The nuclear interactor PYL8/RCAR3 of Fagus sylvatica FsPP2C1 is a positive regulator of abscisic acid signaling in seeds and stress

Type 2C phosphatase 1 of Artemisia annua L. is a negative regulator of ABA signaling

The phytohormone abscisic acid (ABA) plays an important role in plant development and environmental stress response. Additionally, ABA also regulates secondary metabolism such as artemisinin in the medicinal plant Artemisia annua L. Although an earlier study showed that ABA receptor, AaPYL9, plays a positive role in ABA-induced artemisinin content improvement, many components in the ABA signaling pathway remain to be elucidated in Artemisia annua L. To get insight of the function of AaPYL9, we isolated and characterized an AaPYL9-interacting partner, AaPP2C1. The coding sequence of AaPP2C1 encodes a deduced protein of 464 amino acids, with all the features of plant type clade A PP2C. Transcriptional analysis showed that the expression level of AaPP2C1 is increased after ABA, salt, and drought treatments. Yeast two-hybrid and bimolecular fluorescence complementation assays (BiFC) showed that AaPYL9 interacted with AaPP2C1. The P89S, H116A substitution in AaPYL9 as well as G199D substitution or deletion of the third phosphorylation site-like motif in AaPP2C1 abolished this interaction. Furthermore, constitutive expression of AaPP2C1 conferred ABA insensitivity compared with the wild type. In summary, our data reveals that AaPP2C1 is an AaPYL9-interacting partner and involved in the negative modulation of the ABA signaling pathway in A. annua L.

Tomato PYR/PYL/RCAR abscisic acid receptors show high expression in root, differential sensitivity to the abscisic acid agonist quinabactin, and the capability to enhance plant drought resistance

Abscisic acid (ABA) plays a crucial role in the plant's response to both biotic and abiotic stress. Sustainable production of food faces several key challenges, particularly the generation of new varieties with improved water use efficiency and drought tolerance. Different studies have shown the potential applications of Arabidopsis PYR/PYL/RCAR ABA receptors to enhance plant drought resistance. Consequently the functional characterization of orthologous genes in crops holds promise for agriculture. The full set of tomato (Solanum lycopersicum) PYR/PYL/RCAR ABA receptors have been identified here. From the 15 putative tomato ABA receptors, 14 of them could be grouped in three subfamilies that correlated well with corresponding Arabidopsis subfamilies. High levels of expression of PYR/PYL/RCAR genes was found in tomato root, and some genes showed predominant expression in leaf and fruit tissues. Functional characterization of tomato receptors was performed through interaction assays with Arabidopsis and tomato clade A protein phosphatase type 2Cs (PP2Cs) as well as phosphatase inhibition studies. Tomato receptors were able to inhibit the activity of clade A PP2Cs differentially in an ABA-dependent manner, and at least three receptors were sensitive to the ABA agonist quinabactin, which inhibited tomato seed germination. Indeed, the chemical activation of ABA signalling induced by quinabactin was able to activate stress-responsive genes. Both dimeric and monomeric tomato receptors were functional in Arabidopsis plant cells, but only overexpression of monomeric-type receptors conferred enhanced drought resistance. In summary, gene expression analyses, and chemical and transgenic approaches revealed distinct properties of tomato PYR/PYL/RCAR ABA receptors that might have biotechnological implications.

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.

Arabidopsis ABA receptor RCAR1/PYL9 interacts with an R2R3-type MYB transcription factor, AtMYB44

Abscisic acid (ABA) signaling plays important roles in plant growth, development and adaptation to various stresses. RCAR1/PYL9 has been known as a cytoplasm and nuclear ABA receptor in Arabidopsis. To obtain further insight into the regulatory mechanism of RCAR1/PYL9, a yeast two-hybrid approach was performed to screen for RCAR1/PYL9-interacting proteins and an R2R3-type MYB transcription factor, AtMYB44, was identified. The interaction between RCAR1/PYL9 and AtMYB44 was further confirmed by glutathione S-transferase (GST) pull-down and bimolecular fluorescence complementation (BiFC) assays. Gene expression analysis showed that AtMYB44 negatively regulated the expression of ABA-responsive gene RAB18, in contrast to the opposite role reported for RCAR1/PYL9. Competitive GST pull-down assay and analysis of phosphatase activity demonstrated that AtMYB44 and ABI1 competed for binding to RCAR1/PYL9 and thereby reduced the inhibitory effect of RCAR1/PYL9 on ABI1 phosphatase activity in the presence of ABA in vitro. Furthermore, transient activation assay in protoplasts revealed AtMYB44 probably also decreased RCAR1/PYL9-mediated inhibition of ABI1 activity in vivo. Taken together, our work provides a reasonable molecular mechanism of AtMYB44 in ABA signaling.

