Cytochrome P450-Mediated Brassinosteroid Inactivation.

Brassinosteroids are a family of growth-promoting hormones critical for development in plants. Two cytochrome P450s, CYP734A1 (BAS1) and CYP72C1 (SOB7), have been identified as brassinosteroid-inactivating enzymes important for steroid-mediated signal transduction in Arabidopsis. Genetic analysis has demonstrated that the activity of these P450s modulates hypocotyl elongation and cotyledon expansion in seedlings exposed to light, as well as leaf size, floral induction, and fruit size in adult plants. Though these two enzymes act synergistically in the same pathway, genetic and biochemical analysis demonstrates that they do so with different biochemistries. Homology-modeling studies support the hypothesis that BAS1 is a C-26 hydroxylase whereas SOB7 is not. Brassinosteroid binding studies demonstrate that BAS1 binds the biologically active hormones castasterone and brassinolide as well as biosynthetic precursors. In contrast, SOB7 binds castasterone and biosynthetic precursors but not the end product brassinolide. These findings will be discussed in the context of brassinosteroid-inactivating P450s identified in rice.
This research has been supported by grants from the NSF (#0758411) and USDA (2005-35318-16214).