Ghd7蛋白昼夜累积调控水稻抽穗期的分子机制研究

Rice is an important food crop, flowering time (heading date) is one of the most important factors associated with grain yield, determining the seasonal and regional adaptability of varieties in rice. Studying the molecular mechanism of regional adaptability of varieties to different latitude regions is important for artificial breeding. Ghd7 gene controls number of grains per panicle, plant height and heading date of rice varieties. Natural variation in Ghd7 enables rice to adapt to photoperiod conditions in different latitude regions, which expands the regional distribution of rice accessions. Although Ghd7 is an important breeding resource, the molecular mechanism of Ghd7 regulating heading date under various latitudes remains unknown. We previously demonstrated that HAF1, a C3HC4 RING domain-containing E3 ubiquitin ligase, is essential to ensure appropriate photoperiodic responses by mediating the ubiquitination and degradation of OsELF3 and Hd1，indicating their important roles of ubiquitination and degradation mediated by HAF1 in heading date control in rice. Our recent investigation demonstrated that Ghd7 is epistatic to OsELF3 in rice flowering pathway under LDs, and Ghd7 physically interacts with HAF1. Thus, we hypothesized that Ghd7 might be degradated mediated by HAF1-dependent ubiquitination. HAF1 could be participated in the circadian accumulation of Ghd7, which is involved in the heading date control under long-day conditions. The achievements of this project will not only deepen our understanding of the molecular mechanism of Ghd7 for rice heading date regulation, but also may provide theoretical basis and genetic resources for rice breeding.

水稻是重要的粮食作物，开花时间（抽穗期）决定了品种的季节和地区适应性，是影响水稻产量的重要因素之一。研究水稻适应不同纬度条件抽穗的分子机制对人工选育具有重要的指导意义。Ghd7控制着水稻的每穗粒数、株高和抽穗期，其丰富的等位变异使水稻植株能适应不同纬度地区的光周期条件，拓展了水稻的种植区域。虽然Ghd7基因是重要的育种资源，但其调控水稻适应不同纬度条件抽穗的分子机制还鲜有研究。本课题组前期解析了E3泛素连接酶HAF1泛素化降解OsELF3和Hd1调控水稻光周期反应的分子机制，说明泛素化降解机制在水稻抽穗期调控中发挥重要作用。鉴于申请人近期实验结果：Ghd7遗传上位于OsELF3且Ghd7与HAF1相互作用。申请人设想Ghd7可能受到HAF1介导的泛素化降解从而调控水稻长日照条件抽穗。本项目的研究成果不仅将加深人们对Ghd7调控水稻抽穗期分子机制的理解，而且可能为育种提供理论依据和遗传资源。

水稻的开花时间（抽穗期）决定了品种的地域适应性，是影响水稻产量的重要农艺性状。光周期是决定水稻开花时间重要的外源环境信号。水稻作为短日照模式植物，短于其临界日长的光周期条件诱导水稻植株开花，长于其临界日长的光周期条件则延迟水稻开花。水稻中的开花相关因子响应光周期条件从而整合开花时间。Ghd7是光周期调控网络中重要的因子，通过Ehd1途径负调控水稻开花。Ghd7所在复合物通过影响下游基因的表达水平参与水稻抽穗期调控，Ghd7自身的转录水平和蛋白稳定性方面有了一定的研究基础，但Ghd7蛋白的节律性积累模式仍不清楚，Ghd7等位基因在品种中功能强弱的分子机理仍需进一步研究。水稻中近年克隆的E3泛素连接酶HAF1是一个典型的环指类E3泛素连接酶，研究显示HAF1介导的泛素化降解机制在水稻抽穗期调控中起着重要作用。HAF1通过泛素化降解Hd1维持Hd1蛋白的节律性累积从而参与水稻短日照条件下的抽穗期调控。近期研究揭示了HAF1介导OsELF3泛素化降解调控水稻适应长日照条件抽穗的分子机制。虽然已证实HAF1介导OsELF3的泛素化降解调控水稻长日照条件下抽穗期的分子机制，但前期研究中发现在长日照条件下，haf1 oself3双突变体与oself3 抽穗期一致均表现为晚抽穗，OsELF3-OX表现为早抽穗，进一步创制OsELF3-OX/haf1双突变体植株也表现为早抽穗的表型，说明HAF1可能还介导了OsELF3下游某个开花抑制因子的泛素化降解，精细调控水稻长日照条件下的抽穗期。本研究通过遗传学和生化分析发现，HAF1通过OsELF3-Ghd7途径介导长日照条件下的水稻抽穗期调控。HAF1介导Ghd7蛋白的泛素化和降解。在光照条件下未知因子影响了HAF1与Ghd7的结合并抑制HAF1介导Ghd7蛋白泛素化和降解。黑暗条件下，一方面HAF1介导Ghd7蛋白被降解，另一方面OsELF3在转录水平抑制Ghd7基因表达。因此从转录水平和转录后水平精细调控Ghd7的昼夜节律性表达，促进水稻品种在长日照条件下适时抽穗。更进一步地，本研究阐明了Ghd7的不同单倍型的功能强弱受HAF1介导的泛素化和降解所调控，强功能的Ghd7与HAF1的结合能力弱，而弱功能的Ghd7与HAF1的结合能力强。从而导致强功能型品种中的Ghd7蛋白量高于弱功能型品种。