. In addition we re-evaluate the important role of organic acids such as malate and fumarate play in guard cell metabolism in this process.

Intracompartmental and intercompartmental transcriptional networks coordinate the expression of genes for organellar functions

Genes for mitochondrial and chloroplast proteins are distributed between the nuclear and organellar genomes. Organelle biogenesis and metabolism, therefore, require appropriate coordination of gene expression in the different compartments to ensure efficient synthesis of essential multiprotein complexes of mixed genetic origin. Whereas organelle-to-nucleus signaling influences nuclear gene expression at the transcriptional level, organellar gene expression (OGE) is thought to be primarily regulated posttranscriptionally. Here, we show that intracompartmental and intercompartmental transcriptional networks coordinate the expression of genes for organellar functions. Nearly 1,300 ATH1 microarray-based transcriptional profiles of nuclear and organellar genes for mitochondrial and chloroplast proteins in the model plant Arabidopsis (Arabidopsis thaliana) were analyzed. The activity of genes involved in organellar energy production (OEP) or OGE in each of the organelles and in the nucleus is highly coordinated. Intracompartmental networks that link the OEP and OGE gene sets serve to synchronize the expression of nucleus- and organelle-encoded proteins. At a higher regulatory level, coexpression of organellar and nuclear OEP/OGE genes typically modulates chloroplast functions but affects mitochondria only when chloroplast functions are perturbed. Under conditions that induce energy shortage, the intercompartmental coregulation of photosynthesis genes can even override intracompartmental networks. We conclude that dynamic intracompartmental and intercompartmental transcriptional networks for OEP and OGE genes adjust the activity of organelles in response to the cellular energy state and environmental stresses, and we identify candidate cis-elements involved in the transcriptional coregulation of nuclear genes. Regarding the transcriptional regulation of chloroplast genes, novel tentative target genes of σ factors are identified.

The transcription machineries of plant mitochondria and chloroplasts: Composition, function, and regulation

Although genomes of mitochondria and plastids are very small compared to those of their bacterial ancestors, the transcription machineries of these organelles are of surprising complexity. With respect to the number of different RNA polymerases per organelle, the extremes are represented on one hand by chloroplasts of eudicots which use one bacterial-type RNA polymerase and two phage-type RNA polymerases to transcribe their genes, and on the other hand by Physcomitrella possessing three mitochondrial RNA polymerases of the phage type. Transcription of genes/operons is often driven by multiple promoters in both organelles. This review describes the principle components of the transcription machineries (RNA polymerases, transcription factors, promoters) and the division of labor between the different RNA polymerases. While regulation of transcription in mitochondria seems to be only of limited importance, the plastid genes of higher plants respond to exogenous and endogenous cues rather individually by altering their transcriptional activities.

Comparative functional analysis of two hypothetical chloroplast open reading frames (ycf) involved in chlorophyll biosynthesis from Synechocystis sp. PCC6803 and plants

Hypothetical chloroplast open reading frames (ycfs) are highly conserved and interspecifically occurring genes in plastomes of plants and algae with significant functions in gene expression and photosynthesis. However, the function of many ycfs is still in vain so that attention is directed to other chloroplast functions such as metabolism of co-factors, protein translocation and protection against abiotic stress. We provide a comprehensive functional description of ycf53 and ycf59, two genes involved in chlorophyll biosynthesis. While ycf59 encodes an essential enzymatic component of Mg protoporphyrin monomethylester cyclase, ycf53 encodes a posttranslational regulator of chlorophyll biosynthesis. Their roles in tetrapyrrole biosynthesis were compared by using cyanobacterial and plant mutants with modulated expression of these two genes. Our work provides indications for diverse effects of these homologous gene products in plants and cyanobacteria on tetrapyrrole biosynthesis and photosynthesis.

Stochastic dynamics of actin filaments in guard cells regulating chloroplast localization during stomatal movement

Actin filaments and chloroplasts in guard cells play roles in stomatal function. However, detailed actin dynamics vary, and the roles that they play in chloroplast localization during stomatal movement remain to be determined. We examined the dynamics of actin filaments and chloroplast localization in transgenic tobacco expressing green fluorescent protein (GFP)-mouse talin in guard cells by time-lapse imaging. Actin filaments showed sliding, bundling and branching dynamics in moving guard cells. During stomatal movement, long filaments can be severed into small fragments, which can form longer filaments by end-joining activities. With chloroplast movement, actin filaments near chloroplasts showed severing and elongation activity in guard cells during stomatal movement. Cytochalasin B treatment abolished elongation, bundling and branching activities of actin filaments in guard cells, and these changes of actin filaments, and as a result, more chloroplasts were localized at the centre of guard cells. However, chloroplast turning to avoid high light, and sliding of actin fragments near the chloroplast, was unaffected following cytochalasin B treatment in guard cells. We suggest that the sliding dynamics of actin may play roles in chloroplast turning in guard cells. Our results indicate that the stochastic dynamics of actin filaments in guard cells regulate chloroplast localization during stomatal movement.

