Characterization of Effector-Target Interactions in Necrotrophic Pathosystems Reveals Trends and Variation in Host Manipulation

Alterations in Gene Expression during Incompatible Interaction between Amendoim Cavalo Common Bean and Colletotrichum lindemuthianum

Anthracnose, caused by the fungus Colletotrichum lindemuthianum, poses a significant and widespread threat to the common bean crop. The use of plant genetic resistance has proven to be the most effective strategy for managing anthracnose disease. The Amendoim Cavalo (AC) Andean cultivar has resistance against multiple races of C. lindemuthianum, which is conferred by the Co-AC gene. Fine mapping of this resistance gene to common bean chromosome Pv01 enabled the identification of Phvul.001G244300, Phvul.001G244400, and Phvul.001G244500 candidate genes for further validation. In this study, the relative expression of Co-AC candidate genes was assessed, as well as other putative genes in the vicinity of this locus and known resistance genes, in the AC cultivar following inoculation with the race 73 of C. lindemuthianum. Gene expression analysis revealed significantly higher expression levels of Phvul.001G244500. Notably, Phvul.001G244500 encodes a putative Basic Helix-Loop-Helix transcription factor, suggesting its involvement in the regulation of defense responses. Furthermore, a significant modulation of the expression of defense-related genes PR1a, PR1b, and PR2 was observed in a time-course experiment. These findings contribute to the development of improved strategies for breeding anthracnose-resistant common bean cultivars, thereby mitigating the impact of this pathogen on crop yields and ensuring sustainable bean production.

Evolution of the Toxb Gene in Pyrenophora tritici-repentis and Related Species

Pyrenophora tritici-repentis (tan spot) is a destructive foliar pathogen of wheat with global impact. This ascomycete fungus possesses a highly plastic open pangenome shaped by the gain and loss of effector genes. This study investigated the allelic variations in the chlorosis-encoding gene ToxB across 422 isolates representing all identified pathotypes and worldwide origins. To gain better insights into ToxB evolution, we examined its presence and variability in other Pyrenophora spp. A ToxB haplotype network was constructed, revealing the evolutionary relationships of this gene (20 haplotypes) across four Pyrenophora species. Notably, toxb, the homolog of ToxB, was detected for the first time in the barley pathogen Pyrenophora teres. The ToxB/toxb genes display evidence of selection that is characterized by loss of function, duplication, and diverse mutations. Within the ToxB/toxb open reading frame, 72 mutations were identified, including 14 synonymous, 55 nonsynonymous, and 3 indel mutations. Remarkably, a, ∼5.6-kb Copia-like retrotransposon, named Copia-1_Ptr, was found inserted in the toxb gene of a race 3 isolate. This insert disrupted the ToxB gene's function, a first case of effector gene disruption by a transposable element in P. tritici-repentis. Additionally, a microsatellite with 25 nucleotide repeats (0 to 10) in the upstream region of ToxB suggested a potential mechanism influencing ToxB expression and regulation. Exploring ToxB-like protein distribution in other ascomycetes revealed the presence of ToxB-like proteins in 19 additional species, including the Leotiomycetes class for the first time. The presence/absence pattern of ToxB-like proteins defied species relatedness compared with a phylogenetic tree, suggesting a past horizontal gene transfer event during the evolution of the ToxB gene. [Formula: see text] Copyright © 2024 His Majesty the King in Right of Canada, as represented by the Minister of Agriculture and Agri-Food. This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Till death do us pair: Co-evolution of plant-necrotroph interactions

Plants use programmed cell death as a potent defense response against biotrophic pathogens that require living host cells to thrive. However, cell death can promote infection by necrotrophic pathogens. This discrepancy creates specific co-evolutionary dynamics in the interaction between plants and necrotrophs. Necrotrophic pathogens produce diverse cell death-inducing effectors that act redundantly on several plant targets and sometimes suppress plant immune responses as an additional function. Plants use surface receptors that recognize necrotrophic effectors to increase quantitative disease resistance, some of which evolved independently in several plant lineages. Co-evolution has shaped molecular mechanisms involved in plant-necrotroph interactions into robust systems, relying on degenerate and multifunctional modules, general-purpose components, and compartmentalized functioning.