Overexpression of PYL5 in rice enhances drought tolerance, inhibits growth, and modulates gene expression

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

Targeted degradation of abscisic acid receptors is mediated by the ubiquitin ligase substrate adaptor DDA1 in Arabidopsis

CULLIN4-RING E3 ubiquitin ligases (CRL4s) regulate key developmental and stress responses in eukaryotes. Studies in both animals and plants have led to the identification of many CRL4 targets as well as specific regulatory mechanisms that modulate their function. The latter involve COP10-DET1-DDB1 (CDD)-related complexes, which have been proposed to facilitate target recognition by CRL4, although the molecular basis for this activity remains largely unknown. Here, we provide evidence that Arabidopsis thaliana DET1-, DDB1-ASSOCIATED1 (DDA1), as part of the CDD complex, provides substrate specificity for CRL4 by interacting with ubiquitination targets. Thus, we show that DDA1 binds to the abscisic acid (ABA) receptor PYL8, as well as PYL4 and PYL9, in vivo and facilitates its proteasomal degradation. Accordingly, we found that DDA1 negatively regulates ABA-mediated developmental responses, including inhibition of seed germination, seedling establishment, and root growth. All other CDD components displayed a similar regulatory function, although they did not directly interact with PYL8. Interestingly, DDA1-mediated destabilization of PYL8 is counteracted by ABA, which protects PYL8 by limiting its polyubiquitination. Altogether, our data establish a function for DDA1 as a substrate receptor for CRL4-CDD complexes and uncover a mechanism for the desensitization of ABA signaling based on the regulation of ABA receptor stability.

Interactions between soybean ABA receptors and type 2C protein phosphatases

The plant hormone abscisic acid (ABA) plays important roles in regulating plant growth, development, and responses to environmental stresses. Proteins in the PYR/PYL/RCAR family (hereafter referred to as PYLs) are known as ABA receptors. Since most studies thus far have focused on Arabidopsis PYLs, little is known about PYL homologs in crop plants. We report here the characterization of 21 PYL homologs (GmPYLs) in soybean. Twenty-three putative GmPYLs can be found from soybean genome sequence and categorized into three subgroups. GmPYLs interact with AtABI1 and two GmPP2Cs in diverse manners. A lot of the subgroup I GmPYLs interact with PP2Cs in an ABA-dependent manner, whereas most of the subgroup II and III GmPYLs bind to PP2Cs in an ABA-independent manner. The subgroup III GmPYL23, which cannot interact with any of the tested PP2Cs, differs from other GmPYLs. The CL2/gate domain is crucial for GmPYLs-PP2Cs interaction, and a mutation in the conserved proline (P109S) abolishes the interaction between GmPYL1 and AtABI1. Furthermore, the ABA dependence of GmPYLs-PP2Cs interactions are partially correlated with two amino acid residues preceding the CL2/gate domain of GmPYLs. We also show that GmPYL1 interacts with AtABI1 in an ABA-dependent manner in plant cells. Three GmPYLs differentially inhibit AtABI1 and GmPP2C1 in an ABA-dependent or -enhanced manner in vitro. In addition, ectopically expressing GmPYL1 partially restores ABA sensitivity of the Arabidopsis triple mutant pyr1/pyl1/pyl4. Taken together, our results suggest that soybean GmPYLs are ABA receptors that function by interacting and inhibiting PP2Cs.