Tetrapyrrole Metabolism in Arabidopsis thaliana

Higher plants produce four classes of tetrapyrroles, namely, chlorophyll (Chl), heme, siroheme, and phytochromobilin. In plants, tetrapyrroles play essential roles in a wide range of biological activities including photosynthesis, respiration and the assimilation of nitrogen/sulfur. All four classes of tetrapyrroles are derived from a common biosynthetic pathway that resides in the plastid. In this article, we present an overview of tetrapyrrole metabolism in Arabidopsis and other higher plants, and we describe all identified enzymatic steps involved in this metabolism. We also summarize recent findings on Chl biosynthesis and Chl breakdown. Recent advances in this field, in particular those on the genetic and biochemical analyses of novel enzymes, prompted us to redraw the tetrapyrrole metabolic pathways. In addition, we also summarize our current understanding on the regulatory mechanisms governing tetrapyrrole metabolism. The interactions of tetrapyrrole biosynthesis and other cellular processes including the plastid-to-nucleus signal transduction are discussed.

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.

Mg-chelatase H subunit affects ABA signaling in stomatal guard cells, but is not an ABA receptor in Arabidopsis thaliana

Mg-chelatase H subunit (CHLH) is a multifunctional protein involved in chlorophyll synthesis, plastid-to-nucleus retrograde signaling, and ABA perception. However, whether CHLH acts as an actual ABA receptor remains controversial. Here we present evidence that CHLH affects ABA signaling in stomatal guard cells but is not itself an ABA receptor. We screened ethyl methanesulfonate-treated Arabidopsis thaliana plants with a focus on stomatal aperture-dependent water loss in detached leaves and isolated a rapid transpiration in detached leaves 1 (rtl1) mutant that we identified as a novel missense mutant of CHLH. The rtl1 and CHLH RNAi plants showed phenotypes in which stomatal movements were insensitive to ABA, while the rtl1 phenotype showed normal sensitivity to ABA with respect to seed germination and root growth. ABA-binding analyses using (3)H-labeled ABA revealed that recombinant CHLH did not bind ABA, but recombinant pyrabactin resistance 1, a reliable ABA receptor used as a control, showed specific binding. Moreover, we found that the rtl1 mutant showed ABA-induced stomatal closure when a high concentration of extracellular Ca(2+) was present and that a knockout mutant of Mg-chelatase I subunit (chli1) showed the same ABA-insensitive phenotype as rtl1. These results suggest that the Mg-chelatase complex as a whole affects the ABA-signaling pathway for stomatal movements.

Systems biology approaches to abscisic acid signaling

Recent advances in our understanding of abscisic acid (ABA) signaling have identified a core pathway consisting of receptors (PYR/PYL/RCAR), protein phosphatases (PP2C), protein kinases (SnRK2), and several downstream factors that will lead to the next stage of ABA research. Systems biology will be an important concept for further understanding ABA responses in plants. In this review, two practical approaches of systems biology to ABA signaling are presented: the one is 'transcriptome analysis', which covers coding genes as well as unannotated transcripts, and the other is 'phosphoproteomics'. The latter technology will offer an unprecedented overview of the regulatory networks involved in ABA signaling because protein phosphorylation/dephosphorylation is a major center of such regulation. Systematic studies will contribute to our understanding of the network structure and dynamics of ABA signaling; moreover, systems biology will facilitate ABA signaling studies as well as future biotechnological applications in crops or trees.

Transport of ABA from the site of biosynthesis to the site of action

There is substantial evidence that abscisic acid (ABA) moves within plants. ABA has been considered as a root-derived signaling molecule that induces stomatal closure in response to dry soil conditions. It has been also reported that ABA synthesized in vegetative tissues is translocated to the seeds. The transport of ABA is an important factor in determining the endogenous concentrations of the hormone at the site of action, and hence, it is an important process in physiological responses. However, the molecular mechanisms that regulate ABA transport are not fully understood. Recent studies using Arabidopsis indicate that ABA is actively synthesized in leaf vascular tissues in response to drought, and that ABA is subsequently transported to the guard cells to close stomata. Identification of the transporters that mediate ABA export from the inside to the outside of the cells at the site of ABA biosynthesis (vascular tissues) and ABA uptake into the cells at the site of action (guard cells), respectively, in this species indicates an active mechanism to regulate ABA transport. Although Arabidopsis represents only one model plant, these findings are useful to discuss common or different regulatory mechanisms among different species and to improve our total understanding of the regulation of ABA transport.

ABA in bryophytes: how a universal growth regulator in life became a plant hormone?

