Stable predictive markers for Phytophthora sojae avirulence genes that impair infection of soybean uncovered by whole genome sequencing of 31 isolates

A rapid molecular diagnostic tool to discriminate alleles of avirulence genes and haplotypes of Phytophthora sojae using high-resolution melting analysis

Effectors encoded by avirulence genes (Avr) interact with the Phytophthora sojae resistance gene (Rps) products to generate incompatible interactions. The virulence profile of P. sojae is rapidly evolving as a result of the large-scale deployment of Rps genes in soybean. For a successful exploitation of Rps genes, it is recommended that soybean growers use cultivars containing the Rps genes corresponding to Avr genes present in P. sojae populations present in their fields. Determination of the virulence profile of P. sojae isolates is critical for the selection of soybean cultivars. High-resolution melting curve (HRM) analysis is a powerful tool, first applied in medicine, for detecting mutations with potential applications in different biological fields. Here, we report the development of an HRM protocol, as an original approach to discriminate effectors, to differentiate P. sojae haplotypes for six Avr genes. An HRM assay was performed on 24 P. sojae isolates with different haplotypes collected from soybean fields across Canada. The results clearly confirmed that the HRM assay discriminated different virulence genotypes. Moreover, the HRM assay was able to differentiate multiple haplotypes representing small allelic variations. HRM-based prediction was validated by phenotyping assays. This HRM assay provides a unique, cost-effective and efficient tool to predict virulence pathotypes associated with six different Avr (1b, 1c, 1d, 1k, 3a and 6) genes from P. sojae, which can be applied in the deployment of appropriate Rps genes in soybean fields.

RxLR Effectors: Master Modulators, Modifiers and Manipulators

Cytoplasmic effectors with an Arg-any amino acid-Arg-Leu (RxLR) motif are encoded by hundreds of genes within the genomes of oomycete Phytophthora spp. and downy mildew pathogens. There has been a dramatic increase in our understanding of the evolution, function, and recognition of these effectors. Host proteins with a wide range of subcellular localizations and functions are targeted by RxLR effectors. Many processes are manipulated, including transcription, post-translational modifications, such as phosphorylation and ubiquitination, secretion, and intracellular trafficking. This involves an array of RxLR effector modes-of-action, including stabilization or destabilization of protein targets, altering or disrupting protein complexes, inhibition or utility of target enzyme activities, and changing the location of protein targets. Interestingly, approximately 50% of identified host proteins targeted by RxLR effectors are negative regulators of immunity. Avirulence RxLR effectors may be directly or indirectly detected by nucleotide-binding leucine-rich repeat resistance (NLR) proteins. Direct recognition by a single NLR of RxLR effector orthologues conserved across multiple Phytophthora pathogens may provide wide protection of diverse crops. Failure of RxLR effectors to interact with or appropriately manipulate target proteins in nonhost plants has been shown to restrict host range. This knowledge can potentially be exploited to alter host targets to prevent effector interaction, providing a barrier to host infection. Finally, recent evidence suggests that RxLR effectors, like cytoplasmic effectors from fungal pathogen Magnaporthe oryzae, may enter host cells via clathrin-mediated endocytosis. [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.

A global-temporal analysis on Phytophthora sojae resistance-gene efficacy

Plant disease resistance genes are widely used in agriculture to reduce disease outbreaks and epidemics and ensure global food security. In soybean, Rps (Resistance to Phytophthora sojae) genes are used to manage Phytophthora sojae, a major oomycete pathogen that causes Phytophthora stem and root rot (PRR) worldwide. This study aims to identify temporal changes in P. sojae pathotype complexity, diversity, and Rps gene efficacy. Pathotype data was collected from 5121 isolates of P. sojae, derived from 29 surveys conducted between 1990 and 2019 across the United States, Argentina, Canada, and China. This systematic review shows a loss of efficacy of specific Rps genes utilized for disease management and a significant increase in the pathotype diversity of isolates over time. This study finds that the most widely deployed Rps genes used to manage PRR globally, Rps1a, Rps1c and Rps1k, are no longer effective for PRR management in the United States, Argentina, and Canada. This systematic review emphasizes the need to widely introduce new sources of resistance to P. sojae, such as Rps3a, Rps6, or Rps11, into commercial cultivars to effectively manage PRR going forward.

