Abscisic Acid Receptor Research Articles

Phosphorylation of Novel Interactors of Wild Soybean GsSnRK1 Protein Kinase

Plant SnRK1 kinases have been reported to participate in regulating many aspects of plant biology including resistance to biotic and abiotic stresses. Wild soybean (Glycine soja) has strong adaptation abilities to extreme environmental conditions and has four GsSnRK1 genes in its genome. However, the functions of GsSnRK1 are still largely unknown. In this work, we employed yeast two-hybrid and co-immunoprecipitation to determine the physical interactions of GsSnRK1 and three unknown proteins (GsKIP1, GsKIP2, and GsKIP3). To further characterize these proteins, we expressed and purified them in E. coli system. Biochemical analyses indicated that Thr176 of GsSnRK1 could be directly phosphorylated by its upstream kinase GsGRIK1 and the activated GsSnRK1 could phosphorylate its interactors GsKIP1, GsKIP2, GsKIP3, and ABF2 revealed by PhosTag-based approaches and specific phospho-antibodies. Intriguingly, a constituted ABA receptor complex consisting of GsPYL, GsPP2C, could perceive exogenous ABA, and subsequently, the ABA-bound receptor could activate GsSnRK1 to phosphorylate ABF2, suggesting that GsSnRK1 is an essential and bona fide component of ABA signaling pathway. This study is the first time to comprehensively characterize the in vitro functions of wild soybean GsSnRK1 kinase, providing the clues to unveil the novel molecular roles of SnRK1 kinase in cell signaling.

Genome-wide Identification and Characterization of FCS-Like Zinc Finger (FLZ) Family Genes in Maize (Zea mays) and Functional Analysis of ZmFLZ25 in Plant Abscisic Acid Response.

FCS-like zinc finger family proteins (FLZs), a class of plant-specific scaffold of SnRK1 complex, are involved in the regulation of various aspects of plant growth and stress responses. Most information of FLZ family genes was obtained from the studies in Arabidopsis thaliana, whereas little is known about the potential functions of FLZs in crop plants. In this study, 37 maize FLZ (ZmFLZ) genes were identified to be asymmetrically distributed on 10 chromosomes and can be divided into three subfamilies. Protein interaction and subcellular localization assays demonstrated that eight typical ZmFLZs interacted and partially co-localized with ZmKIN10, the catalytic α-subunit of the SnRK1 complex in maize leaf mesophyll cells. Expression profile analysis revealed that several ZmFLZs were differentially expressed across various tissues and actively responded to diverse abiotic stresses. In addition, ectopic overexpression of ZmFLZ25 in Arabidopsis conferred hypersensitivity to exogenous abscisic acid (ABA) and triggered higher expression of ABA-induced genes, pointing to the positive regulatory role of ZmFLZ25 in plant ABA signaling, a scenario further evidenced by the interactions between ZmFLZ25 and ABA receptors. In summary, these data provide the most comprehensive information on FLZ family genes in maize, and shed light on the biological function of ZmFLZ25 in plant ABA signaling.

Two birds with one stone: CEPR2 phosphorylates dual targets in ABA signaling

Abscisic acid (ABA) is a major phytohormone that governs plant development and their responses to many stressors (Chen et al., 2020). After its biosynthesis in specific tissues and cells, ABA is transported throughout the plant and perceived by dedicated receptors. The PYR/PYL/RCAR (pyrabactin resistance1/PYR1-like/regulatory components of ABA receptor) proteins are such ABA receptors and are localized inside the cell. Due to their location, and since ABA is often biosynthesized far from its target cells, ABA transporters are needed to move the phytohormone from the cells that synthesize ABA to the target cells.

