植物NLR免疫受体的识别、免疫激活与信号调控

高等植物进化出大量膜表面和胞内免疫受体以感知各种病原信号,抵御病原物入侵。其中,细胞表面的模式识别受体感知模式分子后激活基础免疫反应,核苷酸结合和富亮氨酸重复蛋白(NLRs)则通过感知病原微生物分泌的效应蛋白激活特异免疫反应,导致超敏反应与细胞死亡。该文主要综述了NLRs对效应蛋白的识别、植物免疫激活及下游信号调控的最新研究进展。     

Quantitative proteomics reveals dynamic changes in the plasma membrane during Arabidopsis immune signaling

The plant plasma membrane is a crucial mediator of the interaction between plants and microbes. Understanding how the plasma membrane proteome responds to diverse immune signaling events will lead to a greater understanding of plant immunity and uncover novel targets for crop improvement. Here we report the results from a large scale quantitative proteomics study of plasma membrane-enriched fractions upon activation of the Arabidopsis thaliana immune receptor RPS2. More than 2300 proteins were identified in total, with 1353 proteins reproducibly identified across multiple replications. Label-free spectral counting was employed to quantify the relative protein abundance between different treatment samples. Over 20% of up-regulated proteins have known roles in plant immune responses. Significantly changing proteins include those involved in calcium and lipid signaling, membrane transport, primary and secondary metabolism, protein phosphorylation, redox homeostasis, and vesicle trafficking. A subset of differentially regulated proteins was independently validated during bacterial infection. This study presents the largest quantitative proteomics data set of plant immunity to date and provides a framework for understanding global plasma membrane proteome dynamics during plant immune responses.     

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.     

Plant immune system: Basal immunity

Plants have an efficient system of innate immunity that is based on the effective detection of potentially harmful microorganisms and rapid induction of defense responses. The first level of plant immunity is basal immunity, which is induced by the conserved molecular structures of microbes, such as bacterial flagellins or fungal chitin, or molecules that result from the interaction of plants with pathogens, for example oligosaccharides and peptides (“danger signals”). Plants recognize these inducers through receptors localized to the plasma membrane, represented mainly by receptor-like protein kinases or receptor-like proteins. Activation of the receptor by a ligand triggers a complex network of signaling events, which eventually cause an array of plant defense responses to prevent further spread of the pathogen.     

Perception of double-stranded RNA in plant antiviral immunity

RNA silencing and antiviral pattern-triggered immunity (PTI) both rely on recognition of double-stranded (ds)RNAs as defence-inducing signals. While dsRNA recognition by dicer-like proteins during antiviral RNA silencing is thoroughly investigated, the molecular mechanisms involved in dsRNA perception leading to antiviral PTI are just about to be untangled. Parallels to antimicrobial PTI thereby only partially facilitate our view on antiviral PTI. PTI against microbial pathogens involves plasma membrane bound receptors; however, dsRNAs produced during virus infection occur intracellularly. Hence, how dsRNA may be perceived during this immune response is still an open question. In this short review, we describe recent discoveries in PTI signalling upon sensing of microbial patterns and endogenous ‘danger’ molecules with emphasis on immune signalling-associated subcellular trafficking processes in plants. Based on these studies, we develop different scenarios how dsRNAs could be sensed during antiviral PTI.     

Enemy at the gates: traffic at the plant cell pathogen interface

The plant apoplast constitutes a space for early recognition of potentially harmful non-self. Basal pathogen recognition operates via dynamic sensing of conserved microbial patterns by pattern recognition receptors or of elicitor-active molecules released from plant cell walls during infection. Recognition elicits defence reactions depending on cellular export via SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex-mediated vesicle fusion or plasma membrane transporter activity. Lipid rafts appear also involved in focusing immunity-associated proteins to the site of pathogen contact. Simultaneously, pathogen effectors target recognition, apoplastic host proteins and transport for cell wall-associated defence. This microreview highlights most recent reports on the arms race for plant disease and immunity at the cell surface.     

