Abstract
The recessive 'tall rice' mutant, elongated uppermost internode (eui), is morphologically normal until its final internode elongates drastically at the heading stage. The stage-specific developmental effect of the eui mutation has been utilized in the breeding of hybrid rice to improve the performance of heading in male sterile cultivars. To determine the molecular mechanisms underlying the tall eui phenotype, we measured endogenous gibberellin (GA) levels in the uppermost internodes of both wild-type and eui plants. GC-MS analysis showed that eui plants contained a much higher level of GA_1 than did wild-type plants. More strikingly, GA_4, the bioactive form in the non-13-hydroxylation pathway, was undetectable in wild-type plants, but was present at an extremely high concentration in the eui mutant. These results indicated that the eui phenotype was caused by elevated levels of bioactive GAs. Map-based cloning revealed that the EUI gene encodes a previously uncharacterized cytochrome P450 monooxygenase. Using heterologous expression in yeast, we found that EUI catalyzed 16α,17-epoxidation of non-13-hydroxylated GAs. Consistent with the tall phenotypes of the eui mutants, 16α,17-epoxidation reduced the biological activity of GA_4 in rice, demonstrating that EUI functions as a GA deactivating enzyme. These results indicate the occurrence of an unrecognized flux of GA synthesis, in which metabolites are deactivated by EUI, during the growth of wild-type internodes.
