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. GA1 has been the most frequently found bioactive GA in various tissues of flowering plants, but the enzymes responsible for GA1 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 CYP714B1orCYP714B2 showsemidwarfism. There was a trend that the levels of 13-OH GAs including GA1 were increased in these transgenic plants. Functional analysis using yeast or insect cells shows that recombinant CYP714B1 and CYP714B2 proteins can convert GA12 into GA53 (13-OH GA12)invitro. Moreover, the levelsof13-OH GAsincluding GA1 were decreased,whereas those of 13-H GAs including GA4 (which is more active than GA1) were increased, in the rice cyp714b1 cyp714b2 double mutant. These results indicate that CYP714B1 and CYP714B2 playa predominant roleinGA 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 finetuning plant growth by decreasing GA bioactivity in rice and that they also participate in GA homeostasis. biosynthesis | plant hormones.