Abstract
The recessive 'tall rice' mutant, elongated uppermost internode (eui), is morphologically normal until its final internode elongates drastically at the heading stage. To determine the molecular mechanisms underlying the tall eui phenotype, we measured endogenous gibberellin (GA) levels in the uppermost internodes of wild-type and eui plants. GC-MS analysis showed that eui internodes contained a much higher level of GA1 and GA4 than did wild-type internodes, indicating 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 GA4 in rice, demonstrating that EUI functions as a GA deactivating enzyme.
