Promotion of shade avoidance by BBX5 involves activation of PIF4 along with auxin biosynthetic and signaling genes

Neighbor proximity triggers changes in light quality that regulate various developmental and physiological processes in plants. phytochrome B (phyB)-PHYTOCHROMEINTERACTING FACTOR 4 (PIF4) module serves as a central regulatory hub enabling plants to accurately perceive and respond to shade cues. Here, we identify B-box PROTEIN 5 (BBX5) as a positive regulator of shade avoidance. phyB interacts with BBX5 and promotes its protein stability. Conversely, the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENICLY 1 (COP1) associates with and destabilizes BBX5 via the 26S proteasome system in shade. BBX5 binds to the PIF4 promoter to upregulate its expression during the early phase of shade exposure, and directly associates with the promoters of auxin biosynthetic and signaling genes YUCCA8 ( YUC8 ) and INDOLE-3-ACETIC ACID INDUCIBLE 19 ( IAA19 ) to activate their expression in shade. Our study reveals that BBX5 acts as a transcriptional activator of PIF4 , YUC8 and IAA19 to promote plant growth and development in response to shade signals.