Arabidopsis PYL8 Plays an Important Role for ABA Signaling and Drought Stress Responses

Plants are frequently exposed to numerous environmental stresses such as dehydration and high salinity, and have developed elaborate mechanisms to counteract the deleterious effects of stress. The phytohormone abscisic acid (ABA) plays a critical role as an integrator of plant responses to water-limited condition to activate ABA signal transduction pathway. Although perception of ABA has been suggested to be important, the function of each ABA receptor remains elusive in dehydration condition. Here, we show that ABA receptor, pyrabactin resistance-like protein 8 (PYL8), functions in dehydration conditions. Transgenic plants overexpressing PYL8 exhibited hypersensitive phenotype to ABA in seed germination, seedling growth and establishment. We found that hypersensitivity to ABA of transgenic plants results in high degrees of stomatal closure in response to ABA leading to low transpiration rates and ultimately more vulnerable to drought than the wild-type plants. In addition, high expression of ABA maker genes also contributes to altered drought tolerance phenotype. Overall, this work emphasizes the importance of ABA signaling by ABA receptor in stomata during defense response to drought stress.

Multiple roles of the transcription factor AtMYBR1/AtMYB44 in ABA signaling, stress responses, and leaf senescence

BACKGROUND

The transcription factor AtMYBR1 (MYB44) is a member of the MYB family of transcription factors and is expressed throughout the plant life cycle and especially in senescing and wounded leaves. It has previously been shown to be involved in responses to abiotic stress and is regulated by phosphorylation.

RESULTS

When MYBR1 was over-expressed under the control of the constitutive 35S promoter in Arabidopsis thaliana (OxMYBR1), leaf senescence was delayed. In contrast loss-of-function mybr1 plants showed more rapid chlorophyll loss and senescence. The MYBR1 promoter strongly drove β-GLUCURONIDASE reporter gene expression in tissues immediately after wounding and many wounding/pathogenesis genes were downregulated in OxMYBR1. OxMYBR1 plants were more susceptible to injury under water stress than wildtype, which was correlated with suppression of many ABA inducible stress genes in OxMYBR1. Conversely, mybr1 plants were more tolerant of water stress and exhibited reduced rates of water loss from leaves. MYBR1 physically interacted with ABA receptor PYR1-LIKE8 (PYL8) suggesting a direct involvement of MYBR1 in early ABA signaling. MYBR1 appears to exhibit partially redundant functions with AtMYBR2 (MYB77) and double mybr1 X mybr2 mutants exhibited stronger senescence and stress related phenotypes than single mybr1 and mybr2 mutants.

CONCLUSIONS

MYBR1 is a negative regulator of ABA, stress, wounding responses and blocks senescence. It appears to have a homeostatic function to maintain growth processes in the event of physical damage or stress.

The role of ABA and MAPK signaling pathways in plant abiotic stress responses

As sessile organisms, plants have developed specific mechanisms that allow them to rapidly perceive and respond to stresses in the environment. Among the evolutionarily conserved pathways, the ABA (abscisic acid) signaling pathway has been identified as a central regulator of abiotic stress response in plants, triggering major changes in gene expression and adaptive physiological responses. ABA induces protein kinases of the SnRK family to mediate a number of its responses. Recently, MAPK (mitogen activated protein kinase) cascades have also been shown to be implicated in ABA signaling. Therefore, besides discussing the role of ABA in abiotic stress signaling, we will also summarize the evidence for a role of MAPKs in the context of abiotic stress and ABA signaling.

Cytokinin oxidase is key enzyme of cytokinin degradation

Cytokinin oxidase (EC 1.5.99.12) is an enzyme that catalyzes the irreversible degradation of cytokinin phytohormones that are extremely necessary for growth, development, and differentiation of plants. Cytokinin oxidase plays an important role in the regulation of quantitative level of cytokinins and their distribution in plant tissues. This review generalizes the available information on the structure, properties, and functional role of this enzyme in plant ontogeny under conditions of normal growth and under the influence of unfavorable environmental factors.

Identification of differentially-expressed genes potentially implicated in drought response in pitaya (Hylocereus undatus) by suppression subtractive hybridization and cDNA microarray analysis

