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

From Lesions to Lessons: Two Decades of Filamentous Plant Pathogen Genomics

Many filamentous microorganisms, such as fungi and oomycetes, have evolved the ability to colonize plants and cause devastating crop diseases. Coevolutionary conflicts with their hosts have shaped the genomes of these plant pathogens. Over the past 20 years, genomics and genomics-enabled technologies have revealed remarkable diversity in genome size, architecture, and gene regulatory mechanisms. Technical and conceptual advances continue to provide novel insights into evolutionary dynamics, diversification of distinct genomic compartments, and facilitated molecular disease diagnostics. In this review, we discuss how genomics has advanced our understanding of genome organization and plant-pathogen coevolution and provide a perspective on future developments in the field. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Computational studies reveal structural characterization and novel families of Puccinia striiformis f. sp. tritici effectors

Understanding the biological functions of Puccinia striiformis f. sp. tritici (Pst) effectors is fundamental for uncovering the mechanisms of pathogenicity and variability, thereby paving the way for developing durable and effective control strategies for stripe rust. However, due to the lack of an efficient genetic transformation system in Pst, progress in effector function studies has been slow. Here, we modeled the structures of 15,201 effectors from twelve Pst races or isolates, a Puccinia striiformis isolate, and one Puccinia striiformis f. sp. hordei isolate using AlphaFold2. Of these, 8,102 folds were successfully predicted, and we performed sequence- and structure-based annotations of these effectors. These effectors were classified into 410 structure clusters and 1,005 sequence clusters. Sequence lengths varied widely, with a concentration between 101-250 amino acids, and motif analysis revealed that 47% and 5.81% of the predicted effectors contain known effector motifs [Y/F/W]xC and RxLR, respectively highlighting the structural conservation across a substantial portion of the effectors. Subcellular localization predictions indicated a predominant cytoplasmic localization, with notable chloroplast and nuclear presence. Structure-guided analysis significantly enhances effector prediction efficiency as demonstrated by the 75% among 8,102 have structural annotation. The clustering and annotation prediction both based on the sequence and structure homologies allowed us to determine the adopted folding or fold families of the effectors. A common feature observed was the formation of structural homologies from different sequences. In our study, one of the comparative structural analyses revealed a new structure family with a core structure of four helices, including Pst27791, PstGSRE4, and PstSIE1, which target key wheat immune pathway proteins, impacting the host immune functions. Further comparative structural analysis showed similarities between Pst effectors and effectors from other pathogens, such as AvrSr35, AvrSr50, Zt-KP4-1, and MoHrip2, highlighting a possibility of convergent evolutionary strategies, yet to be supported by further data encompassing on some evolutionarily distant species. Currently, our initial analysis is the most one on Pst effectors' sequence, structural and annotation relationships providing a novel foundation to advance our future understanding of Pst pathogenicity and evolution.

Technical Advances Drive the Molecular Understanding of Effectors from Wheat and Barley Powdery Mildew Fungi

Pathogens manipulate host physiology through the secretion of virulence factors (effectors) to invade and proliferate on the host. The molecular functions of effectors inside plant hosts have been of interest in the field of molecular plant-microbe interactions. Obligate biotrophic pathogens, such as rusts and powdery mildews, cannot proliferate outside of plant hosts. In addition to the inhibition of the plant's immune components, these pathogens are under particular pressure to extract nutrients efficiently from the host. Understanding the molecular basis of infections mediated by obligate biotrophic pathogens is significant because of their impact in modern agriculture. In addition, powdery mildews serve as excellent models for obligate biotrophic cereal pathogens. Here, we summarize the current knowledge on the effectorome of the barley and wheat powdery mildews and putative molecular virulence functions of effectors. We emphasize the availability of comprehensive genomic, transcriptomic, and proteomic resources and discuss the methodological approaches used for identifying candidate effectors, assessing effector virulence traits, and identifying effector targets in the host. We highlight established and more recently employed methodologies, discuss limitations, and suggest additional strategies. We identify open questions and discuss how addressing them with currently available resources will enhance our understanding of Blumeria candidates for secretor effector proteins. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