Pattern recognition receptors acting in innate immune system of shrimp against pathogen infections

Invertebrates, including shrimp, have developed very complicated innate immune system against pathogens. Much work has been performed on the innate immunity of shrimp, including immune recognition, signal transduction, effector molecules and antiviral responses due to its great economic value. Pattern recognition is the first step of innate immunity. Pattern recognition receptors (PRRs) sense the presence of infection and activate immune responses. The studies on shrimp PRRs revealed the recognition mechanism of shrimp at a certain degree. To date, 11 types of pattern recognition receptors (PRRs) have been identified in shrimp, namely, β-1,3-glucanase-related proteins, β-1,3-glucan-binding proteins, C-type lectins, scavenger receptors, galectins, fibrinogen-related proteins, thioester-containing protein, Down syndrome cell adhesion molecule, serine protease homologs, trans-activation response RNA-binding protein and Toll like receptors. A number of PRRs have been functionally studied and have been found to have different binding specificities and immune functions. The present review aims to summarize the current knowledge on the PRRs of shrimp.     

New faces in plant innate immunity: heterotrimeric G proteins

Co-existence of species seems to inevitably result in origin of parasitism and hence development of molecular mechanisms of attack and defense. Certain similarities between plant and animal defense systems point to an ancient inheritance of the innate immunity. Heterotrimeric G proteins are structurally conserved signaling molecules connecting plasma membrane bound receptors to cytoplasmic effectors. They were found in most eukaryotic organisms. Their role in human pathophysiology and animal diseases was well established. In plants these proteins were also recently implicated in innate immunity. However, molecular mechanisms governed by G proteins and providing resistance against plant pathogens seem to be different from those in animal systems and largely remain elusive. In this review we attempted to sketch current ideas of plant defense system and to present a contemporary status of heterotrimeric G proteins in plant innate immunity.     

模式识别受体的胞内转运及其在植物免疫中的作用

植物利用细胞表面模式识别受体(PRRs)来感知病原相关分子模式(PAMPs), 进而触发自身的免疫反应(PTI)。在植物免疫过程中, PRRs在细胞内的正确定位对其生理功能的发挥至关重要。PRRs蛋白可以在内质网(ER)上合成, 并通过胞吐被分泌到质膜(PM)上。此外, PRRs蛋白也可以通过胞吞进行胞内循环或降解。细胞可以通过胞内转运降解PRRs蛋白以终止信号转导, 也可以通过形成胞内体进行信号传递。该文概述了PRRs蛋白及其配体的研究进展以及PRRs蛋白的胞内转运在植物免疫中的重要作用。     

The INs and OUTs of pattern recognition receptors at the cell surface

Pattern recognition receptors (PRRs) enable plants to sense non-self molecules displayed by microbes to mount proper defense responses or establish symbiosis. In recent years the importance of PRR subcellular trafficking to plant immunity has become apparent. PRRs traffic through the endoplasmatic reticulum (ER) and the Golgi apparatus to the plasma membrane, where they recognize their cognate ligands. At the plasma membrane, PRRs can be recycled or internalized via endocytic pathways. By using genetic and biochemical tools in combination with bioimaging, the trafficking pathways and their role in PRR perception of microbial molecules are now being revealed.     

Rab GTPases regulating receptor trafficking at the late endosome-lysosome membranes

Lysosomes serve key degradative functions for the turnover of membrane lipids and protein components. Its biogenesis is principally dependent on exocytic traffic from the late endosome via the trans-Golgi network, and it also receives cargo to be degraded from the endocytic pathway. Membrane trafficking to the late endosome-lysosome is tightly regulated to maintain the amplitude of signalling events and cellular homeostasis. Key coordinators of lysosomal traffic include members of the Rab small GTPase family. Amongst these, Rab7, Rab9 and the more recently studied Rab22B/31 have all been reported to regulate membrane trafficking processed at the late endosome-lysosome system. We discuss what is known about the roles of these Rab proteins and their interacting partners on the regulation of traffic of important receptor proteins such as the epidermal growth factor receptor (EGFR) and the mannose 6-phosphate receptor (M6PR), in association with the late endosome-lysosome system. Better knowledge of EGFR and M6PR traffic in this regard may aid in understanding the pathological processes, such as oncogenic transformations associated with these receptors. Copyright ? 2012 John Wiley & Sons, Ltd.     

