Light and brassinosteroid signals are integrated via a dark-induced small G protein in etiolated seedling growth

Plant growth and development are regulated through coordinated interactions between light and phytohormones. Here, we demonstrate that a dark-induced small G protein, pea Pra2, regulates a variant cytochrome P450 that catalyz es C-2 hydroxylation in brassinosteroid biosynthesis. The cytochrome P450 i s dark-induced and predominantly expressed in the rapidly elongating zone o f etiolated pea epicotyls, where Pra2 is also most abundant. Transgenic pla nts with reduced Pra2 exhibit a dark-specific dwarfism, which is completely rescued by exogenous brassinolide. Overexpression of the cytochrome P450 r esults in enhanced hypocotyl growth even in the light, which phenocopies th e etiolated hypocotyls. We therefore propose that Pra2 and its orthologs ar e molecular mediators for the cross-talk between light and brassinosteroids in the etiolation process in plants.