Fig. 9.

Model for the roles of CCA1 and ATAF2 in regulating BR inactivation and the incorporation of circadian and photomorphogenic pathways. Both CCA1 and ATAF2 suppress the expression of BR-inactivating genes BAS1 and SOB7 via direct binding to their promoters. BRs promote Arabidopsis hypocotyl growth under light. BAS1 and SOB7 inhibit hypocotyl elongation by catabolizing BRs. Both BAS1 and SOB7 are circadian regulated with higher expression in the dark period. Transcriptionally induced by light, CCA1 plays a role in the oscillation of BAS1 and SOB7. While ATAF2 expression is feedback suppressed by BRs, CCA1 is not subject to such transcriptional regulation. CCA1 suppresses ATAF2 expression in seedlings grown under light but switches to an activator for ATAF2 in etiolated seedlings. CCA1 can also physically interact with ATAF2 at the protein level. Light induces ATAF2 expression in seedlings undergoing a circadian photoperiod but acts as a repressor when seedlings transit from skotomorphogenesis to photomorphogenesis.