Progress and Challenges in Elucidating the Functional Role of Effectors in the Soybean-Phytophthora sojae Interaction

Phytophthora sojae, the agent responsible for stem and root rot, is one of the most damaging plant pathogens of soybean. To establish a compatible-interaction, P. sojae secretes a wide array of effector proteins into the host cell. These effectors have been shown to act either in the apoplastic area or the cytoplasm of the cell to manipulate the host cellular processes in favor of the development of the pathogen. Deciphering effector-plant interactions is important for understanding the role of P. sojae effectors in disease progression and developing approaches to prevent infection. Here, we review the subcellular localization, the host proteins, and the processes associated with P. sojae effectors. We also discuss the emerging topic of effectors in the context of effector-resistance genes interaction, as well as model systems and recent developments in resources and techniques that may provide a better understanding of the soybean-P. sojae interaction.

RXLR effector gene Avr3a from Phytophthora sojae is recognized by Rps8 in soybean

The use of resistance genes in elite soybean cultivars is one of the most widely used methods to manage Phytophthora sojae. This method relies on effector-triggered immunity, where a Resistant to P. sojae (Rps) gene product from the plant recognizes a specific effector from the pathogen, encoded by an avirulence (Avr) gene. Many Avr genes from P. sojae have been identified in the last decade, allowing a better exploitation of this type of resistance. The objective of the present study was to identify the Avr gene triggering immunity derived from the soybean resistance gene Rps8. The analysis of a segregating F2 progeny coupled with a genotyping-by-sequencing approach led to the identification of a putative Avr8 locus. The investigation of this locus using whole-genome sequencing data from 31 isolates of P. sojae identified Avr3a as the likely candidate for Avr8. Long-read sequencing also revealed that P. sojae isolates can carry up to five copies of the Avr3a gene, compared to the four previously reported. Haplotype and transcriptional analyses showed that amino acid changes and absence of Avr3a transcripts from P. sojae isolates caused changes in virulence towards Rps8. Functional analyses using CRISPR/Cas9 knockout and constitutive expression demonstrated that Rps8 interacted with Avr3a. We also showed that a specific allele of Avr3a is recognized by Rps3a but not Rps8. While Rps3a and Rps8 have been previously described as closely linked, this is the first report of a clear distinction hitherto undefined between these two resistance genes.

The SoyaGen Project: Putting Genomics to Work for Soybean Breeders

The SoyaGen project was a collaborative endeavor involving Canadian soybean researchers and breeders from academia and the private sector as well as international collaborators. Its aims were to develop genomics-derived solutions to real-world challenges faced by breeders. Based on the needs expressed by the stakeholders, the research efforts were focused on maximizing realized yield through optimization of maturity and improved disease resistance. The main deliverables related to molecular breeding in soybean will be reviewed here. These include: (1) SNP datasets capturing the genetic diversity within cultivated soybean (both within a worldwide collection of > 1,000 soybean accessions and a subset of 102 short-season accessions (MG0 and earlier) directly relevant to this group); (2) SNP markers for selecting favorable alleles at key maturity genes as well as loci associated with increased resistance to key pathogens and pests (Phytophthora sojae, Heterodera glycines, Sclerotinia sclerotiorum); (3) diagnostic tools to facilitate the identification and mapping of specific pathotypes of P. sojae; and (4) a genomic prediction approach to identify the most promising combinations of parents. As a result of this fruitful collaboration, breeders have gained new tools and approaches to implement molecular, genomics-informed breeding strategies. We believe these tools and approaches are broadly applicable to soybean breeding efforts around the world.

