The stem rust effector protein AvrSr50 escapes Sr50 recognition by a substitution in a single surface-exposed residue

AvrSr27 is a zinc-bound effector with a modular structure important for immune recognition

Effector proteins are central to the success of plant pathogens, while immunity in host plants is driven by receptor-mediated recognition of these effectors. Understanding the molecular details of effector-receptor interactions is key for the engineering of novel immune receptors. Here, we experimentally determined the crystal structure of the Puccinia graminis f. sp. tritici (Pgt) effector AvrSr27, which was not accurately predicted using AlphaFold2. We characterised the role of the conserved cysteine residues in AvrSr27 using in vitro biochemical assays and examined Sr27-mediated recognition using transient expression in Nicotiana spp. and wheat protoplasts. The AvrSr27 structure contains a novel β-strand rich modular fold consisting of two structurally similar domains that bind to Zn2+ ions. The N-terminal domain of AvrSr27 is sufficient for interaction with Sr27 and triggering cell death. We identified two Pgt proteins structurally related to AvrSr27 but with low sequence identity that can also associate with Sr27, albeit more weakly. Though only the full-length proteins, trigger Sr27-dependent cell death in transient expression systems. Collectively, our findings have important implications for utilising protein prediction platforms for effector proteins, and those embarking on bespoke engineering of immunity receptors as solutions to plant disease.

Pooled effector library screening in protoplasts rapidly identifies novel Avr genes

Crop breeding for durable disease resistance is challenging due to the rapid evolution of pathogen virulence. While progress in resistance (R) gene cloning and stacking has accelerated in recent years1-3, the identification of corresponding avirulence (Avr) genes in many pathogens is hampered by the lack of high-throughput screening options. To address this technology gap, we developed a platform for pooled library screening in plant protoplasts to allow rapid identification of interacting R-Avr pairs. We validated this platform by isolating known and novel Avr genes from wheat stem rust (Puccinia graminis f. sp. tritici) after screening a designed library of putative effectors against individual R genes. Rapid Avr gene identification provides molecular tools to understand and track pathogen virulence evolution via genotype surveillance, which in turn will lead to optimized R gene stacking and deployment strategies. This platform should be broadly applicable to many crop pathogens and could potentially be adapted for screening genes involved in other protoplast-selectable traits.

Comparative Analysis of the Avirulence Effectors Produced by the Fungal Stem Rust Pathogen of Wheat

Crops are constantly exposed to pathogenic microbes. Rust fungi are examples of these harmful microorganisms, which have a major economic impact on wheat production. To protect themselves from pathogens like rust fungi, plants employ a multilayered immune system that includes immunoreceptors encoded by resistance genes. Significant efforts have led to the isolation of numerous resistance genes against rust fungi in cereals, especially in wheat. However, the evolution of virulence of rust fungi hinders the durability of resistance genes as a strategy for crop protection. Rust fungi, like other biotrophic pathogens, secrete an arsenal of effectors to facilitate infection, and these are the molecules that plant immunoreceptors target for pathogen recognition and mounting defense responses. When recognized, these effector proteins are referred to as avirulence (Avr) effectors. Despite the many predicted effectors in wheat rust fungi, only five Avr genes have been identified, all from wheat stem rust. Knowledge of the Avr genes and their variation in the fungal population will inform deployment of the most appropriate wheat disease-resistance genes for breeding and farming. The review provides an overview of methodologies as well as the validation techniques that have been used to characterize Avr effectors from wheat stem rust. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Multiplexed effector screening for recognition by endogenous resistance genes using positive defense reporters in wheat protoplasts

Plant resistance (R) and pathogen avirulence (Avr) gene interactions play a vital role in pathogen resistance. Efficient molecular screening tools for crops lack far behind their model organism counterparts, yet they are essential to rapidly identify agriculturally important molecular interactions that trigger host resistance. Here, we have developed a novel wheat protoplast assay that enables efficient screening of Avr/R interactions at scale. Our assay allows access to the extensive gene pool of phenotypically described R genes because it does not require the overexpression of cloned R genes. It is suitable for multiplexed Avr screening, with interactions tested in pools of up to 50 Avr candidates. We identified Avr/R-induced defense genes to create a promoter-luciferase reporter. Then, we combined this with a dual-color ratiometric reporter system that normalizes read-outs accounting for experimental variability and Avr/R-induced cell death. Moreover, we introduced a self-replicative plasmid reducing the amount of plasmid used in the assay. Our assay increases the throughput of Avr candidate screening, accelerating the study of cellular defense signaling and resistance gene identification in wheat. We anticipate that our assay will significantly accelerate Avr identification for many wheat pathogens, leading to improved genome-guided pathogen surveillance and breeding of disease-resistant crops.

