Stepwise deactivation of gibberellins during rice internode elongation.

Bioactive gibberellins (GAs) are a class of plant hormones that regulate various aspects of plant growth and development, and several key GA deactivation enzymes have been identified. In rice, non-13-hydroxylated GAs have been shown to be deactivated via 16α,17-epoxidation by a cytochrome P450 monooxygenase, ELONGATED UPPERMOST INTERNODE (EUI/CYP714D1). Although 16,17-dihydro-16α,17-epoxyGA (16α,17-epoxyGA), the product of EUI from bioactive GA, has shown weak bioactivity on rice seedlings, how 16α,17-epoxyGAs are further deactivated remains elusive. Here, we identify the gene, which regulates internode elongation in rice, using a map-based cloning strategy. encodes an epoxide hydrolase that hydrolyzes 16α,17-epoxyGAs to 16,17-dihydro-16α,17-dihydroxyGAs. The mutants are taller than wild-type plants but are shorter than the mutants. However, the levels of known bioactive GAs in the uppermost internodes are not significantly increased in the mutants. Instead, we show that the mutants accumulate 16α,17-epoxyGA to high levels. We also show that exogenously applied 16α,17-epoxyGA is significantly active in elongating the uppermost internode, although not as potent as GA. Furthermore, we demonstrate that 16α,17-epoxyGA can directly interact with the rice GA receptor, GIBBERELLIN INSENSITIVE DWARF1, in vitro. Taken together, the sequential action of EUI and EUI2 results in the stepwise deactivation of GAs during internode elongation in rice. Our data also suggest that the accumulation of a weakly active GA contributes to the mildly tall phenotype of the mutants.