Mutations in the Promoter and Coding Regions of Avr3a Cause Gain of Virulence of Phytophthora sojae to Rps3a in Soybean

Phytophthora sojae threatens soybean production worldwide, and the cultivation of soybean cultivars carrying Rps genes is the most effective way to control this pathogen. However, DNA mutations in the Avr genes of P. sojae can escape recognization of the corresponding Rps genes, leading to the loss of soybean resistance. In this study, we investigated sequence polymorphism and transcript level of the Avr3a gene in Chinese isolates of P. sojae. Twenty-four mutations resulting in five unique Avr3a alleles were discovered in the Avr3a coding region from 32 P. sojae isolates. The Avr3a transcripts were detectable in the isolates containing Avr3a(I), Avr3a(II), Avr3a(III), and Avr3a(IV) but not in the isolates containing Avr3a(V). Promoter and 5'-UTR sequence analysis revealed eight unique mutations in the promoter region of Avr3a(V), suggesting that the mutations could result in the loss of Avr3a(V) transcription. Virulence tests indicated the isolates containing Avr3a(II) and Avr3a(IV) were virulent, suggesting that the mutations in the coding regions of Avr3a(II) and Avr3a(IV) caused the gain of virulence to Rps3a. Based on DNA mutations of Avr3a in virulent alleles, two SNP markers and one PCR-based marker were developed successfully for detecting the virulence of P. sojae isolates to Rps3a. These findings provide new insights into escape mechanisms of Avr3a and effective support for accurate pathotype identification of P. sojae using molecular methods.

Molecular Assessment of Pathotype Diversity of Phytophthora sojae in Canada Highlights Declining Sources of Resistance in Soybean

The large-scale deployment of resistance to Phytophthora sojae (Rps) genes in soybean has led to the rapid evolution of the virulence profile (pathotype) of P. sojae populations. Determining the pathotypes of P. sojae isolates is important in selecting soybean germplasm carrying the proper Rps, but this process is fastidious and requires specific expertise. In this work, we used a molecular assay to assess the pathotypes of P. sojae isolates obtained throughout the provinces of Québec, Ontario, and Manitoba. In preliminary assays, the molecular tool showed equivalent prediction of the pathotypes as a phenotyping assay and proved to be much faster to apply while eliminating intermediate values. Upon analysis of nearly 300 isolates, 24 different pathotypes were detected in Québec and Ontario, compared with only eight in Manitoba, where soybean culture is more recent. Pathotypes 1a, 1c, and 1d was predominant in Québec, while 1a, 1b, 1c, 1d, and 1k pathotypes were the most common in Manitoba. Overall, the results showed that 98 and 86% of the isolates carried pathotype 1a or 1c, respectively, suggesting that Rps1a and Rps1c were no longer effective in Canada. Based on the history of soybean varieties used in surveyed fields, it was found that 84% of them contained Rps genes that were no longer resistant against the pathotypes of the isolates found in the fields. While highlighting an easier and more precise option to assess pathotypes, this study presents the first pan-Canadian survey of P. sojae and stresses the importance of carefully managing the declining sources of resistance.

Mining germplasm panels and phenotypic datasets to identify loci for resistance to Phytophthora sojae in soybean

Phytophthora sojae causes Phytophthora root and stem rot of soybean and has been primarily managed through deployment of qualitative Resistance to P. sojae genes (Rps genes). The effectiveness of each individual or combination of Rps gene(s) depends on the diversity and pathotypes of the P. sojae populations present. Due to the complex nature of P. sojae populations, identification of more novel Rps genes is needed. In this study, phenotypic data from previous studies of 16 panels of plant introductions (PIs) were analyzed. Panels 1 and 2 consisted of 448 Glycine max and 520 G. soja, which had been evaluated for Rps gene response with a combination of P. sojae isolates. Panels 3 and 4 consisted of 429 and 460 G. max PIs, respectively, which had been evaluated using individual P. sojae isolates with complex virulence pathotypes. Finally, Panels 5-16 (376 G. max PIs) consisted of data deposited in the USDA Soybean Germplasm Collection from evaluations with 12 races of P. sojae. Using these panels, genome-wide association (GWA) analyses were carried out by combining phenotypic and SoySNP50K genotypic data. GWA models identified two, two, six, and seven novel Rps loci with Panels 1, 2, 3, and 4, respectively. A total of 58 novel Rps loci were identified using Panels 5-16. Genetic and phenotypic dissection of these loci may lead to the characterization of novel Rps genes that can be effectively deployed in new soybean cultivars against diverse P. sojae populations.

