 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.


INTRODUCTION
During their life cycles, plants are continuously exposed to environmental challenges including light, heat, cold, flooding, high salinity, and drought stress. Among them, drought stress results in considerable damage to plant growth, and more than 40% of crop production is lost to drought (Guy et al., 1985;May et al., 1998;Hasegawa et al., 2000). Upon exposure to drought stress, plants initiate the expression of resistance genes and subsequent activation of signaling pathways.
Plants have developed complex molecular and signaling mechanisms to adapt to water deficit condition. They respond to drought stress either through osmotic adjustment and regulation of ion homeostasis or by controlling the damage repair system and the detoxification and removal of reactive oxygen species (ROS) (Shinozaki and Yamaguchi-Shinozaki, 2000;Zhu, 2002). The phytohormone abscisic acid (ABA) plays a crucial role in plant physiological processes, regulating many aspects of plant growth and development including seed dormancy, seed maturation, and seedling growth. ABA is also required for drought stress tolerance, which regulates stomatal movement during drought stress and helps plants tolerate extreme waterdeficient conditions Lopez-Molina et al., 2001). However, the molecular and biochemical mechanism of these signaling pathways are not yet fully understood.
The mechanisms of chromatin modification and the constitution of chromatin complexes regulating gene expression are highly conserved in plants, mammals and yeast (Henderson and Jacobsen, 2007). Chromatin structure and its modification form the basic mechanism of genetic and epigenetic regulation of stress-related gene expression (Horn and Peterson, 2002). Histone acetylation is one of the most important features of chromatin remodeling, which removes positive charges by adding an acetyl group to the lysine residues of histone proteins, thereby reducing the histone-DNA affinity and resulting in chromatin decondensation and active transcription (Bannister and Kouzarides, 2011). Histone acetylation levels are dynamically regulated through the combined actions of histone acetyltransferases (HATs) and histone deacetylases (HDACs). HDACs are enzymes conserved in the eukaryotes that function in biological processes including transcription, genome stability, development and in biotic and abiotic stress responses (Haberland et al., 2009;Seto and Yoshida, 2014;Bosch-Presegué and Vaquero, 2015).
Human NCOR1 is a homolog of the Arabidopsis protein powerdress (PWR), which is involved in the floral determinacy network (Yumul et al., 2013). The gene encoding PWR was named Powerdress because of the appearance of the single mutant, which has bulged carpel tips reminiscent of excessively padded suit or dress shoulders. PWR has two conserved SWI3/DAD2/N-CoR/TFIII-B (SANT) domains that together function as a histone-interaction module that couples histone tail binding to enzyme catalysis for the remodeling of nucleosomes. PWR interacts with REDUCED Potassium Dependency Protein 3 (RPD3), a class-1-type Histone Deacetylase 9 (HDA9) and mediates Histone 3 (H3) deacetylation. The complex of PWR-HDA9 also regulates flowering time in Arabidopsis by repressing Agamous-Like 19 (AGL19) transcription (Kim et al., 2016).
HDA9 requires PWR for its nuclear transport and binding to the promoter region of key negative regulator genes involved in leaf senescence (Yumul et al., 2013;Chen et al., 2016). HDA9 interacts with PWR and WRKY53 to regulate leaf aging, and hda9 and pwr loss-of-function mutants exhibit late senescence phenotype (Chen et al., 2016). Furthermore, ABA promotes leaf senescence and loss of function of its receptors PYL8 and PYL9, resulting in delayed leaf senescence (Zhao et al., 2015). To date, approximately 18 Arabidopsis histone deacetylases (HDACs) have been identified. These are divided into three main types. Twelve belong to the reduced Potassium Dependency Protein 3/HDA1 Histone Deacetylase 1 (RPD3) superfamily and are named as HDAs; two are in the histone deacetylase 2 (HD2) family and are named HDTs; and two belong to the silent information regulator protein 2 (SIR2) family and are named SRTs (Pandey et al., 2002;Hollender and Liu, 2008). RPD3-type class 1 HDA6 and HDA19 are involved in the regulation of seed germination, ABA response, salt stress and other abiotic stresses. Unlike hda6 and hda19 mutants, mutants of HDA9 (hda9-1 and hda9-2) are insensitive to ABA and to salt stress during seed germination and root growth (van Zanten et al., 2014;Kang et al., 2015).
