BR-ENHANCED EXPRESSION 1/2/3 fine-tune plant photomorphogenesis by regulating the phyA/B-SPA1 cascade.

BR-ENHANCED EXPRESSION1 (BEE1), BEE2, and BEE3, key transcriptional components of brassinosteroid signaling, have been shown to play critical roles in regulating light signaling. However, the precise molecular mechanism by which BEE1/2/3 repress photomorphogenesis remains elusive. In this study, we found that light triggers the degradation of BEE1/2/3 proteins through the 26S proteasome system. BEE1/2/3 interact with PHYTOCHROME A (phyA) and phyB, and this interaction promotes the degradation of BEE3. Mutations in BEE1/2/3 exhibit short hypocotyls and partially suppress the long hypocotyl phenotype of the phyA phyB double mutant. BEE1/2/3 attenuate the interaction between phyB and PHYTOCHROME-INTERACTING FACTOR 3 (PIF3), leading to PIF3 stabilization and subsequent repression of photomorphogenesis. In addition, BEE1/2/3 interact with SUPPRESSORs OF PHYTOCHROME A (SPAs) to disrupt the phyA/B-SPA1 association, thereby further attenuating the exaggerated light signaling. The COP1-SPAs complexes stabilize BEE1/3 under both dark and light conditions. Mutations in BEE1/2/3 enhance the constitutively photomorphogenic phenotype of spa1/2/4, whereas overexpression of BEE1/3 partially rescue the short hypocotyls of cop1 and spa1/2/4. Collectively, these findings demonstrate that BEE1/2/3 fine-tune the phyA/B-COP1/SPA1 signaling hub to negatively regulate photomorphogenesis in response to excessive light signaling. Our study provides insights into the pivotal roles of BEE1/2/3 as photomorphogenic repressors, facilitating adaptation to complex light conditions.