Drought is one of the most severe threats to the growth, development and yield of plant. In order to unravel the molecular basis underlying the high tolerance of pitaya (Hylocereus undatus) to drought stress, suppression subtractive hybridization (SSH) and cDNA microarray approaches were firstly combined to identify the potential important or novel genes involved in the plant responses to drought stress. The forward (drought over drought-free) and reverse (drought-free over drought) suppression subtractive cDNA libraries were constructed using in vitro shoots of cultivar 'Zihonglong' exposed to drought stress and drought-free (control). A total of 2112 clones, among which half were from either forward or reverse SSH library, were randomly picked up to construct a pitaya cDNA microarray. Microarray analysis was carried out to verify the expression fluctuations of this set of clones upon drought treatment compared with the controls. A total of 309 expressed sequence tags (ESTs), 153 from forward library and 156 from reverse library, were obtained, and 138 unique ESTs were identified after sequencing by clustering and blast analyses, which included genes that had been previously reported as responsive to water stress as well as some functionally unknown genes. Thirty six genes were mapped to 47 KEGG pathways, including carbohydrate metabolism, lipid metabolism, energy metabolism, nucleotide metabolism, and amino acid metabolism of pitaya. Expression analysis of the selected ESTs by reverse transcriptase polymerase chain reaction (RT-PCR) corroborated the results of differential screening. Moreover, time-course expression patterns of these selected ESTs further confirmed that they were closely responsive to drought treatment. Among the differentially expressed genes (DEGs), many are related to stress tolerances including drought tolerance. Thereby, the mechanism of drought tolerance of this pitaya genotype is a very complex physiological and biochemical process, in which multiple metabolism pathways and many genes were implicated. The data gained herein provide an insight into the mechanism underlying the drought stress tolerance of pitaya, as well as may facilitate the screening of candidate genes for drought tolerance.

The PYL4 A194T mutant uncovers a key role of PYR1-LIKE4/PROTEIN PHOSPHATASE 2CA interaction for abscisic acid signaling and plant drought resistance

Because abscisic acid (ABA) is recognized as the critical hormonal regulator of plant stress physiology, elucidating its signaling pathway has raised promise for application in agriculture, for instance through genetic engineering of ABA receptors. PYRABACTIN RESISTANCE1/PYR1-LIKE (PYL)/REGULATORY COMPONENTS OF ABA RECEPTORS ABA receptors interact with high affinity and inhibit clade A phosphatases type-2C (PP2Cs) in an ABA-dependent manner. We generated an allele library composed of 10,000 mutant clones of Arabidopsis (Arabidopsis thaliana) PYL4 and selected mutations that promoted ABA-independent interaction with PP2CA/ABA-HYPERSENSITIVE3. In vitro protein-protein interaction assays and size exclusion chromatography confirmed that PYL4(A194T) was able to form stable complexes with PP2CA in the absence of ABA, in contrast to PYL4. This interaction did not lead to significant inhibition of PP2CA in the absence of ABA; however, it improved ABA-dependent inhibition of PP2CA. As a result, 35S:PYL4(A194T) plants showed enhanced sensitivity to ABA-mediated inhibition of germination and seedling establishment compared with 35S:PYL4 plants. Additionally, at basal endogenous ABA levels, whole-rosette gas exchange measurements revealed reduced stomatal conductance and enhanced water use efficiency compared with nontransformed or 35S:PYL4 plants and partial up-regulation of two ABA-responsive genes. Finally, 35S:PYL4(A194T) plants showed enhanced drought and dehydration resistance compared with nontransformed or 35S:PYL4 plants. Thus, we describe a novel approach to enhance plant drought resistance through allele library generation and engineering of a PYL4 mutation that enhances interaction with PP2CA.

Structure and function of abscisic acid receptors

The phytohormone abscisic acid (ABA) plays a crucial role in adaptive responses to environmental stresses, such as drought and high salinity, as well as in plant development, such as seed maturation and dormancy. PYR/PYL/RCAR has been identified as a bona fide ABA receptor (ABAR) that constitutes the core regulatory component of ABA signaling networks in plants. Here, we review recent structural and functional studies of the ABAR that have elucidated its activation mechanism, early signaling components, and physiological responses. A crucial event in the receptor's activation was found to be an open-to-closed conformational change in the gate loop of the receptor protein. More recent progress has provided strategies for controlling the gate's closure using chemical agonists or protein engineering approaches.

