Comparative Analysis of the Flax Immune Receptors L6 and L7 Suggests an Equilibrium-Based Switch Activation Model

A viral effector blocks the turnover of a plant NLR receptor to trigger a robust immune response

Plant intracellular nucleotide-binding and leucine-rich repeat immune receptors (NLRs) play a key role in activating a strong pathogen defense response. Plant NLR proteins are tightly regulated and accumulate at very low levels in the absence of pathogen effectors. However, little is known about how this low level of NLR proteins is able to induce robust immune responses upon recognition of pathogen effectors. Here, we report that, in the absence of effector, the inactive form of the tomato NLR Sw-5b is targeted for ubiquitination by the E3 ligase SBP1. Interaction of SBP1 with Sw-5b via only its N-terminal domain leads to slow turnover. In contrast, in its auto-active state, Sw-5b is rapidly turned over as SBP1 is upregulated and interacts with both its N-terminal and NB-LRR domains. During infection with the tomato spotted wilt virus, the viral effector NSm interacts with Sw-5b and disrupts the interaction of Sw-5b with SBP1, thereby stabilizing the active Sw-5b and allowing it to induce a robust immune response.

Molecular Basis of Plant-Pathogen Interactions in the Agricultural Context

Biotic stressors pose significant threats to crop yield, jeopardizing food security and resulting in losses of over USD 220 billion per year by the agriculture industry. Plants activate innate defense mechanisms upon pathogen perception and invasion. The plant immune response comprises numerous concerted steps, including the recognition of invading pathogens, signal transduction, and activation of defensive pathways. However, pathogens have evolved various structures to evade plant immunity. Given these facts, genetic improvements to plants are required for sustainable disease management to ensure global food security. Advanced genetic technologies have offered new opportunities to revolutionize and boost plant disease resistance against devastating pathogens. Furthermore, targeting susceptibility (S) genes, such as OsERF922 and BnWRKY70, through CRISPR methodologies offers novel avenues for disrupting the molecular compatibility of pathogens and for introducing durable resistance against them in plants. Here, we provide a critical overview of advances in understanding disease resistance mechanisms. The review also critically examines management strategies under challenging environmental conditions and R-gene-based plant genome-engineering systems intending to enhance plant responses against emerging pathogens. This work underscores the transformative potential of modern genetic engineering practices in revolutionizing plant health and crop disease management while emphasizing the importance of responsible application to ensure sustainable and resilient agricultural systems.

Pathogen perception and signaling in plant immunity

Plant diseases are a constant and serious threat to agriculture and ecological biodiversity. Plants possess a sophisticated innate immunity system capable of detecting and responding to pathogen infection to prevent disease. Our understanding of this system has grown enormously over the past century. Early genetic descriptions of plant disease resistance and pathogen virulence were embodied in the gene-for-gene hypothesis, while physiological studies identified pathogen-derived elicitors that could trigger defense responses in plant cells and tissues. Molecular studies of these phenomena have now coalesced into an integrated model of plant immunity involving cell surface and intracellular detection of specific pathogen-derived molecules and proteins culminating in the induction of various cellular responses. Extracellular and intracellular receptors engage distinct signaling processes but converge on many similar outputs with substantial evidence now for integration of these pathways into interdependent networks controlling disease outcomes. Many of the molecular details of pathogen recognition and signaling processes are now known, providing opportunities for bioengineering to enhance plant protection from disease. Here we provide an overview of the current understanding of the main principles of plant immunity, with an emphasis on the key scientific milestones leading to these insights.

Pattern-Triggered Immunity and Effector-Triggered Immunity: crosstalk and cooperation of PRR and NLR-mediated plant defense pathways during host-pathogen interactions

The elucidation of the molecular basis underlying plant-pathogen interactions is imperative for the development of sustainable resistance strategies against pathogens. Plants employ a dual-layered immunological detection and response system wherein cell surface-localized Pattern Recognition Receptors (PRRs) and intracellular Nucleotide-Binding Leucine-Rich Repeat Receptors (NLRs) play pivotal roles in initiating downstream signalling cascades in response to pathogen-derived chemicals. Pattern-Triggered Immunity (PTI) is associated with PRRs and is activated by the recognition of conserved molecular structures, known as Pathogen-Associated Molecular Patterns. When PTI proves ineffective due to pathogenic effectors, Effector-Triggered Immunity (ETI) frequently confers resistance. In ETI, host plants utilize NLRs to detect pathogen effectors directly or indirectly, prompting a rapid and more robust defense response. Additionally epigenetic mechanisms are participating in plant immune memory. Recently developed technologies like CRISPR/Cas9 helps in exposing novel prospects in plant pathogen interactions. In this review we explore the fascinating crosstalk and cooperation between PRRs and NLRs. We discuss epigenomic processes and CRISPR/Cas9 regulating immune response in plants and recent findings that shed light on the coordination of these defense layers. Furthermore, we also have discussed the intricate interactions between the salicylic acid and jasmonic acid signalling pathways in plants, offering insights into potential synergistic interactions that would be harnessed for the development of novel and sustainable resistance strategies against diverse group of pathogens.

Plant NLR immunity activation and execution: a biochemical perspective

Plants deploy cell-surface and intracellular receptors to detect pathogen attack and trigger innate immune responses. Inside host cells, families of nucleotide-binding/leucine-rich repeat (NLR) proteins serve as pathogen sensors or downstream mediators of immune defence outputs and cell death, which prevent disease. Established genetic underpinnings of NLR-mediated immunity revealed various strategies plants adopt to combat rapidly evolving microbial pathogens. The molecular mechanisms of NLR activation and signal transmission to components controlling immunity execution were less clear. Here, we review recent protein structural and biochemical insights to plant NLR sensor and signalling functions. When put together, the data show how different NLR families, whether sensors or signal transducers, converge on nucleotide-based second messengers and cellular calcium to confer immunity. Although pathogen-activated NLRs in plants engage plant-specific machineries to promote defence, comparisons with mammalian NLR immune receptor counterparts highlight some shared working principles for NLR immunity across kingdoms.

A dominant-negative avirulence effector of the barley powdery mildew fungus provides mechanistic insight into barley MLA immune receptor activation

Nucleotide-binding leucine-rich repeat receptors (NLRs) recognize pathogen effectors to mediate plant disease resistance often involving host cell death. Effectors escape NLR recognition through polymorphisms, allowing the pathogen to proliferate on previously resistant host plants. The powdery mildew effector AVRA13-1 is recognized by the barley NLR MLA13 and activates host cell death. We demonstrate here that a virulent form of AVRA13, called AVRA13-V2, escapes MLA13 recognition by substituting a serine for a leucine residue at the C-terminus. Counterintuitively, this substitution in AVRA13-V2 resulted in an enhanced MLA13 association and prevented the detection of AVRA13-1 by MLA13. Therefore, AVRA13-V2 is a dominant-negative form of AVRA13 and has probably contributed to the breakdown of Mla13 resistance. Despite this dominant-negative activity, AVRA13-V2 failed to suppress host cell death mediated by the MLA13 autoactive MHD variant. Neither AVRA13-1 nor AVRA13-V2 interacted with the MLA13 autoactive variant, implying that the binding moiety in MLA13 that mediates association with AVRA13-1 is altered after receptor activation. We also show that mutations in the MLA13 coiled-coil domain, which were thought to impair Ca2+ channel activity and NLR function, instead resulted in MLA13 autoactive cell death. Our results constitute an important step to define intermediate receptor conformations during NLR activation.

