Rice CYP734As function as multisubstrate and multifunctional enzymes in brassinosteroid catabolism

The concentration of bioactive phytohormones is controlled by the regulation of both, their biosynthesis and catabolism. In Arabidopsis, two P450s CYP734A1/BAS1 and CYP72C1/SOB7 act in the inactivation of brassinosteroids (BRs) by means of C-26 hydroxyation. In this paper, the authors characterized CYP734As orthologs from Oriza sativa (rice). They show that overexpression of rice CYP734As gave typical BR-deficient phenotypes and the transformants were deficient in both bioactive CS and its precursors. In difference with Arabidopsis CYP734As, the rice ortholog genes utilized a broad range of C-22 hydroxylated BRs intermediates as substrates, and in addition the oxidation of 22-hydroxyl-cholesterol. That

suggests that rice CYP734As can control endogenous bioactive BRs not only by direct inactivation of CS by C-26 oxidation (as in A. thaliana), but also by the suppression of CS biosynthesis through decreases in CS precursors. Thus, rice CYP734As possesses multifunctional and multisubstrate properties that enables the regulation of endogenous content of bioactive brassinosteroids.

The Plant Journal (2011) 67, 1-12