Overexpression of the Artemisia orthologue of ABA receptor, AaPYL9, enhances ABA sensitivity and improves artemisinin content in Artemisia annua L

The phytohormone abscisic acid (ABA) plays an important role in plant development and environmental stress response. In this study, we cloned an ABA receptor orthologue, AaPYL9, from Artemisia annua L. AaPYL9 is expressed highly in leaf and flower. AaPYL9 protein can be localized in both nucleus and cytoplasm. Yeast two-hybrid assay shows AaPYL9 can specifically interact with AtABI1 but not with AtABI2, AtHAB1 or AtHAB2. ABA can enhance the interaction between AaPYL9 and AtABI1 while AaPYL9-89 Pro→Ser and AaPYL9-116 His→Ala point mutations abolishes the interaction. BiFC assay shows that AaPYL9 interacts with AtABI1 in nucleus in planta. Transgenic Arabidopsis plants over-expressing AaPYL9 are more sensitive to ABA in the seed germination and primary root growth than wild type. Consistent with this, ABA report genes have higher expression in AaPYL9 overexpressing plants compared to wild type after ABA treatment. Moreover, overexpression of AaPYL9 in A. annua increases not only drought tolerance, but also artemisinin content after ABA treatment, with significant enhancement of the expression of key genes in artemisinin biosynthesis. This study provides a way to develop A. annua with high-yielding artemisinin and high drought resistance.

mRNA-seq analysis of the Gossypium arboreum transcriptome reveals tissue selective signaling in response to water stress during seedling stage

The cotton diploid species, Gossypium arboreum, shows important properties of stress tolerance and good genetic stability. In this study, through mRNA-seq, we de novo assembled the unigenes of multiple samples with 3h H₂O, NaCl, or PEG treatments in leaf, stem and root tissues and successfully obtained 123,579 transcripts of G. arboreum, 89,128 of which were with hits through BLAST against known cotton ESTs and draft genome of G. raimondii. About 36,961 transcripts (including 1,958 possible transcription factor members) were identified with differential expression under water stresses. Principal component analysis of differential expression levels in multiple samples suggested tissue selective signalling responding to water stresses. Venn diagram analysis showed the specificity and intersection of transcripts’ response to NaCl and PEG treatments in different tissues. Self-organized mapping and hierarchical cluster analysis of the data also revealed strong tissue selectivity of transcripts under salt and osmotic stresses. In addition, the enriched gene ontology (GO) terms for the selected tissue groups were differed, including some unique enriched GO terms such as photosynthesis and tetrapyrrole binding only in leaf tissues, while the stem-specific genes showed unique GO terms related to plant-type cell wall biogenesis, and root-specific genes showed unique GO terms such as monooxygenase activity. Furthermore, there were multiple hormone cross-talks in response to osmotic and salt stress. In summary, our multidimensional mRNA sequencing revealed tissue selective signalling and hormone crosstalk in response to salt and osmotic stresses in G. arboreum. To our knowledge, this is the first such report of spatial resolution of transcriptome analysis in G. arboreum. Our study will potentially advance understanding of possible transcriptional networks associated with water stress in cotton and other crop species.

Overexpression of the Mg-chelatase H subunit in guard cells confers drought tolerance via promotion of stomatal closure in Arabidopsis thaliana

The Mg-chelatase H subunit (CHLH) has been shown to mediate chlorophyll biosynthesis, as well as plastid-to-nucleus and abscisic acid (ABA)-mediated signaling. A recent study using a novel CHLH mutant, rtl1, indicated that CHLH specifically affects ABA-induced stomatal closure, but also that CHLH did not serve as an ABA receptor in Arabidopsis thaliana. However, the molecular mechanism by which CHLH engages in ABA-mediated signaling in guard cells remains largely unknown. In the present study, we examined CHLH function in guard cells and explored whether CHLH expression might influence stomatal aperture. Incubation of rtl1 guard cell protoplasts with ABA induced expression of the ABA-responsive genes RAB18 and RD29B, as also observed in wild-type (WT) cells, indicating that CHLH did not affect the expression of ABA-responsive genes. Earlier, ABA was reported to inhibit blue light (BL)-mediated stomatal opening, at least in part through dephosphorylating/inhibiting guard cell H(+)-ATPase (which drives opening). Therefore, we immunohistochemically examined the phosphorylation status of guard cell H(+)-ATPase. Notably, ABA inhibition of BL-induced phosphorylation of H(+)-ATPase was impaired in rtl1 cells, suggesting that CHLH influences not only ABA-induced stomatal closure but also inhibition of BL-mediated stomatal opening by ABA. Next, we generated CHLH-GFP-overexpressing plants using CER6 promoter, which induces gene expression in the epidermis including guard cells. CHLH-transgenic plants exhibited a closed stomata phenotype even when brightly illuminated. Moreover, plant growth experiments conducted under water-deficient conditions showed that CHLH transgenic plants were more tolerant of drought than WT plants. In summary, we show that CHLH is involved in the regulation of stomatal aperture in response to ABA, but not in ABA-induced gene expression, and that manipulation of stomatal aperture via overexpression of CHLH in guard cells improves plant drought tolerance.

