Plant NB-LRR signaling: upstreams and downstreams

Plant disease resistance proteins commonly belong to the nucleotide binding-leucine rich repeat (NB-LRR) protein family. These specialized immune proteins mediate recognition of diverse pathogen-derived effector proteins and initiate potent defense responses. NB-LRRs exhibit a multidomain architecture and each domain appears to have discrete functions depending on the stage of NB-LRR signaling. Novel proteins that were found to interact with the core HSP90 chaperone complex regulate accumulation and activation of NB-LRR immune receptors. Recent studies have also advanced our understanding of how accessory proteins contribute to NB-LRR activation. The dynamic nature of NB-LRR localization to different subcellular compartments before and after activation suggests that NB-LRRs may activate immune responses in multiple parts of the cell. In this review we highlight recent advances in understanding NB-LRR function.     

A RanGAP protein physically interacts with the NB-LRR protein Rx, and is required for Rx-mediated viral resistance

Race-specific disease resistance in plants is mediated by the products of host disease resistance (R) genes. Plant genomes possess hundreds of R gene homologs encoding nucleotide-binding and leucine-rich repeat (NB-LRR) proteins. NB-LRR proteins induce a disease resistance response following recognition of pathogen-encoded avirulence (Avr) proteins. However, little is known about the general mechanisms by which NB-LRR proteins recognize Avr proteins or how they subsequently induce defense responses. The Rx NB-LRR protein of potato confers resistance to potato virus X (PVX). Using a co-purification strategy, we have identified a Ran GTPase-activating protein (RanGAP2) as an Rx-interacting protein. We show by co-immunoprecipitation that this interaction is mediated in planta through the putative signaling domain at the Rx amino terminus. Overexpression of RanGAP2 results in activation of certain Rx derivatives. Likewise, knocking down RanGAP2 expression in Nicotiana benthamiana by virus-induced gene silencing compromises Rx-mediated resistance to PVX. Thus, we have demonstrated a novel role for a RanGAP in the function of a plant disease resistance response.     

Pattern recognition receptors: from the cell surface to intracellular dynamics

Detection of potentially infectious microorganisms is essential for plant immunity. Microbial communities growing on plant surfaces are constantly monitored according to their conserved microbe-associated molecular patterns (MAMPs). In recent years, several pattern-recognition receptors, including receptor-like kinases and receptor-like proteins, and their contribution to disease resistance have been described. MAMP signaling must be carefully controlled and seems to involve receptor endocytosis. As a further surveillance layer, plants are able to specifically recognize microbial effector molecules via nucleotide-binding site leucine-rich repeat receptors (NB-LRR). A number of recent studies show that NB-LRR translocate to the nucleus in order to exert their activity. In this review, current knowledge regarding the recognition of MAMPs by surface receptors, receptor activation, signaling, and subcellular redistribution are discussed.     

Plant NB-LRR immune receptors: from recognition to transcriptional reprogramming

Both plants and animals contain nucleotide-binding domain and leucine-rich repeat (NB-LRR)-type immune receptors that function during defense against pathogens. Unlike animal NB-LRRs that recognize general pathogen or microbe-associated molecular patterns (PAMPs or MAMPs), plant NB-LRR immune receptors have evolved the ability to specifically recognize a wide range of effector proteins from different pathogens. Recent research has revealed that plant NB-LRRs are incredibly adaptive in their ways of pathogen recognition and defense initiation. This review focuses on the remarkable variety of functions, recognition mechanisms, subcellular localizations, and host factors associated with plant NB-LRR immune receptors.     

How to build a pathogen detector: structural basis of NB-LRR function

Many plant disease resistance (R) proteins belong to the family of nucleotide-binding-leucine rich repeat (NB-LRR) proteins. NB-LRRs mediate recognition of pathogen-derived effector molecules and subsequently activate host defence. Their multi-domain structure allows these pathogen detectors to simultaneously act as sensor, switch and response factor. Structure-function analyses and the recent elucidation of the 3D structures of subdomains have provided new insight in how these different functions are combined and what the contribution is of the individual subdomains. Besides interdomain contacts, interactions with chaperones, the proteasome and effector baits are required to keep NB-LRRs in a signalling-competent, yet auto-inhibited state. In this review we explore operational models of NB-LRR functioning based on recent advances in understanding their structure.     