All Roads Lead to Rome: Pathways to Engineering Disease Resistance in Plants

Unlike animals, plants are unable to move and lack specialized immune cells and circulating antibodies. As a result, they are always threatened by a large number of microbial pathogens and harmful pests that can significantly reduce crop yield worldwide. Therefore, the development of new strategies to control them is essential to mitigate the increasing risk of crops lost to plant diseases. Recent developments in genetic engineering, including efficient gene manipulation and transformation methods, gene editing and synthetic biology, coupled with the understanding of microbial pathogenicity and plant immunity, both at molecular and genomic levels, have enhanced the capabilities to develop disease resistance in plants. This review comprehensively explains the fundamental mechanisms underlying the tug-of-war between pathogens and hosts, and provides a detailed overview of different strategies for developing disease resistance in plants. Additionally, it provides a summary of the potential genes that can be employed in resistance breeding for key crops to combat a wide range of potential pathogens and pests, including fungi, oomycetes, bacteria, viruses, nematodes, and insects. Furthermore, this review addresses the limitations associated with these strategies and their possible solutions. Finally, it discusses the future perspectives for producing plants with durable and broad-spectrum disease resistance.

Leveraging coevolutionary insights and AI-based structural modeling to unravel receptor-peptide ligand-binding mechanisms

Secreted signaling peptides are central regulators of growth, development, and stress responses, but specific steps in the evolution of these peptides and their receptors are not well understood. Also, the molecular mechanisms of peptide-receptor binding are only known for a few examples, primarily owing to the limited availability of protein structural determination capabilities to few laboratories worldwide. Plants have evolved a multitude of secreted signaling peptides and corresponding transmembrane receptors. Stress-responsive SERINE RICH ENDOGENOUS PEPTIDES (SCOOPs) were recently identified. Bioactive SCOOPs are proteolytically processed by subtilases and are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) in the model plant Arabidopsis thaliana. How SCOOPs and MIK2 have (co)evolved, and how SCOOPs bind to MIK2 are unknown. Using in silico analysis of 350 plant genomes and subsequent functional testing, we revealed the conservation of MIK2 as SCOOP receptor within the plant order Brassicales. We then leveraged AI-based structural modeling and comparative genomics to identify two conserved putative SCOOP-MIK2 binding pockets across Brassicales MIK2 homologues predicted to interact with the "SxS" motif of otherwise sequence-divergent SCOOPs. Mutagenesis of both predicted binding pockets compromised SCOOP binding to MIK2, SCOOP-induced complex formation between MIK2 and its coreceptor BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE 1, and SCOOP-induced reactive oxygen species production, thus, confirming our in silico predictions. Collectively, in addition to revealing the elusive SCOOP-MIK2 binding mechanism, our analytic pipeline combining phylogenomics, AI-based structural predictions, and experimental biochemical and physiological validation provides a blueprint for the elucidation of peptide ligand-receptor perception mechanisms.

Breeding and management of major resistance genes to stem canker/blackleg in Brassica crops

Blackleg (also known as Phoma or stem canker) is a major, worldwide disease of Brassica crop species, notably B. napus (rapeseed, canola), caused by the ascomycete fungus Leptosphaeria maculans. The outbreak and severity of this disease depend on environmental conditions and management practices, as well as a complex interaction between the pathogen and its hosts. Genetic resistance is a major method to control the disease (and the only control method in some parts of the world, such as continental Europe), but efficient use of genetic resistance is faced with many difficulties: (i) the scarcity of germplasm/genetic resources available, (ii) the different history of use of resistance genes in different parts of the world and the different populations of the fungus the resistance genes are exposed to, (iii) the complexity of the interactions between the plant and the pathogen that expand beyond typical gene-for-gene interactions, (iv) the incredible evolutionary potential of the pathogen and the importance of knowing the molecular processes set up by the fungus to "breakdown' resistances, so that we may design high-throughput diagnostic tools for population surveys, and (v) the different strategies and options to build up the best resistances and to manage them so that they are durable. In this paper, we aim to provide a comprehensive overview of these different points, stressing the differences between the different continents and the current prospects to generate new and durable resistances to blackleg disease.

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.