Phytochrome B increases drought tolerance by enhancing ABA sensitivity in Arabidopsis thaliana

Phytochrome B (phyB) can adjust morphological and physiological responses according to changes in the red:far-red (R:FR) ratio. phyB-driven acclimation of plants to open environments (high R:FR ratio) increases carbon gain at the expense of increased water loss. This behaviour alleviates stressful conditions generated by an excess of light, but increases the chances of desiccation. Here we evaluated how phyB modulates this drought-tolerance response by comparing wild-type Arabidopsis thaliana adult plants to the null phyB in response to water shortage. phyB wilted before the wild type, and this was due to phyB maintaining open stomata under a reduction in soil water availability. Although phyB presented enhanced ABA levels under well-watered conditions, this mutant was less sensitive than the wild type in diminishing stomatal conductance in response to exogenous ABA application. Reduced sensitivity to ABA in phyB correlated with a lower expression of ABCG22, which encodes a putative ABA influx transporter, and PYL5, which encodes a soluble ABA receptor. Furthermore, the expression of RAB18 and RD29A, both typical ABA-induced genes, was lower in phyB than the wild type after ABA treatment. We propose that phyB contributes to the acclimation of plants to open environments by enhancing ABA sensitivity when soil water becomes limiting.

The ABA signal transduction mechanism in commercial crops: learning from Arabidopsis

The phytohormone abscisic acid (ABA) affects a wide range of stages of plant development as well as the plant's response to biotic and abiotic stresses. Manipulation of ABA signaling in commercial crops holds promising potential for improving crop yields. Several decades of research have been invested in attempts to identify the first components of the ABA signaling cascade. It was only in 2009, that two independent groups identified the PYR/PYL/RCAR protein family as the plant ABA receptor. This finding was followed by a surge of studies on ABA signal transduction, many of them using Arabidopsis as their model. The ABA signaling cascade was found to consist of a double-negative regulatory mechanism assembled from three protein families. These include the ABA receptors, the PP2C family of inhibitors, and the kinase family, SnRK2. It was found that ABA-bound PYR/RCARs inhibit PP2C activity, and that PP2Cs inactivate SnRK2s. Researchers today are examining how the elucidation of the ABA signaling cascade in Arabidopsis can be applied to improvements in commercial agriculture. In this article, we have attempted to review recent studies which address this issue. In it, we discuss various approaches useful in identifying the genetic and protein components involved. Finally, we suggest possible commercial applications of genetic manipulation of ABA signaling to improve crop yields.

[ABA signaling mediated by PYR/PYL/RCAR in plants]

Abscisic acid (ABA) is a phytohormone that plays critical roles in numerous developmental stages as well as in adaptive responses to biotic and abiotic stresses. Recent breakthroughs in the field of ABA signaling have indicated that there are three major components, PYR/PYL/RCAR (an ABA receptor), type 2C protein phosphates (PP2C, a negative regulator), and SNF1-related protein kinase 2 (SnRK2, a positive regulator). Further results show that these three proteins construct a double negative regulatory system, PYR/PYL/RCAR-| PP2C-| SnRK2, to regulate ABA signal responses in plant cells. Moreover, the combination patterns of these components in vivo are restricted by spatio-temporal and biochemical determinants and the combinational variation in the ABA signalosome is specific to different ABA signal responses. This review summarizes recent advances of study on the molecular basis and regulatory mechanism of PYR/PYL/RCAR-mediated ABA signaling pathway and PYR/PYL/RCAR-PP2C-SnRK2 complex-mediated ABA signal regulation network in plants. The perspectives related to this study are proposed.

cDNA-AFLP analysis of salt-inducible genes expression in Chrysanthemum lavandulifolium under salt treatment