The Necrotrophic Pathogen Parastagonospora nodorum Is a Master Manipulator of Wheat Defense

Parastagonospora nodorum is a necrotrophic pathogen of wheat that is particularly destructive in major wheat-growing regions of the United States, northern Europe, Australia, and South America. P. nodorum secretes necrotrophic effectors that target wheat susceptibility genes to induce programmed cell death (PCD), resulting in increased colonization of host tissue and, ultimately, sporulation to complete its pathogenic life cycle. Intensive research over the last two decades has led to the functional characterization of five proteinaceous necrotrophic effectors, SnTox1, SnToxA, SnTox267, SnTox3, and SnTox5, and three wheat susceptibility genes, Tsn1, Snn1, and Snn3D-1. Functional characterization has revealed that these effectors, in addition to inducing PCD, have additional roles in pathogenesis, including chitin binding that results in protection from wheat chitinases, blocking defense response signaling, and facilitating plant colonization. There are still large gaps in our understanding of how this necrotrophic pathogen is successfully manipulating wheat defense to complete its life cycle. This review summarizes our current knowledge, identifies knowledge gaps, and provides a summary of well-developed tools and resources currently available to study the P. nodorum-wheat interaction, which has become a model for necrotrophic specialist interactions. Further functional characterization of the effectors involved in this interaction and work toward a complete understanding of how P. nodorum manipulates wheat defense will provide fundamental knowledge about this and other necrotrophic interactions. Additionally, a broader understanding of this interaction will contribute to the successful management of Septoria nodorum blotch disease on wheat. [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.

Spatiotemporal patterns of wheat response to Pyrenophora tritici-repentis in asymptomatic regions revealed by transcriptomic and X-ray fluorescence microscopy analyses

Pathogen attacks elicit dynamic and widespread molecular responses in plants. While our understanding of plant responses has advanced considerably, little is known of the molecular responses in the asymptomatic 'green' regions adjoining lesions. Here, we explore gene expression data and high-resolution elemental imaging to report the spatiotemporal changes in the asymptomatic green region of susceptible and moderately resistant wheat cultivars infected with a necrotrophic fungal pathogen, Pyrenophora tritici-repentis. We show, with improved spatiotemporal resolution, that calcium oscillations are modified in the susceptible cultivar, resulting in 'frozen' host defence signals at the mature disease stage, and silencing of the host's recognition and defence mechanisms that would otherwise protect it from further attacks. In contrast, calcium accumulation and a heightened defence response were observed in the moderately resistant cultivar in the later stage of disease development. Furthermore, in the susceptible interaction, the asymptomatic green region was unable to recover after disease disruption. Our targeted sampling technique also enabled detection of eight previously predicted proteinaceous effectors in addition to the known ToxA effector. Collectively, our results highlight the benefits of spatially resolved molecular analysis and nutrient mapping to provide high-resolution spatiotemporal snapshots of host-pathogen interactions, paving the way for disentangling complex disease interactions in plants.

The effectors of Phytophthora infestans impact host immunity upon regulation of antagonistic hormonal activities

MAIN CONCLUSION

Phytophthora infestans effectors manipulate the antagonism of host hormones to interfere with the immune response of plants at different infection stages. Phytophthora infestans (P. infestans) poses a serious threat to global crop production, and its effectors play an indispensable role in its pathogenicity. However, the function of these effectors during the switch from biotrophy to necrotrophy of P. infestans remains unclear. Further research on the effectors that manipulate the antagonistic response of host hormones is also lacking. In this study, a coexpression analysis and infection assays were performed to identify distinct gene expression changes in both P. infestans and tomato. During the switch from biotrophy to necrotrophy, P. infestans secretes three types of effectors to interfere with host salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and abscisic acid (ABA) levels. The three aforementioned effectors also regulate the host gene expression including NPR1, TGA2.1, PDF1.2, NDR1, ERF3, NCED6, GAI4, which are involved in hormone crosstalk. The changes in plant hormones are mediated by the three types of effectors, which may accelerate infection and drive completion of the P. infestans lifecycle. Our findings provide new insight into plant‒pathogen interactions that may contribute to the prevention growth of hemibiotrophic pathogens.

