寄主植物与病原菌免疫反应的分子遗传基础

近年来,大量的植物抗病基因和病原菌无毒基因被克隆,抗病基因和无毒基因的结构、功能及其互作关系的研究也取得重大进展。在植物中,由病原菌模式分子(pathogen-associated molecular patterns, PAMPs)引发的免疫反应(PAMP-triggered immunity, PTI)和由效应因子引发的免疫反应(effector-triggered immunity, ETI)是植物在长期进化过程中形成的两类抵抗病原物的机制。PTI反应主要通过细胞表面受体(patternrecognition receptors, PRRs)识别并结合PAMPs从而激活下游免疫反应,而在ETI反应中,则通过植物R基因(resistance gene,R)与病原菌无毒基因(avirulence gene, Avr)产物间的直接或间接相互作用来完成免疫反应。本文对植物PTI反应和ETI反应分别进行了概述,重点探讨了植物R基因与病原菌Avr基因之间的互作遗传机理,并对目前植物抗性分子遗传机制研究和抗病育种中的问题进行了探讨和展望。     

植物与病原微生物互作分子基础的研究进展

植物在与病原微生物共同进化过程中形成了复杂的免疫防卫体系。植物的先天免疫系统可大致分为两个层面。第一个层面的免疫基于细胞表面的模式识别受体对病原物相关分子模式的识别,该免疫过程被称为病原物相关分子模式触发的免疫(PAMP-triggered immunity,PTI),能帮助植物抵抗大部分病原微生物;第二个层面的免疫起始于细胞内部,主要依靠抗病基因编码的蛋白产物直接或间接识别病原微生物分泌的效应子并且激发防卫反应,来抵抗那些能够利用效应子抑制第一层面免疫的病原微生物,这一过程被称为效应子触发的免疫(Effector-triggered immunity,ETI)。这两个层面的免疫都是基于植物对"自我"及"非我"的识别,依靠MAPK级联等信号网络,将识别结果传递到细胞核内,调控相应基因的表达,做出适当的免疫应答。本文着重阐述了植物与病原微生物互作过程中不同层面的免疫反应所发生主要事件的分子基础及研究进展。     

参与植物天然免疫的LRR型蛋白

植物含有多种富含亮氨酸重复序列(LRRs)结构的蛋白质,它们在植物天然免疫中发挥着重要作用。参与植物防御反应的LRR型蛋白家族包括:类受体蛋白激酶、抗病基因编码蛋白质、多聚半乳糖醛酸酶抑制蛋白和伸展蛋白家族。最近,人们发现植物免疫系统包含:病原相关分子模式(PAMP)激发的免疫性(PTI),即类受体蛋白激酶识别病原菌PAMPs,启动植物防卫反应;病原菌效应子激发的免疫性(ETI),即抗病基因编码蛋白质识别效应子,启动植物防卫反应。除此之外,细胞壁是植物细胞的天然保护屏障。多聚半乳糖醛酸酶抑制蛋白和伸展蛋白通过维护细胞壁,抵御病原菌入侵。我们综述了植物中LRRs蛋白的结构特征与不同种类的LRR蛋白介导免疫反应的分子机制,讨论了LRR型蛋白在植物免疫过程中的意义及存在的问题,指出搜寻配体和下游信号分子将是LRR型蛋白研究热点。     

植物与病原微生物互作分子基础的研究进展

植物在与病原微生物共同进化过程中形成了复杂的免疫防卫体系。植物的先天免疫系统可大致分为两个层面。第一个层面的免疫基于细胞表面的模式识别受体对病原物相关分子模式的识别, 该免疫过程被称为病原物相关分子模式触发的免疫(PAMP-triggered immunity, PTI), 能帮助植物抵抗大部分病原微生物; 第二个层面的免疫起始于细胞内部, 主要依靠抗病基因编码的蛋白产物直接或间接识别病原微生物分泌的效应子并且激发防卫反应, 来抵抗那些能够利用效应子抑制第一层面免疫的病原微生物, 这一过程被称为效应子触发的免疫(Effector-triggered immunity, ETI)。这两个层面的免疫都是基于植物对“自我”及“非我”的识别, 依靠MAPK级联等信号网络, 将识别结果传递到细胞核内, 调控相应基因的表达, 做出适当的免疫应答。本文着重阐述了植物与病原微生物互作过程中不同层面的免疫反应所发生主要事件的分子基础及研究进展。     