Chrysanthemum lavandulifolium (Fisch. ex Trautv.) Makino is a halophyte species that belongs to the Asteraceae family, and the genus Chrysanthemum. It is one of the ancestors of C.×morifolium Ramatella. Understanding the tolerance mechanism associated with salt stress in C. lavandulifolium could provide important information for explaining the salt tolerance of higher plants and could also help enhancing breeding programs of cultivated Chrysanthemum. In this study, cDNA amplified fragment length polymorphism (cDNA-AFLP) was used to detect differential gene expression in leaves of C. lavandulifolium in response to NaCl treatment. The determination of membrane permeablility, peroxidase activity (POD), malon-dialdehyde (MDA), as well as proline and leaf chlorophyll contents under different NaCl concentrations showed that a 200 mM NaCl treatment was an optimal condition for the cDNA-AFLP experiment. Using this concentration during different times (0, 3 h, 12 h, 24 h and 48 h), we obtained 1930 cDNA fragments using 64 primers. After sequencing 234 randomly chosen cDNA clones and BLASTx analyzing, we got 129 expressed sequence tags (ESTs) which had no significant homology with other sequences, 85 ESTs were homologous to genes with known functions, whereas the rest of ESTs showed homology to unclassified or putative proteins. 25 ESTs that were similar to known functional genes involved in several abiotic and biotic stresses were confirmed by semi-quantitative RT-PCR and qRT-PCR. The expression patterns of these salt-responsive genes not only responded to salt stress but also to plant hormones, such as abscisic acid (ABA), and to other abiotic stresses such as drought and cold. These results indicate an extensive cross-talk among several stresses. Our results provide interesting information for further understanding the molecular mechanisms of salt tolerance in C. lavandulifolium.

Identification of the abscisic acid receptor VvPYL1 in Vitis vinifera

The phytohormone abscisic acid (ABA) plays a central role in many developmental processes and in responses to several abiotic stresses. Identification of the ABA receptor is a first step towards understanding ABA signalling. In this study, using homology analysis, we cloned three genes, named VvPYL1, VvPYL2 and VvPYL3, from Vitis vinifera. An isothermal titration calorimetry assay suggested that VvPYL1 could bind to ABA. A phosphatase activity assay demonstrated that VvPYL1 inhibits phosphatase activity of ABI1, a negative regulator of ABA signalling, in the presence of ABA. Subcellular localisation demonstrates that VvPYL1 is distributed in both the nucleus and cytosol, which is similar to the subcellular localisation of ABA receptors in Arabidopsis. We therefore conclude that VvPYL1 is an ABA receptor that modulates ABA signalling by inhibiting type 2C protein phosphatases (PP2Cs).

Isolation and characterization of novel mutant loci suppressing the ABA hypersensitivity of the Arabidopsis coronatine insensitive 1-16 (coi1-16) mutant during germination and seedling growth

The phytohormone ABA regulates seed germination and stress responses. The identification of clade A protein phosphatase type 2C (PP2C)-interacting proteins PYRABACTIN RESISTANCE 1 (PYR1)/RCAR (REGULATORY COMPONENT OF ABA RECEPTOR) and PYR1-LIKEs (PYLs) as ABA receptors has been a major advance in understanding this process. Here, our aim was to identify additional ABA response loci by suppressor screening of the jasmonate (JA)-insensitive coronatine insensitive 1-16 (coi1-16) mutant using its ABA-hypersensitive phenotype. The identification and genetic characterization of Coi1-16 Resistant to ABA (CRA) loci revealed several unknown and three previously known abi mutants (abi1, abi3 and abi4), thus providing proof-of-concept evidence for this study. The synergistic effect of ABA and JA on seed germination and cotyledon expansion was analyzed in depth and the roles of cra5 coi1-16, cra6 coi1-16, cra7 coi1-16 and cra8 coi1-16 in ABA signaling during seed germination and stress responses were functionally characterized. The cra5 coi1-16 mutant showed resistance to ABA, paclobutrazol, and abiotic stresses during germination and early developmental stages. Furthermore, the cra5 coi1-16 mutation was mapped to the short arm of chromosome V and mutants exhibited differential expression of ABA-responsive genes, suggesting that CRA5 may function as a positive regulator of ABA signaling. Interestingly, cra6 coi1-16, cra7 coi1-16 and cra8 coi1-16 mutants display similar ABA- and abiotic stress-insensitive phenotypes during seed germination and seedling establishment. Taken together, our results demonstrate a key role for CRA genes in regulating the onset of seed germination by ABA, and highlight how cra mutants can be used as powerful tools to analyze novel molecular components of ABA signaling in seeds.

