INTEGRATION OF BRASSINOSTEROID CATABOLISM AND SEEDLING PHOTOMORPHOGENESIS

Hao Peng

Photomorphogenesis refers to light-induced seedling growth regulated by photoreceptors and additional signaling pathways. Multiple hormonal and light signals coordinately regulate plant photomorphogenesis, including brassinosteroids (BRs). BRs are a class of polyhydroxysteroid plant hormones. They are structurally similar to animal and insect steroids. BRs regulate a wide range of plant activities including cell elongation, seed germination, xylem differentiation, stress responses, and disease resistance. 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 cytochrome P450 catabolic enzymes CYP734A1/CYP72B1/BAS1 and CYP72C1/SOB7. In this research, the NAC transcription factor, ATAF2, is found to bind 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 in far-red light. The photomorphogenic phenotypes of ATAF2 loss- and gain-of-function seedlings can be suppressed by treatment with BR biosynthesis inhibitor brassinazole, which indicates an ATAF2-mediated connection between BRs and photomorphogenesis. The expression of ATAF2 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 regulatory pathway. In addition to photomorphogenesis, ATAF2 also modulates root growth by regulating BR catabolism. ATAF2 was previously known to regulate the expression of plant disease resistance and auxin biosynthesis genes. This work suggests that ATAF2 has additional role as an integrator of BR catabolism and seedling photomorphogenesis. In addition to ATAF2, two ATAF subgroup members, ANAC102 and ATAF1, have similar function in regulating BR catabolism. Disruption of ANAC102 or ATAF1 dramatically increases the expression of BAS1 and SOB7 proteins and causes reduced hypocotyl growth responses to BRs in Arabidopsis. The circadian-clock protein CCA1 interacts with both ATAF2 and its promoter, which suggests that ATAFs and CCA1 may form a regulatory network for BR catabolism

INTEGRATION OF BRASSINOSTEROID CATABOLISM AND SEEDLING PHOTOMORPHOGENESIS

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