In the trenches of plant pathogen recognition: Role of NB-LRR proteins

As in nearly every discipline of plant biology, new insights are constantly changing our understanding of plant immunity. It is now clear that plant immunity is controlled by two layers of inducible responses: basal responses triggered by conserved microbial features and specific responses triggered by gene-for-gene recognition of pathogen effector proteins by host resistance (R) proteins. The nucleotide-binding domain leucine-rich repeat (NB-LRR) class of R proteins plays a major role in the combat against a wide range of plant pathogens. The variation that has been generated and is maintained within these conserved proteins has diversified their specificity, subcellular localisations, activation and recognition mechanisms, allowing them to specifically adapt to different plant–pathogen interaction systems. This review addresses recent advances in the molecular role of NB-LRR proteins in pathogen recognition and activation of plant defence responses.     

Plant pathogenic bacterial type III effectors subdue host responses

Like animals, plants sense bacterial pathogens through surface-localized pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat proteins (NB-LRR) and trigger defense responses. Many plant-pathogenic bacteria secrete a large repertoire of effector proteins into host cells to modulate host responses, enabling successful infection and multiplication in plants. A number of these effector proteins target plant innate immunity signaling pathways, while others induce specific host genes to enhance plant susceptibility. Substantial progress has been made in the past two years concerning biochemical function of effectors and their host targets. These advances provide new insights into regulatory mechanisms of plant immunity and host-pathogen co-evolution.     

The coiled-coil and nucleotide binding domains of the Potato Rx disease resistance protein function in pathogen recognition and signaling

Plant genomes encode large numbers of nucleotide binding and leucine-rich repeat (NB-LRR) proteins, some of which mediate the recognition of pathogen-encoded proteins. Following recognition, the initiation of a resistance response is thought to be mediated by the domains present at the N termini of NB-LRR proteins, either a Toll and Interleukin-1 Receptor or a coiled-coil (CC) domain. In order to understand the role of the CC domain in NB-LRR function, we have undertaken a systematic structure-function analysis of the CC domain of the potato (Solanum tuberosum) CC-NB-LRR protein Rx, which confers resistance to Potato virus X. We show that the highly conserved EDVID motif of the CC domain mediates an intramolecular interaction that is dependent on several domains within the rest of the Rx protein, including the NB and LRR domains. Other conserved and nonconserved regions of the CC domain mediate the interaction with the Ran GTPase-activating protein, RanGAP2, a protein required for Rx function. Furthermore, we show that the Rx NB domain is sufficient for inducing cell death typical of hypersensitive plant resistance responses. We describe a model of CC-NB-LRR function wherein the LRR and CC domains coregulate the signaling activity of the NB domain in a recognition-specific manner.     

NRG1, a CC-NB-LRR protein, together with N, a TIR-NB-LRR protein, mediates resistance against tobacco mosaic virus

In animals and plants, innate immunity is regulated by nucleotide binding domain and leucine-rich repeat (NB-LRR) proteins that mediate pathogen recognition and that activate host-cell defense responses. Plant NB-LRR proteins, referred to as R proteins, have amino-terminal domains that contain a coiled coil (CC) or that share similarity with animal Toll and interleukin 1 receptors (TIR). To investigate R protein function, we are using the TIR-NB-LRR protein N that mediates resistance against tobacco mosaic virus (TMV) through recognition of the TMV p50 protein. Here, we describe N requirement gene 1 (NRG1), a novel N-resistance component that was identified by a virus-induced gene silencing (VIGS) screen of a cDNA library. Surprisingly, NRG1 encodes an NB-LRR type R protein that, in contrast to N, contains a CC rather than a TIR domain. Our findings support emerging evidence that many disease-resistance pathways each recruit more than a single NB-LRR protein. The results also indicate that, in addition to the previously recognized role in elicitor recognition, NB-LRR proteins may also function in downstream signaling pathways.     

SIVB 2003 Congress Symposium Proceeding: Plant-Targets of Pathogenic Effectors Can Transduce Both Virulence and Resistance Signals

Summary Pathogens of plants produce effector proteins necessary for successful parasitism. The effectors enhance pathogen virulence by manipulating signaling in the plant. Plants produce resistance (R) proteins that mediate recognition of specific effectors and respond by initiating plant defenses. In many cases, R-proteins perceive effectors indirectly; virulence signaling initiated by the effector is shunted, via the R-protein, into a resistance response. Therefore, by understanding how effectors manipulate virulence targets we will concurrently gain insight into how this signaling elicits R-protein-mediated defense responses.     

