FHY3 Is Involved In The Regulation Of The Expression Of Core Circadian Clock Gene CCA1

The circadian clock provides a time-keeping mechanism for synchronizing various biological activities with the surrounding environment.Arabidopsis CIRCADIAN CLOCK ASSOCIATED1(CCA1),which encodes a myb-related transcription factor,is a key component of the central oscillator with expression peaking at dawn.The molecular mechanisms regulating rhythmic expression of CCA1 still remain elusive.In this study,we show that two essential phytochrome signal transducers,.FAR-RED ELONGATED HYPOCOTYL3(FHY3)and FAR-RED IMPAIRED 1(FAR1),directly bind to the promoter of CCA1 and activate its expression,whereas PHYTOCHROME-INTERACTING FACTOR5(PIF5)directly binds to the promoter of CCA1 and represses its expression.Further,PIF5 and TIMING OF CAB EXPRESSION1(TOC1)physically interact with FHY3 and FAR1 to confer circadian clock-gating of the transcriptional activation activity of FHY3 and FAR1 on CCA1 expression.Our combined results demonstrate that the photosensory signaling pathway integrates with the circadian clock to orchestrate clock gene expression,which may form the molecular basis of clock entrainment in response to daily changes of the light environments,thus increasing plant fitness.