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.

To survive salt stress, plants must maintain a balance between sodium and potassium ions. High-affinity potassium transporters (HKTs) play a key role in reducing Na+ toxicity through K+ uptake. Eutrema parvula (formerly known as Thellungiella parvula), a halophyte closely related to Arabidopsis, has two HKT1 genes that encode EpHKT1;1 and EpHKT1;2. In response to high salinity, the EpHKT1;2 transcript level increased rapidly; by contrast, the EpHKT1;1 transcript increased more slowly in response to salt treatment. Yeast cells expressing EpHKT1;2 were able to tolerate high concentrations of NaCl, whereas EpHKT1;1-expressing yeast cells remained sensitive to NaCl. Amino acid sequence alignment with other plant HKTs showed that EpHKT1;1 contains an asparagine residue (Asn-213) in the second pore-loop domain, but EpHKT1;2 contains an aspartic acid residue (Asp-205) at the same position. Yeast cells expressing EpHKT1;1, in which Asn-213 was substituted with Asp, were able to tolerate high concentrations of NaCl. In contrast, substitution of Asp-205 by Asn in EpHKT1;2 did not enhance salt tolerance and rather resulted in a similar function to that of AtHKT1 (Na+ influx but no K+ influx), indicating that the presence of Asn or Asp determines the mode of cation selectivity of the HKT1-type transporters. Moreover, Arabidopsis plants (Col-gl) overexpressing EpHKT1;2 showed significantly higher tolerance to salt stress and accumulated less Na+ and more K+ compared to those overexpressing EpHKT1;1 or AtHKT1. Taken together, these results suggest that EpHKT1;2 mediates tolerance to Na+ ion toxicity in E. parvula and is a major contributor to its halophytic nature.

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Our ability to overcome the challenges behind metabolic disorders will require a detailed understanding of the regulation of responses to nutrition. The Creb3 transcription factor family appears to have a unique regulatory role that links cellular secretory capacity with development, nutritional state, infection, and other stresses. This role in regulating individual secretory capacity genes could place this family of transcription factors at an important regulatory intersection mediating an animal’s responses to nutrients and other environmental challenges. Interestingly, in both humans and mice, individuals with mutations in Creb3L3/CrebH, one of the Creb3 family members, exhibit hypertriglyceridemia (HTG) thus linking this transcription factor to lipid metabolism. We are beginning to understand how Creb3L3 and related family members are regulated and to dissect the potential redundancy and cross talk between distinct family members, thereby mediating both healthy and pathological responses to the environment. Here, we review the current knowledge on the regulation of Creb3 family transcription factor activity, their target genes, and their role in metabolic disease.