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.

Intrinsically Disordered Kiwellin Protein-Like Effectors Target Plant Chloroplasts and are Extensively Present in Rust Fungi

The effector proteins produced by plant pathogens are one of the essential components of host-pathogen interaction. Despite being important, most of the effector proteins remain unexplored due to the diversity in their primary sequence generated by the high selection pressure of the host immune system. However to maintain the primary function in the infection process, these effectors may tend to maintain their native protein fold to perform the corresponding biological function. In the present study, unannotated candidate secretory effector proteins of sixteen major plant fungal pathogens were analyzed to find the conserved known protein folds using homology, ab initio, and Alpha Fold/Rosetta Fold protein dimensional (3D) structure approaches. Several unannotated candidate effector proteins were found to match various known conserved protein families potentially involved in host defense manipulation in different plant pathogens. Surprisingly a large number of plant Kiwellin proteins fold like secretory proteins (> 100) were found in studied rust fungal pathogens. Many of them were predicted as potential effector proteins. Furthermore, template independent modelling using Alpha Fold/Rosetta Fold analysis and structural comparison of these candidates also predicted them to match with plant Kiwellin proteins. We also found plant Kiwellin matching proteins outside rusts including several non-pathogenic fungi suggesting the broad function of these proteins. One of the highest confidently modeled Kiwellin matching candidates effectors, Pstr_13960 (97.8%), from the Indian P. striiformis race Yr9 was characterized using overexpression, localization, and deletion studies in Nicotiana benthamiana. The Pstr_13960 suppressed the BAX-induced cell death and localized in the chloroplast. Furthermore, the expression of the Kiwellin matching region (Pst_13960_kiwi) alone suppressed the BAX-induced cell death in N. benthamiana despite the change of location to the cytoplasm and nucleus, suggesting the novel function of the Kiwellin core fold in rust fungi. Molecular docking showed that Pstr_13960 can interact with plant Chorismate mutases (CMs) using three loops conserved in plant and rust Kiwellins. Further analysis of Pstr_13960 showed to contain Intrinsically disordered regions (IDRs) in place of the N-terminal β1/β2 region found in plant Kiwellins suggesting the evolution of rust Kiwellins-like effectors (KLEs). Overall, this study reports the presence of a Kiwellin protein-like fold containing a novel effector protein family in rust fungi depicting a classical example of the evolution of effectors at the structure level as Kiwellin effectors show very low significant similarity to plant Kiwellin at the sequence level.

Computational Prediction of Structure, Function, and Interaction of Myzus persicae (Green Peach Aphid) Salivary Effector Proteins

Similar to plant pathogens, phloem-feeding insects such as aphids deliver effector proteins inside their hosts that act to promote host susceptibility and enable feeding and infestation. Despite exciting progress toward identifying and characterizing effector proteins from these insects, their functions remain largely unknown. The recent groundbreaking development in protein structure prediction algorithms, combined with the availability of proteomics and transcriptomic datasets for agriculturally important pests, provides new opportunities to explore the structural and functional diversity of effector repertoires. In this study, we sought to gain insight into the infection strategy used by the Myzus persicae (green peach aphid) by predicting and analyzing the structures of a set of 71 effector candidate proteins. We used two protein structure prediction methods, AlphaFold and OmegaFold, that produced mutually consistent results. We observed a wide continuous spectrum of structures among the effector candidates, from disordered proteins to globular enzymes. We made use of the structural information and state-of-the-art computational methods to predict M. persicae effector protein properties, including function and interaction with host plant proteins. Overall, our investigation provides novel insights into prediction of structure, function, and interaction of M. persicae effector proteins and will guide the necessary experimental characterization to address new hypotheses. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Novel Secreted Effectors Conserved Among Smut Fungi Contribute to the Virulence of Ustilago maydis