The Hypersensitive Response to Plant Viruses

Plant proteins with domains rich in leucine repeats play important roles in detecting pathogens and triggering defense reactions, both at the cellular surface for pattern-triggered immunity and in the cell to ensure effector-triggered immunity. As intracellular parasites, viruses are mostly detected intracellularly by proteins with a nucleotide binding site and leucine-rich repeats but receptor-like kinases with leucine-rich repeats, known to localize at the cell surface, have also been involved in response to viruses. In the present review we report on the progress that has been achieved in the last decade on the role of these leucine-rich proteins in antiviral immunity, with a special focus on our current understanding of the hypersensitive response.

Mutagenesis of the melon Prv gene by CRISPR/Cas9 breaks papaya ringspot virus resistance and generates an autoimmune allele with constitutive defense responses

The majority of plant disease resistance (R) genes encode nucleotide binding-leucine-rich repeat (NLR) proteins. In melon, two closely linked NLR genes, Fom-1 and Prv, were mapped and identified as candidate genes that control resistance to Fusarium oxysporum f.sp. melonis races 0 and 2, and to papaya ringspot virus (PRSV), respectively. In this study, we validated the function of Prv and showed that it is essential for providing resistance against PRSV infection. We generated CRISPR/Cas9 [clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9] mutants using Agrobacterium-mediated transformation of a PRSV-resistant melon genotype, and the T1 progeny proved susceptible to PRSV, showing strong disease symptoms and viral spread upon infection. Three alleles having 144, 154, and ~3 kb deletions, respectively, were obtained, all of which caused loss of resistance. Interestingly, one of the Prv mutant alleles, prvΔ154, encoding a truncated product, caused an extreme dwarf phenotype, accompanied by leaf lesions, high salicylic acid levels, and defense gene expression. The autoimmune phenotype observed at 25 °C proved to be temperature dependent, being suppressed at 32 °C. This is a first report on the successful application of CRISPR/Cas9 to confirm R gene function in melon. Such validation opens up new opportunities for molecular breeding of disease resistance in this important vegetable crop.

Plasma membrane association and resistosome formation of plant helper immune receptors

Intracellular plant immune receptors, termed NLRs (Nucleotide-binding Leucine-rich repeat Receptors), confer effector-triggered immunity. Sensor NLRs are responsible for pathogen effector recognition. Helper NLRs function downstream of sensor NLRs to transduce signaling and induce cell death and immunity. Activation of sensor NLRs that contain TIR (Toll/interleukin-1receptor) domains generates small molecules that induce an association between a downstream heterodimer signalosome of EDS1 (EnhancedDisease Susceptibility 1)/SAG101 (Senescence-AssociatedGene 101) and the helper NLR of NRG1 (NRequired Gene 1). Autoactive NRG1s oligomerize and form calcium signaling channels largely localized at the plasma membrane (PM). The molecular mechanisms of helper NLR PM association and effector-induced NRG1 oligomerization are not well characterized. We demonstrate that helper NLRs require positively charged residues in their N-terminal domains for phospholipid binding and PM association before and after activation, despite oligomerization and conformational changes that accompany activation. We demonstrate that effector activation of a TIR-containing sensor NLR induces NRG1 oligomerization at the PM and that the cytoplasmic pool of EDS1/SAG101 is critical for cell death function. EDS1/SAG101 cannot be detected in the oligomerized NRG1 resistosome, suggesting that additional unknown triggers might be required to induce the dissociation of EDS1/SAG101 from the previously described NRG1/EDS1/SAG101 heterotrimer before subsequent NRG1 oligomerization. Alternatively, the conformational changes resulting from NRG1 oligomerization abrogate the interface for EDS1/SAG101 association. Our data provide observations regarding dynamic PM association during helper NLR activation and underpin an updated model for effector-induced NRG1 resistosome formation.

From Gene-for-Gene to Resistosomes: Flor's Enduring Legacy

The gene-for-gene model proposed by H. H. Flor has been one of the fundamental precepts of plant-pathogen interactions that has underpinned decades of research towards our current concepts of plant immunity. The broad validity of this model as an elegant and accurate genetic description of specific recognition events between the products of plant resistance (R) and pathogen avirulence (Avr) genes has been demonstrated many times over in a wide variety of plant disease systems. In recent years detailed molecular and structural analyses have provided a deep understanding of the principles by which plant immune receptors recognize pathogen effectors, including providing molecular descriptions of many of the genetic loci in flax and flax rust characterized by Flor. Recent advances in molecular and structural understanding of immune receptor recognition and activation mechanisms have brought the field to a new level, where rational design of novel receptors through engineering approaches is becoming a realizable goal. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Altering Specificity and Autoactivity of Plant Immune Receptors Sr33 and Sr50 Via a Rational Engineering Approach

Many resistance genes deployed against pathogens in crops are intracellular nucleotide-binding (NB) leucine-rich repeat (LRR) receptors (NLRs). The ability to rationally engineer the specificity of NLRs will be crucial in the response to newly emerging crop diseases. Successful attempts to modify NLR recognition have been limited to untargeted approaches or depended on previously available structural information or knowledge of pathogen-effector targets. However, this information is not available for most NLR-effector pairs. Here, we demonstrate the precise prediction and subsequent transfer of residues involved in effector recognition between two closely related NLRs without their experimentally determined structure or detailed knowledge about their pathogen effector targets. By combining phylogenetics, allele diversity analysis, and structural modeling, we successfully predicted residues mediating interaction of Sr50 with its cognate effector AvrSr50 and transferred recognition specificity of Sr50 to the closely related NLR Sr33. We created synthetic versions of Sr33 that contain amino acids from Sr50, including Sr33syn, which gained the ability to recognize AvrSr50 with 12 amino-acid substitutions. Furthermore, we discovered that sites in the LRR domain needed to transfer recognition specificity to Sr33 also influence autoactivity in Sr50. Structural modeling suggests these residues interact with a part of the NB-ARC domain, which we named the NB-ARC latch, to possibly maintain the inactive state of the receptor. Our approach demonstrates rational modifications of NLRs, which could be useful to enhance existing elite crop germplasm. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Structural mechanism of heavy metal-associated integrated domain engineering of paired nucleotide-binding and leucine-rich repeat proteins in rice

Plant nucleotide-binding and leucine-rich repeat (NLR) proteins are immune sensors that detect pathogen effectors and initiate a strong immune response. In many cases, single NLR proteins are sufficient for both effector recognition and signaling activation. These proteins possess a conserved architecture, including a C-terminal leucine-rich repeat (LRR) domain, a central nucleotide-binding (NB) domain, and a variable N-terminal domain. Nevertheless, many paired NLRs linked in a head-to-head configuration have now been identified. The ones carrying integrated domains (IDs) can recognize pathogen effector proteins by various modes; these are known as sensor NLR (sNLR) proteins. Structural and biochemical studies have provided insights into the molecular basis of heavy metal-associated IDs (HMA IDs) from paired NLRs in rice and revealed the co-evolution between pathogens and hosts by combining naturally occurring favorable interactions across diverse interfaces. Focusing on structural and molecular models, here we highlight advances in structure-guided engineering to expand and enhance the response profile of paired NLR-HMA IDs in rice to variants of the rice blast pathogen MAX-effectors (Magnaporthe oryzae AVRs and ToxB-like). These results demonstrate that the HMA IDs-based design of rice materials with broad and enhanced resistance profiles possesses great application potential but also face considerable challenges.

