Abstract

Significance Phytochrome B (phyB) is the predominant red light photoreceptor that transduces red light signals to downstream signaling. On red light exposure, photoactivated phyB interacts with a transcription factor termed PHYTOCHROME INTERACTING FACTOR 3 (PIF3), a repressor of red light signaling, triggering its rapid phosphorylation and subsequent degradation. Thus, phyB-PIF3 defines a critical regulatory hub for red light-mediated seedling development. In this study, we show that B-BOX CONTAINING PROTEIN 4 (BBX4) is a key component involved in the phyB-PIF3 regulatory module. phyB directly interacts with BBX4 and positively controls the abundance of BBX4 in red light. Accumulated BBX4 directly interacts with PIF3 to inhibit its transcriptional activation activity toward target genes, thereby promoting photomorphogenesis. Phytochrome B (phyB) absorbs red light signals and subsequently initiates a set of molecular events in plant cells to promote photomorphogenesis. Here we show that phyB directly interacts with B-BOX CONTAINING PROTEIN 4 (BBX4), a positive regulator of red light signaling, and positively controls its abundance in red light. BBX4 associates with PHYTOCHROME INTERACTING FACTOR 3 (PIF3) and represses PIF3 transcriptional activation activity and PIF3-controlled gene expression. The degradation of BBX4 in darkness is dependent on CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and the 26S proteasome system. Collectively, BBX4 acts as a key component of the phyB-PIF3–mediated signaling module and fine tunes the red light action. phyB promotes the accumulation of BBX4, which in turn serves to repress PIF3 action through direct physical interaction to promote photomorphogenic development in red light.