Abscisic Acid (ABA) and the AP2/ERF-type transcription factor called ABA Insensitive 4 (ABI4) play pivotal roles in the plant growth responses to environmental stress. An analysis of seedling development in Arabidopsis ABA hypersensitive mutants suggested that OXS3, OXS3b, O3L3, O3L4, and O3L6 were negative regulators of ABI4 expression. We therefore characterized the roles of the Oxidative Stress 3 (OXS3) family members in ABA signaling. All five OXS3 family proteins were found to interact with AFP1 in yeast two hybrid assays. Seven OXS3 family members were found to interact with histone H2A.X, although OXS3b, O3L3, and O3L5 showed weaker interactions. ChIP-qPCR analysis showed that the absence of some these OXS3 family proteins was associated with increased occupancy of histone γ-H2A.X at the ABI4 promoter which also corresponded to the de-repression of ABI4 expression. Repression of ABI4 expression, however, required both AFP1 and OXS3, OXS3b or O3L6. We conclude that in the absence of stress, OXS3 family proteins regulate γ-H2A.X deposition at the ABI4 promoter and that together with AFP1, OXS3 family proteins serve to prevent an ABA-induced growth arrest by co-repressing ABI4 through decreased promoter occupancy of histone γ-H2A.X.

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