CYP714B1 and CYP714B2 encode gibberellin 13-oxidases that reduce gibberellin activity in rice

Bioactive gibberellins (GAs) control many aspects of growth and development in plants. GA 1 has been the most frequently found bioactive GA in various tissues of flowering plants, but the enzymes responsible for GA 1 biosynthesis have not been fully elucidated due to the enzymes catalyzing the 13-hydroxylation step not being identified. Because of the lack of mutants defective in this enzyme, biological significance of GA 13-hydroxylation has been unknown. Here, we report that two cytochrome P450 genes, CYP714B1 and CYP714B2 , encode GA 13-oxidase in rice. Transgenic Arabidopsis plants that overexpress CYP714B1 or CYP714B2 show semidwarfism. There was a trend that the levels of 13-OH GAs including GA 1 were increased in these transgenic plants. Functional analysis using yeast or insect cells shows that recombinant CYP714B1 and CYP714B2 proteins can convert GA 12 into GA 53 (13-OH GA 12 ) in vitro. Moreover, the levels of 13-OH GAs including GA 1 were decreased, whereas those of 13-H GAs including GA 4 (which is more active than GA 1 ) were increased, in the rice cyp714b1 cyp714b2 double mutant. These results indicate that CYP714B1 and CYP714B2 play a predominant role in GA 13-hydroxylation in rice. The double mutant plants appear phenotypically normal until heading, but show elongated uppermost internode at the heading stage. Moreover, CYP714B1 and CYP714B2 expression was up-regulated by exogenous application of bioactive GAs. Our results suggest that GA 13-oxidases play a role in fine-tuning plant growth by decreasing GA bioactivity in rice and that they also participate in GA homeostasis.