Subcellular localization requirements and specificities for plant immune receptor Toll-interleukin-1 receptor signaling

Recent work shed light on how plant intracellular immune receptors of the nucleotide-binding leucine-rich repeat (NLR) family are activated upon pathogen effector recognition to trigger immune responses. Activation of Toll-interleukin-1 receptor (TIR) domain-containing NLRs (TNLs) induces receptor oligomerization and close proximity of the TIR domain, which is required for TIR enzymatic activity. TIR-catalyzed small signaling molecules bind to EDS1 family heterodimers and subsequently activate downstream helper NLRs, which function as Ca²⁺ permeable channel to activate immune responses eventually leading to cell death. Subcellular localization requirements of TNLs and signaling partners are not well understood, although they are required to understand fully the mechanisms underlying NLR early signaling. TNLs show diverse subcellular localization while EDS1 shows nucleocytosolic localization. Here, we studied the impact of TIR and EDS1 mislocalization on the signaling activation of different TNLs. In Nicotiana benthamiana, our results suggest that close proximity of TIR domains isolated from flax L6 and Arabidopsis RPS4 and SNC1 TNLs drives signaling activation from different cell compartments. Nevertheless, both Golgi-membrane anchored L6 and nucleocytosolic RPS4 have the same requirements for EDS1 subcellular localization in Arabidopsis thaliana. By using mislocalized variants of EDS1, we found that autoimmune L6 and RPS4 TIR domain can induce seedling cell death when EDS1 is present in the cytosol. However, when EDS1 is restricted to the nucleus, both induce a stunting phenotype but no cell death. Our data point out the importance of thoroughly investigating the dynamics of TNLs and signaling partners subcellular localization to understand TNL signaling fully.

Orthologous genes Pm12 and Pm21 from two wild relatives of wheat show evolutionary conservation but divergent powdery mildew resistance

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is a devastating disease that threatens wheat production worldwide. Pm12, which originated from Aegilops speltoides, a wild relative of wheat, confers strong resistance to powdery mildew and therefore has potential use in wheat breeding. Using susceptible mutants induced by gamma irradiation, we physically mapped and isolated Pm12 and showed it to be orthologous to Pm21 from Dasypyrum villosum, also a wild relative of wheat. The resistance function of Pm12 was validated via ethyl methanesulfonate mutagenesis, virus-induced gene silencing, and stable genetic transformation. Evolutionary analysis indicates that the Pm12/Pm21 loci in wheat species are relatively conserved but dynamic. Here, we demonstrated that the two orthologous genes, Pm12 and Pm21, possess differential resistance against the same set of Bgt isolates. Overexpression of the coiled-coil domains of both PM12 and PM21 induces cell death in Nicotiana benthamiana leaves. However, their full-length forms display different cell death-inducing activities caused by their distinct intramolecular interactions. Cloning of Pm12 will facilitate its application in wheat breeding programs. This study also gives new insight into two orthologous resistance genes, Pm12 and Pm21, which show different race specificities and intramolecular interaction patterns.

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.

Connecting the dots between cell surface- and intracellular-triggered immune pathways in plants

Plants can detect microbial molecules via surface-localized pattern-recognition receptors (PRRs) and intracellular immune receptors from the nucleotide-binding, leucine-rich repeat receptor (NLR) family. The corresponding pattern-triggered (PTI) and effector-triggered (ETI) immunity were long considered separate pathways, although they converge on largely similar cellular responses, such as calcium influx and overlapping gene reprogramming. A number of studies recently uncovered genetic and molecular interconnections between PTI and ETI, highlighting the complexity of the plant immune network. Notably, PRR- and NLR-mediated immune responses require and potentiate each other to reach an optimal immune output. How PTI and ETI connect to confer robust immunity in different plant species, including crops will be an exciting future research area.

Direct recognition of pathogen effectors by plant NLR immune receptors and downstream signalling

Plants deploy extracellular and intracellular immune receptors to sense and restrict pathogen attacks. Rapidly evolving pathogen effectors play crucial roles in suppressing plant immunity but are also monitored by intracellular nucleotide-binding, leucine-rich repeat immune receptors (NLRs), leading to effector-triggered immunity (ETI). Here, we review how NLRs recognize effectors with a focus on direct interactions and summarize recent research findings on the signalling functions of NLRs. Coiled-coil (CC)-type NLR proteins execute immune responses by oligomerizing to form membrane-penetrating ion channels after effector recognition. Some CC-NLRs function in sensor-helper networks with the sensor NLR triggering oligomerization of the helper NLR. Toll/interleukin-1 receptor (TIR)-type NLR proteins possess catalytic activities that are activated upon effector recognition-induced oligomerization. Small molecules produced by TIR activity are detected by additional signalling partners of the EDS1 lipase-like family (enhanced disease susceptibility 1), leading to activation of helper NLRs that trigger the defense response.

Large-scale mutational analysis of wheat powdery mildew resistance gene Pm21

Wheat powdery mildew is a devastating disease leading to severe yield loss. The powdery mildew resistance gene Pm21, encoding a nucleotide-binding leucine-rich repeat receptor (NLR) protein, confers broad-spectrum resistance to powdery mildew and has great potential for controlling this disease. In this study, a large-scale mutagenesis was conducted on wheat cultivar (cv.) Yangmai 18 carrying Pm21. As a result, a total of 113 independent mutant lines susceptible to powdery mildew were obtained, among which, only one lost the whole Pm21 locus and the other 112 harbored one- (107) or two-base (5) mutations in the encoding region of Pm21. From the 107 susceptible mutants containing one-base change, we found that 25 resulted in premature stop codons leading to truncated proteins and 82 led to amino acid changes involving in 59 functional sites. We determined the mutations per one hundred amino acids (MPHA) indexes of different domains, motifs, and non-domain and non-motif regions of PM21 protein and found that the loss-of-function mutations occurred in a tendentious means. We also observed a new mutation hotspot that was closely linked to RNBS-D motif of the NB-ARC domain and relatively conserved in different NLRs of wheat crops. In addition, we crossed all the susceptible mutants with Yangmai 18 carrying wild-type Pm21, subsequently phenotyped their F₁ plants and revealed that the variant E44K in the coiled-coil (CC) domain could lead to dominant-negative effect. This study revealed key functional sites of PM21 and their distribution characteristics, which would contribute to understanding the relationship of resistance and structure of Pm21-encoded NLR.

A genetically linked pair of NLR immune receptors shows contrasting patterns of evolution

Throughout their evolution, plant nucleotide-binding leucine-rich-repeat receptors (NLRs) have acquired widely divergent unconventional integrated domains that enhance their ability to detect pathogen effectors. However, the functional dynamics that drive the evolution of NLRs with integrated domains (NLR-IDs) remain poorly understood. Here, we reconstructed the evolutionary history of an NLR locus prone to unconventional domain integration and experimentally tested hypotheses about the evolution of NLR-IDs. We show that the rice (Oryza sativa) NLR Pias recognizes the effector AVR-Pias of the blast fungal pathogen Magnaporthe oryzae. Pias consists of a functionally specialized NLR pair, the helper Pias-1 and the sensor Pias-2, that is allelic to the previously characterized Pia pair of NLRs: the helper RGA4 and the sensor RGA5. Remarkably, Pias-2 carries a C-terminal DUF761 domain at a similar position to the heavy metal-associated (HMA) domain of RGA5. Phylogenomic analysis showed that Pias-2/RGA5 sensor NLRs have undergone recurrent genomic recombination within the genus Oryza, resulting in up to six sequence-divergent domain integrations. Allelic NLRs with divergent functions have been maintained transspecies in different Oryza lineages to detect sequence-divergent pathogen effectors. By contrast, Pias-1 has retained its NLR helper activity throughout evolution and is capable of functioning together with the divergent sensor-NLR RGA5 to respond to AVR-Pia. These results suggest that opposite selective forces have driven the evolution of paired NLRs: highly dynamic domain integration events maintained by balancing selection for sensor NLRs, in sharp contrast to purifying selection and functional conservation of immune signaling for helper NLRs.

