TCP21 negatively regulates photomorphogenesis by integrating phyB ‐ and HY5 ‐mediated signaling pathways in Arabidopsis

SUMMARY The phytochrome B (phyB)‐transcription factor signaling module plays a critical role in regulating photomorphogenesis. In this study, TCP21, a member of the Class I TCP transcription factor family, was identified as a direct interacting partner of phyB. TCP21 negatively regulates photomorphogenesis through downregulating HY5 expression and repressing the transcriptional activity of HY5. Moreover, phyB physically interacts with TCP21 and inhibits its transcriptional activity. Notably, phyB outcompetes HY5 for binding to TCP21, thereby attenuating the interaction between TCP21 and HY5 and alleviating TCP21‐mediated repression of HY5 DNA‐binding ability. Collectively, these findings establish TCP21 as a key component within the phyB‐HY5 signaling module, contributing to plant adaptation to dynamic light environments.