Genome-wide association mapping of resistance to the foliar diseases septoria nodorum blotch and tan spot in a global winter wheat collection

Septoria nodorum blotch (SNB) and tan spot, caused by the necrotrophic fungal pathogens Parastagonospora nodorum and Pyrenophora tritici-repentis, respectively, often occur together as a leaf spotting disease complex on wheat (Triticum aestivum L.). Both pathogens produce necrotrophic effectors (NEs) that contribute to the development of disease. Here, genome-wide association analysis of a diverse panel of 264 winter wheat lines revealed novel loci on chromosomes 5A and 5B associated with sensitivity to the NEs SnTox3 and SnTox5 in addition to the known sensitivity genes for NEs Ptr/SnToxA, SnTox1, SnTox3, and SnTox5. Sensitivity loci for SnTox267 and Ptr ToxB were not detected. Evaluation of the panel with five P. nodorum isolates for SNB development indicated the Snn3-SnTox3 and Tsn1-SnToxA interactions played significant roles in disease development along with additional QTL on chromosomes 2A and 2D, which may correspond to the Snn7-SnTox267 interaction. For tan spot, the Tsc1-Ptr ToxC interaction was associated with disease caused by two isolates, and a novel QTL on chromosome 7D was associated with a third isolate. The Tsn1-ToxA interaction was associated with SNB but not tan spot. Therefore some, but not all, of the previously characterized host gene-NE interactions in these pathosystems play significant roles in disease development in winter wheat. Based on these results, breeders should prioritize the selection of resistance alleles at the Tsc1, Tsn1, Snn3, and Snn7 loci as well as the 2A and 7D QTL to obtain good levels of resistance to SNB and tan spot in winter wheat.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01400-5.

Genetic and physical localization of a major susceptibility gene to Pyrenophora teres f. maculata in barley

KEY MESSAGE

Genetic characterization of a major spot form net blotch susceptibility locus to using linkage mapping to identify a candidate gene and user-friendly markers in barley. Spot form net blotch (SFNB), caused by the necrotrophic fungal pathogen Pyrenophora teres f. maculata (Ptm), is an economically important foliar diseases in barley. Although various resistance loci have been identified, breeding for SFNB-resistant varieties has been hampered due to the complex virulence profile of Ptm populations. One resistance locus in the host may be effective against one specific isolate, but it may confer susceptibility to other isolates. A major susceptibility QTL on chromosome 7H, named Sptm1, was consistently identified in many studies. In the present study, we conduct fine mapping to localize Sptm1 with high resolution. A segregating population was developed from selected F₂ progenies of the cross Tradition (S) × PI 67381 (R), in which the disease phenotype was determined by the Sptm1 locus alone. Disease phenotypes of critical recombinants were confirmed in the following two consecutive generations. Genetic mapping anchored the Sptm1 gene to an ⁓400 kb region on chromosome 7H. Gene prediction and annotation identified six protein-coding genes in the delimited Sptm1 region, and the gene encoding a putative cold-responsive protein kinase was selected as a strong candidate. Therefore, providing fine localization and candidate of Sptm1 for functional validation, our study will facilitate the understanding of susceptibility mechanism underlying the barley-Ptm interaction and offers a potential target for gene editing to develop valuable materials with broad-spectrum resistance to SFNB.

The woody plant-degrading pathogen Lasiodiplodia theobromae effector LtCre1 targets the grapevine sugar-signaling protein VvRHIP1 to suppress host immunity

Lasiodiplodia theobromae is a causal agent of Botryosphaeria dieback, which seriously threatens grapevine production worldwide. Plant pathogens secrete diverse effectors to suppress host immune responses and promote the progression of infection, but the mechanisms underlying the manipulation of host immunity by L. theobromae effectors are poorly understood. In this study, we characterized LtCre1, which encodes a L. theobromae effector that suppresses BAX-triggered cell death in Nicotiana benthamiana. RNAi-silencing and overexpression of LtCre1 in L. theobromae showed impaired and increased virulence, respectively, and ectopic expression in N. benthamiana increased susceptibility. These results suggest that LtCre1 is as an essential virulence factor for L. theobromae. Protein-protein interaction studies revealed that LtCre1 interacts with grapevine RGS1-HXK1-interacting protein 1 (VvRHIP1). Ectopic overexpression of VvRHIP1 in N. benthamiana reduced infection, suggesting that VvRHIP1 enhances plant immunity against L. theobromae. LtCre1 was found to disrupt the formation of the VvRHIP1-VvRGS1 complex and to participate in regulating the plant sugar-signaling pathway. Thus, our results suggest that L. theobromae LtCre1 targets the grapevine VvRHIP1 protein to manipulate the sugar-signaling pathway by disrupting the association of the VvRHIP1-VvRGS1 complex.