Innate immunity: structure and function of TLRs

The innate immune system provides the first line of defence against infection. Through a limited number of germline-encoded receptors called pattern recognition receptors (PRRs), innate cells recognize and are activated by highly conserved structures expressed by large group of microorganisms called pathogen-associated molecular patterns (PAMPs). PRRs are involved either in recognition (scavenger receptors, C-type lectins) or in cell activation (Toll-like receptors or TLR, helicases and NOD molecules). TLRs play a pivotal role in cell activation in response to PAMPs. TLR are type I transmembrane proteins characterized by an intracellular Toll/IL 1 receptor homology domain that are expressed by innate immune cells (dendritic cells, macrophages, NK cells), cells of the adaptive immunity (T and B lymphocytes) and non immune cells (epithelial and endothelial cells, fibroblasts). In all the cell types analyzed, TLR agonists, alone or in combination with costimulatory molecules, induce cell activation. The crucial role played by TLR in immune cell activation has been detailed in dendritic cells. A TLR-dependent activation of dendritic cells is required to induce their maturation and migration to regional lymph nodes and to activate na?ve T cells. The ability of different cell types to respond to TLR agonists is related to the pattern of expression of the TLRs and its regulation as well as their intracellular localization. Recent studies suggest that the nature of the endocytic and signaling receptors engaged by PAMPs may determine the nature of the immune response generated against the microbial molecules, highlighting the role of TLRs as molecular interfaces between innate and adaptive immunity. In this review are summarized the main biological properties of the TLR molecules.     

Characterization of the plasma membrane proteins and receptor-like kinases associated with secondary vascular differentiation in poplar

The constituents of plasma membrane proteins, particularly the integral membrane proteins, are closely associated with the differentiation of plant cells. Secondary vascular differentiation, which gives rise to the increase in plant stem diameter, is the key process by which the volume of the plant body grows. However, little is known about the plasma membrane proteins that specifically function in the vascular differentiation process. Proteomic analysis of the membrane proteins in poplar differentiating secondary vascular tissues led to the identification 226 integral proteins in differentiating xylem and phloem tissues. A majority of the integral proteins identified were receptors (55 proteins), transporters (34 proteins), cell wall formation related (27 proteins) or intracellular trafficking (17 proteins) proteins. Gene expression analysis in developing vascular cells further demonstrated that cambium differentiation involves the expression of a group of receptor kinases which mediate an array of signaling pathways during secondary vascular differentiation. This paper provides an outline of the protein composition of the plasma membrane in differentiating secondary vascular tissues and sheds light on the role of receptor kinases during secondary vascular development.     

The alliance of sphingosine-1-phosphate and its receptors in immunity

Sphingosine-1-phosphate (S1P) is a biologically active metabolite of plasma-membrane sphingolipids that is essential for immune-cell trafficking. Its concentration is increased in many inflammatory conditions, such as asthma and autoimmunity. Much of the immune function of S1P results from the engagement of a family of G-protein-coupled receptors (S1PR1-S1PR5). Recent findings on the role of S1P in immunosurveillance, the discovery of regulatory mechanisms in S1P-mediated immune-cell trafficking and new advances in understanding the mechanism by which S1P affects immune-cell function indicate that the alliance between S1P and its receptors has a fundamental role in immunity.     

Immunity functions of Arabidopsis pathogenesis-related 1 are coupled but not confined to its C-terminus processing and trafficking

The pathogenesis-related 1 (PR1) proteins are members of the cross-kingdom conserved CAP superfamily (from Cysteine-rich secretory protein, Antigen 5, and PR1 proteins). PR1 mRNA expression is frequently used for biotic stress monitoring in plants; however, the molecular mechanisms of its cellular processing, localization, and function are still unknown. To analyse the localization and immunity features of Arabidopsis thaliana PR1, we employed transient expression in Nicotiana benthamiana of the tagged full-length PR1 construct, and also disrupted variants with C-terminal truncations or mutations. We found that en route from the endoplasmic reticulum, the PR1 protein transits via the multivesicular body and undergoes partial proteolytic processing, dependent on an intact C-terminal motif. Importantly, only nonmutated or processing-mimicking variants of PR1 are secreted to the apoplast. The C-terminal proteolytic cleavage releases a protein fragment that acts as a modulator of plant defence responses, including localized cell death control. However, other parts of PR1 also have immunity potential unrelated to cell death. The described modes of the PR1 contribution to immunity were found to be tissue-localized and host plant ontogenesis dependent.     