ABA signalling and metabolism are not essential for dark-induced stomatal closure but affect response speed

Stomata are microscopic pores that open and close, acting to balance CO2 uptake with water loss. Stomata close in response to various signals including the drought hormone abscisic acid (ABA), microbe-associated-molecular-patterns, high CO2 levels, and darkness. The signalling pathways underlying ABA-induced stomatal closure are well known, however, the mechanism for dark-induced stomatal closure is less clear. ABA signalling has been suggested to play a role in dark-induced stomatal closure, but it is unclear how this occurs. Here we investigate the role of ABA in promoting dark-induced stomatal closure. Tracking stomatal movements on the surface of leaf discs we find, although steady state stomatal apertures are affected by mutations in ABA signalling and metabolism genes, all mutants investigated close in response to darkness. However, we observed a delayed response to darkness for certain ABA signalling and metabolism mutants. Investigating this further in the quadruple ABA receptor mutant (pyr1pyl1pyl2pyl4), compared with wild-type, we found reduced stomatal conductance kinetics. Although our results suggest a non-essential role for ABA in dark-induced stomatal closure, we show that ABA modulates the speed of the dark-induced closure response. These results highlight the role of ABA signalling and metabolic pathways as potential targets for enhancing stomatal movement kinetics.

Identification of ABA-Mediated Genetic and Metabolic Responses to Soil Flooding in Tomato (Solanum lycopersicum L. Mill).

Soil flooding is a compound abiotic stress that alters soil properties and limits atmospheric gas diffusion (O2 and CO2) to the roots. The involvement of abscisic acid (ABA) in the regulation of soil flooding-specific genetic and metabolic responses has been scarcely studied despite its key importance as regulator in other abiotic stress conditions. To attain this objective, wild type and ABA-deficient tomatoes were subjected to short-term (24 h) soil waterlogging. After this period, gas exchange parameters were reduced in the wild type but not in ABA-deficient plants that always had higher E and gs. Transcript and metabolite alterations were more intense in waterlogged tissues, with genotype-specific variations. Waterlogging reduced the ABA levels in the roots while inducing PYR/PYL/RCAR ABA receptors and ABA-dependent transcription factor transcripts, of which induction was less pronounced in the ABA-deficient genotype. Ethylene/O2-dependent genetic responses (ERFVIIs, plant anoxia survival responses, and genes involved in the N-degron pathway) were induced in hypoxic tissues independently of the genotype. Interestingly, genes encoding a nitrate reductase and a phytoglobin involved in NO biosynthesis and scavenging and ERFVII stability were induced in waterlogged tissues, but to a lower extent in ABA-deficient tomato. At the metabolic level, flooding-induced accumulation of Ala was enhanced in ABA-deficient lines following a differential accumulation of Glu and Asp in both hypoxic and aerated tissues, supporting their involvement as sources of oxalacetate to feed the tricarboxylic acid cycle in waterlogged tissues and constituting a potential advantage upon long periods of soil waterlogging. The promoter analysis of upregulated genes indicated that the production of oxalacetate from Asp via Asp oxidase, energy processes such as acetyl-CoA, ATP, and starch biosynthesis, and the lignification process were likely subjected to ABA regulation. Taken together, these data indicate that ABA depletion in waterlogged tissues acts as a positive signal, inducing several specific genetic and metabolic responses to soil flooding.

Biochar’s Leacheates Affect the Abscisic Acid Pathway in Rice Seedlings Under Low Temperature

Organic molecules of biochar’s leacheates are known to increase the cold resistance of rice seedlings. Yet, it remains unclear whether the organic molecules of biochar leacheates can interact with the abscisic acid (ABA) signaling pathway associated with low temperature. This study used experiments and bioinformatics (molecular docking) to determine which of the organic molecules of biochar’s leacheates could influence the ABA signaling pathway. Specifically, we investigated whether these molecules affected ABA, a plant hormone linked to cold resistance. The contents of endogenous ABA and its precursor carotenoids were determined under low-temperature stress (10°C) and treatment with different concentrations of biochar leacheates. With increased leacheate concentrations, the endogenous ABA and carotenoid contents also increased, as did the expression of ABA- and cold-related genes. When rice seedlings were instead treated with exogenous ABA, it also affected the above-measured indexes; hence, we surmised that certain water-soluble organic molecules of biochar could exert a similar effect as ABA. We first used gas chromatography/mass spectrometry (GC/MS) to identify the organic molecules in the biochar extract, and then we used molecular docking software Autodock to show how they interact. We found that the molecule (1R, 2R, 4S)-2-(6-chloropyridin-3-yl)-7-azabicyclo(2.2.1)heptane was simplified, as Cyah could dock with the ABA receptor protein OsPYL2 in rice, which shows Cyah in biochar is probably an analog of ABA, with a similar function. Based on these results, we conclude that organic molecules of biochar’s leacheates could enter into rice plants and interact with ABA-related proteins to affect the ABA signaling pathway, thereby improving the cold stress resistance of plants.

