Unlike activation of target genes in response to abscisic acid (ABA), how MYB96 transcription factor represses ABA-repressible genes to further enhance ABA responses remains unknown. Here, we show MYB96 interacts with the histone modifier HDA15 to suppress negative regulators of early ABA signaling. The MYB96-HDA15 complex co-binds to the promoters of a subset of RHO GTPASE OF PLANTS ( ROP ) genes, ROP6 , ROP10 , and ROP11 , and represses their expression by removing acetyl groups of histone H3 and H4 from the cognate regions, particularly in the presence of ABA. In support, HDA15 -deficient mutants display reduced ABA sensitivity and are susceptible to drought stress with derepression of the ROP genes, as observed in the myb96-1 mutant. Biochemical and genetic analyses show that MYB96 and HDA15 are interdependent in the regulation of ROP suppression. Thus, MYB96 confers maximal ABA sensitivity by regulating both positive and negative regulators of ABA signaling through distinctive molecular mechanisms.

The phytohormone abscisic acid (ABA) regulates plant responses to various environmental challenges. Controlled protein turnover is an important component of ABA signalling. Here we show that the RING-type E3 ligase MYB30-INTERACTING E3 LIGASE 1 (MIEL1) regulates ABA sensitivity by promoting MYB96 turnover in Arabidopsis. Germination of MIEL1-deficient mutant seeds is hypersensitive to ABA, whereas MIEL1-overexpressing transgenic seeds are less sensitive. MIEL1 can interact with MYB96, a regulator of ABA signalling, and stimulate its ubiquitination and degradation. Genetic analysis shows that MYB96 is epistatic to MIEL1 in the control of ABA sensitivity in seeds. While MIEL1 acts primarily via MYB96 in seed germination, MIEL1 regulates protein turnover of both MYB96 and MYB30 in vegetative tissues. We find that ABA regulates the expression of MYB30-responsive genes during pathogen infection and this regulation is partly dependent on MIEL1. These results suggest that MIEL1 may facilitate crosstalk between ABA and biotic stress signalling.

Circadian clocks regulate the rhythms of biological activities with a period of approximately 24-hours and synchronize plant metabolism and physiology with the environmental cycles. The clock also gates responses to environmental stresses to maximize fitness advantages. Here we report that the MYB96 transcription factor is connected with the clock oscillator to shape the circadian gating of abscisic acid (ABA) responses. MYB96 directly binds to the TIMING OF CAB EXPRESSION 1 (TOC1) promoter to positively regulate its expression. The use of myb96 mutant plants shows that this regulation is essential for the gated induction of TOC1 by ABA. In turn, MYB96 induction by ABA is also altered in toc1-3 mutant plants. The increased tolerance to drought of MYB96 over-expressing plants is decreased in the toc1-3 mutant background, suggesting that MYB96 and TOC1 intersect the circadian clock and ABA signaling. The MYB96-TOC1 function might be also regulated by the clock component CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1), which binds to the MYB96 promoter and alters its circadian expression. Thus, a complex circuitry of CCA1-MYB96-TOC1 regulatory interactions provides the mechanistic basis underlying the connection between circadian and stress signaling to optimize plant fitness to ambient stresses.

Floral transition is influenced by environmental factors such as light and temperature. Plants are capable of integrating photoperiod and ambient temperature signaling into their developmental program. Despite extensive investigations on individual genetic pathways, little is known about the molecular components that integrate both pathways. Here, we demonstrate that the RING finger–containing E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) acts as an integrator of photoperiod and ambient temperature signaling. In addition to the role in photoperiodic destabilization of CONSTANS (CO), COP1 also regulates temperature sensitivity by controlling the degradation of GIGANTEA (GI). COP1-impaired mutants showed reduced sensitivity to low ambient temperature. Notably, COP1 is more stabilized at low temperature and accelerates GI turnover in a 26S proteasome-dependent manner. The direct association of GI with the promoter of FLOWERING LOCUS T (FT) was reduced because of its ambient temperature-dependent protein stability control, and thus COP1-triggered GI turnover delays flowering at low temperatures via a CO-independent pathway. Taken together, our findings indicate that environmental conditions regulate the stability of COP1, and conditional specificity of its target selection stimulates proper developmental responses and ensures reproductive success.

Seed germination is a key developmental transition that initiates the plant life cycle. The timing of germination is determined by the coordinated action of two phytohormones, gibberellin and abscisic acid (ABA). In particular, ABA plays a key role in integrating environmental information and inhibiting the germination process. The utilization of embryonic lipid reserves contributes to seed germination by acting as an energy source, and ABA suppresses lipid degradation to modulate the germination process. Here, we report that the ABA-responsive R2R3-type MYB transcription factor MYB96, which is highly expressed in embryo, regulates seed germination by controlling the expression of ABSCISIC ACID-INSENSITIVE4 (ABI4) in Arabidopsis (Arabidopsis thaliana). In the presence of ABA, germination was accelerated in MYB96-deficient myb96-1 seeds, whereas the process was significantly delayed in MYB96-overexpressing activation-tagging myb96-ox seeds. Consistently, myb96-1 seeds degraded a larger extent of lipid reserves even in the presence of ABA, while reduced lipid mobilization was observed in myb96-ox seeds. MYB96 directly regulates ABI4, which acts as a repressor of lipid breakdown, to define its spatial and temporal expression. Genetic analysis further demonstrated that ABI4 is epistatic to MYB96 in the control of seed germination. Taken together, the MYB96-ABI4 module regulates lipid mobilization specifically in the embryo to ensure proper seed germination under suboptimal conditions.