Plant immunity: a lesson from pathogenic bacterial effector proteins

Phytopathogenic bacteria inject an array of effector proteins into host cells to alter host physiology and assist the infection process. Some of these effectors can also trigger disease resistance as a result of recognition in the plant cell by cytoplasmic immune receptors. In addition to effector-triggered immunity, plants immunity can be triggered upon the detection of Pathogen/Microbe-Associated Molecular Patterns by surface-localized immune receptors. Recent progress indicates that many bacterial effector proteins use a variety of biochemical properties to directly attack key components of PAMP-triggered immunity and effector-triggered immunity, providing new insights into the molecular basis of plant innate immunity. Emerging evidence indicate that the evolution of disease resistance in plants is intimately linked to the mechanism by which bacterial effectors promote parasitism. This review focuses on how these studies have conceptually advanced our understanding of plant–pathogen interactions.     

All hands on deck—the role of chloroplasts, endoplasmic reticulum, and the nucleus in driving plant innate immunity

Plant innate immunity is mediated by cell membrane and intracellular immune receptors that function in distinct and overlapping cell-signaling pathways to activate defense responses. It is becoming increasingly evident that immune receptors rely on components from multiple organelles for the generation of appropriate defense responses. This review analyzes the defense-related functions of the chloroplast, nucleus, and endoplasmic reticulum (ER) during plant innate immunity. It details the role of the chloroplasts in synthesizing defense-specific second messengers and discusses the retrograde signal transduction pathways that exist between the chloroplast and nucleus. Because the activities of immune modulators are regulated, in part, by their subcellular localization, the review places special emphasis on the dynamics and nuclear–cytoplasmic transport of immune receptors and regulators and highlights the importance of this process in generating orderly events during an innate immune response. The review also covers the recently discovered contributions of the ER quality-control pathways in ensuring the signaling competency of cell surface immune receptors or immune regulators.     

Identification and characterization of a new class of trafficking motifs for controlling clathrin-independent internalization and recycling

Plasma membrane proteins such as receptors and ion channels allow a cell to communicate with its environment and regulate many intracellular activities. Thus, the proper control of the surface number of these proteins is essential for maintaining the structural and functional homeostasis of a cell. Internalization and recycling plays a key role in determining the surface density of receptors and channels. Whereas the clathrin-mediated internalization and its associated recycling have been the focus of research in this field, recent studies have revealed that an increasing number of receptors and channels enter a cell via clathrin-independent pathways. However, little is known about the trafficking motifs involved in controlling clathrin-independent internalization and various associated recycling pathways. By using a potassium channel as a model system, we identified a class of trafficking motifs that function along a clathrin-independent pathway to increase the surface density of a membrane protein by preventing its rapid internalization and/or facilitating its recycling via the ADP-ribosylation factor 6-dependent recycling pathway. Moreover our data suggest that these motifs may enhance the association of membrane proteins with the EFA6 family of guanine nucleotide exchange factors for ADP-ribosylation factor 6.     

Apoplastic invasion patterns triggering plant immunity: plasma membrane sensing at the frontline

Plants are able to effectively cope with invading pathogens by activating an immune response based on the detection of invasion patterns (IPs) originating from the pathogen or released by the plant after infection. At a first level, this perception takes place at the plasma membrane through cell surface immune receptors and although the involvement of proteinaceous pattern recognition receptors (PRRs) is well established, increasing data are also pointing out the role of membrane lipids in the sensing of IPs. In this review, we discuss the evolution of various conceptual models describing plant immunity and present an overview of well-characterized IPs from different natures and origins. We summarize the current knowledge on how they are perceived by plants at the plasma membrane, highlighting the increasingly apparent diversity of sentinel-related systems in plants.