Cytoplasmic male sterile (CMS) rice Zhenshan 97A (ZS97A) has been widely used in hybrid rice production in China. However, ZS97A suffers from serious panicle enclosure, which blocks normal pollination and greatly reduces seed production of hybrid rice. Little is known about the cause of panicle closure in ZS97A. In this study, it was found that the occurrence of cytoplasmic male sterility caused a deficiency of indole-3-acetic acid (IAA) in ZS97A panicles, and less IAA was provided to the uppermost internode (UI). Further, it was found that the decreased panicle-derived IAA caused a gibberellin A(1)(GA(1)) deficiency in the UI by the down-regulation of OsGA3ox2 transcript level. Reduced GA(1) level in the UI led to decreases of both cell number and cell elongation, resulting in a shortened UI. The shortened UI was unable to push the panicle out of the flag leaf sheath that remained normal, which resulted in panicle enclosure in ZS97A. These findings suggest that decreased panicle-derived IAA reduces the GA(1) level in the UI, causing panicle enclosure in CMS rice ZS97A.

Elongation of rice internodes is one of the most important agronomic traits, which determines the plant height and underlies the grain yield. It has been shown that the elongation of internodes is under genetic control and various factors are implicated in the process. Here, we report a detailed characterization of an elongated uppermost internode1 (eui1) mutant, which has been used in hybrid rice breeding. In the eui1-2 mutant, the cell lengths in the uppermost internodes are significantly longer than that of wild type and thus give rise to the elongated uppermost internode. It was found that the level of active gibberellic acids (GAs) was elevated in the mutant, whereas its growth in response to GA was similar to that of wild type, suggesting the higher level accumulation of GA in the eui1 causes the abnormal elongation of the uppermost internode. Consistently, the expression levels of several genes, which encode gibberellin biosynthesis enzymes, were altered. We cloned the EUI1 gene by map-based cloning, which encodes a putative cytochrome P450 monooxygenase, and found that EUI1 was weakly expressed in most tissues, but preferentially in young panicles. To confirm its function, transgenic experiments with different constructs of EUI1 were conducted. Overexpression of EUI1 gave rise to the GA-deficient like phenotypes, which could be partially reversed by the supplement of GA. Furthermore, apart from the alteration of expression levels of the GA biosynthesis genes, accumulation of SLR1 protein was found in the overexpression transgenic plants, indicating expression level of EUI1 is implicated in both GA-mediated SLR1 destruction and a feedback regulation in GA biosynthesis. Therefore, we proposed that EUI1 plays a negative role in GA-mediated regulation of cell elongation in the uppermost internode of rice.

Elongation of rice internodes is one of the most important agronomic traits, which determines the plant height and underlies the grain yield. It has been shown that the elongation of internodes is under genetic control and various factors are implicated in the process. Here, we report a detailed characterization of an elongated uppermost internode1 (eui1) mutant, which has been used in hybrid rice breeding. In the eui1-2 mutant, the cell lengths in the uppermost internodes are significantly longer than that of wild type and thus give rise to the elongated uppermost internode. It was found that the level of active gibberellic acids (GAs) was elevated in the mutant, whereas its growth in response to GA was similar to that of wild type, suggesting the higher level accumulation of GA in the eui1 causes the abnormal elongation of the uppermost internode. Consistently, the expression levels of several genes, which encode gibberellin biosynthesis enzymes, were altered. We cloned the EUI1 gene by map-based cloning, which encodes a putative cytochrome P450 monooxygenase, and found that EUI1 was weakly expressed in most tissues, but preferentially in young panicles. To confirm its function, transgenic experiments with different constructs of EUI1 were conducted. Overexpression of EUI1 gave rise to the GA-deficient like phenotypes, which could be partially reversed by the supplement of GA. Furthermore, apart from the alteration of expression levels of the GA biosynthesis genes, accumulation of SLR1 protein was found in the overexpression transgenic plants, indicating expression level of EUI1 is implicated in both GA-mediated SLR1 destruction and a feedback regulation in GA biosynthesis. Therefore, we proposed that EUI1 plays a negative role in GA-mediated regulation of cell elongation in the uppermost internode of rice.