Transcription factor CCA1 interacts with RGL1 to regulate flowering in Arabidopsis.

Flowering time is regulated by both external environmental cues and internal factors. CCA1(CIRCADIAN CLOCK ASSOCIATED 1) is a core component of the plant circadian clock. While previous studies have implicated CCA1 in flowering regulation, its underlying mechanism has remained incompletely characterized. Here, we demonstrate that CCA1 physically interacts with RGL1(RGA-LIKE 1), a key repressor in the gibberellin (GA) signaling pathway. Genetic analyses revealed that the early-flowering phenotype of cca1 rgl1-1 double mutant was more pronounced than those of cca1 or rgl1-1 single mutants. In addition, CCA1-OE rgl1-1 flowered earlier than CCA1-OE but later than rgl1-1. Gene expression and transcriptional activation assays further indicated that the CCA1-RGL1 complex can repress both expression and function of APETALA1 (AP1), a master regulator of floral primordia identity. Together, these results suggest that CCA1 may modulate GA-mediated flowering in Arabidopsis in part by directly repressing AP1 transcription through a regulatory module with RGL1.