Fungal pathogens deploy a set of molecules (proteins, specialized metabolites, and sRNAs), so-called effectors, to aid the infection process. In comparison to other plant pathogens, smut fungi have small genomes and secretomes of 20 Mb and around 500 proteins, respectively. Previous comparative genomic studies have shown that many secreted effector proteins without known domains, i.e., novel, are conserved only in the Ustilaginaceae family. By analyzing the secretomes of 11 species within Ustilaginaceae, we identified 53 core homologous groups commonly present in this lineage. By collecting existing mutants and generating additional ones, we gathered 44 Ustilago maydis strains lacking single core effectors as well as 9 strains containing multiple deletions of core effector gene families. Pathogenicity assays revealed that 20 of these 53 mutant strains were affected in virulence. Among the 33 mutants that had no obvious phenotypic changes, 13 carried additional, sequence-divergent, structurally similar paralogs. We report a virulence contribution of seven previously uncharacterized single core effectors and of one effector family. Our results help to prioritize effectors for understanding U. maydis virulence and provide genetic resources for further characterization. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Development of an NLR-ID Toolkit and Identification of Novel Disease-Resistance Genes in Soybean

The recognition of pathogen effectors through the nucleotide-binding leucine-rich repeat receptor (NLR) family is an important component of plant immunity. In addition to typical domains such as TIR, CC, NBS, and LRR, NLR proteins also contain some atypical integrated domains (IDs), the roles of which are rarely investigated. Here, we carefully screened the soybean (Glycine max) genome and identified the IDs that appeared in the soybean TNL-like proteins. Our results show that multiple IDs (36) are widely present in soybean TNL-like proteins. A total of 27 Gm-TNL-ID genes (soybean TNL-like gene encoding ID) were cloned and their antiviral activity towards the soybean mosaic virus (SMV)/tobacco mosaic virus (TMV) was verified. Two resistance (R) genes, SRA2 (SMV resistance gene contains AAA_22 domain) and SRZ4 (SMV resistance gene contains zf-RVT domain), were identified to possess broad-spectrum resistance characteristics towards six viruses including SMV, TMV, plum pox virus (PPV), cabbage leaf curl virus (CaLCuV), barley stripe mosaic virus (BSMV), and tobacco rattle virus (TRV). The effects of Gm-TNL-IDX (the domain of the Gm-TNL-ID gene after the TN domain) on the antiviral activity of a R protein SRC7TN (we previously reported the TN domain of the soybean broad-spectrum resistance gene SRC7) were validated, and most of Gm-TNL-IDX inhibits antiviral activity mediated by SRC7TN, possibly through intramolecular interactions. Yeast-two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that seven Gm-TNL-IDX interacted with SMV-component proteins. Truncation analysis on a broad-spectrum antiviral protein SRZ4 indicated that SRZ4TIR is sufficient to mediate antiviral activity against SMV. Soybean cDNA library screening on SRZ4 identified 48 interacting proteins. In summary, our results indicate that the integration of IDs in soybean is widespread and frequent. The NLR-ID toolkit we provide is expected to be valuable for elucidating the functions of atypical NLR proteins in the plant immune system and lay the foundation for the development of engineering NLR for plant-disease control in the future.

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.

The synthetic NLR RGA5HMA5 requires multiple interfaces within and outside the integrated domain for effector recognition

Some plant sensor nucleotide-binding leucine-rich repeat (NLR) receptors detect pathogen effectors through their integrated domains (IDs). Rice RGA5 sensor NLR recognizes its corresponding effectors AVR-Pia and AVR1-CO39 from the blast fungus Magnaporthe oryzae through direct binding to its heavy metal-associated (HMA) ID to trigger the RGA4 helper NLR-dependent resistance in rice. Here, we report a mutant of RGA5 named RGA5HMA5 that confers complete resistance in transgenic rice plants to the M. oryzae strains expressing the noncorresponding effector AVR-PikD. RGA5HMA5 carries three engineered interfaces, two of which lie in the HMA ID and the other in the C-terminal Lys-rich stretch tailing the ID. However, RGA5 variants having one or two of the three interfaces, including replacing all the Lys residues with Glu residues in the Lys-rich stretch, failed to activate RGA4-dependent cell death of rice protoplasts. Altogether, this work demonstrates that sensor NLRs require a concerted action of multiple surfaces within and outside the IDs to both recognize effectors and activate helper NLR-mediated resistance, and has implications in structure-guided designing of sensor NLRs.