Structural and functional analysis of a plant resistance protein TIR domain reveals interfaces for self-association, signaling, and autoregulation

The Toll/interleukin-1 receptor (TIR) domain occurs in animal and plant immune receptors. In the animal Toll-like receptors, homodimerization of the intracellular TIR domain is required for initiation of signaling cascades leading to innate immunity. By contrast, the role of the TIR domain in cytoplasmic nucleotide-binding/leucine-rich repeat (NB-LRR) plant immune resistance proteins is poorly understood. L6 is a TIR-NB-LRR resistance protein from flax (Linum usitatissimum) that confers resistance to the flax rust phytopathogenic fungus (Melampsora lini). We determine the crystal structure of the L6 TIR domain and show that, although dispensable for pathogenic effector protein recognition, the TIR domain alone is both necessary and sufficient for L6 immune signaling. We demonstrate that the L6 TIR domain self-associates, most likely forming?a homodimer. Analysis of the structure combined with site-directed mutagenesis suggests that self-association is a requirement for immune signaling and reveals distinct surface regions involved in self-association, signaling, and autoregulation.     

A molecular evolutionary concept connecting nonhost resistance, pathogen host range, and pathogen speciation

Any given pathogenic microbial species typically colonizes a limited number of plant species. Plant species outside of this host range mount nonhost disease resistance to attempted colonization by the, in this case, non-adapted pathogen. The underlying mechanism of nonhost immunity and host immunity involves the same non-self detection systems, the combined action of nucleotide-binding and leucine-rich repeat (NB-LRR) proteins and pattern recognition receptors (PRRs). Here we hypothesize that the relative contribution of NB-LRR- and PRR-triggered immunity to nonhost resistance changes as a function of phylogenetic divergence time between host and nonhost. Similarly, changes in pathogen host range, e.g. host range expansions, appear to be driven by variation in pathogen effector repertoires, in turn leading to reproductive isolation and subsequent pathogen speciation.     

Activation of a plant nucleotide binding-leucine rich repeat disease resistance protein by a modified self protein

Nucleotide binding-leucine rich repeat (NB-LRR) proteins function as intracellular receptors for the detection of pathogens in both plants and animals. Despite their central role in innate immunity, the molecular mechanisms that govern NB-LRR activation are poorly understood. The Arabidopsis NB-LRR protein RPS5 detects the presence of the Pseudomonas syringae effector protein AvrPphB by monitoring the status of the Arabidopsis protein kinase PBS1. AvrPphB is a cysteine protease that targets PBS1 for cleavage at a single site within the activation loop of PBS1. Using a transient expression system in the plant Nicotiana benthamiana and stable transgenic Arabidopsis plants we found that both PBS1 cleavage products are required to activate RPS5 and can do so in the absence of AvrPphB. We also found, however, that the requirement for cleavage of PBS1 could be bypassed simply by inserting five amino acids at the PBS1 cleavage site, which is located at the apex of the activation loop of PBS1. Activation of RPS5 did not require PBS1 kinase function, and thus RPS5 appears to sense a subtle conformational change in PBS1, rather than cleavage. This finding suggests that NB-LRR proteins may function as fine-tuned sensors of alterations in the structures of effector targets.     

Physical association of the NB-LRR resistance protein Rx with a Ran GTPase-activating protein is required for extreme resistance to Potato virus X

Nucleotide binding leucine-rich repeat (NB-LRR) proteins play an important role in plant and mammalian innate immunity. In plants, these resistance proteins recognize specific pathogen-derived effector proteins. Recognition subsequently triggers a rapid and efficient defense response often associated with the hypersensitive response and other poorly understood processes that suppress the pathogen. To investigate mechanisms associated with the activation of disease resistance responses, we investigated proteins binding to the potato (Solanum tuberosum) NB-LRR protein Rx that confers extreme resistance to Potato virus X (PVX) in potato and Nicotiana benthamiana. By affinity purification experiments, we identified an endogenous N. benthamiana Ran GTPase-Activating Protein2 (RanGAP2) as an Rx-associated protein in vivo. Further characterization confirmed the specificity of this interaction and showed that the association occurs through their N-terminal domains. By specific virus-induced gene silencing of RanGAP2 in N. benthamiana carrying Rx, we demonstrated that this interaction is required for extreme resistance to PVX and suggest that RanGAP2 is part of the Rx signaling complex. These results implicate RanGAP-mediated cellular mechanisms, including nucleocytoplasmic trafficking, in the activation of disease resistance.     