Comparative Transcriptome Analysis of Early- and Late-Bolting Traits in Chinese Cabbage (Brassica rapa).

Chinese cabbage is one of the most important and widely consumed vegetables in China. The developmental transition from the vegetative to reproductive phase is a crucial process in the life cycle of flowering plants. In spring-sown Chinese cabbage, late bolting is desirable over early bolting. In this study, we analyzed double haploid (DH) lines of late bolting (“Y410-1” and “SY2004”) heading Chinese cabbage (Brassica rapa var. pekinensis) and early-bolting Chinese cabbage (“CX14-1”) (B. rapa ssp. chinensis var. parachinensis) by comparative transcriptome profiling using the Illumina RNA-seq platform. We assembled 721.49 million clean high-quality paired-end reads into 47,363 transcripts and 47,363 genes, including 3,144 novel unigenes. There were 12,932, 4,732, and 4,732 differentially expressed genes (DEGs) in pairwise comparisons of Y410-1 vs. CX14-1, SY2004 vs. CX14-1, and Y410-1 vs. SY2004, respectively. The RNA-seq results were confirmed by reverse transcription quantitative real-time PCR (RT-qPCR). A Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of DEGs revealed significant enrichment for plant hormone and signal transduction as well as starch and sucrose metabolism pathways. Among DEGs related to plant hormone and signal transduction, six unigenes encoding the indole-3-acetic acid-induced protein ARG7 (BraA02g009130), auxin-responsive protein SAUR41 (BraA09g058230), serine/threonine-protein kinase BSK11 (BraA07g032960), auxin-induced protein 15A (BraA10g019860), and abscisic acid receptor PYR1 (BraA08g012630 and BraA01g009450), were upregulated in both late bolting Chinese cabbage lines (Y410-1 and SY2004) and were identified as putative candidates for the trait. These results improve our understanding of the molecular mechanisms underlying flowering in Chinese cabbage and provide a foundation for studies of this key trait in related species.

Abscisic acid receptors maintain abscisic acid homeostasis by modulating UGT71C5 glycosylation activity.

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

Fig fruit ripening is regulated by the interaction between ethylene and abscisic acid.

Fleshy fruit ripening is typically regulated by ethylene in climacteric fruits and abscisic acid (ABA) in non-climacteric fruits. Common fig (Ficus carica) shows a dual-ripening mechanism, which is not fully understood. Here, we detected separate peaks of ethylene and ABA in fig fruits at the onset- and on-ripening stages, in conjunction with a sharp rise in glucose and fructose contents. In a newly-designed split-fruit system, exogenous ethylene failed to rescue fluridone-inhibited fruit ripening, whereas exogenous ABA rescued 2-amino-ethoxy-vinyl glycine (AVG)-inhibited fruit ripening. Transcriptome analysis revealed changes in the expression of genes key to both ABA and ethylene biosynthesis and perception during fig fruit ripening. At the de-greening stage, downregulation of FcACO2 or FcPYL8 retarded ripening, but downregulation of FcETR1/2 did not; unexpectedly, downregulation of FcAAO3 promoted ripening, but it inhibited ripening only before the de-greening stage. Furthermore, we detected an increase in ethylene emissions in the FcAAO3-RNAi ripening fruit and a decrease in ABA levels in the FcACO2-RNAi unripening fruit. Importantly, FcPYL8 can bind to ABA, suggesting that it functions as an ABA receptor. Our findings support the hypothesis that ethylene regulates the fig fruit ripening in an ABA-dependent manner. We propose a model for the role of the ABA-ethylene interaction in climacteric/non-climacteric processes.

