Rice CYP734A cytochrome P450s inactivate brassinosteroids in Arabidopsis

Endogenous brassinosteroid concentrations are an important target for optimizing the growth of crop plants because these hormones influence yield and stress tolerance. The CYP734A subfamily of cytochrome P450 enzymes has been shown to inactivate brassinosteroid hormones in Arabidopsis and tomato. Rice has three genes for CYP734A enzymes whose expression appears to be up-regulated by exogenous brassinolide. The amino acids predicted to be in the active site of the rice enzymes vary when compared with the Arabidopsis protein sequence, suggesting that there could be differences in their ability to inactivate the hormone. We have cloned three CYP734A rice genes and expressed them in Arabidopsis to assess their efficacy as brassinosteroid-inactivating enzymes. We found that incorrect transcript splicing can complicate the expression of monocot genomic clones in a eudicot. However, the Arabidopsis system allowed us to characterize an atypical splice variant in one of the rice genes. cDNA clones produced high levels of expression and conferred the brassinosteroid inactivation phenotype. This study shows that Arabidopsis is a useful heterologous system for testing plant genes predicted to act in biochemical pathways that are conserved between monocots and eudicots.