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。     

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。     

Dual Regulation of Gene Expression Mediated by Extended MAPK Activation and Salicylic Acid Contributes to Robust Innate Immunity in Arabidopsis thaliana

Network robustness is a crucial property of the plant immune signaling network because pathogens are under a strong selection pressure to perturb plant network components to dampen plant immune responses. Nevertheless, modulation of network robustness is an area of network biology that has rarely been explored. While two modes of plant immunity, Effector-Triggered Immunity (ETI) and Pattern-Triggered Immunity (PTI), extensively share signaling machinery, the network output is much more robust against perturbations during ETI than PTI, suggesting modulation of network robustness. Here, we report a molecular mechanism underlying the modulation of the network robustness in Arabidopsis thaliana. The salicylic acid (SA) signaling sector regulates a major portion of the plant immune response and is important in immunity against biotrophic and hemibiotrophic pathogens. In Arabidopsis, SA signaling was required for the proper regulation of the vast majority of SA-responsive genes during PTI. However, during ETI, regulation of most SA-responsive genes, including the canonical SA marker gene PR1, could be controlled by SA-independent mechanisms as well as by SA. The activation of the two immune-related MAPKs, MPK3 and MPK6, persisted for several hours during ETI but less than one hour during PTI. Sustained MAPK activation was sufficient to confer SA-independent regulation of most SA-responsive genes. Furthermore, the MPK3 and SA signaling sectors were compensatory to each other for inhibition of bacterial growth as well as for PR1 expression during ETI. These results indicate that the duration of the MAPK activation is a critical determinant for modulation of robustness of the immune signaling network. Our findings with the plant immune signaling network imply that the robustness level of a biological network can be modulated by the activities of network components.     

水稻免疫机制研究进展

植物先天免疫主要由两部分组成：一类是通过细胞膜上的病原菌分子模式识别受体识别病原微生物表面存在的分子特征激发的免疫反应（PTI）;另一类是专化性的抗病R蛋白识别病原微生物的效应蛋白,从而激发下游的病原菌小种特异性的防卫反应过程（ETI）．随着水稻抗病信号途径中越来越多的抗病基因以及关键的调控基因被克隆和功能鉴定,同时多种水稻病原菌效应蛋白的发现,水稻抗病机理的研究也越来越深入．本文阐述了水稻的PTI,ETI及其下游参与免疫信号转导的关键性组分,从而形成一个初步的水稻免疫调控网络．     

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.     

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.     

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.     

Network Properties of Robust Immunity in Plants

Two modes of plant immunity against biotrophic pathogens, Effector Triggered Immunity (ETI) and Pattern-Triggered Immunity (PTI), are triggered by recognition of pathogen effectors and Microbe-Associated Molecular Patterns (MAMPs), respectively. Although the jasmonic acid (JA)/ethylene (ET) and salicylic acid (SA) signaling sectors are generally antagonistic and important for immunity against necrotrophic and biotrophic pathogens, respectively, their precise roles and interactions in ETI and PTI have not been clear. We constructed an Arabidopsis dde2/ein2/pad4/sid2-quadruple mutant. DDE2, EIN2, and SID2 are essential components of the JA, ET, and SA sectors, respectively. The pad4 mutation affects the SA sector and a poorly characterized sector. Although the ETI triggered by the bacterial effector AvrRpt2 (AvrRpt2-ETI) and the PTI triggered by the bacterial MAMP flg22 (flg22-PTI) were largely intact in plants with mutations in any one of these genes, they were mostly abolished in the quadruple mutant. For the purposes of this study, AvrRpt2-ETI and flg22-PTI were measured as relative growth of Pseudomonas syringae bacteria within leaves. Immunity to the necrotrophic fungal pathogen Alternaria brassicicola was also severely compromised in the quadruple mutant. Quantitative measurements of the immunity levels in all combinatorial mutants and wild type allowed us to estimate the effects of the wild-type genes and their interactions on the immunity by fitting a mixed general linear model. This signaling allocation analysis showed that, contrary to current ideas, each of the JA, ET, and SA signaling sectors can positively contribute to immunity against both biotrophic and necrotrophic pathogens. The analysis also revealed that while flg22-PTI and AvrRpt2-ETI use a highly overlapping signaling network, the way they use the common network is very different: synergistic relationships among the signaling sectors are evident in PTI, which may amplify the signal; compensatory relationships among the sectors dominate in ETI, explaining the robustness of ETI against genetic and pathogenic perturbations.     