The structural repertoire of Fusarium oxysporum f. sp. lycopersici effectors revealed by experimental and computational studies

Plant pathogens secrete proteins, known as effectors, that function in the apoplast or inside plant cells to promote virulence. Effector recognition by cell-surface or cytosolic receptors results in the activation of defence pathways and plant immunity. Despite their importance, our general understanding of fungal effector function and recognition by immunity receptors remains poor. One complication often associated with effectors is their high sequence diversity and lack of identifiable sequence motifs precluding prediction of structure or function. In recent years, several studies have demonstrated that fungal effectors can be grouped into structural classes, despite significant sequence variation and existence across taxonomic groups. Using protein X-ray crystallography, we identify a new structural class of effectors hidden within the secreted in xylem (SIX) effectors from Fusarium oxysporum f. sp. lycopersici (Fol). The recognised effectors Avr1 (SIX4) and Avr3 (SIX1) represent the founding members of the Fol dual-domain (FOLD) effector class, with members containing two distinct domains. Using AlphaFold2, we predicted the full SIX effector repertoire of Fol and show that SIX6 and SIX13 are also FOLD effectors, which we validated experimentally for SIX6. Based on structural prediction and comparisons, we show that FOLD effectors are present within three divisions of fungi and are expanded in pathogens and symbionts. Further structural comparisons demonstrate that Fol secretes effectors that adopt a limited number of structural folds during infection of tomato. This analysis also revealed a structural relationship between transcriptionally co-regulated effector pairs. We make use of the Avr1 structure to understand its recognition by the I receptor, which leads to disease resistance in tomato. This study represents an important advance in our understanding of Fol-tomato, and by extension plant-fungal interactions, which will assist in the development of novel control and engineering strategies to combat plant pathogens.

Barley MLA3 recognizes the host-specificity effector Pwl2 from Magnaporthe oryzae

Plant nucleotide-binding leucine-rich repeat (NLRs) immune receptors directly or indirectly recognize pathogen-secreted effector molecules to initiate plant defense. Recognition of multiple pathogens by a single NLR is rare and usually occurs via monitoring for changes to host proteins; few characterized NLRs have been shown to recognize multiple effectors. The barley (Hordeum vulgare) NLR gene Mildew locus a (Mla) has undergone functional diversification, and the proteins encoded by different Mla alleles recognize host-adapted isolates of barley powdery mildew (Blumeria graminis f. sp. hordei [Bgh]). Here, we show that Mla3 also confers resistance to the rice blast fungus Magnaporthe oryzae in a dosage-dependent manner. Using a forward genetic screen, we discovered that the recognized effector from M. oryzae is Pathogenicity toward Weeping Lovegrass 2 (Pwl2), a host range determinant factor that prevents M. oryzae from infecting weeping lovegrass (Eragrostis curvula). Mla3 has therefore convergently evolved the capacity to recognize effectors from diverse pathogens.

Resistance that stacks up: engineering rust and mildew disease control in the cereal crops wheat and barley

Staying ahead of the arms race against rust and mildew diseases in cereal crops is essential to maintain and preserve food security. The methodological challenges associated with conventional resistance breeding are major bottlenecks for deploying resistance (R) genes in high-yielding crop varieties. Advancements in our knowledge of plant genomes, structural mechanisms, innovations in bioinformatics, and improved plant transformation techniques have alleviated this bottleneck by permitting rapid gene isolation, functional studies, directed engineering of synthetic resistance and precise genome manipulation in elite crop cultivars. Most cloned cereal R genes encode canonical immune receptors which, on their own, are prone to being overcome through selection for resistance-evading pathogenic strains. However, the increasingly large repertoire of cloned R genes permits multi-gene stacking that, in principle, should provide longer-lasting resistance. This review discusses how these genomics-enabled developments are leading to new breeding and biotechnological opportunities to achieve durable rust and powdery mildew control in cereals.

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.

A pathogen-induced putative NAC transcription factor mediates leaf rust resistance in barley

Leaf rust, caused by Puccinia hordei, is one of the most widespread and damaging foliar diseases affecting barley. The barley leaf rust resistance locus Rph7 has been shown to have unusually high sequence and haplotype divergence. In this study, we isolate the Rph7 gene using a fine mapping and RNA-Seq approach that is confirmed by mutational analysis and transgenic complementation. Rph7 is a pathogen-induced, non-canonical resistance gene encoding a protein that is distinct from other known plant disease resistance proteins in the Triticeae. Structural analysis using an AlphaFold2 protein model suggests that Rph7 encodes a putative NAC transcription factor with a zinc-finger BED domain with structural similarity to the N-terminal DNA-binding domain of the NAC transcription factor (ANAC019) from Arabidopsis. A global gene expression analysis suggests Rph7 mediates the activation and strength of the basal defence response. The isolation of Rph7 highlights the diversification of resistance mechanisms available for engineering disease control in crops.