Ca2+ signals in plant immunity

Calcium ions function as a key second messenger ion in eukaryotes. Spatially and temporally defined cytoplasmic Ca²⁺ signals are shaped through the concerted activity of ion channels, exchangers, and pumps in response to diverse stimuli; these signals are then decoded through the activity of Ca²⁺ -binding sensor proteins. In plants, Ca²⁺ signaling is central to both pattern- and effector-triggered immunity, with the generation of characteristic cytoplasmic Ca²⁺ elevations in response to potential pathogens being common to both. However, despite their importance, and a long history of scientific interest, the transport proteins that shape Ca²⁺ signals and their integration remain poorly characterized. Here, we discuss recent work that has both shed light on and deepened the mysteries of Ca²⁺ signaling in plant immunity.

Indirect recognition of pathogen effectors by NLRs

To perceive pathogen threats, plants utilize both plasma membrane-localized and intracellular receptors. Nucleotide-binding domain leucine-rich repeat containing (NLR) proteins are key receptors that can recognize pathogen-derived intracellularly delivered effectors and activate downstream defense. Exciting recent findings have propelled our understanding of the various recognition and activation mechanisms of plant NLRs. Some NLRs directly bind to effectors, but others can perceive effector-induced changes on targeted host proteins (guardees), or non-functional host protein mimics (decoys). Such guarding strategies are thought to afford the host more durable resistance to quick-evolving and diverse pathogens. Here, we review classic and recent examples of indirect effector recognition by NLRs and discuss strategies for the discovery and study of new NLR-decoy/guardee systems. We also provide a perspective on how executor NLRs and helper NLRs (hNLRs) provide recognition for a wider range of effectors through sensor NLRs and how this can be considered an expanded form of indirect recognition. Furthermore, we summarize recent structural findings on NLR activation and resistosome formation upon indirect recognition. Finally, we discuss existing and potential applications that harness NLR indirect recognition for plant disease resistance and crop resilience.

SGT1-Specific Domain Mutations Impair Interactions with the Barley MLA6 Immune Receptor in Association with Loss of NLR Protein

The Mla (Mildew resistance locus a) of barley (Hordeum vulgare L.) is an effective model for cereal immunity against fungal pathogens. Like many resistance proteins, variants of the MLA coiled-coil nucleotide-binding leucine-rich repeat (CC-NLR) receptor often require the HRS complex (HSP90, RAR1, and SGT1) to function. However, functional analysis of Sgt1 has been particularly difficult, as deletions are often lethal. Recently, we identified rar3 (required for Mla6 resistance 3), an in-frame Sgt1_ΔKL308-309 mutation in the SGT1-specific domain, that alters resistance conferred by MLA but without lethality. Here, we use autoactive MLA6 and recombinant yeast-two-hybrid strains with stably integrated HvRar1 and HvHsp90 to determine that this mutation weakens but does not entirely disrupt the interaction between SGT1 and MLA. This causes a concomitant reduction in MLA6 protein accumulation below the apparent threshold required for effective resistance. The ΔKL308-309 deletion had a lesser effect on intramolecular interactions than alanine or arginine substitutions, and MLA variants that display diminished interactions with SGT1 appear to be disproportionately affected by the SGT1_ΔKL308-309 mutation. We hypothesize that those dimeric plant CC-NLRs that appear unaffected by Sgt1 silencing are those with the strongest intermolecular interactions with it. Combining our data with recent work in CC-NLRs, we propose a cyclical model of the MLA-HRS resistosome interactions.[Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Structural basis of NLR activation and innate immune signalling in plants

Animals and plants have NLRs (nucleotide-binding leucine-rich repeat receptors) that recognize the presence of pathogens and initiate innate immune responses. In plants, there are three types of NLRs distinguished by their N-terminal domain: the CC (coiled-coil) domain NLRs, the TIR (Toll/interleukin-1 receptor) domain NLRs and the RPW8 (resistance to powdery mildew 8)-like coiled-coil domain NLRs. CC-NLRs (CNLs) and TIR-NLRs (TNLs) generally act as sensors of effectors secreted by pathogens, while RPW8-NLRs (RNLs) signal downstream of many sensor NLRs and are called helper NLRs. Recent studies have revealed three dimensional structures of a CNL (ZAR1) including its inactive, intermediate and active oligomeric state, as well as TNLs (RPP1 and ROQ1) in their active oligomeric states. Furthermore, accumulating evidence suggests that members of the family of lipase-like EDS1 (enhanced disease susceptibility 1) proteins, which are uniquely found in seed plants, play a key role in providing a link between sensor NLRs and helper NLRs during innate immune responses. Here, we summarize the implications of the plant NLR structures that provide insights into distinct mechanisms of action by the different sensor NLRs and discuss plant NLR-mediated innate immune signalling pathways involving the EDS1 family proteins and RNLs.

Functional diversification gave rise to allelic specialization in a rice NLR immune receptor pair

Cooperation between receptors from the nucleotide-binding, leucine-rich repeats (NLR) superfamily is important for intracellular activation of immune responses. NLRs can function in pairs that, upon pathogen recognition, trigger hypersensitive cell death and stop pathogen invasion. Natural selection drives specialization of host immune receptors towards an optimal response, whilst keeping a tight regulation of immunity in the absence of pathogens. However, the molecular basis of co-adaptation and specialization between paired NLRs remains largely unknown. Here, we describe functional specialization in alleles of the rice NLR pair Pik that confers resistance to strains of the blast fungus Magnaporthe oryzae harbouring AVR-Pik effectors. We revealed that matching pairs of allelic Pik NLRs mount effective immune responses, whereas mismatched pairs lead to autoimmune phenotypes, a hallmark of hybrid necrosis in both natural and domesticated plant populations. We further showed that allelic specialization is largely underpinned by a single amino acid polymorphism that determines preferential association between matching pairs of Pik NLRs. These results provide a framework for how functionally linked immune receptors undergo co-adaptation to provide an effective and regulated immune response against pathogens. Understanding the molecular constraints that shape paired NLR evolution has implications beyond plant immunity given that hybrid necrosis can drive reproductive isolation.

Regulation of Cell Death and Signaling by Pore-Forming Resistosomes

Nucleotide-binding leucine-rich repeat receptors (NLRs) are the largest class of immune receptors in plants. They play a key role in the plant surveillance system by monitoring pathogen effectors that are delivered into the plant cell. Recent structural biology and biochemical analyses have uncovered how NLRs are activated to form oligomeric resistosomes upon the recognition of pathogen effectors. In the resistosome, the signaling domain of the NLR is brought to the center of a ringed structure to initiate immune signaling and regulated cell death (RCD). The N terminus of the coiled-coil (CC) domain of the NLR protein HOPZ-ACTIVATED RESISTANCE 1 likely forms a pore in the plasma membrane to trigger RCD in a way analogous to animal pore-forming proteins that trigger necroptosis or pyroptosis. NLRs that carry TOLL-INTERLEUKIN1-RECEPTOR as a signaling domain may also employ pore-forming resistosomes for RCD execution. In addition, increasing evidence supports intimate connections between NLRs and surface receptors in immune signaling. These new findings are rapidly advancing our understanding of the plant immune system.