The NLR immune receptor ADR1 and lipase-like proteins EDS1 and PAD4 mediate stomatal immunity in Nicotiana benthamiana and Arabidopsis

In the presence of pathogenic bacteria, plants close their stomata to prevent pathogen entry. Intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors recognize pathogenic effectors and activate effector-triggered immune responses. However, the regulatory and molecular mechanisms of stomatal immunity involving NLR immune receptors are unknown. Here, we show that the Nicotiana benthamiana RPW8-NLR central immune receptor ACTIVATED DISEASE RESISTANCE 1 (NbADR1), together with the key immune proteins ENHANCED DISEASE SUSCEPTIBILITY 1 (NbEDS1) and PHYTOALEXIN DEFICIENT 4 (NbPAD4), plays an essential role in bacterial pathogen- and flg22-induced stomatal immunity by regulating the expression of salicylic acid (SA) and abscisic acid (ABA) biosynthesis or response-related genes. NbADR1 recruits NbEDS1 and NbPAD4 in stomata to form a stomatal immune response complex. The transcription factor NbWRKY40e, in association with NbEDS1 and NbPAD4, modulates the expression of SA and ABA biosynthesis or response-related genes to influence stomatal immunity. NbADR1, NbEDS1, and NbPAD4 are required for the pathogen infection-enhanced binding of NbWRKY40e to the ISOCHORISMATE SYNTHASE 1 promoter. Moreover, the ADR1-EDS1-PAD4 module regulates stomatal immunity in Arabidopsis (Arabidopsis thaliana). Collectively, our findings show the pivotal role of the core intracellular immune receptor module ADR1-EDS1-PAD4 in stomatal immunity, which enables plants to limit pathogen entry.

Allelic compatibility in plant immune receptors facilitates engineering of new effector recognition specificities

Engineering the plant immune system offers genetic solutions to mitigate crop diseases caused by diverse agriculturally significant pathogens and pests. Modification of intracellular plant immune receptors of the nucleotide-binding leucine-rich repeat (NLR) receptor superfamily for expanded recognition of pathogen virulence proteins (effectors) is a promising approach for engineering disease resistance. However, engineering can cause NLR autoactivation, resulting in constitutive defense responses that are deleterious to the plant. This may be due to plant NLRs associating in highly complex signaling networks that coevolve together, and changes through breeding or genetic modification can generate incompatible combinations, resulting in autoimmune phenotypes. The sensor and helper NLRs of the rice (Oryza sativa) NLR pair Pik have coevolved, and mismatching between noncoevolved alleles triggers constitutive activation and cell death. This limits the extent to which protein modifications can be used to engineer pathogen recognition and enhance disease resistance mediated by these NLRs. Here, we dissected incompatibility determinants in the Pik pair in Nicotiana benthamiana and found that heavy metal-associated (HMA) domains integrated in Pik-1 not only evolved to bind pathogen effectors but also likely coevolved with other NLR domains to maintain immune homeostasis. This explains why changes in integrated domains can lead to autoactivation. We then used this knowledge to facilitate engineering of new effector recognition specificities, overcoming initial autoimmune penalties. We show that by mismatching alleles of the rice sensor and helper NLRs Pik-1 and Pik-2, we can enable the integration of synthetic domains with novel and enhanced recognition specificities. Taken together, our results reveal a strategy for engineering NLRs, which has the potential to allow an expanded set of integrations and therefore new disease resistance specificities in plants.

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.

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.

Effector protein structures: a tale of evolutionary relationship

Effector proteins are highly diverse, often lacking similarity in their protein sequences, making it challenging to determine their biological function. Using AlphaFold2 (AF2), Seong and Krasileva recently found that effector structures, but not sequences, share commonality. This helps further understanding of effector evolution across fungal species and reveals unique sequence-unrelated, structurally similar, effector families.