Plant and pathogen genomics: essential approaches for stem rust resistance gene stacks in wheat

The deployment of disease resistance genes is currently the most economical and environmentally sustainable method of crop protection. However, disease resistance genes can rapidly break down because of constant pathogen evolution, particularly when they are deployed singularly. Polygenic resistance is, therefore, considered the most durable, but combining and maintaining these genes by breeding is a laborious process as effective genes are usually unlinked. The deployment of polygenic resistance with single-locus inheritance is a promising innovation that overcomes these difficulties while enhancing resistance durability. Because of major advances in genomic technologies, increasing numbers of plant resistance genes have been cloned, enabling the development of resistance transgene stacks (RTGSs) that encode multiple genes all located at a single genetic locus. Gene stacks encoding five stem rust resistance genes have now been developed in transgenic wheat and offer both breeding simplicity and potential resistance durability. The development of similar genomic resources in phytopathogens has advanced effector gene isolation and, in some instances, enabled functional validation of individual resistance genes in RTGS. Here, the wheat stem rust pathosystem is used as an illustrative example of how host and pathogen genomic advances have been instrumental in the development of RTGS, which is a strategy applicable to many other agricultural crop species.

Structural polymorphisms within a common powdery mildew effector scaffold as a driver of coevolution with cereal immune receptors

In plants, host-pathogen coevolution often manifests in reciprocal, adaptive genetic changes through variations in host nucleotide-binding leucine-rich repeat immune receptors (NLRs) and virulence-promoting pathogen effectors. In grass powdery mildew (PM) fungi, an extreme expansion of a RNase-like effector family, termed RALPH, dominates the effector repertoire, with some members recognized as avirulence (AVR) effectors by cereal NLR receptors. We report the structures of the sequence-unrelated barley PM effectors AVRA6, AVRA7, and allelic AVRA10/AVRA22 variants, which are detected by highly sequence-related barley NLRs MLA6, MLA7, MLA10, and MLA22 and of wheat PM AVRPM2 detected by the unrelated wheat NLR PM2. The AVR effectors adopt a common scaffold, which is shared with the RNase T1/F1 family. We found striking variations in the number, position, and length of individual structural elements between RALPH AVRs, which is associated with a differentiation of RALPH effector subfamilies. We show that all RALPH AVRs tested have lost nuclease and synthetase activities of the RNase T1/F1 family and lack significant binding to RNA, implying that their virulence activities are associated with neo-functionalization events. Structure-guided mutagenesis identified six AVRA6 residues that are sufficient to turn a sequence-diverged member of the same RALPH subfamily into an effector specifically detected by MLA6. Similar structure-guided information for AVRA10 and AVRA22 indicates that MLA receptors detect largely distinct effector surface patches. Thus, coupling of sequence and structural polymorphisms within the RALPH scaffold of PMs facilitated escape from NLR recognition and potential acquisition of diverse virulence functions.

Pathways to engineering plant intracellular NLR immune receptors

Factors including climate change and increased global exchange are set to escalate the prevalence of plant diseases, posing an unprecedented threat to global food security and making it more challenging to meet the demands of an ever-growing population. As such, new methods of pathogen control are essential to help with the growing danger of crop losses to plant diseases. The intracellular immune system of plants utilizes nucleotide-binding leucine-rich repeat (NLR) receptors to recognize and activate defense responses to pathogen virulence proteins (effectors) delivered to the host. Engineering the recognition properties of plant NLRs toward pathogen effectors is a genetic solution to plant diseases with high specificity, and it is more sustainable than several current methods for pathogen control that frequently rely on agrochemicals. Here, we highlight the pioneering approaches toward enhancing effector recognition in plant NLRs and discuss the barriers and solutions in engineering the plant intracellular immune system.