A Comparative Overview of the Intracellular Guardians of Plants and Animals: NLRs in Innate Immunity and Beyond

Nucleotide-binding domain leucine-rich repeat receptors (NLRs) play important roles in the innate immune systems of both plants and animals. Recent breakthroughs in NLR biochemistry and biophysics have revolutionized our understanding of how NLR proteins function in plant immunity. In this review, we summarize the latest findings in plant NLR biology and draw direct comparisons to NLRs of animals. We discuss different mechanisms by which NLRs recognize their ligands in plants and animals. The discovery of plant NLR resistosomes that assemble in a comparable way to animal inflammasomes reinforces the striking similarities between the formation of plant and animal NLR complexes. Furthermore, we discuss the mechanisms by which plant NLRs mediate immune responses and draw comparisons to similar mechanisms identified in animals. Finally, we summarize the current knowledge of the complex genetic architecture formed by NLRs in plants and animals and the roles of NLRs beyond pathogen detection.

Disentangling cause and consequence: genetic dissection of the DANGEROUS MIX2 risk locus, and activation of the DM2h NLR in autoimmunity

Nucleotide-binding domain-leucine-rich repeat-type immune receptors (NLRs) protect plants against pathogenic microbes through intracellular detection of effector proteins. However, this comes at a cost, as NLRs can also induce detrimental autoimmunity in genetic interactions with foreign alleles. This may occur when independently evolved genomes are combined in inter- or intraspecific crosses, or when foreign alleles are introduced by mutagenesis or transgenesis. Most autoimmunity-inducing NLRs are encoded within highly variable NLR gene clusters with no known immune functions, which were termed autoimmune risk loci. Whether risk NLRs differ from sensor NLRs operating in natural pathogen resistance and how risk NLRs are activated in autoimmunity is unknown. Here, we analyzed the DANGEROUS MIX2 risk locus, a major autoimmunity hotspot in Arabidopsis thaliana. By gene editing and heterologous expression, we show that a single gene, DM2h, is necessary and sufficient for autoimmune induction in three independent cases of autoimmunity in accession Landsberg erecta. We focus on autoimmunity provoked by an EDS1-yellow fluorescent protein (YFP)NLS fusion protein to characterize DM2h functionally and determine features of EDS1-YFPNLS activating the immune receptor. Our data suggest that risk NLRs function in a manner reminiscent of sensor NLRs, while autoimmunity-inducing properties of EDS1-YFPNLS in this context are unrelated to the protein's functions as an immune regulator. We propose that autoimmunity, at least in some cases, may be caused by spurious, stochastic interactions of foreign alleles with coincidentally matching risk NLRs.

Recent Advances in Effector-Triggered Immunity in Plants: New Pieces in the Puzzle Create a Different Paradigm

Plants rely on multiple immune systems to protect themselves from pathogens. When pattern-triggered immunity (PTI)-the first layer of the immune response-is no longer effective as a result of pathogenic effectors, effector-triggered immunity (ETI) often provides resistance. In ETI, host plants directly or indirectly perceive pathogen effectors via resistance proteins and launch a more robust and rapid defense response. Resistance proteins are typically found in the form of nucleotide-binding and leucine-rich-repeat-containing receptors (NLRs). Upon effector recognition, an NLR undergoes structural change and associates with other NLRs. The dimerization or oligomerization of NLRs signals to downstream components, activates "helper" NLRs, and culminates in the ETI response. Originally, PTI was thought to contribute little to ETI. However, most recent studies revealed crosstalk and cooperation between ETI and PTI. Here, we summarize recent advancements in our understanding of the ETI response and its components, as well as how these components cooperate in the innate immune signaling pathways. Based on up-to-date accumulated knowledge, this review provides our current perspective of potential engineering strategies for crop protection.

Cross Kingdom Immunity: The Role of Immune Receptors and Downstream Signaling in Animal and Plant Cell Death

Both plants and animals are endowed with sophisticated innate immune systems to combat microbial attack. In these multicellular eukaryotes, innate immunity implies the presence of cell surface receptors and intracellular receptors able to detect danger signal referred as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Membrane-associated pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), receptor-like kinases (RLKs), and receptor-like proteins (RLPs) are employed by these organisms for sensing different invasion patterns before triggering antimicrobial defenses that can be associated with a form of regulated cell death. Intracellularly, animals nucleotide-binding and oligomerization domain (NOD)-like receptors or plants nucleotide-binding domain (NBD)-containing leucine rich repeats (NLRs) immune receptors likely detect effectors injected into the host cell by the pathogen to hijack the immune signaling cascade. Interestingly, during the co-evolution between the hosts and their invaders, key cross-kingdom cell death-signaling macromolecular NLR-complexes have been selected, such as the inflammasome in mammals and the recently discovered resistosome in plants. In both cases, a regulated cell death located at the site of infection constitutes a very effective mean for blocking the pathogen spread and protecting the whole organism from invasion. This review aims to describe the immune mechanisms in animals and plants, mainly focusing on cell death signaling pathways, in order to highlight recent advances that could be used on one side or the other to identify the missing signaling elements between the perception of the invasion pattern by immune receptors, the induction of defenses or the transmission of danger signals to other cells. Although knowledge of plant immunity is less advanced, these organisms have certain advantages allowing easier identification of signaling events, regulators and executors of cell death, which could then be exploited directly for crop protection purposes or by analogy for medical research.

Hybrid Incompatibility of the Plant Immune System: An Opposite Force to Heterosis Equilibrating Hybrid Performances

Hybridization is a core element in modern rice breeding as beneficial combinations of two parental genomes often result in the expression of heterosis. On the contrary, genetic incompatibility between parents can manifest as hybrid necrosis, which leads to tissue necrosis accompanied by compromised growth and/or reduced reproductive success. Genetic and molecular studies of hybrid necrosis in numerous plant species revealed that such self-destructing symptoms in most cases are attributed to autoimmunity: plant immune responses are inadvertently activated in the absence of pathogenic invasion. Autoimmunity in hybrids predominantly occurs due to a conflict involving a member of the major plant immune receptor family, the nucleotide-binding domain and leucine-rich repeat containing protein (NLR; formerly known as NBS-LRR). NLR genes are associated with disease resistance traits, and recent population datasets reveal tremendous diversity in this class of immune receptors. Cases of hybrid necrosis involving highly polymorphic NLRs as major causes suggest that diversified R gene repertoires found in different lineages would require a compatible immune match for hybridization, which is a prerequisite to ensure increased fitness in the resulting hybrids. In this review, we overview recent genetic and molecular findings on hybrid necrosis in multiple plant species to provide an insight on how the trade-off between growth and immunity is equilibrated to affect hybrid performances. We also revisit the cases of hybrid weakness in which immune system components are found or implicated to play a causative role. Based on our understanding on the trade-off, we propose that the immune system incompatibility in plants might play an opposite force to restrict the expression of heterosis in hybrids. The antagonism is illustrated under the plant fitness equilibrium, in which the two extremes lead to either hybrid necrosis or heterosis. Practical proposition from the equilibrium model is that breeding efforts for combining enhanced disease resistance and high yield shall be achieved by balancing the two forces. Reverse breeding toward utilizing genomic data centered on immune components is proposed as a strategy to generate elite hybrids with balanced immunity and growth.