基于模式识别受体和人工设计诱饵蛋白扩展植物识别特异性

病原/微生物相关分子模式(PAMPs/MAMPs)被位于宿主细胞表面的模式识别受体(PRRs)识别并激活免疫反应.这种病原相关分子模式触发的免疫反应(PTI)能够帮助植物抵抗大部分致病微生物的侵入,因此利用基因工程技术在植物中表达PRRs,以增强植物对病原微生物的免疫识别是一种非常有潜力的植物抗病性改良的策略.植物病原微生物分泌的效应蛋白通常利用多种多样的生化机制直接靶向和抑制PTI信号通路的关键组分,从而抑制PTI.一些植物进化出与效应蛋白的靶标类似的诱饵蛋白,并诱导效应蛋白的错误靶向.这种识别的结果不抑制PTI免疫反应,反而诱导效应蛋白激活的免疫反应(ETI).这种机制提示了人工设计的诱饵蛋白在特定植物中产生新的识别特异性的可能性.本综述总结了PRRs对PAMPs的识别,以及诱饵蛋白对效应蛋白监控方面的研究进展.利用转基因异源表达EFR或PBS1诱饵蛋白在实验室条件下成功扩展了植物的识别特异性,体现了对PRRs和人工设计的诱饵蛋白在植物对病原识别特异性的扩展和抗病性改良方面的潜力,突显了分离和鉴定新的PRRs和诱饵蛋白的必要性.     

植物抗病基因(R)与病原物无毒基因(Avr)相互作用机制的研究进展

PTI和ETI是植物在长期进化过程中形成的两类抵抗病原物的机制。基因对基因假说的抗病方式属于ETI抗性机制的一种,该假说认为具有保守NB-LRR结构域的R蛋白识别病原物非保守的无毒蛋白效应子(Avr),激活防卫反应信号途径,导致过敏性坏死。植物抗病基因(R)与病原菌无毒基因(Avr)产物间的直接或间接相互作用而产生的基因对基因抗性是植物抗病性的重要形式,该文对植物抗病蛋白与无毒蛋白相互作用机制进行了综述。其中,间接相互作用模式是主要方式。     

两类免疫受体强强联手筑牢植物免疫防线

植物先天免疫系统在抵御病原菌入侵过程中发挥至关重要的作用, 主要包括两个层次, 即病原菌相关分子模式和效应因子分别触发的PTI和ETI免疫反应。PTI和ETI分别由植物细胞膜表面模式识别受体(PRRs)和胞内免疫受体(NLRs)激活, 具有特异的激活机制, 但是两者激活的下游免疫事件相互重叠。PTI和ETI是否为泾渭分明的两道防线, 以及ETI与PTI下游事件为何如此相似, 一直是植物免疫领域最受关注的问题之一。最近, 中国科学院分子植物科学卓越创新中心辛秀芳团队与合作者利用拟南芥(Arabidopsis thaliana)与丁香假单胞杆菌(Pseudomonas syringae)互作系统对PTI和ETI在机制上的联系进行了研究。他们发现PRRs和共受体参与ETI, 而活性氧的产生是联系PRRs和NLRs所介导的免疫早期信号事件。他们还发现NLRs信号能够迅速增强PTI关键因子的转录和蛋白水平, PTI的增强在ETI免疫反应中不可或缺。该研究从机制上解析了植物免疫领域中长期悬而未决的PTI与ETI相似性之谜, 是该领域的一项突破性进展, 为未来作物分子设计育种提供了新的启示。     

植物抗病蛋白研究进展

为了应对外界复杂的环境变化, 植物进化出一套复杂而精细的免疫应答调控机制。植物抗病蛋白能够特异地识别病原微生物分泌的效应蛋白, 触发免疫响应以对抗病原微生物的侵扰。该文综述了植物抗病蛋白的结构与功能及对病原菌的识别方式、在免疫响应过程中抗病蛋白的动态平衡机制及其介导的防御反应信号转导。开展植物抗病蛋白研究可为定向培育抗病作物奠定理论基础。     

《Molecular Plant》文章中文摘要 2010年第3卷第5期

（http：//mplant.oxfordjournals.org/content/vol3/issue5/index.dtl）1 Zhang J,Zhou JM（2010）.Plant immumty triggered by microbial molecular signatures.Mol Plant,3（5）：783～793题目：微生物分子特征激活的植物免疫（综述）摘要：病原体/微生物相关分子模式（pathogen/microbe-associated molecular patterns,PAMPs/MAMPs）被位于宿主细胞表面的模式识别受体（pattern-recognition receptors,PRRs）识别来激活植物免疫。病原相关分子模式诱导的免疫反应（PAMP-triggered immunity,PTI）是植物限制病原菌增殖的第一层防卫反应。PTI信号传导元件往往被多种Pseudomonas syringae毒性效应蛋白作为攻击靶点,     