Role of ABA in Overcoming Environmental Stress: Sensing, Signaling and Crosstalk

The Abscisic Acid (ABA) is an isoprenoid phytohormone, regulating various physiological processes ranging from stomatal opening to protein storage. Moreover, it provides adaptation to drought, salt and cold stresses acts also as a signaling mediator during the plant’s adaptive response to environmental conditions. In addition, numbers of transcription factors are involved in regulating the expression of ABA responsive genes by interacting with their respective cis-acting elements. ABA signal transduction initiates signal perception by ABA receptors and transfer via downstream proteins, including protein kinases and phosphatases. Hence, for improvement in plants-stress-tolerance capacity, it is necessary to understand the mechanism behind it. On this ground, this article lightens the importance and also the role of ABA signaling with regard to various stresses as well as regulation of ABA biosynthetic pathway along with the transcription factors for stress tolerance.

Grafting Delays Watermel on Fruit Ripening by Altering Gene Expression of ABA Centric Phytohormone Signaling

Grafting cultivation is implemented worldwide mainly to resist abiotic and biotic stresses and is an effective method to improve watermelon production. However, grafting may affect fruit development and quality. In our experiment, pumpkin-grafted (PG) watermelon fruits developed slower and the ripening period was extended compared to self-grafted (SG) fruits. We found that the concentrations of abscisic acid (ABA) among endogenous phytohormones were dramatically reduced by pumpkin grafting. In order to understand these changes at the gene expression level, we performed a comprehensive analysis of the fruit flesh transcriptomes between PG and SG during fruit development and ripening. A total of 1,675 and 4,102 differentially expressed genes (DEGs) were identified between PG and SG. Further functional enrichment analysis revealed that these DEGs were associated with carbohydrate biosynthesis, phytohormone signaling transmission, and cell wall metabolism categories. ABA centric phytohormone signaling and fruit quality-related genes including ABA receptor, PP2C proteins, AP2-EREBP transcription factors, sucrose transporter, and carotenoid isomerase were co-expressed with fruit ripening. These results provide the valuable resource for understanding the mechanism of pumpkin grafting effect on watermelon fruit ripening and quality development.

Cytokinin and auxin modulate cucumber parthenocarpy fruit development

Parthenocarpy (fruit development without pollination) is an important agricultural trait, which determines the production of horticultural crops like tomato, eggplant and cucumber. Parthenocarpy ability varies widely among cucumber lines. To better understand the genetic regulators underlying this variation, hormone and transcriptome profiles and the expressions of hormone-related genes in non-pollinated fruit of a highly parthenocarpic line DDX and a weakly parthenocarpic line ZK were compared at 1 day after anthesis. The cytokinin and gibberellic acid contents were higher in DDX than in ZK, while the abscisic acid content was less. Exogenous application of cytokinins, auxin and gibberellic acid 4 + 7 successfully induced the parthenocarpic fruit set of ZK. The transcript levels of several cytokinin biosynthesis genes (CsCYP735A1, CsCYP735A2 and CsLOG1), cytokinin-responsive genes (CsRR8/9b, CsRR8/9d, CsRR8/9e and CsRR16/17) and auxin signal transduction genes (Aux/IAAs) were greater in DDX than in ZK, while the level of an abscisic acid receptor (CsPYR1) was lower. These results suggest that high expression levels of cytokinin- and auxin-related genes in DDX contribute to its high parthenocarpy ability.