A natural diversity screen in Arabidopsis thaliana reveals determinants for HopZ1a recognition in the ZAR1-ZED1 immune complex

Pathogen pressure on hosts can lead to the evolution of genes regulating the innate immune response. By characterizing naturally occurring polymorphisms in immune receptors, we can better understand the molecular determinants of pathogen recognition. ZAR1 is an ancient Arabidopsis thaliana NLR (Nucleotide-binding [NB] Leucine-rich-repeat [LRR] Receptor) that recognizes multiple secreted effector proteins from the pathogenic bacteria Pseudomonas syringae and Xanthomonas campestris through its interaction with receptor-like cytoplasmic kinases (RLCKs). ZAR1 was first identified for its role in recognizing P. syringae effector HopZ1a, through its interaction with the RLCK ZED1. To identify additional determinants of HopZ1a recognition, we performed a computational screen for ecotypes from the 1001 Genomes project that were likely to lack HopZ1a recognition, and tested ~300 ecotypes. We identified ecotypes containing polymorphisms in ZAR1 and ZED1. Using our previously established Nicotiana benthamiana transient assay and Arabidopsis ecotypes, we tested for the effect of naturally occurring polymorphisms on ZAR1 interactions and the immune response. We identified key residues in the NB or LRR domain of ZAR1 that impact the interaction with ZED1. We demonstrate that natural diversity combined with functional assays can help define the molecular determinants and interactions necessary to regulate immune induction in response to pathogens.

Genetics of autoimmunity in plants: an evolutionary genetics perspective

Autoimmunity in plants has been found in numerous hybrids as a form of hybrid necrosis and mutant panels. Uncontrolled cell death is a main cellular outcome of autoimmunity, which negatively impacts growth. Its occurrence highlights the vulnerable nature of the plant immune system. Genetic investigation of autoimmunity in hybrid plants revealed that extreme variation in the immune receptor repertoire is a major contributor, reflecting an evolutionary conundrum that plants face in nature. In this review, we discuss natural variation in the plant immune system and its contribution to fitness. The value of autoimmunity genetics lies in its ability to identify combinations of a natural immune receptor and its partner that are predisposed to triggering autoimmunity. The network of immune components for autoimmunity becomes instrumental in revealing mechanistic details of how immune receptors recognize cellular invasion and activate signaling. The list of autoimmunity-risk variants also allows us to infer evolutionary processes contributing to their maintenance in the natural population. Our approach to autoimmunity, which integrates mechanistic understanding and evolutionary genetics, has the potential to serve as a prognosis tool to optimize immunity in crops.

A Truncated Singleton NLR Causes Hybrid Necrosis in Arabidopsis thaliana

Hybrid necrosis in plants arises from conflict between divergent alleles of immunity genes contributed by different parents, resulting in autoimmunity. We investigate a severe hybrid necrosis case in Arabidopsis thaliana, where the hybrid does not develop past the cotyledon stage and dies 3 weeks after sowing. Massive transcriptional changes take place in the hybrid, including the upregulation of most NLR (nucleotide-binding site leucine-rich repeat) disease-resistance genes. This is due to an incompatible interaction between the singleton TIR-NLR gene DANGEROUS MIX 10 (DM10), which was recently relocated from a larger NLR cluster, and an unlinked locus, DANGEROUS MIX 11 (DM11). There are multiple DM10 allelic variants in the global A. thaliana population, several of which have premature stop codons. One of these, which has a truncated LRR-PL (leucine-rich repeat [LRR]-post-LRR) region, corresponds to the DM10 risk allele. The DM10 locus and the adjacent genomic region in the risk allele carriers are highly differentiated from those in the nonrisk carriers in the global A. thaliana population, suggesting that this allele became geographically widespread only relatively recently. The DM11 risk allele is much rarer and found only in two accessions from southwestern Spain-a region from which the DM10 risk haplotype is absent-indicating that the ranges of DM10 and DM11 risk alleles may be nonoverlapping.

Direct pathogen-induced assembly of an NLR immune receptor complex to form a holoenzyme

Direct or indirect recognition of pathogen-derived effectors by plant nucleotide-binding leucine-rich repeat (LRR) receptors (NLRs) initiates innate immune responses. The Hyaloperonospora arabidopsidis effector ATR1 activates the N-terminal Toll-interleukin-1 receptor (TIR) domain of Arabidopsis NLR RPP1. We report a cryo-electron microscopy structure of RPP1 bound by ATR1. The structure reveals a C-terminal jelly roll/Ig-like domain (C-JID) for specific ATR1 recognition. Biochemical and functional analyses show that ATR1 binds to the C-JID and the LRRs to induce an RPP1 tetrameric assembly required for nicotinamide adenine dinucleotide hydrolase (NADase) activity. RPP1 tetramerization creates two potential active sites, each formed by an asymmetric TIR homodimer. Our data define the mechanism of direct effector recognition by a plant NLR leading to formation of a signaling-active holoenzyme.

Advancement of research on plant NLRs evolution, biochemical activity, structural association, and engineering

In this review, we have included evolution of plant intracellular immune receptors, oligomeric complex formation, enzymatic action, engineering, and mechanisms of immune inspection for appropriate defense outcomes. NLR (Nucleotide binding oligomerization domain containing leucine-rich repeat) proteins are the intracellular immune receptors that recognize pathogen-derived virulence factors to confer effector-triggered immunity (ETI). Activation of plant defense by the NLRs are often conveyed through N-terminal Toll-like/ IL-1 receptor (TIR) or non-TIR (coiled-coils or CC) domains. Homodimerization or self-association property of CC/ TIR domains of plant NLRs contribute to their auto-activity and induction of in planta ectopic cell death. High resolution crystal structures of Arabidopsis thaliana RPS4^TIR, L6^TIR, SNC1^TIR, RPP1^TIR and Muscadinia rotundifolia RPV1^TIR showed that interaction is mediated through one or two distinct interfaces i.e., αA and αE helices comprise AE interface and αD and αE helices were found to form DE interface. By contrast, conserved helical regions were determined for CC domains of plant NLRs. Evolutionary history of NLRs diversification has shown that paired forms were originated from NLR singletons. Plant TIRs executed NAD⁺ hydrolysis activity for cell death promotion. Plant NLRs were found to form large oligomeric complexes as observed in animal inflammasomes. We have also discussed different protein engineering methods includes domain shuffling, and decoy modification that increase effector recognition spectrum of plant NLRs. In summary, our review highlights structural basis of perception of the virulence factors by NLRs or NLR pairs to design novel classes of plant immune receptors.

Role of AT1G72910, AT1G72940, and ADR1-LIKE 2 in Plant Immunity under Nonsense-Mediated mRNA Decay-Compromised Conditions at Low Temperatures

Nonsense-mediated mRNA decay (NMD) removes aberrant transcripts to avoid the accumulation of truncated proteins. NMD regulates nucleotide-binding, leucine-rich repeat (NLR) genes to prevent autoimmunity; however, the function of a large number of NLRs still remains poorly understood. Here, we show that three NLR genes (AT1G72910, AT1G72940, and ADR1-LIKE 2) are important for NMD-mediated regulation of defense signaling at lower temperatures. At 16 °C, the NMD-compromised up-frameshift protein1 (upf1) upf3 mutants showed growth arrest that can be rescued by the artificial miRNA-mediated knockdown of the three NLR genes. mRNA levels of these NLRs are induced by Pseudomonas syringae inoculation and exogenous SA treatment. Mutations in AT1G72910, AT1G72940, and ADR1-LIKE 2 genes resulted in increased susceptibility to Pseudomonas syringae, whereas their overexpression resulted in severely stunted growth, which was dependent on basal disease resistance genes. The NMD-deficient upf1 upf3 mutants accumulated higher levels of NMD signature-containing transcripts from these NLR genes at 16 °C. Furthermore, mRNA degradation kinetics showed that these NMD signature-containing transcripts were more stable in upf1 upf3 mutants. Based on these findings, we propose that AT1G72910, AT1G72940, and ADR1-LIKE 2 are directly regulated by NMD in a temperature-dependent manner and play an important role in modulating plant immunity at lower temperatures.