Silent control: microbial plant pathogens evade host immunity without coding sequence changes

Both animals and plants have evolved a robust immune system to surveil and defeat invading pathogenic microbes. Evasion of host immune surveillance is the key for pathogens to initiate successful infection. To evade the host immunity, plant pathogens evolved a variety of strategies such as masking themselves from host immune recognitions, blocking immune signaling transductions, reprogramming immune responses and adapting to immune microenvironmental changes. Gain of new virulence genes, sequence and structural variations enables plant pathogens to evade host immunity through changes in the genetic code. However, recent discoveries demonstrated that variations at the transcriptional, post-transcriptional, post-translational and glycome level enable pathogens to cope with the host immune system without coding sequence changes. The biochemical modification of pathogen associated molecular patterns and silencing of effector genes emerged as potent ways for pathogens to hide from host recognition. Altered processing in mRNA activities provide pathogens with resilience to microenvironment changes. Importantly, these hiding variants are directly or indirectly modulated by catalytic enzymes or enzymatic complexes and cannot be revealed by classical genomics alone. Unveiling these novel host evasion mechanisms in plant pathogens enables us to better understand the nature of plant disease and pinpoints strategies for rational diseases management in global food protection.

Silicon influences the localization and expression of Phytophthora sojae effectors in interaction with soybean

In plant-pathogen interactions, expression and localization of effectors in the aqueous apoplastic region play a crucial role in the establishment or suppression of pathogen development. Silicon (Si) has been shown to protect plants in several host-pathogen interactions, but its mode of action remains a source of debate. Its deposition in the apoplastic area of plant cells suggests that it might interfere with receptor-effector recognition. In this study, soybean plants treated or not with Si were inoculated with Phytophthora sojae and differences in the ensuing infection process were assessed through different microscopy techniques, transcript analysis of effector and defense genes, and effector (Avr6) localization through immunolocalization and fluorescence labeling. In plants grown without Si, the results showed the rapid (4 d post-inoculation) host recognition by P. sojae through the development of haustorium-like bodies, followed by expression and release of effectors into the apoplastic region. In contrast, Si treatment resulted in limited pathogen development, and significantly lower expression and presence of Avr6 in the apoplastic region. Based on immunolocalization and quantification of Avr6 through fluorescence labeling, our results suggest that the presence of Si in the apoplast interferes with host recognition and/or limits receptor-effector interactions, which leads to an incompatible interaction.

Identification of Resistance Genes to Phytophthora sojae in Domestic Soybean Cultivars from China Using Particle Bombardment

Phytophthora root and stem rot caused by Phytophthora sojae is a destructive disease that afflicts soybean plants throughout the world. The use of resistant soybean cultivars is the primary means of managing this disease, as well as the most effective and economical approach. There are abundant soybean germplasm resources in China that could be deployed for breeding programs; however, the resistance genes (Rps genes) in most cultivars are unknown, leading to uncertainty concerning which are resistant cultivars for use. The resistance genes Rps1a, Rps1c, and Rps1k prevent root and stem rot caused by most P. sojae isolates within a Chinese field population. This study identified three Rps genes in Chinese domestic soybean cultivars using three related avirulence genes by particle bombardment. The complex genetic diversity of soybean cultivars and P. sojae strains has made it difficult to define single Rps genes without molecular involvement. Gene cobombardment is a method for identifying Rps genes quickly and specifically. We showed that cultivars Dongnong 60 and Henong 72 contained Rps1a, while Hedou 19, Henong 76, 75-3, Wandou 21020, Zheng 196, Wandou 28, Heinong 71, and Wandou 29 all contained Rps1c. The cultivars Jidou 12, Henong 72, Heinong 71, and Wandou 29 contained Rps1k. The cultivar Henong 72 contained both Rps1a and Rps1k, while Wandou 29 and Heinong 71 contained both Rps1c and Rps1k. We then evaluated the phenotype of 11 domestic soybean cultivars reacting to P. sojae using the isolates P6497 and Ps1. The 11 domestic cultivars were all resistant to P6497 and Ps1. This research provides source materials and parent plant strains containing Rps1a, Rps1c, and Rps1k for soybean breeding programs.