A ras-related small GTP-binding protein, RabE1c, regulates stomatal movements and drought stress responses by mediating the interaction with ABA receptors

Drought represents a leading constraint over crop productivity worldwide. The plant response to this stress is centered on the behavior of the cell membrane, where the transduction of abscisic acid (ABA) signaling occurs. Here, the Ras-related small GTP-binding protein RabE1c has been shown able to bind to an ABA receptor in the Arabidopsis thaliana plasma membrane, thereby positively regulating ABA signaling. RabE1c is highly induced by drought stress and expressed abundantly in guard cells. In the loss-of-function rabe1c mutant, both stomatal closure and the whole plant drought stress response showed a reduced sensitivity to ABA treatment, demonstrating that RabE1c is involved in the control over transpirative water loss through the stomata. Impairment of RabE1c’s function suppressed the accumulation of the ABA receptor PYL4. The over-expression of RabE1c in A. thaliana enhanced the plants’ ability to tolerate drought, and a similar phenotypic effect was achieved by constitutively expressing the gene in Chinese cabbage (Brassica rapassp. pekinensis). The leading conclusion was that RabE1c promotes the degradation of PYL4, suggesting a possible genetic strategy to engineer crop plants to better withstand drought stress.

Characterization on the physiological traits of plants and yield formation capacity upon water- and N-saving conditions in wheat (T. aestivum L.)

Improving water and nitrogen (N) use efficiencies of the Triticum aestivum species contributes to the sustainable agriculture worldwide. In this study, we investigated the physiological traits of plants and the yield formation capacity of winter wheat treated with deficit irrigation together with N-saving condition. Compared with OI-ON (optimum irrigation, OI together with optimum nitrogen, ON), treatment DI-DN (deficit irrigation, DI combined with deficient N (DN, N120) led to decreased contents of chlorophyll, nitrogen (N), and osmolytes, Pn, and the activities of SOD and POD in leaves at various stages together with reduced yields in Jimai 585 and Shimai 22, two cultivars with contrast drought stress responses. The moisture contents in soil profile planted by tested cultivars showed significant reduction under OI-ON with respect to those under DI-DN, suggesting the positive impact of elevated soil water consumption on plant physiological and agronomic traits. The drought-tolerant cultivar Shimai 22 displayed much more improvement on the physiological and agronomic traits, WUE, and NUE than drought-sensitive cultivar Jimai 585 under DI-DN, due to effective usage for soil water and N storage. Yields and grain weights under DI-DN were positively correlated with chlorophyll and osmolytes contents, Pn, and antioxidant enzyme activities, suggesting that the improved physiological traits under DI-DN contributed to the elevated yield for the drought-tolerant cultivar. Two nitrate transporter (NRT) family ones TaNRT2.1 and TaNRT2.2 and TaPYR3 and TaPYR6, two abscisic acid receptor (PYR) family genes exhibited induced transcripts upon DI-DN in wheat cultivars, with more abundance in Shimai 22 than in Jimai 585. Transgenic analysis on the differential genes confirmed their functions in mediating plant drought and N deprivation responses, suggesting that distinct PYR and NRT family members mediate drought and N deprivation adaptation in wheat plants. Our investigation provided insights into high WUE and NUE cultivation of winter wheat under the limited-water and N-saving management.

The different root apex zones contribute to drought priming induced tolerance to a reoccurring drought stress in wheat

Drought priming is a promising approach to improve tolerance to further drought in wheat. The root apex plays important roles in drought however, its contribution to drought priming remains unknown. To provide mechanistic insights into this process, the transcriptomes and proteomes at three different zones along the root axis under drought stress were analyzed. Physiological assessment of root growth indicated that priming augmented roots growth in response to drought and also the levels of protective proline and glycine betaine. Scanning across the proximal to the distal zones of the root apex indicated increases the transcription of genes involved in primary and secondary metabolism. Conversely, genes related to translation, transcription, folding, sorting and degradation, replication and repair were increased in the apex compared to the proximal zone. A single drought episode suppressed their expression but prior drought priming served to maintain expression with recurrent drought stress. The differentially primed responses genes were mainly involved in the pathways related to plant hormone signaling, stress defense and cell wall modification. The prediction of regulatory hubs using Cytoscape implicated signaling components such as the ABA receptor PYL4 as influencing antioxidant status and the cell cycle. Based our integrative transcriptomic-proteomic assessments we present a model for drought priming protected plant hormone signaling transduction pathways to drive the cell cycle and cell wall loosening to confer beneficial effects on roots to counter the effects of drought. This model provides a theoretical basis for improvement of drought tolerance in wheat, via an increased understanding of drought priming induced drought tolerance.