The rice NLR pair Pikp-1/Pikp-2 initiates cell death through receptor cooperation rather than negative regulation

Plant NLR immune receptors are multidomain proteins that can function as specialized sensor/helper pairs. Paired NLR immune receptors are generally thought to function via negative regulation, where one NLR represses the activity of the second and detection of pathogen effectors relieves this repression to initiate immunity. However, whether this mechanism is common to all NLR pairs is not known. Here, we show that the rice NLR pair Pikp-1/Pikp-2, which confers resistance to strains of the blast pathogen Magnaporthe oryzae (syn. Pyricularia oryzae) expressing the AVR-PikD effector, functions via receptor cooperation, with effector-triggered activation requiring both NLRs to trigger the immune response. To investigate the mechanism of Pikp-1/Pikp-2 activation, we expressed truncated variants of these proteins, and made mutations in previously identified NLR sequence motifs. We found that any domain truncation, in either Pikp-1 or Pikp-2, prevented cell death in the presence of AVR-PikD, revealing that all domains are required for activity. Further, expression of individual Pikp-1 or Pikp-2 domains did not result in cell death. Mutations in the conserved P-loop and MHD sequence motifs in both Pikp-1 and Pikp-2 prevented cell death activation, demonstrating that these motifs are required for the function of the two partner NLRs. Finally, we showed that Pikp-1 and Pikp-2 associate to form homo- and hetero-complexes in planta in the absence of AVR-PikD; on co-expression the effector binds to Pikp-1 generating a tri-partite complex. Taken together, we provide evidence that Pikp-1 and Pikp-2 form a fine-tuned system that is activated by AVR-PikD via receptor cooperation rather than negative regulation.

Single residues in the LRR domain of the wheat PM3A immune receptor can control the strength and the spectrum of the immune response

The development of improved plant nucleotide-binding, leucine-rich repeat (LRR) immune receptors (NLRs) has mostly been based on random mutagenesis or on structural information available for specific receptors complexed with the recognized pathogen effector. Here, we use a targeted mutagenesis approach based on the natural diversity of the Pm3 powdery mildew resistance alleles present in different wheat (Triticum aestivum) genotypes. In order to understand the functional importance of the amino acid polymorphisms between the active immune receptor PM3A and the inactive ancestral variant PM3CS, we exchanged polymorphic regions and residues in the LRR domain of PM3A with the corresponding segments of PM3CS. These novel variants were functionally tested for recognition of the corresponding AVRPM3^A2/F2 avirulence protein in Nicotiana benthamiana. We identified polymorphic residues in four regions of PM3A that enhance the immune response, but also residues that reduce it or result in complete loss of function. We found that the identified critical residues in PM3A modify its activation threshold towards different protein variants of AVRPM3^A2/F2 . PM3A variants with a lowered threshold gave a stronger overall response and gained an extended recognition spectrum. One of these variant proteins with a single amino acid change was stably transformed into wheat, where it conferred race-specific resistance to mildew. This is a proof of concept that improved PM3A variants with an enlarged recognition spectrum can be engineered based on natural diversity by exchanging single or multiple residues that modulate resistance function.

Resistosome and inflammasome: platforms mediating innate immunity

The nucleotide-binding domain (NBD) and leucine-rich repeat (LRR) containing (NLR) proteins are intracellular immune receptors that sense pathogens or stress-associated signals in animals and plants. Direct or indirect binding of these stimuli to NLRs results in formation of higher-order large protein complexes termed inflammasomes in animals and resistosomes in plants to mediate immune signaling. Here we focus on plant NLRs and discuss the activation mechanism of the ZAR1 resistosome from Arabidopsis thaliana. We also outline the analogies and differences between the ZAR1 resistosome and the NLR inflammasomes, and discuss how the structural and biochemical information available on these two large types of protein complexes sheds light on signaling mechanisms of other plant NLRs.

Induced proximity of a TIR signaling domain on a plant-mammalian NLR chimera activates defense in plants

Animal NLRs form wheel-like structures called inflammasomes upon perception of pathogen-associated molecules. The induced proximity of the signaling domains at the center of the wheel is hypothesized to recruit caspases for the first step of immune signal transduction. We expressed a plant-animal NLR fusion to demonstrate that induced proximity of TIR signaling domains from plant NLRs is sufficient to activate plant immune signaling. This demonstrates that a signaling-competent inflammasome can be formed from known, minimal components. The intrinsic NADase activity of plant TIRs is necessary for immune signaling, but fusions to a bacterial or a mammalian TIR domain with NADase activity, which also lead to accumulation of NAD⁺ hydrolysis products (e.g. cyclic ADP-ribose), were unable to activate immune signaling.

Plant and animal intracellular nucleotide-binding, leucine-rich repeat (NLR) immune receptors detect pathogen-derived molecules and activate defense. Plant NLRs can be divided into several classes based upon their N-terminal signaling domains, including TIR (Toll-like, Interleukin-1 receptor, Resistance protein)- and CC (coiled-coil)-NLRs. Upon ligand detection, mammalian NAIP and NLRC4 NLRs oligomerize, forming an inflammasome that induces proximity of its N-terminal signaling domains. Recently, a plant CC-NLR was revealed to form an inflammasome-like hetero-oligomer. To further investigate plant NLR signaling mechanisms, we fused the N-terminal TIR domain of several plant NLRs to the N terminus of NLRC4. Inflammasome-dependent induced proximity of the TIR domain in planta initiated defense signaling. Thus, induced proximity of a plant TIR domain imposed by oligomerization of a mammalian inflammasome is sufficient to activate authentic plant defense. Ligand detection and inflammasome formation is maintained when the known components of the NLRC4 inflammasome is transferred across kingdoms, indicating that NLRC4 complex can robustly function without any additional mammalian proteins. Additionally, we found NADase activity of a plant TIR domain is necessary for plant defense activation, but NADase activity of a mammalian or a bacterial TIR is not sufficient to activate defense in plants.

Pm21 CC domain activity modulated by intramolecular interactions is implicated in cell death and disease resistance

Nucleotide-binding (NB) leucine-rich repeat (LRR) receptors (NLRs) provide resistance against several plant pathogens. We previously cloned the wheat powdery mildew resistance gene Pm21, which encodes a coiled-coil (CC) NLR that confers broad-spectrum resistance against Blumeria graminis f. sp. tritici. Here, we report comprehensive biochemical and functional analyses of Pm21 CC domain in Nicotiana benthamiana. Transient overexpression assay suggested that only the extended CC (eCC, amino acid residues 1-159) domain has cell-death-inducing activity, whereas the CC-containing truncations, including CC-NB and CC-NB-LRR, do not induce cell-death responses. Coimmunoprecipitation (Co-IP) assay showed that the eCC domain self-associates and interacts with the NB and LRR domains in planta. These results imply that the activity of the eCC domain is inhibited by the intramolecular interactions of different domains in the absence of pathogens. We found that the LRR domain plays a crucial role in D491V-mediated full-length (FL) Pm21 autoactivation. Some mutations in the CC domain leading to the loss of Pm21 resistance to powdery mildew impaired the CC activity of cell-death induction. Two mutations (R73Q and E80K) interfered with D491V-mediated Pm21 autoactivation without affecting the cell-death-inducing activity of the eCC domain. Notably, some susceptible mutants harbouring mutations in the CC domain still exhibited cell-death-inducing activity. Taken together, these results implicate the CC domain of Pm21 in cell-death signalling and disease-resistance signalling, which are potentially independent of each other.

