Abstract
We investigated CYP72B1,an Arabidopsis cytochrome P450,to determine the biochemical and physiological functions of this enzyme. Using a yeast functional assay, we have demonstrated that CYP72B1 is a steroid C-26 hydroxylase that converts brassinolide (BL) to 26-hydroxybrassinolide (26-OHBL) and castasterone (CS) to 26-hydroxycastasterone (26-OHCS). We tested the ability of an Arabidopsis CYP72B1-null mutant, the wild type, and a CYP72B1 over-expressor to metabolize BL or CS. Reduced levels of 26-OHBL and 26-OHCS were detected in the null mutant, and increased levels were detected in the over-expressor, demonstrating that 26-hydroxylation of BRs is an endogenous biochemical function of CYP72B1. Bioassays with BL and 26-OHBL have provided evidence that 26-hydroxylation is an inactivation step. We also showed that CYP72B1-mediated brassinosteroid inactivation provides positive modulation of photomorphogenesis.