The Physiological and Molecular Mechanism of Abscisic Acid in Regulation of Fleshy Fruit Ripening.

The ripening of fleshy fruits is coupled with the degradation of both chlorophyll and cell walls, as well as changes in the metabolism of phenylpropanoids, flavonoids, starch/sucrose, and carotenoids. These processes are controlled by phytohormones and other factors, including abscisic acid (ABA), ethylene, auxin, polyamines, sugar, and reactive oxygen species. The ripening of climacteric fruits is controlled by ethylene and non-climacteric fruit ripening is regulated mainly by ABA. Also, ABA and ethylene may interact in both types of fruit ripening. ABA concentrations in fleshy fruits are regulated in response to developmental and environmental cues and are controlled by the relative rates of ABA biosynthesis and catabolism, the former mainly via 9-cis-epoxycarotenoid dioxygenases (NCEDs) and β-glucosidases and the latter via ABA 8'-hydroxylases (CYP707As) and β-glycosyltransferases. In strawberry fruit ripening, ABA is perceived via at least two receptors, Pyrabactin resistance (PYR)/PYR-like (PYL) and putative abscisic acid receptor (ABAR), which are linked separately to the conserved signaling pathway ABA-FaPYR1-FaABIl-FaSnRK2 and the novel signaling pathway ABA-FaABAR-FaRIPK1-FaABI4. Downstream signaling components include important transcription factors, such as AREB (ABA responsive element binding protein)/ABF (ABRE binding factors ABA responsive factor), ethylene response factor (ERF), and V-myb Myeloblastosis viral oncogene homolog (MYB), as well as ripening-related genes. Finally, a comprehensive model of ABA linked to ethylene, sugar, polyamines, auxin and reactive oxygen species in the regulation of strawberry fruit ripening is proposed. Next, new integrated mechanisms, including two ABA signaling pathways, ABA and ethylene signaling pathways, and ABA/ethylene to other phytohormones are interesting and important research topics in ripening, especially in non-climacteric fruits.

Cloning and expression analyses of a Pyrabactin Resistance 1 (PYR1) gene from Magnolia sieboldii K. Koch

ABSTRACT Magnolia sieboldii K. Koch is endemic to China and has high medicinal and ornamental values. However, its seed exhibits morphophysiological dormancy, and the molecular mechanisms of which are not clearly understood. To reveal the regulation mechanism of the ABA signal in seed dormancy, the M. sieboldii ABA receptor Pyrabactin Resistance 1 (PYR1) gene was cloned and analyzed. Analysis of the MsPYR1 sequence analysis showed that the full-length cDNA contained a complete open reading frame of 987 bp and encoded a predicted protein of 204 amino acid residues. The protein had a relative molecular weight of 22.661 kDa and theoretical isoelectric point of 5.01. The transcript levels of MsPYR1 were immediately upregulated at 16 DAI and then decreased at 40 DAI. The highest transcript level of MsPYR1 was found in the dry seeds, indicating that the MsPYR1 gene may play an important role in the regulation of dormancy. The MsPYR1 gene cDNA was successfully expressed in E. coli Rosetta (DE3), and the protein bands were consistent with the prediction. The Anti-MsPYR1antibody could detect the expression of MsPYR1 in M. sieboldii. The results provided a foundation for further study of the function of the MsPYR1 gene. ABBREVIATIONS ABA: Abscisic acid; MPD: morphophysiological; PYR1: Pyrabactin Resistance1; PYL: Pyr1-Like; RCAR: Regulatory Components of Aba Receptors; PP2C: protein phosphatases 2C; SnRK2: sucrose non-fermenting1-related protein kinase2; DAI: day after imbibition; NCBI: National Center for Biotechnology Information; BCA: Bicinchoninic acid; CDD: Conserved Domains.

