Abstract
Brz (brassinazole) was synthesized as the first specific inhibitor of brassinosteroid biosynthesis. Target of Brz220 is the cytochrome P450 enzyme encoded by DWF4. In order to analyze in detail the mechanisms of brassinosteroid signal transduction, we screened for mutants that showed longer hypocotyls than wild type when grown with Brz220 in the dark, and designated bil mutants (Brz-insensitive-long hypocotyls). We identified a semidominant mutant, bil5, from fast neutron-treated lines. Hypocotyl elongation of these plants on Brz medium was less than that of bill-D, but still at least twice that of the wild type. Interestingly, adult bil5 plants showed pale green, thin stems, thin leaves and shortened stem length. In the light, plants treated with Brz display dwarfism with the dark-green leaves. Brassinosteroid deficiency in the dark with Brz caused not only the deetiolation but also the acceleration to chloroplast from etioplast after the short-term-light treatment to cotyledon that germinated in the dark with Brz. The pale-green leaves of bil5 were in contrast to the dark-green leaves of brassinosteroid-deficient mutants, and chloroplast gene expression was lower in bil5 than in the wild type. BIL5 may be a key protein of brassinosteroid regulation in not only a cell development but also chloroplast differentiation.