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     

Dynamics of Defense Responses and Cell Fate Change during Arabidopsis-Pseudomonas syringae Interactions

Plant-pathogen interactions involve sophisticated action and counteraction strategies from both parties. Plants can recognize pathogen derived molecules, such as conserved pathogen associated molecular patterns (PAMPs) and effector proteins, and subsequently activate PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI), respectively. However, pathogens can evade such recognitions and suppress host immunity with effectors, causing effector-triggered susceptibility (ETS). The differences among PTI, ETS, and ETI have not been completely understood. Toward a better understanding of PTI, ETS, and ETI, we systematically examined various defense-related phenotypes of Arabidopsis infected with different Pseudomonas syringae pv. maculicola ES4326 strains, using the virulence strain DG3 to induce ETS, the avirulence strain DG34 that expresses avrRpm1 (recognized by the resistance protein RPM1) to induce ETI, and HrcC^- that lacks the type three secretion system to activate PTI. We found that plants infected with different strains displayed dynamic differences in the accumulation of the defense signaling molecule salicylic acid, expression of the defense marker gene PR1, cell death formation, and accumulation/localization of the reactive oxygen species, H₂O₂. The differences between PTI, ETS, and ETI are dependent on the doses of the strains used. These data support the quantitative nature of PTI, ETS, and ETI and they also reveal qualitative differences between PTI, ETS, and ETI. Interestingly, we observed the induction of large cells in the infected leaves, most obviously with HrcC^- at later infection stages. The enlarged cells have increased DNA content, suggesting a possible activation of endoreplication. Consistent with strong induction of abnormal cell growth by HrcC^-, we found that the PTI elicitor flg22 also activates abnormal cell growth, depending on a functional flg22-receptor FLS2. Thus, our study has revealed a comprehensive picture of dynamic changes of defense phenotypes and cell fate determination during Arabidopsis-P. syringae interactions, contributing to a better understanding of plant defense mechanisms.     

CPR5 positively regulates pattern-triggered immunity via a mediator protein

Plant cells possess a two-layered immune system consisting of pattern-triggered immunity(PTI)and effector-triggered immunity(ETI), mediated by cell surface pattern-recognition receptors and intracellular nucleotide-binding leucine-rich repeat receptors(NLRs), respectively. The CONSTITUTIVE EXPRESSION OF PR GENES 5(CPR5) nuclear pore complex protein negatively regulates ETI, including ETI-associated hypersensitive response. Here, we show that CPR5 is essential for the activation of various PTI re...     

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.     

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.     

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.     

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。     

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。     

Recent advances in plant immunity: recognition, signaling, response, and evolution

Innate immune system is employed by plants to defend against phytopathogenic microbes through specific perception of non-self molecules and subsequent initiation of resistance responses. Current researches elucidate that plants mostly rely on cell surface-located pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat proteins (NB-LRRs) to recognize pathogen-associated molecular patterns (PAMPs) and effector proteins from microbial pathogens, initiating PAMP- and effector-triggered immunity (PTI and ETI), respectively. Some pathogenic bacterial effector proteins are usually secreted into plant cells and play a virulence function by suppressing plant PTI, implying an evolutionary process of plant immunity from PTI to ETI. In the past several years, a great progress has been achieved to reveal fascinating molecular mechanisms underlying the pathogenic recognition, resistance signaling transduction, and plant immunity evolution. Here, we summarized the latest breakthroughs about these topics, and offered an integral understanding of plant molecular immunity.     

A novel module regulating ROS in NLR-mediated immunity

The regulatory mechanisms of apoplastic reactive oxygen species (ROS) production during pattern-triggered immunity (PTI) are well known. However, how ROS levels are regulated during effector-triggered immunity (ETI) remains largely unknown. Recently, Zhang et al. discovered that MAPK-Alfin-like 7 module enhances nucleotide-binding, leucine-rich repeat receptor (NLR)-mediated immunity by negatively regulating genes encoding ROS scavenging enzymes, deepening our understanding of ROS control during ETI in plants.     

Plant targets for Pseudomonas syringae type III effectors: virulence targets or guarded decoys?

The phytopathogenic bacterium Pseudomonas syringae can suppress both pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI) by the injection of type III effector (T3E) proteins into host cells. T3Es achieve immune suppression using a variety of strategies including interference with immune receptor signaling, blocking RNA pathways and vesicle trafficking, and altering organelle function. T3Es can be recognized indirectly by resistance proteins monitoring specific T3E targets resulting in ETI. It is presently unclear whether the monitored targets represent bona fide virulence targets or guarded decoys. Extensive overlap between PTI and ETI signaling suggests that T3Es may suppress both pathways through common targets and by possessing multiple activities.