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 core oscillator of Arabidopsis circadian clock with peaking expression in the morning. The molecular mechanisms regulating light induction and rhythmic expression of CCA1 still remain elusive. In this study, we show that two phytochrome signal transducers, FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and its paralogue FAR-RED IMPAIRED RESPONSE1 (FAR1) are essential for light induction of CCA1 expression by direct binding to the promoter of CCA1 and activating its expression, whereas PHYTOCHROME INTERACTING FACTOR 5 (PIF5) directly binds to the promoter of CCA1 to repress its expression. Further, PIF5 and TIMING OF CAB EXPRESSION1 (TOC1) physically interact with FHY3 and FAR1 to repress the transcriptional activation activity of FHY3 and FAR1 on CCA1 expression. Our combined results demonstrate that the photosensory signaling pathway integrates with the circadian oscillators to orchestrate clock gene expression, which may form the molecular basis of light regulation of clock system in response to daily changes in the light environment, thus increasing plant fitness.

该生物钟提供了一种计时机制，用于使各种生物活动与周围环境同步。编码与Myb相关的转录因子的拟南芥CIRCADIAN CLOCK ASSOCIATED1（CCA1）是拟南芥生物钟的核心振荡器的关键组成部分，其早晨表达高峰。调节CCA1的光诱导和节律性表达的分子机制仍然不清楚。在这项研究中，我们显示了两个植物色素信号转导子，FAR-RED加长HYPOCOTYL3（FHY3）及其旁系同源物FAR-RED受损反应1（FAR1）通过直接结合CCA1的启动子并激活CCA1的启动子对于光诱导CCA1表达至关重要表达，而植物染色体相互作用因子5（PIF5）直接与CCA1的启动子结合以抑制其表达。进一步，PIF5和CAB EXPRESSION1（TOC1）的时序与FHY3和FAR1物理相互作用，以抑制FHY3和FAR1对CCA1表达的转录激活活性。我们的综合结果表明，光传感信号通路与生物钟振荡器整合在一起，协调时钟基因的表达，这可能形成时钟系统的光调节的分子基础，以响应光环境的日常变化，从而提高植物的适应性。