Evolution of Plant NLRs: From Natural History to Precise Modifications

Nucleotide-binding leucine-rich repeat receptors (NLRs) monitor the plant intracellular environment for signs of pathogen infection. Several mechanisms of NLR-mediated immunity arose independently across multiple species. These include the functional specialization of NLRs into sensors and helpers, the independent emergence of direct and indirect recognition within NLR subfamilies, the regulation of NLRs by small RNAs, and the formation of NLR networks. Understanding the evolutionary history of NLRs can shed light on both the origin of pathogen recognition and the common constraints on the plant immune system. Attempts to engineer disease resistance have been sparse and rarely informed by evolutionary knowledge. In this review, we discuss the evolution of NLRs, give an overview of previous engineering attempts, and propose how to use evolutionary knowledge to advance future research in the generation of novel disease-recognition capabilities.

Structural Insights into the Plant Immune Receptors PRRs and NLRs

Recent progresses made in structural analysis of plant PRRs and NLRs show the advancements in cryo-EM structural biology.

Atypical Resistance Protein RPW8/HR Triggers Oligomerization of the NLR Immune Receptor RPP7 and Autoimmunity

In certain plant hybrids, immunity signaling is initiated when immune components interact in the absence of a pathogen trigger. In Arabidopsis thaliana, such autoimmunity and cell death are linked to variants of the NLR RPP7 and the RPW8 proteins involved in broad-spectrum resistance. We uncover the molecular basis for this autoimmunity and demonstrate that a homolog of RPW8, HR4Fei-0, can trigger the assembly of a higher-order RPP7 complex, with autoimmunity signaling as a consequence. HR4Fei-0-mediated RPP7 oligomerization occurs via the RPP7 C-terminal leucine-rich repeat (LRR) domain and ATP-binding P-loop. RPP7 forms a higher-order complex only in the presence of HR4Fei-0 and not with the standard HR4 variant, which is distinguished from HR4Fei-0 by length variation in C-terminal repeats. Additionally, HR4Fei-0 can independently form self-oligomers, which directly kill cells in an RPP7-independent manner. Our work provides evidence for a plant resistosome complex and the mechanisms by which RPW8/HR proteins trigger cell death.

Plant and Animal Innate Immunity Complexes: Fighting Different Enemies with Similar Weapons

Both animals and plants express intracellular innate immunity receptors known as NLR (NOD-like receptors or nucleotide-binding domain and leucine-rich repeat receptors, respectively). For various mammalian systems, the specific formation of macromolecular structures, such as inflammasomes by activated NLR receptors, has been extensively reported. However, for plant organisms, the formation of such structures was an open scientific question for many years. This year, the first plant 'resistosome' structure was reported, revealing significant structural similarities to mammalian apoptosome and inflammasome structures. In this review, we summarize the key components comprising the mammalian apoptosome/inflammasome structures and the newly discovered plant resistosome, highlighting their commonalities and differences.

Structure-function analysis of ZAR1 immune receptor reveals key molecular interactions for activity

NLR (nucleotide-binding [NB] leucine-rich repeat [LRR] receptor) proteins are critical for inducing immune responses in response to pathogen proteins, and must be tightly modulated to prevent spurious activation in the absence of a pathogen. The ZAR1 NLR recognizes diverse effector proteins from Pseudomonas syringae, including HopZ1a, and Xanthomonas species. Receptor-like cytoplasmic kinases (RLCKs) such as ZED1, interact with ZAR1 and provide specificity for different effector proteins, such as HopZ1a. We previously developed a transient expression system in Nicotiana benthamiana that allowed us to demonstrate that ZAR1 function is conserved from the Brassicaceae to the Solanaceae. Here, we combined structural modelling of ZAR1, with molecular and functional assays in our transient system, to show that multiple intramolecular and intermolecular interactions modulate ZAR1 activity. We identified determinants required for the formation of the ZARCC oligomer and its activity. Lastly, we characterized intramolecular interactions between ZAR1 subdomains that participate in keeping ZAR1 immune complexes inactive. This work identifies molecular constraints on immune receptor function and activation.

The Plant "Resistosome": Structural Insights into Immune Signaling

Plant innate immunity is triggered via direct or indirect recognition of pathogen effectors by the NLR family immune receptors. Mechanistic understanding of plant NLR function has relied on structural information from individual NLR domains and inferences from studies on animal NLRs. Recent reports of the cryo-EM structures of the Arabidopsis plant immune receptor ZAR1 in monomeric inactive and transition states, as well as the active oligomeric state or the "resistosome," have afforded a quantum leap in our understanding of how plant NLRs function. In this Review, we outline the recent structural findings and examine their implications for the activation of plant immune receptors more broadly. We also discuss how NLR signaling in plants, as illustrated by the ZAR1 structure, is analogous to innate immune receptor signaling mechanisms across kingdoms, drawing particular attention to the concept of signaling by cooperative assembly formation.

A CC-NBS-LRR gene induces hybrid lethality in cotton

Hybrid lethality forms a reproductive barrier that has been found in many eukaryotes. Most cases follow the Bateson-Dobzhansky-Muller genetic incompatibility model and involve two or more loci. In this study, we demonstrate that a coiled-coil nucleotide-binding site leucine-rich repeat (CC-NBS-LRR) gene is the causal gene underlying the Le4 locus for interspecific hybrid lethality between Gossypium barbadense and G. hirsutum (cotton). Silencing this CC-NBS-LRR gene can restore F1 plants from a lethal to a normal phenotype. A total of 11 099 genes were differentially expressed between the leaves of normal and lethal F1 plants, of which genes related to autoimmune responses were highly enriched. Genes related to ATP-binding and ATPase were up-regulated before the lethal syndrome appeared; this may result in the conversion of Le4 into an active state and hence trigger immune signals in the absence of biotic/abiotic stress. We discuss our results in relation to the evolution and domestication of Sea Island cottons and the molecular mechanisms of hybrid lethality associated with autoimmune responses. Our findings provide new insights into reproductive isolation and may benefit cotton breeding.

Double autoinhibition mechanism of signal transduction ATPases with numerous domains (STAND) with a tetratricopeptide repeat sensor

Upon triggering by their inducer, signal transduction ATPases with numerous domains (STANDs), initially in monomeric resting forms, multimerize into large hubs that activate target macromolecules. This process requires conversion of the STAND conserved core (the NOD) from a closed form encasing an ADP molecule to an ATP-bound open form prone to multimerize. In the absence of inducer, autoinhibitory interactions maintain the NOD closed. In particular, in resting STAND proteins with an LRR- or WD40-type sensor domain, the latter establishes interactions with the NOD that are disrupted in the multimerization-competent forms. Here, we solved the first crystal structure of a STAND with a tetratricopeptide repeat sensor domain, PH0952 from Pyrococcus horikoshii, revealing analogous NOD-sensor contacts. We use this structural information to experimentally demonstrate that similar interactions also exist in a PH0952 homolog, the MalT STAND archetype, and actually contribute to the MalT autoinhibition in vitro and in vivo. We propose that STAND activation occurs by stepwise release of autoinhibitory contacts coupled to the unmasking of inducer-binding determinants. The MalT example suggests that STAND weak autoinhibitory interactions could assist the binding of inhibitory proteins by placing in register inhibitor recognition elements born by two domains.