NLR家族蛋白网络在水稻免疫系统中的功能研究

Rice is one of the world's most important food crops, rice research has a major impact on human beings, and improvement of disease resistance in rice is a critical research issue. The NOD-like receptor (NLR) gene family is the most variable and one of the largest gene families in plants. NLR family proteins [also called Resistance (R) proteins] are known to act as immune receptors in both plants and animals, and also to be associated with hybrid necrosis, development, and drought tolerance in plants. However, while NLR family R proteins are clearly vital in inducing an immune response by recognizing a variety of pathogens, their molecular mechanism of action remains elusive.In previous studies, we identified the small GTPase OsRac1 as a master regulator of rice immunity that induces both the production of reactive oxygen species (ROS) and hypersensitive cell death. We also discovered that OsRac1 acts as a switch protein, working downstream of an NLR family R protein, Pit-1, which recognizes the rice blast fungus, the most serious disease agent for reducing rice yield. Although NLR family R proteins were originally thought to work alone, recent analyses have identified various NLR family R proteins whose function is dependent on the formation of a pair with a different NLR family R protein. We subsequently revealed how one such pair of NLR family proteins, RGA4 and RGA5, perceive the ligand protein AVR-Pia and thereby trigger disease resistance to rice blast fungus.A recent series of immunoprecipitation experiments demonstrated that Pit-1 forms complexes with four NLR family proteins other than Pit-2, and that RGA4 associates with another NLR family protein as well as with RGA5. These results suggest that a single NLR family R protein can form pairs with many other NLR family R proteins, and it is probable that pairs of NLR family R proteins participate in NLR family protein networks. Until now, there has been no documented case of a single NLR family R protein pairing with multiple other NLR family proteins to form such an 'NLR family protein network’; however, we believe that NLR family protein networks underlie the functional diversity of NLR family proteins. In this study, we will conduct experiments using the rice immunity system to decipher NLR family protein networks. Since NLR family proteins are highly conserved as innate immune receptors from plants to animals, these new data will represent a potentially major contribution to our understanding of plant as well as animal immunity. Moreover, we can apply our new knowledge for the improvement of disease resistance to pathogens, growth and drought tolerance, not only in rice but also in various food crops such as tomato, wheat and potato.

水稻是重要的粮食作物之一，改善水稻抗病性是一个非常重要的研究课题。NLR家族蛋白作为已知的免疫受体，与杂交坏死、发育、干旱胁迫密切相关。NLR家族中的R蛋白在免疫应答过程中发挥重要的作用，但其中的分子机制还不清楚。我们之前的研究发现G蛋白OsRac1是水稻天然免疫的关键调控因子，作为一个开关蛋白作用于识别稻瘟病菌的R蛋白Pit-1的下游。我们通过免疫沉降实验还发现Pit-1能够与NLR家族的其他R蛋白配对组成复合体来调节植物对病菌的免疫反应。单个NLR蛋白与其它NLR蛋白配对组合进而构成一个复杂的“NLR家族蛋白网络”，目前尚未有与此相关的报道。我们认为“NLR家族蛋白网络”是NLR家族蛋白功能多样性的基础。此项目旨在揭示NLR家族蛋白网络在水稻免疫系统中的功能，这些结果将为改善各种粮食作物的抗病性研究提供更多理论依据。

核苷酸结合域和富含亮氨酸的重复序列（NLR）的R蛋白家族是多种物种中重要的细胞内免疫受体。与其他物种不同的是植物体内具有大量的NLR蛋白，不同物种中的NLR数目范围约为50至1000个。最初认为NLR蛋白可以单独起作用，但最近的报道揭示了多种NLR蛋白其功能依赖于一个或多个NLR蛋白，它们可以成对或形成网络以启动效应子触发的免疫反应。成对的NLR蛋白在免疫诱导中一般表现出拮抗关系。通过基因复制产生的NLR基因的进化导致增加NLR蛋白的复杂性。然而，尽管自从鉴定出第一个NLR型R基因以来已经过去了25年，但阐明复制的NLR基因如何在进化过程中产生这种复杂性以协调植物的免疫力仍不清楚。 .通过与他人合作，我们解析了一对NLR蛋白RGA4和RGA5抵抗水稻稻瘟病菌抗病性。同时发现，小激蛋白OsRac1及其激活因子OsSPK1的直接下游靶标是NLR型R蛋白Pit1，同时诱导了对稻瘟病的抗病性。这些是理解NLR蛋白激活机制的重要里程碑式成果。在本研究中，我们发现一对R基因Pit1及其旁系同源Pit2通过祖先Pit基因串联复制，并鉴定了Pit1和Pit2之间的决定命运的残基。Pit1和Pit2具有拮抗作用：Pit1充当免疫执行者，Pit2抑制Pit1诱导的免疫。Pit2中的P300和F415的两个突变导致两个蛋白的功能差异，并消除了质膜的定位并赋予了新的功能，使Pit2可以捕获细胞质中的Pit1。本研究首次鉴定一对NLR蛋白的决定命运的残基并解释这些残基如何使NLR蛋白具有拮抗功能的研究。我们相信该研究加深了我们对NLR基因如何进化和协调植物免疫力的理解。

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