A photomorphogenesis-promoting B-box protein, BReaks into the dark

The steroid hormone brassinosteroid promotes skotomorphogenesis in the dark. Light, on the other hand, induces photomorphogenic development, including inhibition of hypocotyl elongation, cotyledon opening, and greening. The molecular players that orchestrate the antagonistic interplay between brassinosteroid and light signaling pathways are limited. Here, we identify the B-box protein BBX13 as a new signal integrator of the light signaling and brassinosteroid pathway, modulating the transition from dark to light. Seedlings overexpressing BBX13 exhibit open cotyledons in the dark, while bbx13 is hyposensitive to BRZ-mediated cotyledon opening, suggesting that BBX13 inhibits brassinosteroid-mediated cotyledon closure. BBX13 interacts with BZR1 and antagonizes its cotyledon closure activity in the dark. BBX13 promotes the accumulation of protochlorophyllide in darkness. HY5 activates BBX13 by directly binding to its promoter to promote photomorphogenesis. In conclusion, BBX13 inhibits brassinosteroid-mediated skotomorphogenesis in the dark and promotes photomorphogenesis in the light. This counterplay might allow plants to optimize their morphology for efficient photosynthesis in response to changing conditions during de-etiolation.