Plant Hormone Regulation On Panicle Enclosure In Cytoplasmic Male Sterile Rice

Cytoplasmic male sterile (CMS) lines are widely used in hybrid rice seed production. However, most of the CMS lines suffer from panicle enclosure. Panicle enclosure means that the panicle is partly or fully enclosed within the flag leaf sheath, which blocks normal pollination and greatly reduces seed production of hybrid rice seed.Up to now, the cause of panicle enclosure is still unclear. In this study, Zhenshan 97A (ZS97A), a widely used CMS line, and its maintainer line Zhenshan 97B (ZS97B) were used to investigate the cause of panicle through molecular and physiological methodsIn this study, we compared differences in panicle differentiation and internode elongation between ZS97A and ZS97B. Our result indicated that, before ZS97A pollen abortion, no difference in panicle differentiation and internode elongation between ZS97A and ZS97B was observed. But after the abortion, the development of ZS97A pollen was impaired and the elongation rate decreased, leading to panicle enclosure by shortening the uppermost internode (UI) length.Analysis result of endogenous hormone indicated that gibberellin A1 (GA₁) level in ZS97A UI was only one-third of that in ZS97B UI. At the early heading stage, exogenous GA₁ application could eliminate panicle enclosure in ZS97A by promoting UI elongation, while exogenous uniconazole (GA biosynthesis inbhibitor) application could cause panicle enclosure in ZS97B by shortening the UI length. These results suggested that GA₁ plays an important role in UI elongation, and that GA₁-deficiency is the caused of panicle enclosure in ZS97A.Active GA level in plant was regulated not only by biosynthesis but also by catabolism. In order to find out the caused of GA₁-deficiency in ZS97A UI, expression levels of some genes involving in GA metabolism pathway were observed. The result showed that, in intact plant, no difference of OsGA2ox1 (converts bioactive GA to inactive GA) transcript level was detected between ZS97A and ZS97B UI, but, OsGA3ox2 transcript level in ZS97A UI decreased obviously. This result suggested that GA₁-deficiency in ZS97A UI might be resulted from the impaired GA biosynthesis by down-regulating OsGA3ox2 transcript level.In order to find out the relationship between ZS97A pollen abortion and GA₁-deficeciency in UI, Indole-3-acetic acid (IAA) level in panicle and UI was analyzed. Our result indicated that IAA level in panicle and UI of ZS97A was much lower than that of ZS97B. Treatment of auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) could inhibited IAA transport from panicle to UI, which caused a strong decrease in IAA level in UI, leading to decreases in GA₁ level and in UI elongation rate. Decapitation (remove the panicle) could inhibit IAA transport from panicle to UI completely, which caused a significant decrease in IAA level in UI, leading to significant decreases in GA₁ level and in UI elongation rate. These results indicated that IAA in UI is mainly transported from panicle, and suggested that IAA-deficiency might be the cause of GA₁-deficiency in ZS97A UI.Moreover, we analyzed the relationship between IAA and GA₁ in regulation of UI elongation. Decapitation caused a decrease in IAA level in UI, which led to down-regulation of OsGA3ox2 and up-regulation of OsGA2ox1, resulting in a decrease of GA₁ level. In decapitated plant, exogenous IAA application could up-regulate OsGA3ox2 transcript level and down-regulated OsGA2oxl transcript level, leading to an increasing in GA₁ level.In this study, we also analyzed IAA oxidase activity. The result indicated that, IAA oxidase activity in ZS97A panicle was much higher than that in ZS97B panicle either at pollen development stage or at heading stage. This result suggested that higher IAA oxidase activity might be contributed to IAA-deficiency in ZS97A panicle.