OsPP2C09 Is a Bifunctional Regulator in Both ABA-Dependent and Independent Abiotic Stress Signaling Pathways.

Clade A Type 2C protein phosphatases (PP2CAs) negatively regulate abscisic acid (ABA) signaling and have diverse functions in plant development and in response to various stresses. In this study, we showed that overexpression of the rice ABA receptor OsPYL/RCAR3 reduces the growth retardation observed in plants exposed to osmotic stress. By contrast, overexpression of the OsPYL/RCAR3-interacting protein OsPP2C09 rendered plant growth more sensitive to osmotic stress. We tested whether OsPP2CAs activate an ABA-independent signaling cascade by transfecting rice protoplasts with luciferase reporters containing the drought-responsive element (DRE) or ABA-responsive element (ABRE). We observed that OsPP2CAs activated gene expression via the cis-acting drought-responsive element. In agreement with this observation, transcriptome analysis of plants overexpressing OsPP2C09 indicated that OsPP2C09 induces the expression of genes whose promoters contain DREs. Further analysis showed that OsPP2C09 interacts with DRE-binding (DREB) transcription factors and activates reporters containing DRE. We conclude that, through activating DRE-containing promoters, OsPP2C09 positively regulates the drought response regulon and activates an ABA-independent signaling pathway.

Tomato protein phosphatase 2C influences the onset of fruit ripening and fruit glossiness

Abscisic acid (ABA) plays a vital role in coordinating physiological processes during fresh fruit ripening. Binding of ABA to receptors facilitates the interaction and inhibition of type 2C phosphatase (PP2C) co-receptors. However, the exact mechanism of PP2C during fruit ripening is unclear. In this study, we determined the role of the tomato ABA co-receptor type 2C phosphatase SlPP2C3, a negative regulator of ABA signaling and fruit ripening. SlPP2C3 selectively interacted with monomeric ABA receptors and SlSnRK2.8 kinase in both yeast and tobacco epidermal cells. Expression of SlPP2C3 was ABA-inducible, which was negatively correlated with fruit ripening. Tomato plants with suppressed SlPP2C3 expression exhibited enhanced sensitivity to ABA, while plants overexpressing SlPP2C3 were less sensitive to ABA. Importantly, lack of SlPP2C3 expression accelerated the onset of fruit ripening and affected fruit glossiness by altering the outer epidermis structure. There was a significant difference in the expression of cuticle-related genes in the pericarp between wild-type and SlPP2C3-suppressed lines based on RNA sequencing (RNA-seq) analysis. Taken together, our findings demonstrate that SlPP2C3 plays an important role in the regulation of fruit ripening and fruit glossiness in tomato.

Transcriptional Regulation of Protein Phosphatase 2C Genes to Modulate Abscisic Acid Signaling.

The plant hormone abscisic acid (ABA) triggers cellular tolerance responses to osmotic stress caused by drought and salinity. ABA controls the turgor pressure of guard cells in the plant epidermis, leading to stomatal closure to minimize water loss. However, stomatal apertures open to uptake CO2 for photosynthesis even under stress conditions. ABA modulates its signaling pathway via negative feedback regulation to maintain plant homeostasis. In the nuclei of guard cells, the clade A type 2C protein phosphatases (PP2Cs) counteract SnRK2 kinases by physical interaction, and thereby inhibit activation of the transcription factors that mediate ABA-responsive gene expression. Under osmotic stress conditions, PP2Cs bind to soluble ABA receptors to capture ABA and release active SnRK2s. Thus, PP2Cs function as a switch at the center of the ABA signaling network. ABA induces the expression of genes encoding repressors or activators of PP2C gene transcription. These regulators mediate the conversion of PP2C chromatins from a repressive to an active state for gene transcription. The stress-induced chromatin remodeling states of ABA-responsive genes could be memorized and transmitted to plant progeny; i.e., transgenerational epigenetic inheritance. This review focuses on the mechanism by which PP2C gene transcription modulates ABA signaling.