Polymorphism of Avirulence Genes and Adaptation to Brassica Resistance Genes Is Gene-Dependent in the Phytopathogenic Fungus Leptosphaeria maculans

The fungal phytopathogen Leptosphaeria maculans, which causes stem canker (blackleg) of rapeseed (Brassica napus), is mainly controlled worldwide by genetic resistance, which includes major resistance genes (Rlm). This model is one of those for which the highest number of avirulence genes (AvrLm) has been cloned. In many systems, including the L. maculans-B. napus interaction, intense use of resistance genes exerts strong selection pressure on the corresponding avirulent isolates, and the fungi may rapidly escape resistance through various molecular events which modify the avirulence genes. In the literature, the study of polymorphism at avirulence loci is often focused on single genes under selection pressure. In this study, we investigate allelic polymorphism at 11 avirulence loci in a French population of 89 L. maculans isolates collected on a trap cultivar in four geographic locations in the 2017-2018 cropping season. The corresponding Rlm genes have been (i) used for a long time, (ii) recently used, or (iii) unused in agricultural practice. The sequence data generated indicate an extreme diversity of situations. For example, genes submitted to an ancient selection may have either been deleted in populations (AvrLm1) or replaced by a single-nucleotide mutated virulent version (AvrLm2, AvrLm5-9). Genes that have never been under selection may either be nearly invariant (AvrLm6, AvrLm10A, AvrLm10B), exhibit rare deletions (AvrLm11, AvrLm14), or display a high diversity of alleles and isoforms (AvrLmS-Lep2). These data suggest that the evolutionary trajectory of avirulence/virulence alleles is gene-dependent and independent of selection pressure in L. maculans. [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.

Pathogen protein modularity enables elaborate mimicry of a host phosphatase

Pathogens produce diverse effector proteins to manipulate host cellular processes. However, how functional diversity is generated in an effector repertoire is poorly understood. Many effectors in the devastating plant pathogen Phytophthora contain tandem repeats of the "(L)WY" motif, which are structurally conserved but variable in sequences. Here, we discovered a functional module formed by a specific (L)WY-LWY combination in multiple Phytophthora effectors, which efficiently recruits the serine/threonine protein phosphatase 2A (PP2A) core enzyme in plant hosts. Crystal structure of an effector-PP2A complex shows that the (L)WY-LWY module enables hijacking of the host PP2A core enzyme to form functional holoenzymes. While sharing the PP2A-interacting module at the amino terminus, these effectors possess divergent C-terminal LWY units and regulate distinct sets of phosphoproteins in the host. Our results highlight the appropriation of an essential host phosphatase through molecular mimicry by pathogens and diversification promoted by protein modularity in an effector repertoire.

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 pathogen effector FOLD diversified in symbiotic fungi

Pathogenic fungi use secreted effector proteins to suppress immunity and support their infection, but effectors have also been reported from fungi that engage in nutritional symbioses with plants. Sequence-based effector comparisons between pathogens and symbiotic arbuscular mycorrhizal (AM) fungi are hampered by the huge diversity of effector sequences even within closely related microbes. To find sequence-divergent but structurally similar effectors shared between symbiotic and pathogenic fungi, we compared secreted protein structure models of the AM fungus Rhizophagus irregularis to known pathogen effectors. We identified proteins with structural similarity to known Fusarium oxysporum f. sp. lycopersici dual domain (FOLD) effectors, which occur in low numbers in several fungal pathogens. Contrastingly, FOLD genes from AM fungi (MycFOLDs) are found in enlarged and diversified gene families with higher levels of positive selection in their C-terminal domains. Our structure model comparison suggests that MycFOLDs are similar to carbohydrate-binding motifs. Different MycFOLD genes are expressed during colonisation of different hosts and MycFOLD-17 transcripts accumulate in plant intracellular arbuscules. The exclusive presence of MycFOLDs across unrelated plant-colonising fungi, their inducible expression, lineage-specific sequence diversification and transcripts in arbuscules suggest that FOLD proteins act as effectors during plant colonisation of symbiotic and pathogenic fungi.