Genetics and breeding for resistance against four leaf spot diseases in wheat (Triticum aestivum L.)

In wheat, major yield losses are caused by a variety of diseases including rusts, spike diseases, leaf spot and root diseases. The genetics of resistance against all these diseases have been studied in great detail and utilized for breeding resistant cultivars. The resistance against leaf spot diseases caused by each individual necrotroph/hemi-biotroph involves a complex system involving resistance (R) genes, sensitivity (S) genes, small secreted protein (SSP) genes and quantitative resistance loci (QRLs). This review deals with resistance for the following four-leaf spot diseases: (i) Septoria nodorum blotch (SNB) caused by Parastagonospora nodorum; (ii) Tan spot (TS) caused by Pyrenophora tritici-repentis; (iii) Spot blotch (SB) caused by Bipolaris sorokiniana and (iv) Septoria tritici blotch (STB) caused by Zymoseptoria tritici.

Combined pangenomics and transcriptomics reveals core and redundant virulence processes in a rapidly evolving fungal plant pathogen

BACKGROUND

Studying genomic variation in rapidly evolving pathogens potentially enables identification of genes supporting their "core biology", being present, functional and expressed by all strains or "flexible biology", varying between strains. Genes supporting flexible biology may be considered to be "accessory", whilst the "core" gene set is likely to be important for common features of a pathogen species biology, including virulence on all host genotypes. The wheat-pathogenic fungus Zymoseptoria tritici represents one of the most rapidly evolving threats to global food security and was the focus of this study.

RESULTS

We constructed a pangenome of 18 European field isolates, with 12 also subjected to RNAseq transcription profiling during infection. Combining this data, we predicted a "core" gene set comprising 9807 sequences which were (1) present in all isolates, (2) lacking inactivating polymorphisms and (3) expressed by all isolates. A large accessory genome, consisting of 45% of the total genes, was also defined. We classified genetic and genomic polymorphism at both chromosomal and individual gene scales. Proteins required for essential functions including virulence had lower-than average sequence variability amongst core genes. Both core and accessory genomes encoded many small, secreted candidate effector proteins that likely interact with plant immunity. Viral vector-mediated transient in planta overexpression of 88 candidates failed to identify any which induced leaf necrosis characteristic of disease. However, functional complementation of a non-pathogenic deletion mutant lacking five core genes demonstrated that full virulence was restored by re-introduction of the single gene exhibiting least sequence polymorphism and highest expression.

CONCLUSIONS

These data support the combined use of pangenomics and transcriptomics for defining genes which represent core, and potentially exploitable, weaknesses in rapidly evolving pathogens.

Optimization of highly efficient exogenous-DNA-free Cas9-ribonucleoprotein mediated gene editing in disease susceptibility loci in wheat (Triticum aestivum L.)

The advancement of precision engineering for crop trait improvement is important in the face of rapid population growth, climate change, and disease. To this end, targeted double-stranded break technology using RNA-guided Cas9 has been adopted widely for genome editing in plants. Agrobacterium or particle bombardment-based delivery of plasmids encoding Cas9 and guide RNA (gRNA) is common, but requires optimization of expression and often results in random integration of plasmid DNA into the plant genome. Recent advances have described gene editing by the delivery of Cas9 and gRNA as pre-assembled ribonucleoproteins (RNPs) into various plant tissues, but with moderate efficiency in resulting regenerated plants. In this report we describe significant improvements to Cas9-RNP mediated gene editing in wheat. We demonstrate that Cas9-RNP assays in protoplasts are a fast and effective tool for rational selection of optimal gRNAs for gene editing in regenerable immature embryos (IEs), and that high temperature treatment enhances gene editing rates in both tissue types. We also show that Cas9-mediated editing persists for at least 14 days in gold particle bombarded wheat IEs. The regenerated edited wheat plants in this work are recovered at high rates in the absence of exogenous DNA and selection. With this method, we produce knockouts of a set of three homoeologous genes and two pathogenic effector susceptibility genes, engineering insensitivity to corresponding necrotrophic effectors produced by Parastagonospora nodorum. The establishment of highly efficient, exogenous DNA-free gene editing technology holds promise for accelerated trait diversity production in an expansive array of crops.