微丝骨架调控植物免疫机制的研究进展

植物病害是威胁农业生产的重要因素之一,会造成严重的粮食安全问题以及经济损失.植物对病原微生物的抵抗依赖于自身的先天免疫系统(plant innate immunity),主要包括分子模式触发免疫(pattern-triggered immunity, PTI)和效应因子触发免疫(effector-triggered immunity, ETI)两个层次.研究表明,微丝骨架在植物免疫中扮演重要角色,其通过自身动态重排来应对病原微生物的侵染,破坏宿主微丝会显著降低植物的抗病能力.本文重点介绍了植菌互作过程中的微丝骨架动态、参与调控植物免疫的微丝结合蛋白、调控微丝骨架的上游免疫信号以及微丝骨架动态在植物免疫中的生物学功能等的相关研究,并对微丝调控植物免疫的未来研究方向提出了展望.     

Toward understanding of rice innate immunity against Magnaporthe oryzae

The blast fungus, Magnaporthe oryzae, causes serious disease on a wide variety of grasses including rice, wheat and barley. The recognition of pathogens is an amazing ability of plants including strategies for displacing virulence effectors through the adaption of both conserved and variable pathogen elicitors. The pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) were reported as two main innate immune responses in plants, where PTI gives basal resistance and ETI confers durable resistance. The PTI consists of extracellular surface receptors that are able to recognize PAMPs. PAMPs detect microbial features such as fungal chitin that complete a vital function during the organism’s life. In contrast, ETI is mediated by intracellular receptor molecules containing nucleotide-binding (NB) and leucine rich repeat (LRR) domains that specifically recognize effector proteins produced by the pathogen. To enhance crop resistance, understanding the host resistance mechanisms against pathogen infection strategies and having a deeper knowledge of innate immunity system are essential. This review summarizes the recent advances on the molecular mechanism of innate immunity systems of rice against M. oryzae. The discussion will be centered on the latest success reported in plant–pathogen interactions and integrated defense responses in rice.     

Role of pathogen's effectors in understanding host-pathogen interaction

Pathogens can pose challenges to plant growth and development at various stages of their life cycle. Two interconnected defense strategies prevent the growth of pathogens in plants, i.e., molecular patterns triggered immunity (PTI) and pathogenic effector-triggered immunity (ETI) that often provides resistance when PTI no longer functions as a result of pathogenic effectors. Plants may trigger an ETI defense response by directly or indirectly detecting pathogen effectors via their resistance proteins. A typical resistance protein is a nucleotide-binding receptor with leucine-rich sequences (NLRs) that undergo structural changes as they recognize their effectors and form associations with other NLRs. As a result of dimerization or oligomerization, downstream components activate “helper” NLRs, resulting in a response to ETI. It was thought that ETI is highly dependent on PTI. However, recent studies have found that ETI and PTI have symbiotic crosstalk, and both work together to create a robust system of plant defense. In this article, we have summarized the recent advances in understanding the plant's early immune response, its components, and how they cooperate in innate defense mechanisms. Moreover, we have provided the current perspective on engineering strategies for crop protection based on up-to-date knowledge.     

Plant immunity: Evolutionary insights from PBS1, Pto and RIN4

Two layers of plant immune systems are used by plants to defend against phytopathogens. The first layer is pathogen-associate molecular patterns (PAMPs)-triggered immunity (PTI), which is activated by plant cell-surface pattern recognition receptors (PRRs) upon perception of microbe general elicitors. The second layer is effector-triggered immunity (ETI), which is initiated by specific recognition of pathogen type III secreted effectors (T3SEs) with plant intracellular resistance (R) proteins. Current opinions agree that ETI was evolved from PTI, and the impetus for the evolution of plant immunity is pathogen T3SEs, which exhibit virulence functions through blocking PTI, but show avirulence functions for triggering ETI. A decoy model was put forward and explained that the avirulence targets of pathogen T3SEs were evolved as decoys to compete with the virulence targets for binding with pathogen T3SEs. However, little direct evidence for the evolutionary mode has been offered. Here we reviewed the recent progresses about Pto, PBS1 and RIN4 to present our viewpoints about the evolution of plant immunity.Key words: plant immunity, evolution, Pto, PBS1, RIN4     

Virus perception at the cell surface: revisiting the roles of receptor-like kinases as viral pattern recognition receptors

Activation of antiviral innate immune responses depends on the recognition of viral components or viral effectors by host receptors. This virus recognition system can activate two layers of host defence, pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI). While ETI has long been recognized as an efficient plant defence against viruses, the concept of antiviral PTI has only recently been integrated into virus–host interaction models, such as the RNA silencing-based defences that are triggered by viral dsRNA PAMPs produced during infection. Emerging evidence in the literature has included the classical PTI in the antiviral innate immune arsenal of plant cells. Therefore, our understanding of PAMPs has expanded to include not only classical PAMPS, such as bacterial flagellin or fungal chitin, but also virus-derived nucleic acids that may also activate PAMP recognition receptors like the well-documented phenomenon observed for mammalian viruses. In this review, we discuss the notion that plant viruses can activate classical PTI, leading to both unique antiviral responses and conserved antipathogen responses. We also present evidence that virus-derived nucleic acid PAMPs may elicit the NUCLEAR SHUTTLE PROTEIN-INTERACTING KINASE 1 (NIK1)-mediated antiviral signalling pathway that transduces an antiviral signal to suppress global host translation.     

