Virulent Soybean Cyst Nematode Research Articles

Fortifying nematode resistance through susceptibility gene inactivation

The predominant genetic defense mechanism against soybean cyst nematode (SCN) in 95% of the North America market is under threat by virulent SCN populations. Usovsky et al. identified GmSNAP02 as an SCN susceptibility gene through fine-mapping of unique bi-parental populations. Loss-of-function of GmSNAP02 confers enhanced resistance to more virulent SCN.

Interactions Among Heterodera glycines, Macrophomina phaseolina, and Soybean Genotype.

Heterodera glycines, the soybean cyst nematode (SCN), and fungal pathogen Macrophomina phaseolina are economically important soybean pathogens that may coinfest fields. Resistance remains the most effective management tactic for SCN, and the rhg1-b resistance allele derived from plant introduction 88788 is most commonly deployed in the northern United States. The concomitant effects of SCN and M. phaseolina on soybean performance, as well as the effect of the rhg1-b allele in two different genetic backgrounds, were evaluated in three environments (during 2013 to 2015) and a greenhouse bioassay. Within two soybean populations, half of the lines had the rhg1-b allele, and the other half had the susceptible allele in the backgrounds of the cultivars IA3023 and LD00-3309. Significant interactions between soybean rhg1-b allele and M. phaseolina-infested plots were observed in 2014. In all experiments, initial SCN populations (Pi) and M. phaseolina in roots were associated with reduced soybean yield. SCN reproduction factor (RF = final population/Pi) was affected by SCN Pi, rhg1-b, and genetic background. A background-by-genotype interaction on yield was observed only in 2015, with a stronger rhg1-b effect in the LD00-3309 background, which suggested that the susceptible parent 'IA3023' is tolerant to SCN. SCN female index from greenhouse experiments was compared with field RF, and Lin's concordance and Pearson's correlation coefficients decreased with increasing field SCN Pi in soil. In this study, both SCN and M. phaseolina reduced soybean yield asymptomatically, and the impact of SCN rhg1-b resistance was dependent on SCN virulence but also population density.

Epistatic interaction between Rhg1-a and Rhg2 in PI 90763 confers resistance to virulent soybean cyst nematode populations

Key messageAn epistatic interaction between SCN resistance loci rhg1-a and rhg2 in PI 90763 imparts resistance against virulent SCN populations which can be employed to diversify SCN resistance in soybean cultivars.With more than 95% of the $46.1B soybean market dominated by a single type of genetic resistance, breeding for soybean cyst nematode (SCN)-resistant soybean that can effectively combat the widespread increase in virulent SCN populations presents a significant challenge. Rhg genes (for Resistance to Heterodera glycines) play a key role in resistance to SCN; however, their deployment beyond the use of the rhg1-b allele has been limited. In this study, quantitative trait loci (QTL) were mapped using PI 90763 through two biparental F3:4 recombinant inbred line (RIL) populations segregating for rhg1-a and rhg1-b alleles against a SCN HG type 1.2.5.7 (Race 2) population. QTL located on chromosome 18 (rhg1-a) and chromosome 11 (rhg2) were determined to confer SCN resistance in PI 90763. The rhg2 gene was fine-mapped to a 169-Kbp region pinpointing GmSNAP11 as the strongest candidate gene. We demonstrated a unique epistatic interaction between rhg1-a and rhg2 loci that not only confers resistance to multiple virulent SCN populations. Further, we showed that pyramiding rhg2 with the conventional mode of resistance, rhg1-b, is ineffective against these virulent SCN populations. This highlights the importance of pyramiding rhg1-a and rhg2 to maximize the impact of gene pyramiding strategies toward management of SCN populations virulent on rhg1-b sources of resistance. Our results lay the foundation for the next generation of soybean resistance breeding to combat the number one pathogen of soybean.