Automated Quantitative Measurement of Yellow Halos Suggests Activity of Necrotrophic Effectors in Septoria tritici Blotch

Many necrotrophic plant pathogens utilize host-selective toxins or necrotrophic effectors during the infection process. We hypothesized that the chlorotic yellow halos frequently observed around necrotic lesions caused by the wheat pathogen Zymoseptoria tritici could result from the activity of necrotrophic effectors interacting with the products of toxin sensitivity genes. As an initial step toward testing this hypothesis, we developed an automated image analysis (AIA) workflow that could quantify the degree of yellow halo formation occurring in wheat leaves naturally infected by a highly diverse pathogen population under field conditions. This AIA based on statistical learning was applied to more than 10,000 naturally infected leaves collected from 335 wheat cultivars grown in a replicated field experiment. We estimated a high heritability (h² = 0.71) for the degree of yellow halo formation, suggesting that this quantitative trait has a significant genetic component. Using genome-wide association mapping, we identified six chromosome segments significantly associated with the yellow halo phenotype. Most of these segments contained candidate genes associated with targets of necrotrophic effectors in other necrotrophic pathogens. Our findings conform with the hypothesis that toxin sensitivity genes could account for a significant fraction of the observed variation in quantitative resistance to Septoria tritici blotch. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Genetics of resistance to septoria nodorum blotch in wheat

Septoria nodorum blotch (SNB) is a foliar disease of wheat caused by the necrotrophic fungal pathogen Parastagonospora nodorum. Research over the last two decades has shown that the wheat-P. nodorum pathosystem mostly follows an inverse gene-for-gene model. The fungus produces necrotrophic effectors (NEs) that interact with specific host gene products encoded by dominant sensitivity (S) genes. When a compatible interaction occurs, a 'defense response' in the host leads to programmed cell death thereby provided dead/dying cells from which the pathogen, being a necrotroph, can acquire nutrients allowing it to grow and sporulate. To date, nine S gene-NE interactions have been characterized in this pathosystem. Five NE-encoding genes, SnTox1, SnTox3, SnToxA, SnTox5, and SnTox267, have been cloned along with three host S genes, Tsn1, Snn1, and Snn3-D1. Studies have shown that P. nodorum hijacks multiple and diverse host targets to cause disease. SNB resistance is often quantitative in nature because multiple compatible interactions usually occur concomitantly. NE gene expression plays a key role in disease severity, and the effect of each compatible interaction can vary depending on the other existing compatible interactions. Numerous SNB-resistance QTL have been identified in addition to the known S genes, and more research is needed to understand the nature of these resistance loci. Marker-assisted elimination of S genes through conventional breeding practices and disruption of S genes using gene editing techniques are both effective strategies for the development of SNB-resistant wheat cultivars, which will become necessary as the global demand for sustenance grows.

Host and pathogen genetics reveal an inverse gene-for-gene association in the P. teres f. maculata-barley pathosystem

Pathogen and host genetics were used to uncover an inverse gene-for-gene interaction where virulence genes from the pathogen Pyrenophora teres f. maculata target barley susceptibility genes, resulting in disease. Although models have been proposed to broadly explain how plants and pathogens interact and coevolve, each interaction evolves independently, resulting in various scenarios of host manipulation and plant defense. Spot form net blotch is a foliar disease of barley caused by Pyrenophora teres f. maculata. We developed a barley population (Hockett × PI 67381) segregating for resistance to a diverse set of P. teres f. maculata isolates. Quantitative trait locus analysis identified major loci on barley chromosomes (Chr) 2H and 7H associated with resistance/susceptibility. Subsequently, we used avirulent and virulent P. teres f. maculata isolates to develop a pathogen population, identifying two major virulence loci located on Chr1 and Chr2. To further characterize this host-pathogen interaction, progeny from the pathogen population harboring virulence alleles at either the Chr1 or Chr2 locus was phenotyped on the Hockett × PI 67381 population. Progeny harboring only the Chr1 virulence allele lost the barley Chr7H association but maintained the 2H association. Conversely, isolates harboring only the Chr2 virulence allele lost the barley Chr2H association but maintained the 7H association. Hockett × PI 67381 F₂ individuals showed susceptible/resistant ratios not significantly different than 15:1 and results from F₂ inoculations using the single virulence genotypes were not significantly different from a 3:1 (S:R) ratio, indicating two dominant susceptibility genes. Collectively, this work shows that P. teres f. maculata virulence alleles at the Chr1 and Chr2 loci are targeting the barley 2H and 7H susceptibility alleles in an inverse gene-for-gene manner to facilitate colonization.