Physical association of pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) immune receptors in Arabidopsis

Plants possess two distinct types of immune receptor. The first type, pattern recognition receptors (PRRs), recognizes microbe-associated molecular patterns (MAMPs) and initiates pattern-triggered immunity (PTI) on recognition. FLS2 is a PRR, which recognizes a part of bacterial flagellin. The second type, resistance (R) proteins, recognizes pathogen effectors and initiates effector-triggered immunity (ETI) on recognition. RPM1, RPS2 and RPS5 are R proteins. Here, we provide evidence that FLS2 is physically associated with all three R proteins. Our findings suggest that signalling interactions occur between PTI and ETI at very early stages and/or that FLS2 forms a PTI signalling complex, some components of which are guarded by R proteins.     

A plant effector‐triggered immunity signaling sector is inhibited by pattern‐triggered immunity

Since signaling machineries for two modes of plant‐induced immunity, pattern‐triggered immunity (PTI) and effector‐triggered immunity (ETI), extensively overlap, PTI and ETI signaling likely interact. In an Arabidopsis quadruple mutant, in which four major sectors of the signaling network, jasmonate, ethylene, PAD4, and salicylate, are disabled, the hypersensitive response (HR) typical of ETI is abolished when the Pseudomonas syringae effector AvrRpt2 is bacterially delivered but is intact when AvrRpt2 is directly expressed in planta. These observations led us to discovery of a network‐buffered signaling mechanism that mediates HR signaling and is strongly inhibited by PTI signaling. We named this mechanism the ETI‐Mediating and PTI‐Inhibited Sector (EMPIS). The signaling kinetics of EMPIS explain apparently different plant genetic requirements for ETI triggered by different effectors without postulating different signaling machineries. The properties of EMPIS suggest that information about efficacy of the early immune response is fed back to the immune signaling network, modulating its activity and limiting the fitness cost of unnecessary immune responses.     

The zig-zag-zig in oomycete–plant interactions

In addition to a range of preformed barriers, plants defend themselves against microbial invasion by detecting conserved, secreted molecules, called pathogen-associated molecular patterns (PAMPs). PAMP-triggered immunity (PTI) is the first inducible layer of plant defence that microbial pathogens must navigate by the delivery of effector proteins that act to suppress or otherwise manipulate key components of resistance. Effectors may themselves be targeted by a further layer of defence, effector-triggered immunity (ETI), as their presence inside or outside host cells may be detected by resistance proteins. This 'zig-zag-zig' of tightly co-evolving molecular interactions determines the outcome of attempted infection. In this article, we consider the complex molecular interplay between plants and plant pathogenic oomycetes, drawing on recent literature to illustrate what is known about oomycete PAMPs and elicitors of defence responses, the effectors they utilize to suppress PTI, and the phenomenal molecular 'battle' between effector and resistance ( R ) genes that dictates the establishment or evasion of ETI.     

Of PAMPs and effectors: the blurred PTI-ETI dichotomy

Typically, pathogen-associated molecular patterns (PAMPs) are considered to be conserved throughout classes of microbes and to contribute to general microbial fitness, whereas effectors are species, race, or strain specific and contribute to pathogen virulence. Both types of molecule can trigger plant immunity, designated PAMP-triggered and effector-triggered immunity (PTI and ETI, respectively). However, not all microbial defense activators conform to the common distinction between PAMPs and effectors. For example, some effectors display wide distribution, while some PAMPs are rather narrowly conserved or contribute to pathogen virulence. As effectors may elicit defense responses and PAMPs may be required for virulence, single components cannot exclusively be referred to by one of the two terms. Therefore, we put forward that the distinction between PAMPs and effectors, between PAMP receptors and resistance proteins, and, therefore, also between PTI and ETI, cannot strictly be maintained. Rather, as illustrated by examples provided here, there is a continuum between PTI and ETI. We argue that plant resistance is determined by immune receptors that recognize appropriate ligands to activate defense, the amplitude of which is likely determined by the level required for effective immunity.