Soybean cyst nematode (Heterodera glycines) resistant cultivar rotation system impacts nematode population density, virulence, and yield

Soybean cyst nematode (SCN) is the most economically important pathogen of soybeans in the United States. The most effective SCN management practice is the use of resistant cultivars; however, continuous use of cultivars derived from a single line of resistance results in selection for SCN virulent populations. A four-year field study (2017–2020) was conducted in Michigan to evaluate the impact of six soybean rotation systems with cultivars derived from PI548402 (5DV) and PI88788 (8DV) on SCN population development, virulence, and soybean yield. The rotation systems were: 1) 8DV/5DV/8DV/5DV, 2) 5DV/5DV/5DV/5DV, 3) 8DV/Susceptible (SDV)/5DV/8DV, 4) 5DV/SDV/8DV/5DV, 5) 8DV/8DV/8DV/8DV, and 6) 5DV/SDV/SDV/SDV. SCN population levels were significantly lower in the 5DV/8DV rotation in 2018 compared to the 5DV/SDV rotation. In 2020, SCN population levels were the lowest in the rotation system 8DV/5DV/8DV/5DV and the highest in the continuous 5DV soybeans. In 2017, there were no substantial soybean yield differences among the differing systems, but for the remainder of the trial, the 5DV/8DV rotations had the highest yields. HG type tests showed that rotation of resistance sources can alter the virulence of the SCN population. The initial 2017 SCN population was HG type 1.2. The final populations in 2020 for the rotation systems 8DV/5DV/8DV/5DV, 5DV/5DV/5DV/5DV, and 8DV/SDV/5DV/8DV was HG type 1.2.5.7, and HG type 1.2.3.5.6.7 for the other rotation systems 5DV/SDV/8DV/5DV, 8DV/8DV/8DV/8DV, and 5DV/SDV/SDV/SDV. Based on the results of the field trial, a greenhouse trial was conducted where we found that a Michigan SCN population from continuous PI88788 sources of resistance can also reproduce on Peking. These findings indicate rotating PI548402- and PI88788-derived cultivars can reduce SCN density and maintain high soybean yields; thus rotating these two sources of resistance with non-host rotations might delay the aggressiveness of SCN populations further, reducing soybean yield loss.

Resistance Gene Pyramiding and Rotation to Combat Widespread Soybean Cyst Nematode Virulence

Soybean cyst nematode (SCN) is an important pathogen of soybean causing >$1 billion in yield losses annually in the United States. Planting SCN-resistant soybean cultivars is the primary management strategy. Resistance genes derived from the plant introduction (PI) 88788 (rhg1-b) and PI 548402 (Peking; rhg1-a and Rhg4) are the main types of resistance available in commercial cultivars. The PI 88788 rhg1-b resistance allele is found in the majority of SCN-resistant cultivars in the north central United States. The widespread use of PI 88788 rhg1-b has led to limited options for farmers to rotate resistance sources to manage SCN. Consequently, overreliance on a single type of resistance has resulted in the selection of SCN populations that have adapted to reproduce on these resistant cultivars. Here we evaluated the effectiveness of rotating soybean lines with different combinations of resistance genes to determine the best strategy for combating the widespread increase in virulent SCN and limit future nematode adaptation to resistant cultivars. Eight SCN populations were developed by continuous selection of a virulent SCN field population (Heterodera glycines [HG] type 1.2.5.7) on a single resistance source or in rotation with soybean pyramiding different resistance gene alleles derived from PI 88788 (rhg1-b), PI 437654 (rhg1-a and Rhg4), PI 468916 (cqSCN-006 and cqSCN-007), and PI 567516C (Chr10). SCN population densities were determined for eight generations. HG type tests were conducted after the eighth generation to evaluate population shifts. The continued use of rhg1-b or 006/007 had limited effectiveness for reducing SCN type 1.2.5.7 population density, whereas rotation to the use of rhg1-a/Rhg4 resistance significantly reduced SCN population density but selected for broader SCN virulence (HG type 1.2.3.5.6.7). A rotation of rhg1-a/Rhg4 with a pyramid of rhg1-b/006/007/Chr10 was the most effective combination at both reducing population density and minimizing selection pressure. Our results provide guidance for implementation of a strategic SCN resistance rotation plan to manage the widespread virulence on PI 88788 and sustain the future durability of SCN resistance genes.

Genomic Profiling of Virulence in the Soybean Cyst Nematode Using Single-Nematode Sequencing.