Effector Identification in Plant Pathogens

Effectors play a central role in determining the outcome of plant-pathogen interactions. As key virulence proteins, effectors are collectively indispensable for disease development. By understanding the virulence mechanisms of effectors, fundamental knowledge of microbial pathogenesis and disease resistance have been revealed. Effectors are also considered double-edged swords because some of them activate immunity in disease resistant plants after being recognized by specific immune receptors, which evolved to monitor pathogen presence or activity. Characterization of effector recognition by their cognate immune receptors and the downstream immune signaling pathways is instrumental in implementing resistance. Over the past decades, substantial research effort has focused on effector biology, especially concerning their interactions with virulence targets or immune receptors in plant cells. A foundation of this research is robust identification of the effector repertoire from a given pathogen, which depends heavily on bioinformatic prediction. In this review, we summarize methodologies that have been used for effector mining in various microbial pathogens which use different effector delivery mechanisms. We also discuss current limitations and provide perspectives on how recently developed analytic tools and technologies may facilitate effector identification and hence generation of a more complete vision of host-pathogen interactions. [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.

Genome-Wide Characterization of Effector Protein-Encoding Genes in Sclerospora graminicola and Its Validation in Response to Pearl Millet Downy Mildew Disease Stress

Pearl millet [Pennisetum glaucum (L.) R. Br.] is the essential food crop for over ninety million people living in drier parts of India and South Africa. Pearl millet crop production is harshly hindered by numerous biotic stresses. Sclerospora graminicola causes downy mildew disease in pearl millet. Effectors are the proteins secreted by several fungi and bacteria that manipulate the host cell structure and function. This current study aims to identify genes encoding effector proteins from the S. graminicola genome and validate them through molecular techniques. In silico analyses were employed for candidate effector prediction. A total of 845 secretory transmembrane proteins were predicted, out of which 35 proteins carrying LxLFLAK (Leucine–any amino acid–Phenylalanine–Leucine–Alanine–Lysine) motif were crinkler, 52 RxLR (Arginine, any amino acid, Leucine, Arginine), and 17 RxLR-dEER putative effector proteins. Gene validation analysis of 17 RxLR-dEER effector protein-producing genes was carried out, of which 5genes were amplified on the gel. These novel gene sequences were submitted to NCBI. This study is the first report on the identification and characterization of effector genes in Sclerospora graminicola. This dataset will aid in the integration of effector classes that act independently, paving the way to investigate how pearl millet responds to effector protein interactions. These results will assist in identifying functional effector proteins involving the omic approach using newer bioinformatics tools to protect pearl millet plants against downy mildew stress. Considered together, the identified effector protein-encoding functional genes can be utilized in screening oomycetes downy mildew diseases in other crops across the globe.

Ab Initio Modelling of the Structure of ToxA-like and MAX Fungal Effector Proteins

Pathogenic fungal diseases in crops are mediated by the release of effector proteins that facilitate infection. Characterising the structure of these fungal effectors is vital to understanding their virulence mechanisms and interactions with their hosts, which is crucial in the breeding of plant cultivars for disease resistance. Several effectors have been identified and validated experimentally; however, their lack of sequence conservation often impedes the identification and prediction of their structure using sequence similarity approaches. Structural similarity has, nonetheless, been observed within fungal effector protein families, creating interest in validating the use of computational methods to predict their tertiary structure from their sequence. We used Rosetta ab initio modelling to predict the structures of members of the ToxA-like and MAX effector families for which experimental structures are known to validate this method. An optimised approach was then used to predict the structures of phenotypically validated effectors lacking known structures. Rosetta was found to successfully predict the structure of fungal effectors in the ToxA-like and MAX families, as well as phenotypically validated but structurally unconfirmed effector sequences. Interestingly, potential new effector structural families were identified on the basis of comparisons with structural homologues and the identification of associated protein domains.

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.

NLRscape: an atlas of plant NLR proteins

NLRscape is a webserver that curates a collection of over 80 000 plant protein sequences identified in UniProtKB to contain NOD-like receptor signatures, and hosts in addition a number of tools aimed at the exploration of the complex sequence landscape of this class of plant proteins. Each entry gathers sequence information, domain and motif annotations from multiple third-party sources but also in-house advanced annotations aimed at addressing caveats of the existing broad-based annotations. NLRscape provides a top-down perspective of the NLR sequence landscape but also services for assisting a bottom-up approach starting from a given input sequence. Sequences are clustered by their domain organization layout, global homology and taxonomic spread-in order to allow analysis of how particular traits of an NLR family are scattered within the plant kingdom. Tools are provided for users to locate their own protein of interest in the overall NLR landscape, generate custom clusters centered around it and perform a large number of sequence and structural analyses using included interactive online instruments. Amongst these, we mention: taxonomy distribution plots, homology cluster graphs, identity matrices and interactive MSA synchronizing secondary structure and motif predictions. NLRscape can be found at: https://nlrscape.biochim.ro/.

