Light Activates Brassinosteroid Biosynthesis to Promote Hook Opening and Petiole Development in Arabidopsis thaliana.

Although brassinosteroids (BRs) have been proposed to be negative regulators of photomorphogenesis, their physiological role therein has remained elusive. We studied light-induced photomorphogenic development in the presence of the BR-biosynthesis inhibitor, brassinazole (Brz). Hook opening was inhibited in the presence of Brz; this inhibition was reversed in the presence of brassinolide. Hook opening was accompanied by cell expansion on the inner (concave) side of the hook. This cell expansion was inhibited in the presence of Brz, but was restored upon addition of brassinolide. We then evaluated light-induced organ-specific expression of three BR biosynthesis genes, DWF4, BR6ox1, BR6ox2, and a BR-responsive gene, SAUR-AC1, during photomorphogenesis of Arabidopsis. Expression of these genes was induced, particularly in the hook region, in response to illumination. The induction peaked after 3 h light exposure and preceded hook opening. Phytochrome-deficient mutants, hy1, hy2, and phyAphyB, and a light-signaling mutant, hy5, were defective in light-induced expression of BR6ox1, BR6ox2 and SAUR-AC1. Light induced both expression of BR6ox genes and petiole development. Petiole development was inhibited in the presence of Brz. Our results largely contradict the early view that BRs are negative regulators of photomorphogenesis. Our data collectively suggest that light activates expression of BR biosynthesis genes in the hook region via a phytochrome-signaling pathway and HY5, and that BR biosynthesis is essential for hook opening and petiole development during photomorphogenesis.