SignificancePhenotypic adaptations of plants in response to changes in climate are well known to be mediated by molecular mechanisms, including activation or suppression of transcription factors that control target gene expression. However, the chromatin changes that are essential for the binding of transcription factors are much less understood. Gene derepression at the chromatin level is considered to be the starting point for gene transcription. We report a mechanism of gene derepression through which HOS15 promotes the degradation of histone deacetylase HD2C in a cold-dependent manner that correlates with increased levels of acetylated histones on COR gene chromatin. Moreover, HOS15 directly promotes COR gene transcription by association of CBF transcription factors with the “open” state of the target COR chromatin.
Dehydrating stresses trigger the accumulation of abscisic acid (ABA), a key plant stresssignaling hormone that activates Snf1-Related Kinases (SnRK2s) to mount adaptive responses.However, the regulatory circuits that terminate the SnRK2s signal relay after acclimation or poststress conditions remain to be defined. Here, we show that the desensitization of the ABA-signal is achieved by the regulation of OST1 (SnRK2.6) protein stability via the E3-ubiquitin-ligase HOS15. Upon ABA signal, HOS15-induced degradation of OST1 is inhibited and stabilized OST1 promotes the stress-response. When the ABA signal terminates, protein phosphatases ABI1/2 recruit HOS15 to OST1 to promote the rapid degradation of OST1. Notably, we found that even in the presence of ABA, OST1 levels were also depleted within hours of ABA signal onset. The unexpected dynamics of OST1 abundance was resolved by a systematic mathematical modeling demonstrating a desensitizing feedback loop by which OST1-induced up-regulation of ABI1/2 leads to the degradation of OST1. This model illustrates the complex rheostat dynamics underlying the ABA-induced stress response and desensitization. signaling components. Several members of the PYR/PYL/RCAR family of ABA receptors are specifically recognized by different E3 ubiquitin ligases and targeted for degradation through proteasome action (Irigoyen et al., 2014). ABI1, a PP2C phosphatase that inhibits ABA-related SnRK2 kinases such as OST1, is ubiquitinated by PUB12/PUB13 (U-box E3 ligases) and also degraded by the proteasome in the presence of ABA signal (Kong et al., 2015), which then facilitates the activation of SnRK2 kinases and of their downstream transcription factors (TFs).Eventually, the TFs that accumulate in response to ABA need to be degraded when the signal ceases. When ABA signaling stops, ABI FIVE BINDING PROTEIN1 (AFP1) and KEG (KEEP ON GOING) facilitate UPS-mediated proteolysis of ABI5 and ABF1/ABF3 (Lopez-Molina et al., 2003;Stone et al., 2006;Chen et al., 2013;Liu et al., 2013). In addition, DWA1/DWA2 (DWD HYPERSENSITIVE TO ABA1/2), and ABD1 (ABA-HYPERSENSITIVE DCAF1), substrate receptors for the DDB1 CULLIN4-based E3 ligases, command the degradation of ABI5 (Seo et al., 2014;Lee et al., 2010). The positive signaling effectors SnRK2.2, SnRK2.3 and SnRK2.6/OST1 are known to be degraded by an ubiquitination-and proteasome-dependent mechanism, but the mechanism involved has not been identified with the exception of SnKR2.3 that was shown to be degraded by AtPP2-B11 (Kim et al., 2013;Cheng et al., 2017). In summary, the degradation of positive signaling effectors leads to deactivation of the ABA signal pathway.The ubiquitin-26S proteasome system (UPS) proceeds via sequential reactions performed by three distinct sets of enzymes: ubiquitin activating enzymes (E1), ubiquitin conjugating enzymes (E2) and ubiquitin protein ligases (E3). Because target specificity is conferred by the E3 ligases, plant genomes encode hundreds of E3 ligases that recruit specific target proteins in multiple biological pro...
Drought stress adversely affects plant growth and development and significantly reduces crop productivity and yields. The phytohormone abscisic acid (ABA) rapidly accumulates in response to drought stress and mediates the expression of stress-responsive genes that help the plant to survive dehydration. The protein Powerdress (PWR), which interacts with Histone Deacetylase 9 (HDA9), has been identified as a critical component regulating plant growth and development, flowering time, floral determinacy, and leaf senescence. However, the role and function of PWR and HDA9 in abiotic stress response had remained elusive. Here we report that a complex of PWR and HDA9 interacts with ABI4 and epigenetically regulates drought signaling in plants. T-DNA insertion mutants of PWR and HDA9 are insensitive to ABA and hypersensitive to dehydration. Furthermore, the expression of ABA-responsive genes (RD29A, RD29B, and COR15A) is also downregulated in pwr and hda9 mutants. Yeast two-hybrid assays showed that PWR and HDA9 interact with ABI4. Transcript levels of genes that are normally repressed by ABI4, such as CYP707A1, AOX1a and ACS4, are increased in pwr. More importantly, during dehydration stress, PWR and HDA9 regulate the acetylation status of the CYP707A1, which encodes a major enzyme of ABA catabolism. Taken together, our results indicate that PWR, in association with HDA9 and ABI4, regulates the chromatin modification of genes responsible for regulation of both the ABA-signaling and ABA-catabolism pathways in response to ABA and drought stress.
Drought is one of the most critical environmental stresses limiting plant growth and crop productivity. The synthesis and signaling of abscisic acid (ABA), a key phytohormone in the drought stress response, is under photoperiodic control. GIGANTEA (GI), a key regulator of photoperiod-dependent flowering and the circadian rhythm, is also involved in the signaling pathways for various abiotic stresses. In this study, we isolated ENHANCED EM LEVEL (EEL)/basic Leu zipper 12, a transcription factor involved in ABA signal responses, as a GI interactor in Arabidopsis (Arabidopsis thaliana). The diurnal expression of 9-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3), a rate-limiting ABA biosynthetic enzyme, was reduced in the eel, gi-1, and eel gi-1 mutants under normal growth conditions. Chromatin immunoprecipitation and electrophoretic mobility shift assays revealed that EEL and GI bind directly to the ABA-responsive element motif in the NCED3 promoter. Furthermore, the eel, gi-1, and eel gi-1 mutants were hypersensitive to drought stress due to uncontrolled water loss. The transcript of NCED3, endogenous ABA levels, and stomatal closure were all reduced in the eel, gi-1, and eel gi-1 mutants under drought stress. Our results suggest that the EEL-GI complex positively regulates diurnal ABA synthesis by affecting the expression of NCED3, and contributes to the drought tolerance of Arabidopsis.
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