EUI1, Encoding a Putative Cytochrome P450 Monooxygenase, Regulates Internode Elongation by Modulating Gibberellin Responses in Rice

Elongation of rice internodes is one of the most impor-tant agronomic traits, which determines the plant heightand underlies the grain yield. It has been shown that theelongation of internodes is under genetic control, and vari-ous factors are implicated in the process. Here, we report adetailed characterization of an elongated uppermostinternode1 (eui1) mutant, which has been used in hybridrice breeding. In the eui1-2 mutant, the cell lengths in theuppermost internodes are significantly longer than that ofwild type and thus give rise to the elongated uppermostinternode. It was found that the level of active gibberellinwas elevated in the mutant, whereas its growth in responseto gibberellin is similar to that of the wild type, suggestingthat the higher level accumulation of gibberellin in the eui1mutant causes the abnormal elongation of the uppermostinternode. Consistently, the expression levels of severalgenes which encode gibberellin biosynthesis enzymes werealtered. We cloned the EUI1 gene, which encodes a puta-tive cytochrome P450 monooxygenase, by map-based clon-ing and found that EUI1 was weakly expressed in mosttissues, but preferentially in young panicles. To confirm itsfunction, transgenic experiments with different constructsof EUI1 were conducted. Overexpression of EUI1 gave riseto the gibberellin-deficient-like phenotypes, which could bepartially reversed by supplementation with gibberellin.Furthermore, apart from the alteration of expression levelsof the gibberellin biosynthesis genes, accumulation of SLR1protein was found in the overexpressing transgenic plants,indicating that the expression level of EUI1 is implicated inboth gibberellin-mediated SLR1 destruction and a feed-back regulation in gibberellin biosynthesis. Therefore, weproposed that EUI1 plays a negative role in gibberellin-mediated regulation of cell elongation in the uppermostinternode of rice.Keywords: Cytochrome P450 monooxygenase — Elongateduppermost internode 1 — Gibberellin — Internode elongation— Rice.