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 and functional analysis of the cerato-platanin-like protein Cpl1 suggests diverging functions in smut fungi

Plant-pathogenic fungi are causative agents of the majority of plant diseases and can lead to severe crop loss in infected populations. Fungal colonization is achieved by combining different strategies, such as avoiding and counteracting the plant immune system and manipulating the host metabolome. Of major importance are virulence factors secreted by fungi, which fulfil diverse functions to support the infection process. Most of these proteins are highly specialized, with structural and biochemical information often absent. Here, we present the atomic structures of the cerato-platanin-like protein Cpl1 from Ustilago maydis and its homologue Uvi2 from Ustilago hordei. Both proteins adopt a double-Ψβ-barrel architecture reminiscent of cerato-platanin proteins, a class so far not described in smut fungi. Our structure-function analysis shows that Cpl1 binds to soluble chitin fragments via two extended grooves at the dimer interface of the two monomer molecules. This carbohydrate-binding mode has not been observed previously and expands the repertoire of chitin-binding proteins. Cpl1 localizes to the cell wall of U. maydis and might synergize with cell wall-degrading and decorating proteins during maize infection. The architecture of Cpl1 harbouring four surface-exposed loop regions supports the idea that it might play a role in the spatial coordination of these proteins. While deletion of cpl1 has only mild effects on the virulence of U. maydis, a recent study showed that deletion of uvi2 strongly impairs U. hordei virulence. Our structural comparison between Cpl1 and Uvi2 reveals sequence variations in the loop regions that might explain a diverging function.

Precision genome editing of crops for improved disease resistance

Genome editing (GE) technologies allow rapid trait manipulation in crop plants. Disease resistance is one of the best test cases for this technology because it is usually monogenic and under constant challenge by rapidly evolving pathogens. Classical methods suffer from severe bottlenecks in discovery of new resistance (R) genes and their incorporation into elite varieties, largely because they are identified in landraces and species with limited sexual compatibility, and may last only a few years before losing effectiveness. Most plant R genes encode receptors located externally on the plasma membrane (receptor proteins and receptor kinases) or internally as NOD-like receptors (NLR). Both have well defined molecular interactions with activating pathogen ligands which are virulence proteins known as effectors. As structural data for R-effector interactions accumulate, promising strategies for rational manipulation of binding specificities are emerging. This offers the potential to change elite varieties directly rather than through 10-20 years of crossing. Successful application of GE is already evident in mutation of susceptibility (S) genes required for infection. GE is in its infancy with only four modified organisms grown currently in the US. The Anglosphere and Japan seem more open to deployment of these technologies, with the European Union, Switzerland and New Zealand being notably more conservative. Consumers are not well informed on the differences between GE and classical genetic modification (GM). The possibility that minor GE changes will not be regulated as GM offers the hope that current bottlenecks to resistance breeding can be eased.

A split GAL4 RUBY assay for visual in planta detection of protein-protein interactions

Protein-protein interactions (PPIs) are a fundamental process in cellular biogenesis. Here we have developed a split GAL4 RUBY assay that enables macroscopically visual PPI detection in plant leaves in real time. Candidate interacting protein partners are fused to specific domains of the yeast GAL4 and herpes simplex virus VP16 transcription factors and transiently expressed in Nicotiana benthamina leaves by Agrobacterium infiltration. PPI, that may be either direct or indirect, results in transcriptional activation of a RUBY reporter gene leading to the production of the highly visual metabolite, betalain, in leaf tissue of living plants. Samples require no processing for in planta visual qualitative assessment, but with very simple processing steps the assay is quantitative. Its accuracy is demonstrated using a series of known interacting protein partners and mutant derivatives including transcription factors, signalling molecules and plant resistance proteins with cognate pathogen effectors. Using this assay, association between the wheat Sr27 stem rust disease resistance protein and corresponding AvrSr27 avirulence effector family produced by the rust pathogen is detected. Interaction is also observed between this resistance protein and the effector encoded by the corresponding avrSr27-3 virulence allele. However, this association appears weaker in the split GAL4 RUBY assay, which coupled with lower avrSr27-3 expression during stem rust infection, likely enables virulent races of the rust pathogen to avoid Sr27-mediated detection.

Rapid evolution of an RNA virus to escape recognition by a rice nucleotide-binding and leucine-rich repeat domain immune receptor

Viral diseases are a major limitation for crop production, and their control is crucial for sustainable food supply. We investigated by a combination of functional genetics and experimental evolution the resistance of rice to the rice yellow mottle virus (RYMV), which is among the most devastating rice pathogens in Africa, and the mechanisms underlying the extremely fast adaptation of the virus to its host. We found that the RYMV3 gene that protects rice against the virus codes for a nucleotide-binding and leucine-rich repeat domain immune receptor (NLRs) from the Mla-like clade of NLRs. RYMV3 detects the virus by forming a recognition complex with the viral coat protein (CP). The virus escapes efficiently from detection by mutations in its CP, some of which interfere with the formation of the recognition complex. This study establishes that NLRs also confer in monocotyledonous plants immunity to viruses, and reveals an unexpected functional diversity for NLRs of the Mla clade that were previously only known as fungal disease resistance proteins. In addition, it provides precise insight into the mechanisms by which viruses adapt to plant immunity and gives important knowledge for the development of sustainable resistance against viral diseases of cereals.