A rice orthologue of the ABA receptor, OsPYL/RCAR5, is a positive regulator of the ABA signal transduction pathway in seed germination and early seedling growth

Abscisic acid (ABA) is a phytohormone that positively regulates seed dormancy and stress tolerance. PYL/RCARs were identified an intracellular ABA receptors regulating ABA-dependent gene expression in Arabidopsis thaliana. However, their function in monocot species has not been characterized yet. Herein, it is demonstrated that PYL/RCAR orthologues in Oryza sativa function as a positive regulator of the ABA signal transduction pathway. Transgenic rice plants expressing OsPYL/RCAR5, a PYL/RCAR orthologue of rice, were found to be hypersensitive to ABA during seed germination and early seedling growth. A rice ABA signalling unit composed of OsPYL/RCAR5, OsPP2C30, SAPK2, and OREB1 for ABA-dependent gene regulation was further identified, via interaction assays and a transient gene expression assay. Thus, a core signalling unit for ABA-responsive gene expression modulating seed germination and early seedling growth in rice has been unravelled. This study provides substantial contributions toward understanding the ABA signal transduction pathway in rice.

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.

Molecular basis of the core regulatory network in ABA responses: sensing, signaling and transport

ABA is a major phytohormone that regulates a broad range of plant traits and is especially important for adaptation to environmental conditions. Our understanding of the molecular basis of ABA responses in plants improved dramatically in 2009 and 2010, banner years for ABA research. There are three major components; PYR/PYL/ RCAR (an ABA receptor), type 2C protein phosphatase (PP2C; a negative regulator) and SNF1-related protein kinase 2 (SnRK2; a positive regulator), and they offer a double negative regulatory system, [PYR/PYL/RCAR-| PP2C-| SnRK2]. In the absence of ABA, PP2C inactivates SnRK2 by direct dephosphorylation. In response to environmental or developmental cues, ABA promotes the interaction of PYR/PYL/RCAR and PP2C, resulting in PP2C inhibition and SnRK2 activation. This signaling complex can work in both the nucleus and cytosol, as it has been shown that SnRK2 phosphorylates basic-domain leucine zipper (bZIP) transcription factors or membrane proteins. Several structural analyses of PYR/PYL/RCAR have provided the mechanistic basis for this 'core signaling' model, by elucidating the mechanism of ABA binding of receptors, or the 'gate-latch-lock' mechanism of interaction with PP2C in inhibiting activity. On the other hand, intercellular ABA transport had remained a major issue, as had intracellular ABA signaling. Recently, two plasma membrane-type ABC transporters were identified and shed light on the influx/efflux system of ABA, resolving how ABA is transported from cell to cell in plants. Our knowledge of ABA responses in plants has been greatly expanded from intracellular signaling to intercellular transport of ABA.

ABA receptors: the START of a new paradigm in phytohormone signalling

The phytohormone abscisic acid (ABA) plays a central role in plant development and in plant adaptation to both biotic and abiotic stressors. In recent years, knowledge of ABA metabolism and signal transduction has advanced rapidly to provide detailed glimpses of the hormone's activities at the molecular level. Despite this progress, many gaps in understanding have remained, particularly at the early stages of ABA perception by the plant cell. The search for an ABA receptor protein has produced multiple candidates, including GCR2, GTG1, and GTG2, and CHLH. In addition to these candidates, in 2009 several research groups converged on a novel family of Arabidopsis proteins that bind ABA, and thereby interact directly with a class of protein phosphatases that are well known as critical players in ABA signal transduction. The PYR/PYL/RCAR receptor family is homologous to the Bet v 1-fold and START domain proteins. It consists of 14 members, nearly all of which appear capable of participating in an ABA receptor-signal complex that responds to the hormone by activating the transcription of ABA-responsive genes. Evidence is provided here that PYR/PYL/RCAR receptors can also drive the phosphorylation of the slow anion channel SLAC1 to provide a fast and timely response to the ABA signal. Crystallographic studies have vividly shown the mechanics of ABA binding to PYR/PYL/RCAR receptors, presenting a model that bears some resemblance to the binding of gibberellins to GID1 receptors. Since this ABA receptor family is highly conserved in crop species, its discovery is likely to usher a new wave of progress in the elucidation and manipulation of plant stress responses in agricultural settings.