Antagonistic regulation of PIF4 and HY5 activity by BBX28 regulates thermomorphogenesis.

ELONGATED HYPOCOTYL 5 (HY5) and PHYTOCHROME INTERACTING FACTOR 4 (PIF4) compete for binding to common promoter elements of growth-related genes. While HY5 suppresses these genes to inhibit elongation, under high ambient temperature, PIF4 promotes their expression. The molecular regulation of this differential binding is critical for adaptive growth. Under elevated temperature (28°C), BBX28 expression and accumulation are enhanced, and bbx28 seedlings exhibit hyposensitivity to temperature-mediated hypocotyl elongation. Our genetic analyses indicate that BBX28 and PIF4 act in the same pathway and form a positive feed-forward loop. PIF4 promotes BBX28 expression under elevated temperature. BBX28 in turn interacts with PIF4 and promotes PIF4-mediated activation of auxin signaling genes by binding to their promoters. Under elevated temperature, BBX28 interacts with HY5 and inhibits its binding to the same sites on the promoters of IAA19 and SAUR19. BBX28 therefore regulates the HY5-PIF4-mediated antagonistic regulation of growth-related genes as per environmental conditions. This potential of BBX28 to fine-tune adaptive morphological plasticity may contribute to enhancing climate resilience.