Discriminant haplotypes of avirulence genes of Phytophthora sojae lead to a molecular assay to predict phenotypes

The soybean-Phytophthora sojae interaction operates on a gene-for-gene relationship, where the product of a resistance gene (Rps) in the host recognizes that of an avirulence gene (Avr) in the pathogen to generate an incompatible reaction. To exploit this form of resistance, one must match with precision the appropriate Rps gene with the corresponding Avr gene. Currently, this association is evaluated by phenotyping assays that are labour-intensive and often imprecise. To circumvent this limitation, we sought to develop a molecular assay that would reveal the avirulence allele of the seven main Avr genes (Avr1a, Avr1b, Avr1c, Avr1d, Avr1k, Avr3a, and Avr6) in order to diagnose with precision the pathotypes of P. sojae isolates. For this purpose, we analysed the genomic regions of these Avr genes in 31 recently sequenced isolates with different virulence profiles and identified discriminant mutations between avirulence and virulence alleles. Specific primers were designed to generate amplicons of a distinct size, and polymerase chain reaction conditions were optimized in a final assay of two parallel runs. When tested on the 31 isolates of known virulence, the assay accurately revealed all avirulence alleles. The test was further assessed and compared to a phenotyping assay on 25 isolates of unknown virulence. The two assays matched in 97% (170/175) of the interactions studied. Interestingly, the sole cases of discrepancy were obtained with Avr3a, which suggests a possible imperfect interaction with Rps3a. This molecular assay offers a powerful and reliable tool to exploit and study with greater precision soybean resistance against P. sojae.

An Improved Method for the Identification of Soybean Resistance to Phytophthora sojae Applied to Germplasm Resources from the Huanghuaihai and Dongbei Regions of China

Phytophthora root and stem rot (PRR) caused by Phytophthora sojae is a destructive disease afflicting soybean. The use of resistant cultivars is the most effective method to combat PRR. PRR resistance was assessed in 223 soybean cultivars from Huanghuaihai and Dongbei, major soybean-producing regions in east central and northeastern China. To evaluate levels of soybean resistance to P. sojae, we used eight representative P. sojae isolates and a modified etiolated hypocotyl-slit inoculation method. The cultivars Wandou21020, Xu9302-A, Kedou10, and Lidi055 showed resistance to all eight isolates; 14 cultivars showed intermediate resistance to all eight P. sojae isolates, and 53 cultivars were resistant to seven isolates. Thirty-three cultivars (15%) were susceptible only to the highly virulent PsJS2 isolate, which is consistent with the reactions of the Chapman differential line that carries Rps3a. The diverse reaction patterns seen in germplasm from different regions (provinces/cities) in this study reflect the variety of PRR-resistant soybean sources in China. Our research indicates that sources of P. sojae resistance are present in the major soybean production areas of China. This study provides useful information for soybean breeding programs.

Whole Genome Re-sequencing Reveals Natural Variation and Adaptive Evolution of Phytophthora sojae

Due to the monocultural basis of agricultural crops, mutated plant microbes with increased pathogenicity can easily spread in the field and lead to serious yield losses. As a major threat to a wide range of crop plants, oomycete pathogens continuously undergo adaptive evolution to overcome plant defense barriers. However, the genetic basis of their evolution at the molecular level remains largely unknown. Here, we investigated the nature variation and the population genomics of the soybean pathogen Phytophthora sojae by high-throughput genome re-sequencing. Genomic variation analysis revealed uneven “two-speed” evolutionary pattern with genes in gene-sparse regions (GSRs) showing higher rates of structural polymorphisms and positive selection. GSRs are enriched in effector genes and transposase-related genes. Our results also suggested that the NADH oxidase and MIP transporter gene families undergo rapid and diversifying selection. Furthermore, we demonstrated that P. sojae isolates possess varying numbers of RxLR effectors with diverse sequences, totaling 471 members. Among them, 42 core RxLR effectors are assumed to be important for infection. Finally, we observed that Avr genes exhibit abundant sequence variation in P. sojae isolates. Several novel variants lead to the evading of host resistance, including a complete deletion in Avr3c and amino acid mutations in Avr1a. Taken together, our results provide an adaptive landscape of P. sojae at single-nucleotide resolution, as well as resources for further resistance breeding and disease prevention against this important plant pathogen.