Plant endogenous ABA concentration is determined by the rate of ABA metabolism (i.e., biosynthesis, and catabolism). The molecular mechanisms of ABA signaling, biosynthesis, and catabolism have been characterized using genetic and biochemical approaches (Nambara, 2005). Arabidopsis contains several NCED family genes, including AtNCED3, which plays the central role in ABA biosynthesis in response to drought stress (Iuchi et al., 2001). The transcript level of AtNCED3 rapidly increases in response to drought stress, while a nced3 mutant carrying a T-DNA insertion is defective in accumulation of endogenous ABA under drought stress and impaired in drought stress tolerance. For ABA catabolism, at least two crucial pathways have been characterized: the oxidative pathway and the sugar-conjugation pathway (Nambara, 2005). The oxidative pathway is stimulated by CYP707A-induced hydroxylation of ABA C-80 to phaseic acid (Kushiro et al., 2004;Saito et al., 2004). The four Arabidopsis CYP707A-family genes encoding ABA 8hydroxylases are induced by exogenous ABA, as well as by dehydration and other abiotic stresses (Kushiro et al., 2004;Saito et al., 2004). By contrast, CYP707A2 transcripts predominantly accumulate in dry seeds, and the gene is immediately upregulated after seed imbibition. The cyp707a2 mutant maintains a high level of ABA and exhibits enhanced seed dormancy as compared to the wild type (WT) (Kushiro et al., 2004). These reports indicate that CYP707A2 is a component in ABA catabolism during seed germination and regulation of seed dormancy. However, the physiological role of other CYP707A genes remained unclear.
The interaction and binding of ABA with PYL/PYR1/RCAR receptors results in the deactivation of protein phosphatase type-2C (PP2C) proteins (ABI1, ABI2, HAB1 and HAB2), thereby releasing SNF1-related protein kinase (SnRK2) kinases. The ABA-mediated disassociation of PP2C from SnRK2s leads to autophosphorylation and subsequently to transphosphorylation, activation of the downstream targets (such as ABI3, ABI4, and ABI5) and ultimately regulation of downstream signaling pathways (Fujii et al., 2009;Umezawa et al., 2009;Antoni et al., 2012). ABI4 is an important transcription factor that was initially identified as a member of the AP2/ERF family and that binds to ABA-responsive cis-regulatory elements (CREs), ABRE and regulates the expression of genes in response to abiotic stresses (Mizoi et al., 2012). ABI4 is also a versatile activator and a repressor of several genes. Beside inducing the expression of genes involved in seed dormancy, ABA signaling, salt stress and floral transition (Söderman et al., 2000;Nakabayashi et al., 2005;Koussevitzky et al., 2007;Bossi et al., 2009;Giraud et al., 2009;Reeves et al., 2011;Shu et al., 2018), ABI4 also represses the expression of genes involved in ABA catabolism (CYP707A genes), ethylene biosynthesis (ACS genes) and retrograde signaling (AOX1a), genes encoding Arabidopsis response regulators (ARRs) (Huang et al., 2017), as well as genes involved in fatty acid biosynthesis, photosynthesis, pigment and wax metabolic processes (Shu et al., 2013;Dong et al., 2016), by directly binding to their promoters.
PWR also interacts with HOS15 (HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 15), a homolog of human transducin-β-like protein 1 (TBL1). HOS15 contains a LisH and a WD40-repeat domain and is involved in histone modification and deacetylation during abiotic stresses. Furthermore, HOS15 also interacts with HISTONE DEACETYLASE 9 (HDA9), as determined by affinity purification of HOS15-interacting proteins (Park et al., 2018). Loss-offunction hos15-2 mutant plants are hypersensitive to ABA during germination and extremely tolerant to drought stress, indicating the importance of HOS15 as a negative regulator (Ali et al., 2019). On the other hand, the function of PWR in abiotic stresses is largely unknown.
Here we report that T