番茄E3泛素连接酶LeSL1的免疫调控作用机制研究

Along with the endless process of interplay between plants and microbial pathogens, plants have evolved elaborate defense strategies to prevent diseases caused by pathogens they encounter. Recognition and resistance to pathogen often results in hypersensitive response-based programmed cell death, which limits pathogen establishment and spread by killing both pathogen and infected host cell. Given the important role of this defense reaction in plant immunity, it has potential as a target for pathogen attack and plant regulation. Considerable breakthroughs have recently been made on triggerring of plant immunity. However, the mechanisms underlying the down regulation of the defense, especially hypersensitive response-based programmed cell death, still remain unknown. In the present work, tomato LeSL1, which possesses E3 ubiquitin ligase activity, is found to strongly repress diverse R proteins mediated hypersensitive reaction. Meanwhile, LeSL1 interacts with AvrPtoB, an effector protein of pathogen P. syringae pv tomoto. Whether LeSL1 is hijacked by pathogen or works as a plant trap after pathogen recognition, and the regulated target proteins by LeSL1 need thorough investigations. In this study, multidisciplinary techniques are going to be employed to unravel the immune regulation function of tomato E3 ligase LeSL1 to further illustrate the molecular mechanisms on plant defense-growth transition and plant-pathogen interaction.

植物在与微生物无止境的相互较量中进化形成了复杂精细的防御策略避免病害的发生。对病原微生物的识别防御经常会导致植物细胞程序死亡超敏反应，通过同时杀死病原菌和感染宿主细胞限制病原的定植扩散。由于这种防御反应在植物免疫中的重要作用，它可能成为病原攻击和植物调控的靶标。近年来在植物免疫启动过程已取得较多进展，但对于如何关闭免疫应答，尤其是细胞死亡超敏反应尚不清楚。本项目中，番茄LeSL1具有E3泛素连接酶活性，对多个R蛋白介导免疫反应均有很强抑制作用。同时，LeSL1还与病原菌P. syringae pv tomato效应蛋白AvrPtoB直接互作。LeSL1是否被病原菌“劫持”利用，或在病原菌存在时被植物作为“陷阱”，以及LeSL1的调控靶标，均有待深入研究。因此，本工作将解析番茄E3连接酶LeSL1的免疫调控功能，进一步阐释植物防御-生长状态转换、病原微生物-植物相互作用分子机制。

番茄是世界上最重要的蔬菜作物之一，也是研究果实品质以及植物-病原微生物相互作用的模式植物。植物的抗病防御策略之一就是形成细胞死亡超敏反应以限制病原侵染。本项目检测了番茄LeSL1及家族成员依赖于RING结构域的E3泛素连接酶活性，并证明它们在抗病免疫超敏反应中发挥作用。进一步利用酵母双杂交对LeSL1家族成员的互作靶标基因进行筛选，并对重要候补靶标基因SlBSD1进行功能分析。SlBSD1是具有BSD结构域的转录因子。BSD (BTF2-like transcription factors, synapse-associated proteins, and DOS2-like proteins)是存在于多种生物中的保守结构域，但其功能还未充分研究。SlBSD1下调植株显示营养生长迟缓，叶片早衰，而SlBSD1过表达植株表型则相反。这种对叶片衰老的负调控作用也与转录组分析数据相符。功能缺失及过表达遗传表型分析结果表明SlBSD1是番茄营养生长与叶片衰老的新的调控因子。此外，LeSL1与病原菌效应蛋白相互作用。AvrPto/AvrPtoB-Pto途径转录因子Pti4/5/6也被泛素化途径所降解。Pti过表达番茄株系中病程相关基因表达升高，对病原菌Pst DC3000抗性增强。同时，Pti4/5/6在果实成熟中发挥作用，转基因果实中防御相关酶活增加，腐败过程放缓，而对其它生长发育及果实农艺特性没有明显不利影响，具有作物遗传改良应用潜力。总之，本项目所揭示的番茄E3连接酶LeSL1的免疫调控功能，有助于我们对植物防御-生长状态转换及植物-病原微生物相互作用分子机制的认识。