Prediction of effector protein structures from fungal phytopathogens enables evolutionary analyses

Elucidating the similarity and diversity of pathogen effectors is critical to understand their evolution across fungal phytopathogens. However, rapid divergence that diminishes sequence similarities between putatively homologous effectors has largely concealed the roots of effector evolution. Here we modelled the structures of 26,653 secreted proteins from 14 agriculturally important fungal phytopathogens, six non-pathogenic fungi and one oomycete with AlphaFold 2. With 18,000 successfully predicted folds, we performed structure-guided comparative analyses on two aspects of effector evolution: uniquely expanded sequence-unrelated structurally similar (SUSS) effector families and common folds present across the fungal species. Extreme expansion of lineage-specific SUSS effector families was found only in several obligate biotrophs, Blumeria graminis and Puccinia graminis. The highly expanded effector families were the source of conserved sequence motifs, such as the Y/F/WxC motif. We identified new classes of SUSS effector families that include known virulence factors, such as AvrSr35, AvrSr50 and Tin2. Structural comparisons revealed that the expanded structural folds further diversify through domain duplications and fusion with disordered stretches. Putatively sub- and neo-functionalized SUSS effectors could reconverge on regulation, expanding the functional pools of effectors in the pathogen infection cycle. We also found evidence that many effector families could have originated from ancestral folds conserved across fungi. Collectively, our study highlights diverse effector evolution mechanisms and supports divergent evolution as a major force in driving SUSS effector evolution from ancestral proteins.

Wheat Genes Associated with Different Types of Resistance against Stem Rust (Puccinia graminis Pers.)

Stem rust is one wheat's most dangerous fungal diseases. Yield losses caused by stem rust have been significant enough to cause famine in the past. Some races of stem rust are considered to be a threat to food security even nowadays. Resistance genes are considered to be the most rational environment-friendly and widely used way to control the spread of stem rust and prevent yield losses. More than 60 genes conferring resistance against stem rust have been discovered so far (so-called Sr genes). The majority of the Sr genes discovered have lost their effectiveness due to the emergence of new races of stem rust. There are some known resistance genes that have been used for over 50 years and are still effective against most known races of stem rust. The goal of this article is to outline the different types of resistance against stem rust as well as the effective and noneffective genes, conferring each type of resistance with a brief overview of their origin and usage.

A wheat resistosome defines common principles of immune receptor channels

Plant intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) detect pathogen effectors to trigger immune responses1. Indirect recognition of a pathogen effector by the dicotyledonous Arabidopsis thaliana coiled-coil domain containing NLR (CNL) ZAR1 induces the formation of a large hetero-oligomeric protein complex, termed the ZAR1 resistosome, which functions as a calcium channel required for ZAR1-mediated immunity2-4. Whether the resistosome and channel activities are conserved among plant CNLs remains unknown. Here we report the cryo-electron microscopy structure of the wheat CNL Sr355 in complex with the effector AvrSr356 of the wheat stem rust pathogen. Direct effector binding to the leucine-rich repeats of Sr35 results in the formation of a pentameric Sr35-AvrSr35 complex, which we term the Sr35 resistosome. Wheat Sr35 and Arabidopsis ZAR1 resistosomes bear striking structural similarities, including an arginine cluster in the leucine-rich repeats domain not previously recognized as conserved, which co-occurs and forms intramolecular interactions with the 'EDVID' motif in the coiled-coil domain. Electrophysiological measurements show that the Sr35 resistosome exhibits non-selective cation channel activity. These structural insights allowed us to generate new variants of closely related wheat and barley orphan NLRs that recognize AvrSr35. Our data support the evolutionary conservation of CNL resistosomes in plants and demonstrate proof of principle for structure-based engineering of NLRs for crop improvement.

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.

Seeing is believing: Exploiting advances in structural biology to understand and engineer plant immunity

Filamentous plant pathogens cause disease in numerous economically important crops. These pathogens secrete virulence proteins, termed effectors, that modulate host cellular processes and promote infection. Plants have evolved immunity receptors that detect effectors and activate defence pathways, resulting in resistance to the invading pathogen. This leads to an evolutionary arms race between pathogen and host that is characterised by highly diverse effector repertoires in plant pathogens. Here, we review the recent advances in understanding host-pathogen co-evolution provided by the structural determination of effectors alone, and in complex with immunity receptors. We highlight the use of recent advances in structural prediction within this field and its role for future development of designer resistance proteins.