Transcription Factors FHY3 and FAR1 Regulate Light-Induced Gene Expression in Arabidopsis.

The circadian clock provides a time-keeping mechanism that synchronizes various biological activities with the surrounding environment. Arabidopsis () (), encoding a MYB-related transcription factor, is a key component of the core oscillator of the circadian clock, with peak expression in the morning. The molecular mechanisms regulating the light induction and rhythmic expression of remain elusive. In this study, we show that two phytochrome signaling proteins, FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and its paralog FAR-RED IMPAIRED RESPONSE1 (FAR1), are essential for the light-induced expression of FHY3 and FAR1 directly bind to the promoter and activate its expression, whereas PHYTOCHROME INTERACTING FACTOR5 (PIF5) directly binds to its promoter and represses its expression. Furthermore, PIF5 and TIMING OF CAB EXPRESSION1 physically interact with FHY3 and FAR1 to repress their transcriptional activation activity on expression. These findings demonstrate that the photosensory-signaling pathway integrates with circadian oscillators to orchestrate clock gene expression. This mechanism might form the molecular basis of the regulation of the clock system by light in response to daily changes in the light environment, thus increasing plant fitness.