ATAF2 integrates Arabidopsis brassinosteroid inactivation and seedling photomorphogenesis.

The Arabidopsis thaliana hypocotyl is a robust system for studying the interplay of light and plant hormones, such as brassinosteroids (BRs), in the regulation of plant growth and development. Since BRs cannot be transported between plant tissues, their cellular levels must be appropriate for given developmental fates. BR homeostasis is maintained in part by transcriptional feedback regulation loops that control the expression of key metabolic enzymes, including the BR-inactivating enzymes BAS1 (CYP734A1, formerly CYP72B1) and SOB7 (CYP72C1). Here, we find that the NAC transcription factor (TF) ATAF2 binds the promoters of BAS1 and SOB7 to suppress their expression. ATAF2 restricts the tissue-specific expression of BAS1 and SOB7 in planta. ATAF2 loss- and gain-of-function seedlings have opposite BR-response phenotypes for hypocotyl elongation. ATAF2 modulates hypocotyl growth in a light-dependent manner, with the photoreceptor phytochrome A playing a major role. The photomorphogenic phenotypes of ATAF2 loss- and gain-of-function seedlings are suppressed by treatment with the BR biosynthesis inhibitor brassinazole. Moreover, the disruption of BAS1 and SOB7 abolishes the short-hypocotyl phenotype of ATAF2 loss-of-function seedlings in low fluence rate white light, demonstrating an ATAF2-mediated connection between BR catabolism and photomorphogenesis. ATAF2 expression is suppressed by both BRs and light, which demonstrates the existence of an ATAF2-BAS1/SOB7-BR-ATAF2 feedback regulation loop, as well as a light-ATAF2-BAS1/SOB7-BR-photomorphogenesis pathway. ATAF2 also modulates root growth by regulating BR catabolism. As it is known to regulate plant defense and auxin biosynthesis, ATAF2 therefore acts as a central regulator of plant defense, hormone metabolism and light-mediated seedling development.