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.     

Initiation of RPS2-specified disease resistance in Arabidopsis is coupled to the AvrRpt2-directed elimination of RIN4

Plants have evolved a sophisticated innate immune system to recognize invading pathogens and to induce a set of host defense mechanisms resulting in disease resistance. Pathogen recognition is often mediated by plant disease resistance (R) proteins that respond specifically to one or a few pathogen-derived molecules. This specificity has led to suggestions of a receptor-ligand mode of R protein function. Delivery of the bacterial effector protein AvrRpt2 by Pseudomonas syringae specifically induces disease resistance in Arabidopsis plants expressing the RPS2 R protein. We demonstrate that RPS2 physically interacts with Arabidopsis RIN4 and that AvrRpt2 causes the elimination of RIN4 during activation of the RPS2 pathway. AvrRpt2-mediated RIN4 elimination also occurs in the rps2, ndr1, and Atrar1 mutant backgrounds, demonstrating that this activity can be achieved independent of an RPS2-mediated signaling pathway. Therefore, we suggest that RPS2 initiates signaling based upon perception of RIN4 disappearance rather than direct recognition of AvrRpt2.     

Molecular and spatial constraints on NB-LRR receptor signaling

In plants, a large polymorphic family of intracellular NB-LRR receptors lies at the heart of robust resistance to diverse pathogens and mechanisms by which these versatile molecular switches operate in effector-triggered immunity are beginning to emerge. We outline recent advances in our understanding of NB-LRR receptor signaling leading to disease resistance. Themes covered are (i) NB-LRR molecular constraining forces and their intimate relationship with receptor activation in different parts of the cell, (ii) cooperativity between NB-LRR proteins and the formation of higher order NB-LRR signaling complexes, and (iii) the spatial separation of different resistance branches within cells. Finally, we examine evidence for dynamic signaling across cell compartments in coordinating diverse immune outputs.     

Membrane microdomain may be a platform for immune signaling

Arabidopsis RPS2 is a typical disease resistance (R) protein with nucleotide-binding leucine-rich repeats (NB-LRR). Previously, we reported that RPS2 is physically associated with some Arabidopsis hypersensitive induced reaction (AtHIR) proteins, which are enriched in membrane microdomains. Biochemical and genetic analyses suggested that members of the AtHIR gene family have a function in RPS2-mediated immune signaling. Here, we provide evidence that the pattern recognition receptor (PRR) FLS2 is also physically associated with AtHIR2 in a N. benthamiana transient expression system. We thus speculate that PM microdomains provide a platform for both types of immune receptors, R proteins and PRRs, and that the activation of the receptors is facilitated by AtHIR proteins.     

植物TIR-NB-LRR类型抗病基因各结构域的研究进展

植物抗病反应是一个多基因调控的复杂过程,在这个过程中R基因发挥了非常重要的作用。根据其氨基酸基序组成以及跨膜结构域的不同,R基因可以分为多种类型,其中NBS-LRR类型是植物基因组中最大的基因家族之一。TIR-NB-LRR类型的抗病基因又是NB-LRR类型中的一大类,也是目前抗病基因研究的热点。该文总结了TIR-NB-LRR类型抗病基因各个结构域的功能和相关的研究进展。相关研究表明,TIR结构域主要通过自身或异源的二聚体化介导抗性信号的转导,但也有部分研究表明,该结构域可能参与病原菌的特异性识别。NBS结构域常被认为具有"分子开关"的功能,它可以通过结合ADP或ATP来调节植物抗病蛋白的构象变化,从而调节下游抗病信号的传导。LRR结构域在植物与病原菌互作的过程中可以通过与病原菌的无毒蛋白直接或间接互作来特异识别病原菌。也有研究发现,LRR结构域具有调节信号传导的功能。这些信息将为研究植物抗病机理提供理论依据,也为将来通过基因编辑技术对作物进行定向抗病育种提供思路。