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.

Although a role for microRNA399 (miR399) in plant responses to phosphate (Pi) starvation has been indicated, the regulatory mechanism underlying miR399 gene expression is not clear. Here, we report that AtMYB2 functions as a direct transcriptional activator for miR399 in Arabidopsis (Arabidopsis thaliana) Pi starvation signaling. Compared with untransformed control plants, transgenic plants constitutively overexpressing AtMYB2 showed increased miR399f expression and tissue Pi contents under high Pi growth and exhibited elevated expression of a subset of Pi starvation-induced genes. Pi starvation-induced root architectural changes were more exaggerated in AtMYB2-overexpressing transgenic plants compared with the wild type. AtMYB2 directly binds to a MYB-binding site in the miR399f promoter in vitro, as well as in vivo, and stimulates miR399f promoter activity in Arabidopsis protoplasts. Transcription of AtMYB2 itself is induced in response to Pi deficiency, and the tissue expression patterns of miR399f and AtMYB2 are similar. Both genes are expressed mainly in vascular tissues of cotyledons and in roots. Our results suggest that AtMYB2 regulates plant responses to Pi starvation by regulating the expression of the miR399 gene.Phosphorus (P) is an essential component of all organisms, as it is found, among other compounds, in nucleic acids, ATP, and membrane phospholipids. It is an essential nutrient for plants. P can be acquired by plants only as inorganic phosphate (Pi). Therefore, most of the P content of soils is unavailable for plant growth and development (Hinsinger, 2001). To overcome the problem of Pi limitation, plants have developed a variety of adaptive responses that conserve internal P while activating mechanisms that enhance the accessibility and uptake of external P. The accompanying gene expression changes produce changes in root architecture, enhanced Pi uptake activity, secretion of organic acids, and secretion of phosphatases (Raghothama, 1999;Poirier and Bucher, 2002;Yuan and Liu, 2008;Péret et al., 2011). The synchronization of Pi availability with plant growth and development is orchestrated by several phytohormones, including abscisic acid, ethylene, auxin, and cytokinin (Hillwig et al., 2008;Devaiah et al., 2009;Lei et al., 2011).A few transcription factors have been characterized that appear to regulate subsets of the response to Pi stress, either positively or negatively. PHOSPHATE STARVATION RESPONSE1 (PHR1) is a MYB transcription factor that initiates the up-regulation of Pi starvation-responsive genes in plants and unicellular algae (Rubio et al., 2001). WRKY75, a WRKY transcription factor family member, has been identified as a key regulator of Pi acquisition and root architecture in response to Pi starvation (Devaiah et al., 2007a). MYB62, an R2R3-type MYB transcription factor, connects Pi homeostasis and GA signaling during Pi starvation (Devaiah et al., 2009). ZAT6, a C2H2-type zinc finger transcription factor, regulates Pi homeostasis and exerts some control over root development (...

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, a major environmental factor, significantly affects plant growth and reproduction. Plants have evolved complex molecular mechanisms to tolerate drought stress. In this study, we investigated the function of the Arabidopsis thaliana RPD3-type HISTONE DEACETYLASE 9 (HDA9) in response to drought stress. The loss-of-function mutants hda9-1 and hda9-2 were insensitive to abscisic acid (ABA) and sensitive to drought stress. The ABA content in the hda9-1 mutant was reduced in wild type (WT) plant. Most histone deacetylases in animals and plants form complexes with other chromatin-remodeling components, such as transcription factors. In this study, we found that HDA9 interacts with the ABA INSENSITIVE 4 (ABI4) transcription factor using a yeast two-hybrid assay and coimmunoprecipitation. The expression of CYP707A1 and CYP707A2, which encode (+)-ABA 8′-hydroxylases, key enzymes in ABA catabolic pathways, was highly induced in hda9-1, hda9-2, abi4, and hda9-1 abi4 mutants upon drought stress. Chromatin immunoprecipitation and quantitative PCR showed that the HDA9 and ABI4 complex repressed the expression of CYP707A1 and CYP707A2 by directly binding to their promoters in response to drought stress. Taken together, these data suggest that HDA9 and ABI4 form a repressive complex to regulate the expression of CYP707A1 and CYP707A2 in response to drought stress in Arabidopsis.

The Pseudomonas syringae effector protein AvrRpm1 activates the Arabidopsis (Arabidopsis thaliana) intracellular innate immune receptor protein RESISTANCE TO PSEUDOMONAS MACULICOLA1 (RPM1) via modification of a second Arabidopsis protein, RPM1-INTERACTING PROTEIN4 (AtRIN4). Prior work has shown that AvrRpm1 induces phosphorylation of AtRIN4, but homology modeling indicated that AvrRpm1 may be an ADP-ribosyl transferase. Here, we show that AvrRpm1 induces ADP-ribosylation of RIN4 proteins from both Arabidopsis and soybean (Glycine max) within two highly conserved nitrateinduced (NOI) domains. It also ADP ribosylates at least 10 additional Arabidopsis NOI domain-containing proteins. The ADPribosylation activity of AvrRpm1 is required for subsequent phosphorylation on Thr-166 of AtRIN4, an event that is necessary and sufficient for RPM1 activation. We also show that the C-terminal NOI domain of AtRIN4 interacts with the exocyst subunits EXO70B1, EXO70E1, EXO70E2, and EXO70F1. Mutation of either EXO70B1 or EXO70E2 inhibited secretion of callose induced by the bacterial flagellin-derived peptide flg22. Substitution of RIN4 Thr-166 with Asp enhanced the association of AtRIN4 with EXO70E2, which we posit inhibits its callose deposition function. Collectively, these data indicate that AvrRpm1 ADP-ribosyl transferase activity contributes to virulence by promoting phosphorylation of RIN4 Thr-166, which inhibits the secretion of defense compounds by promoting the inhibitory association of RIN4 with EXO70 proteins.