A Role of Swi/snf Complex in Aba-Dependent Drought Responsive Gene Expression in Arabidopsis Thaliana

The survival of plants as sessile organisms depends on their ability to cope with environmental challenges. Of key importance in this regard is the phytohormone abscisic acid (ABA). ABA not only promotes seed dormancy but also triggers growth arrest in postgermination embryos that encounter water stress. This is accompanied by increased desiccation tolerance. Postgermination ABA responses in Arabidopsis thaliana are mediated in large part by the ABA-induced basic domain/leucine zipper transcription factor ABA INSENSITIVE5 (ABI5). Here, I show that loss of function of the SWI/SNF chromatin remodeling ATPase BRAHMA (BRM) causes ABA hypersensitivity during postgermination growth arrest. ABI5 expression was derepressed in brm mutants in the absence of exogenous ABA and accumulated to high levels upon ABA sensing. This effect was likely direct; chromatin immunoprecipitation revealed BRM binding to the ABI5 locus. Moreover, loss of BRM activity led to destabilization of a nucleosome likely to repress ABI5 transcription. Genetic interaction revealed that the abi5 null mutant was epistatic to BRM in postgermination growth arrest. In addition, vegetative growth defects typical of brm mutants in the absence of ABA treatment could be partially overcome by reduction of ABA responses, and brm mutants displayed increased drought tolerance. I propose a role for BRM in the balance between growth or stress responses. Intriguingly, BRM resides at the ABI5 promoter both in the absence and presence of the stress signal. I found that BRM interacts with the core components of abscisic acid signaling transduction pathway. Moreover, the C-terminus of BRM can be phosphorylated in an ABA dependent manner in vitro. It is therefore likely that stress sensing inactivates the BRM complex to allow ABI5 upregulation