Soybean cyst nematode (SCN) is one of the most important diseases in soybean. Currently, the main management strategy relies on planting resistant cultivars. However, the overuse of a single resistance source has led to the selection of virulent SCN populations, although the mechanisms by which the nematode overcomes the resistance genes remain unknown. In this study, we used a nematode-adapted single-cell RNA-seq approach to identify SCN genes potentially involved in resistance breakdown in Peking and PI 88788 parental soybean lines. We established for the first time the full transcriptome of single SCN individuals allowing us to identify a list of putative virulence genes against both major SCN resistance sources. Our analysis identified 48 differentially expressed putative effectors (secreted proteins required for infection) alongside 40 effectors showing evidence of novel structural variants, and 11 effector genes containing phenotype-specific sequence polymorphisms. Additionally, a differential expression analysis revealed an interesting phenomenon of coexpressed gene regions with some containing putative effectors. The selection of virulent SCN individuals on Peking resulted in a profoundly altered transcriptome, especially for genes known to be involved in parasitism. Several sequence polymorphisms were also specific to these virulent nematodes and could potentially play a role in the acquisition of nematode virulence. On the other hand, the transcriptome of virulent individuals on PI 88788 was very similar to avirulent ones with the exception of a few genes, which suggest a distinct virulence strategy to Peking.

Targeted suppression of soybean BAG6-induced cell death in yeast by soybean cyst nematode effectors.

While numerous effectors that suppress plant immunity have been identified from bacteria, fungi, and oomycete pathogens, relatively little is known for nematode effectors. Several dozen effectors have been reported from the soybean cyst nematode (SCN). Previous studies suggest that a hypersensitive response‐like programmed cell death is triggered at nematode feeding sites in soybean during an incompatible interaction. However, virulent SCN populations overcome this incompatibility using unknown mechanisms. A soybean BAG6 (Bcl‐2 associated anthanogene 6) gene previously reported by us to be highly up‐regulated in degenerating feeding sites induced by SCN in a resistant soybean line was attenuated in response to a virulent SCN population. We show that GmBAG6‐1 induces cell death in yeast like its Arabidopsis homolog AtBAG6 and also in soybean. This led us to hypothesize that virulent SCN may target GmBAG6‐1 as part of their strategy to overcome soybean defence responses during infection. Thus, we used a yeast viability assay to screen SCN effector candidates for their ability to specifically suppress GmBAG6‐1‐induced cell death. We identified several effectors that strongly suppressed cell death mediated by GmBAG6‐1. Two effectors identified as suppressors showed direct interaction with GmBAG6‐1 in yeast, suggesting that one mechanism of cell death suppression may occur through an interaction with this host protein.

Genome-wide association study and genomic selection for tolerance of soybean biomass to soybean cyst nematode infestation.

Soybean cyst nematode (SCN), Heterodera glycines Ichinohe, is one of the most devastating pathogens affecting soybean production in the U.S. and worldwide. The use of SCN-resistant soybean cultivars is one of the most affordable strategies to cope with SCN infestation. Because of the limited sources of SCN resistance and changes in SCN virulence phenotypes, host resistance in current cultivars has increasingly been overcome by the pathogen. Host tolerance has been recognized as an additional tool to manage the SCN. The objectives of this study were to conduct a genome-wide association study (GWAS), to identify single nucleotide polymorphism (SNP) markers, and to perform a genomic selection (GS) study for SCN tolerance in soybean based on reduction in biomass. A total of 234 soybean genotypes (lines) were evaluated for their tolerance to SCN in greenhouse using four replicates. The tolerance index (TI = 100 × Biomass of a line in SCN infested / Biomass of the line without SCN) was used as phenotypic data of SCN tolerance. GWAS was conducted using a total of 3,782 high quality SNPs. GS was performed based upon the whole set of SNPs and the GWAS-derived SNPs, respectively. Results showed that (1) a large variation in soybean TI to SCN infection among the soybean genotypes was identified; (2) a total of 35, 21, and 6 SNPs were found to be associated with SCN tolerance using the models SMR, GLM (PCA), and MLM (PCA+K) with 6 SNPs overlapping between models; (3) GS accuracy was SNP set-, model-, and training population size-dependent; and (4) genes around Glyma.06G134900, Glyma.15G097500.1, Glyma.15G100900.3, Glyma.15G105400, Glyma.15G107200, and Glyma.19G121200.1 (Table 4). Glyma.06G134900, Glyma.15G097500.1, Glyma.15G100900.3, Glyma.15G105400, and Glyma.19G121200.1 are best candidates. To the best of our knowledge, this is the first report highlighting SNP markers associated with tolerance index based on biomass reduction under SCN infestation in soybean. This research opens a new approach to use SCN tolerance in soybean breeding and the SNP markers will provide a tool for breeders to select for SCN tolerance.