The Venturia inaequalis effector repertoire is dominated by expanded families with predicted structural similarity, but unrelated sequence, to avirulence proteins from other plant-pathogenic fungi

BACKGROUND

Scab, caused by the biotrophic fungus Venturia inaequalis, is the most economically important disease of apples worldwide. During infection, V. inaequalis occupies the subcuticular environment, where it secretes virulence factors, termed effectors, to promote host colonization. Consistent with other plant-pathogenic fungi, many of these effectors are expected to be non-enzymatic proteins, some of which can be recognized by corresponding host resistance proteins to activate plant defences, thus acting as avirulence determinants. To develop durable control strategies against scab, a better understanding of the roles that these effector proteins play in promoting subcuticular growth by V. inaequalis, as well as in activating, suppressing, or circumventing resistance protein-mediated defences in apple, is required.

RESULTS

We generated the first comprehensive RNA-seq transcriptome of V. inaequalis during colonization of apple. Analysis of this transcriptome revealed five temporal waves of gene expression that peaked during early, mid, or mid-late infection. While the number of genes encoding secreted, non-enzymatic proteinaceous effector candidates (ECs) varied in each wave, most belonged to waves that peaked in expression during mid-late infection. Spectral clustering based on sequence similarity determined that the majority of ECs belonged to expanded protein families. To gain insights into function, the tertiary structures of ECs were predicted using AlphaFold2. Strikingly, despite an absence of sequence similarity, many ECs were predicted to have structural similarity to avirulence proteins from other plant-pathogenic fungi, including members of the MAX, LARS, ToxA and FOLD effector families. In addition, several other ECs, including an EC family with sequence similarity to the AvrLm6 avirulence effector from Leptosphaeria maculans, were predicted to adopt a KP6-like fold. Thus, proteins with a KP6-like fold represent another structural family of effectors shared among plant-pathogenic fungi.

CONCLUSIONS

Our study reveals the transcriptomic profile underpinning subcuticular growth by V. inaequalis and provides an enriched list of ECs that can be investigated for roles in virulence and avirulence. Furthermore, our study supports the idea that numerous sequence-unrelated effectors across plant-pathogenic fungi share common structural folds. In doing so, our study gives weight to the hypothesis that many fungal effectors evolved from ancestral genes through duplication, followed by sequence diversification, to produce sequence-unrelated but structurally similar proteins.

A potato late blight resistance gene protects against multiple Phytophthora species by recognizing a broadly conserved RXLR-WY effector

Species of the genus Phytophthora, the plant killer, cause disease and reduce yields in many crop plants. Although many Resistance to Phytophthora infestans (Rpi) genes effective against potato late blight have been cloned, few have been cloned against other Phytophthora species. Most Rpi genes encode nucleotide-binding domain, leucine-rich repeat-containing (NLR) immune receptor proteins that recognize RXLR (Arg-X-Leu-Arg) effectors. However, whether NLR proteins can recognize RXLR effectors from multiple Phytophthora species has rarely been investigated. Here, we identified a new RXLR-WY effector AVRamr3 from P. infestans that is recognized by Rpi-amr3 from a wild Solanaceae species Solanum americanum. Rpi-amr3 associates with AVRamr3 in planta. AVRamr3 is broadly conserved in many different Phytophthora species, and the recognition of AVRamr3 homologs by Rpi-amr3 activates resistance against multiple Phytophthora pathogens, including the tobacco black shank disease and cacao black pod disease pathogens P. parasitica and P. palmivora. Rpi-amr3 is thus the first characterized resistance gene that acts against P. parasitica or P. palmivora. These findings suggest a novel path to redeploy known R genes against different important plant pathogens.