Abstract
Brassinosteroids (BRs) are inactivated by cytochrome P450 monooxygenases in plants. It is known that Arabidopsis CYP734A1(BAS1) and tomato CYP734A7 inactivate BRs by C26 hydroxylation of castasterone and brassinolide. Rice (Oriza sativa) has four CYP734A homologues (CYP734A2, A4, A5, and 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 BR intermediates located at the upstream of BR biosynthetic pathway. These results suggest that BR inactivation mechanism in rice is different from that in Arabidopsis and tomato.