Prediction of effector proteins and their implications in pathogenicity of phytopathogenic filamentous fungi: A review

Plant pathogenic fungi encode and secrete effector proteins to promote pathogenesis. In recent years, the important role of effector proteins in fungi and plant host interactions has become increasingly prominent. In this review, the functional characterization and molecular mechanisms by which fungal effector proteins modulate biological processes and suppress the defense of plant hosts are discussed, with an emphasis on cell localization during fungal infection. This paper also provides a comprehensive review of bioinformatic and experimental methods that are currently available for the identification of fungal effector proteins. We additionally summarize the secretion pathways and the methods for verifying the presence effector proteins in plant host cells. For future research, comparative genomic studies of different pathogens with varying life cycles will allow comprehensive and systematic identification of effector proteins. Additionally, functional analysis of effector protein interactions with a wider range of hosts (especially non-model crops) will provide more detailed repertoires of fungal effectors. Identifying effector proteins and verifying their functions will improve our understanding of their role in causing disease and in turn guide future strategies for combatting fungal infections.

The Parastagonospora nodorum necrotrophic effector SnTox5 targets the wheat gene Snn5 and facilitates entry into the leaf mesophyll

Parastagonospora nodorum is an economically important necrotrophic fungal pathogen of wheat. Parastagonospora nodorum secretes necrotrophic effectors that target wheat susceptibility genes to induce programmed cell death (PCD). In this study, we cloned and functionally validated SnTox5 and characterized its role in pathogenesis. We used whole genome sequencing, genome-wide association study (GWAS) mapping, CRISPR-Cas9-based gene disruption, gain-of-function transformation, quantitative trait locus (QTL) analysis, haplotype and isoform analysis, protein modeling, quantitative PCR, and laser confocal microscopy to validate SnTox5 and functionally characterize SnTox5. SnTox5 is a mature 16.26 kDa protein with high structural similarity to SnTox3. Wild-type and mutant P. nodorum strains and wheat genotypes of SnTox5 and Snn5, respectively, were used to show that SnTox5 not only targets Snn5 to induce PCD but also facilitates the colonization of the mesophyll layer even in the absence of Snn5. Here we show that SnTox5 facilitates the efficient colonization of the mesophyll tissue and elicits PCD specific to host lines carrying Snn5. The homology to SnTox3 and the ability of SnTox5 to facilitate the colonizing of the mesophyll also suggest a role in the suppression of host defense before PCD induction.

Characterization of a Secretory YML079-like Cupin Protein That Contributes to Sclerotinia sclerotiorum Pathogenicity

Sclerotinia sclerotiorum causes devastating diseases in many agriculturally important crops, including oilseed rape and sunflower. However, the mechanisms of Sclerotinia sclerotiorum pathogenesis remain poorly understood. In this study, we characterized a YML079-like cupin protein (SsYCP1) from Sclerotinia sclerotiorum. We showed that SsYCP1 is strongly expressed and secreted during Sclerotinia sclerotiorum infection. Sclerotinia sclerotiorum infection was promoted by SsYCP1 overexpression and inhibited by silencing this gene with synthetic double-stranded RNA. These results collectively indicate SsYCP1 as a putative effector protein that contributes to Sclerotinia sclerotiorum pathogenicity. These findings extend our understanding of effector-mediated Sclerotinia sclerotiorum pathogenesis and suggest a novel role for YML079-like cupin proteins in plant-pathogen interactions.