Abstract
Brz (brassinazole) was synthesized as the first specific inhibitor of brassinosteroid biosynthesis by Asami et al. Target of Brz220 is the cytochrome P450 enzyme encoded by [I]DWF4[/I]. 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 [I]bil[/I] mutants ([I]Brz-insensitive-long hypocotyl[/I]). We identified a recessive mutant, [I]bil5[/I], from fast neutron-treated lines. Hypocotyl elongation of these plants on Brz medium was less than that of [I]bil1-D[/I], but still at least twice that of the wild type. Interestingly, adult [I]bil5[/I] plants showed pale green, thin stems, thin leaves and shortened stem length. The pale-green leaves of [I]bil5[/I] were in contrast to the dark-green leaves of brassinosteroid-deficient mutants, and chloroplast gene expression was lower in [I]bil5[/I] than in the wild type.