Dynamics of Population Density and Virulence Phenotype of the Soybean Cyst Nematode as Influenced by Resistance Source Sequence and Tillage.

The soybean cyst nematode (SCN), Heterodera glycines, is the most damaging pathogen of soybean. Use of resistant cultivars is an effective strategy to manage SCN, but it also selects for virulent populations over time. A 12-year field experiment was initiated in 2003 to study how tillage and 11 different sequences of four cultivars impact SCN population dynamics and virulence. An SCN-susceptible cultivar and three resistant cultivars (R1, R2, and R3 derived from cultivars PI 88788, Peking, and PI 437654, respectively) were used. Tillage had minimal effect on SCN population density. Compared with no till, conventional tillage resulted in a faster increase of SCN virulence to Peking when the SCN was selected by R2 and virulence to PI 88788 by R3. Among the three SCN-resistant cultivars, R1 supported the greatest population density, R2 supported intermediate population density, and R3 supported the least SCN population density. The SCN populations selected by R1 overcame the resistance in PI 88788 but not in Peking and PI 437654. R2 selected SCN populations that overcame the resistance in Peking but not in PI 88788 and PI 437654. In contrast, R3 selected SCN populations that overcame both PI 88788 and Peking sources of resistance. There was no increase of virulence to PI 437654 in any cultivar sequence. R1 in rotation with R2 or R3 had a negative effect on female index on Peking. Susceptible soybean reduced SCN virulence to Peking, indicating that there was fitness cost of the Peking virulent SCN type. These results suggest that rotation of Peking with PI 88788 is a good strategy for managing the SCN, and susceptible cultivar and no till may reduce SCN virulence selection pressure in some rotations.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Impaired folate binding of serine hydroxymethyltransferase 8 from soybean underlies resistance to the soybean cyst nematode

Management of the agricultural pathogen soybean cyst nematode (SCN) relies on the use of SCN-resistant soybean cultivars, a strategy that has been failing in recent years. An underutilized source of resistance in the soybean genotype Peking is linked to two polymorphisms in serine hydroxy-methyltransferase 8 (SHMT8). SHMT is a pyridoxal 5'-phosphate-dependent enzyme that converts l-serine and (6S)-tetrahydrofolate to glycine and 5,10-methylenetetrahydrofolate. Here, we determined five crystal structures of the 1884-residue SHMT8 tetramers from the SCN-susceptible cultivar (cv.) Essex and the SCN-resistant cv. Forrest (whose resistance is derived from the SHMT8 polymorphisms in Peking); the crystal structures were determined in complex with various ligands at 1.4-2.35 Å resolutions. We find that the two Forrest-specific polymorphic substitutions (P130R and N358Y) impact the mobility of a loop near the entrance of the (6S)-tetrahydrofolate-binding site. Ligand-binding and kinetic studies indicate severely reduced affinity for folate and dramatically impaired enzyme activity in Forrest SHMT8. These findings imply widespread effects on folate metabolism in soybean cv. Forrest that have implications for combating the widespread increase in virulent SCN.

Active and inactive forms of biotin synthase occur in Heterodera glycines.

Heterodera glycines, the soybean cyst nematode (SCN), is a plant-parasitic nematode capable of manipulating host plant biochemistry and development. Many studies have suggested that the nematode has acquired genes from bacteria via horizontal gene transfer events (HGTs) that have the potential to enhance nematode parasitism. A recent allelic imbalance analysis identified two candidate virulence genes, which also appear to have entered the SCN genome through HGTs. One of the candidate genes, H. glycines biotin synthase (HgBioB), contained sequence polymorphisms between avirulent and virulent inbred SCN strains. To test the function of these HgBioB alleles, a complementation experiment using biotin synthase-deficient Escherichia coli was conducted. Here, we report that avirulent nematodes produce an active biotin synthase while virulent ones contain an inactive form of the enzyme. Moreover, sequencing analysis of HgBioB genes from SCN field populations indicates the presence of diverse mixture of HgBioB alleles with the virulent form being the most prevalent. We hypothesize that the mutations in the inactive HgBioB allele within the virulent SCN could result in a change in protein function that in some unknown way bolster its parasitic lifestyle.

Genetics and Adaptation of Soybean Cyst Nematode to Broad Spectrum Soybean Resistance.

