Abstract
Brassinosteroids (BRs) are plant steroid hormones that influence of important developmental processes. In BR biosynthesis pathway, several cytochrome P450 monooxygenases (P450) catalyze essential oxidative reactions, C-22 and C-23 hydroxylation of the BR side-chain and C-6 oxidation of 6-deoxointermediates. These P450 genes have been identified from Arabidopsis, tomato and rice. Phylogenitic analysis of plant P450s revealed that these P450s are closely related and clustered. In this cluster, there are several P450 genes whose functions are still unknown. Among them, we isolated a tomato cDNA encoding a novel P450 homologous to CYP90s. To investigate the biochemical properties of the novel P450, the recombinant P450 protein was expressed using a baculovirus expression system in insect cells. The enzyme activity was measured in an assay mixture reconstituted with the recombinant P450 protein, NADPH-P450 reductase and NADPH generating system. We have identified that the novel P450 catalyzes C-22 hydroxylation of BRs.