Abscisic acid (ABA) is not a plant-specific compound but one found in organisms across kingdoms from bacteria to animals, suggesting that it is a ubiquitous and versatile substance that can modulate physiological functions of various organisms. Recent studies have shown that plants developed an elegant system for ABA sensing and early signal transduction mechanisms to modulate responses to environmental stresses for survival in terrestrial conditions. ABA-induced increase in stress tolerance has been reported not only in vascular plants but also in non-vascular bryophytes. Since bryophytes are the key group of organisms in the context of plant evolution, clarification of their ABA-dependent processes is important for understanding evolutionary adaptation of land plants. Molecular approaches using Physcomitrella patens have revealed that ABA plays a role in dehydration stress tolerance in mosses, which comprise a major group of bryophytes. Furthermore, we recently reported that signaling machinery for ABA responses is also conserved in liverworts, representing the most basal members of extant land plant lineage. Conservation of the mechanism for ABA sensing and responses in angiosperms and basal land plants suggests that acquisition of this mechanism for stress tolerance in vegetative tissues was one of the critical evolutionary events for adaptation to the land. This review describes the role of ABA in basal land plants as well as non-land plant organisms and further elaborates on recent progress in molecular studies of model bryophytes by comparative and functional genomic approaches.

Abscisic acid signal off the STARting block

The year 2009 marked a real turnaround in our understanding of the mode of abscisic acid (ABA) action. Nearly 25 years had elapsed since the first biochemical detection of ABA-binding proteins in the plasmalemma of Vicia guard cells was reported. This recent--and laudable--achievement is owed largely to the discovery of the soluble ABA receptors whose major interacting proteins happen to be some of the most well-established components of earliest steps in ABA signaling. These soluble receptors, with the double name of PYRABACTIN RESISTANCE (PYR) or REGULATORY COMPONENT OF ABA RECEPTOR (RCAR), are a family of Arabidopsis proteins of about 150-200 amino acids that share a conserved START domain. The ABA signal transduction circuitry under non-stress conditions is muted by the clade A protein phosphatases 2C (PP2C) (notably HAB1, ABI1, and ABI2). During the initial steps of ABA signaling, the binding of the hormone to the receptor induces a conformational change in the latter that allows it to sequester the PP2Cs. This excludes them from the negative regulation of the downstream ABA-activated kinases (OST1/SnRK2.6/SRK2E, SnRK2.2, and SnRK2.3), thus unleashing the pathway by freeing them to phosphorylate downstream targets that now include several b-ZIP transcription factors, ion channels (SLAC1, KAT1), and a NADPH oxidase (AtrbohF). The discovery of this family of soluble receptors and the rich insight already gained from structural studies of their complexes with different isoforms of ABA, PP2C, and the synthetic agonist pyrabactin lay the foundation towards rational design of chemical switches that can bolster drought hardiness in plants.

ABA signaling in stomatal guard cells: lessons from Commelina and Vicia

Abscisic acid (ABA) signaling mechanisms have been studied in a broad variety of plant species using complementary analyses, taking advantage of different methodologies suitable for each plant species. Early studies on ABA biosynthesis using Solanum lycopersicum mutants suggested an importance of ABA synthesis in stomatal closure. To understand ABA signaling in guard cells, cellular, biochemical and electrophysiological studies in Vicia faba and Commelina communis have been conducted, providing fundamental knowledge that was further reconfirmed by molecular genetic studies of Arabidopsis. In this article, examples of stomatal studies in several plants and prospects in ABA research are discussed.

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.

New ABA-hypersensitive Arabidopsis mutants are affected in loci mediating responses to water deficit and Dickeya dadantii infection

On water deficit, abscisic acid (ABA) induces stomata closure to reduce water loss by transpiration. To identify Arabidopsis thaliana mutants which transpire less on drought, infrared thermal imaging of leaf temperature has been used to screen for suppressors of an ABA-deficient mutant (aba3-1) cold-leaf phenotype. Three novel mutants, called hot ABA-deficiency suppressor (has), have been identified with hot-leaf phenotypes in the absence of the aba3 mutation. The defective genes imparted no apparent modification to ABA production on water deficit, were inherited recessively and enhanced ABA responses indicating that the proteins encoded are negative regulators of ABA signalling. All three mutants showed ABA-hypersensitive stomata closure and inhibition of root elongation with little modification of growth and development in non-stressed conditions. The has2 mutant also exhibited increased germination inhibition by ABA, while ABA-inducible gene expression was not modified on dehydration, indicating the mutated gene affects early ABA-signalling responses that do not modify transcript levels. In contrast, weak ABA-hypersensitivity relative to mutant developmental phenotypes suggests that HAS3 regulates drought responses by both ABA-dependent and independent pathways. has1 mutant phenotypes were only apparent on stress or ABA treatments, and included reduced water loss on rapid dehydration. The HAS1 locus thus has the required characteristics for a targeted approach to improving resistance to water deficit. In contrast to has2, has1 exhibited only minor changes in susceptibility to Dickeya dadantii despite similar ABA-hypersensitivity, indicating that crosstalk between ABA responses to this pathogen and drought stress can occur through more than one point in the signalling pathway.