Abstract
Brassinosteroids (BRs) are inactivated by cytochrome P450 monooxygenases in plants. It is known that Arabidopsis CYP734A1 and tomato CYP734A7 inactivate BRs by C26-hydroxylation of castasterone and brassinolide. Rice has four CYP734A homologues (CYP734A2/A4/A5/A6). Transgenic rice plants overexpressing these CYP734As, except for CYP734A5, showed a severe dwarf phenotype, and their endogenous BR levels were decreased. In order to investigate BR inactivation mechanism in rice, we have prepared the recombinant CYP734As expressed using a baculovirus insect cells system, and performed the enzyme assays with various BR-biosynthetic intermediates as a substrate. Rice CYP734As showed wide substrate specificities for BRs and exhibited high activity for early intermediates. Furthermore, rice CYP734As was found to catalyze three-step oxidations to form the carboxylated-BRs. These results indicate that BR inactivation mechanism in rice is different from those in Arabidopsis and tomato.