The soybean cyst nematode (SCN) Heterodera glycines is a major threat to soybean production, made more challenging by the current limitations of natural resistance for managing this pathogen. The use of resistant host cultivars is effective, but, over time, results in the generation of virulent nematode populations able to robustly parasitize the resistant host. In order to understand how virulence develops in SCN, we utilized a single backcross BC1F2 strategy to mate a highly virulent inbred population (TN20), capable of reproducing on all current sources of resistance, with an avirulent one (PA3), unable to reproduce on any of the resistant soybean lines. The offspring were then investigated to determine how virulence is inherited on the main sources of SCN resistance, derived from soybean lines Peking, PI 88788, PI 90763, and the broad spectrum resistance source PI 437654. Significantly, our results suggest virulence on PI 437654 is a multigenic recessive trait that allows the nematode to reproduce on all current sources of resistance. In addition, we examined how virulence on different sources of resistance interact by placing virulent SCN populations under secondary selection, and identified a strong counter-selection between virulence on PI 88788- and PI 90763-derived resistances, while no such counter-selection existed between virulence on Peking and PI 88788 resistance sources. Our results suggest that the genes responsible for virulence on PI 88788 and PI 90763 may be different alleles at a common locus. If so, rotation of cultivars with resistance from these two sources may be an effective management protocol.

Increase in Soybean Cyst Nematode Virulence and Reproduction on Resistant Soybean Varieties in Iowa From 2001 to 2015 and the Effects on Soybean Yields

Management of the soybean cyst nematode (SCN) relies heavily on use of SCN-resistant soybean varieties to limit nematode reproduction and minimize yield loss. For Iowa, almost all SCN-resistant soybean varieties contain SCN resistance genes from a breeding line named Plant Introduction (PI) 88788. Iowa State University conducts experiments to evaluate numerous SCN-resistant and three to four SCN-susceptible soybean varieties in up to nine field experiments across Iowa each year. Data on SCN population density, virulence (SCN race and HG type), soybean yield, precipitation, and growing degree days from more than 25,000 four-row plots in field experiments conducted from 2001 to 2015 were analyzed to determine how these factors affected SCN reproduction and yield. SCN population densities were positively correlated with temperatures and negatively associated with precipitation during the growing seasons, indicating that SCN reproduction was greatest in hot, dry years. Over the years, virulence of SCN populations on PI 88788 increased in the fields in which the experiments were conducted, resulting in increased end-of-season SCN population densities and reduced yields of SCN-resistant soybean varieties with the PI 88788 source of resistance. These results indicate that soybean yield loss caused by SCN on resistant varieties with the common PI 88788 source of resistance likely will increase as virulence of SCN populations increases unless new sources of resistance become widely available and used in the future.

A SNARE-Like Protein and Biotin Are Implicated in Soybean Cyst Nematode Virulence.

Phytoparasitic nematodes that are able to infect and reproduce on plants that are considered resistant are referred to as virulent. The mechanism(s) that virulent nematodes employ to evade or suppress host plant defenses are not well understood. Here we report the use of a genetic strategy (allelic imbalance analysis) to associate single nucleotide polymorphisms (SNPs) with nematode virulence genes in Heterodera glycines, the soybean cyst nematode (SCN). To accomplish this analysis, a custom SCN SNP array was developed and used to genotype SCN F3-derived populations grown on resistant and susceptible soybean plants. Three SNPs reproducibly showed allele imbalances between nematodes grown on resistant and susceptible plants. Two candidate SCN virulence genes that were tightly linked to the SNPs were identified. One SCN gene encoded biotin synthase (HgBioB), and the other encoded a bacterial-like protein containing a putative SNARE domain (HgSLP-1). The two genes mapped to two different linkage groups. HgBioB contained sequence polymorphisms between avirulent and virulent nematodes. However, the gene encoding HgSLP-1 had reduced copy number in virulent nematode populations and appears to produce multiple forms of the protein via intron retention and alternative splicing. We show that HgSLP-1 is an esophageal-gland protein that is secreted by the nematode during plant parasitism. Furthermore, in bacterial co-expression experiments, HgSLP-1 co-purified with the SCN resistance protein Rhg1 α-SNAP, suggesting that these two proteins physically interact. Collectively our data suggest that multiple SCN genes are involved in SCN virulence, and that HgSLP-1 may function as an avirulence protein and when absent it helps SCN evade host defenses.