Advances in Omics Approaches for Abiotic Stress Tolerance in Tomato

Tomato, one of the most important crops worldwide, has a high demand in the fresh fruit market and processed food industries. Despite having considerably high productivity, continuous supply as per the market demand is hard to achieve, mostly because of periodic losses occurring due to biotic as well as abiotic stresses. Although tomato is a temperate crop, it is grown in almost all the climatic zones because of widespread demand, which makes it challenge to adapt in diverse conditions. Development of tomato cultivars with enhanced abiotic stress tolerance is one of the most sustainable approaches for its successful production. In this regard, efforts are being made to understand the stress tolerance mechanism, gene discovery, and interaction of genetic and environmental factors. Several omics approaches, tools, and resources have already been developed for tomato growing. Modern sequencing technologies have greatly accelerated genomics and transcriptomics studies in tomato. These advancements facilitate Quantitative trait loci (QTL) mapping, genome-wide association studies (GWAS), and genomic selection (GS). However, limited efforts have been made in other omics branches like proteomics, metabolomics, and ionomics. Extensive cataloging of omics resources made here has highlighted the need for integration of omics approaches for efficient utilization of resources and a better understanding of the molecular mechanism. The information provided here will be helpful to understand the plant responses and the genetic regulatory networks involved in abiotic stress tolerance and efficient utilization of omics resources for tomato crop improvement.

A Molecular Assay Allows the Simultaneous Detection of 12 Fungi Causing Fruit Rot in Cranberry

Cranberry fruit rot (CFR) is arguably one of the most limiting factors of cranberry (Vaccinium macrocarpon) production throughout its growing areas. The disease is caused by a group of closely related fungi that require identification using long and cumbersome steps of isolation and microscopic observations of structural features. The objective of this study was to develop a molecular assay to simultaneously detect and discriminate 12 of the most important fungal species reported to be pathogenic on cranberry fruit to facilitate the diagnosis of CFR. As the first approach, internal transcribed spacers and large subunit regions of all fungi were sequenced and confirmed with sequences available in the NCBI database. These data were used to develop primers able to differentiate seven of the 12 species. The five remaining species, including three in the Phacidiaceae family and two in the Glomerellaceae family, were differentiated on the basis of a more discriminant marker, the translation elongation factor 1-α. Two PCR reactions were optimized to clearly delineate the 12 species. The multiplex test was first validated using pure fungal cultures; it was subsequently validated using fruit collected in cranberry beds in eastern Canada. In the latter case, the test was rigorous enough to clearly discriminate the fungal pathogens from contaminants. Within the tested samples, Physalospora vaccinii and Coleophoma empetri were most commonly found. This molecular test offers scientists, diagnosticians, and growers a powerful tool that can rapidly and precisely identify fungi causing CFR so they can implement appropriate control methods.

Pathogen enrichment sequencing (PenSeq) enables population genomic studies in oomycetes

The oomycete pathogens Phytophthora infestans and P. capsici cause significant crop losses world-wide, threatening food security. In each case, pathogenicity factors, called RXLR effectors, contribute to virulence. Some RXLRs are perceived by resistance proteins to trigger host immunity, but our understanding of the demographic processes and adaptive evolution of pathogen virulence remains poor. Here, we describe PenSeq, a highly efficient enrichment sequencing approach for genes encoding pathogenicity determinants which, as shown for the infamous potato blight pathogen Phytophthora infestans, make up < 1% of the entire genome. PenSeq facilitates the characterization of allelic diversity in pathogen effectors, enabling evolutionary and population genomic analyses of Phytophthora species. Furthermore, PenSeq enables the massively parallel identification of presence/absence variations and sequence polymorphisms in key pathogen genes, which is a prerequisite for the efficient deployment of host resistance genes. PenSeq represents a cost-effective alternative to whole-genome sequencing and addresses crucial limitations of current plant pathogen population studies, which are often based on selectively neutral markers and consequently have limited utility in the analysis of adaptive evolution. The approach can be adapted to diverse microbes and pathogens.