Recombination suppression at the dominant Rhg1/Rfs2 locus underlying soybean resistance to the cyst nematode

Characterization of Pingliang xiaoheidou (ZDD 11047), a soybean variety with resistance to soybean cyst nematode Heterodera glycines

KEY MESSAGE

A novel QTL (qSCN-PL10) for SCN resistance and related candidate genes were identified in the soybean variety Pingliang xiaoheidou, and plant basal immunity seems to contribute to the SCN resistance. Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is one of the most devastating soybean pests worldwide. The development of host plant resistance represents an effective strategy to control SCN. However, owing to the lack of diversity of resistance genes in soybean varieties, further investigation is necessary to identify new SCN resistance genes. By analyzing the resistance phenotypes of soybean variety Pingliang xiaoheidou (Pingliang, ZDD 11047), we found that it exhibited the different resistance phenotypes from PI 88788 and Peking varieties. Because Pingliang variety contains the Rhg1-a (low copy) haplotype and lacks the resistant Rhg4 haplotype, novel quantitative trait locus might account for their SCN resistance. After sequencing parental lines (Magellan and Pingliang) and 200 F2:3 progenies, a high-density genetic map was constructed using the specific length amplified fragment sequencing method and qSCN-PL10 was identified as a novel locus for SCN resistance. Candidate genes were predicted by RNA sequencing (RNA-seq) in the qSCN-PL10 locus region. The RNA-seq analysis performed also indicated that plant basal immunity plays an important role in the resistance of Pingliang to SCN. These results lay a foundation for the use of marker-assisted breeding to enhance the resistance to SCN.

Comparative genomic analyses of two segregating mutants reveal seven genes likely involved in resistance to Fusarium equiseti in soybean via whole genome re-sequencing

KEY MESSAGE

The candidate genes involved in resistance to Fusarium equiseti in soybean PI 437654 were identified through comparative genomic analyses of mutants via whole genome re-sequencing. The fungus Fusarium infects each stage of the growth and development of soybean and causes soybean (Glycine max (L.)) seed and root rot and seedling damping-off and wilt with a large quantity of annual yield loss worldwide. It is very important to identify the resistant genes in soybean to prevent and control this pathogen. One Fusarium equiseti isolate was previously identified to be incompatible with 'PI 437654' but compatible with a Chinese soybean cultivar 'Zhonghuang 13'. In this study, with the infection of this isolate on the seedling roots of developed PI 437654 mutants, 6 mutants were identified from 500 mutants to significantly alter their phenotypes to F. equiseti compared to wild-type PI 437654. Then, two identified segregating mutants were selected to directly perform whole genome re-sequencing. Finally, through comparative genomic analyses 7 genes including one cluster of 4 nucleotide binding site-leucine-rich repeat genes on one genomic region of chromosome 7, a 60S ribosomal protein L12 gene and 2 uncharacterized genes were identified to be likely involved in the resistance to F. equiseti. These genes will facilitate the breeding of resistant germplasm resources and the identification of resistance of soybean to Fusarium spp.

Effective identification of soybean candidate genes involved in resistance to soybean cyst nematode via direct whole genome re-sequencing of two segregating mutants

KEY MESSAGE

Three soybean candidate genes involved in resistance to soybean cyst nematode race 4 were identified via direct whole genome re-sequencing of two segregating mutants. The genes conferring resistance to soybean cyst nematode (SCN) race 4 (Hg type 1.2.3.5.7) in soybean (Glycine max L. Merr.) remains unknown. Next generation sequencing-based methods identify a wide range of targets, it is difficult to identify genes underlying traits. Use of the MutMap and QTL-seq methods to identify trait candidate genes needs backcrossing and is very time-consuming. Here we report a simple method to effectively identify candidate genes involved in resistance to SCN race 4. Two ethane methylsulfonate mutagenized mutants of soybean 'PI 437654', whose SCN race 4-infection phenotype altered, were selected. Six relevant whole genomes were re-sequenced, and then calling of genomic variants (SNPs and InDels) was conducted and compared to 'Williams 82'. The comparison eliminated many genomic variants from the mutant lines that overlapped two non-phenotypic but mutant progeny plants, wild-type PI 437654 and 'Zhonghuang 13'. Finally, only 27 mutations were found among 10 genes. Of these 10 genes, 3 genes, Glyma.09g054000, Glyma.16g065700 and Glyma.18g192200 were overlapped between two phenotypic mutant progeny plants. Therefore, the three genes may be the candidate genes involved in resistance of PI 437654 to soybean cyst nematode race 4. This method simplifies the effective identification of candidate genes.

Four receptor-like cytoplasmic kinases regulate development and immunity in rice

Receptor-like cytoplasmic kinases (RLCKs) represent a large family of proteins in plants. However, few RLCKs have been well characterized. Here, we report the functional characterization of four rice RLCKs - OsRLCK57, OsRLCK107, OsRLCK118 and OsRLCK176 from subfamily VII. These OsRLCKs interact with the rice brassinosteroid receptor, OsBRI1 in yeast cell, but not the XA21 immune receptor. Transgenic lines silenced for each of these genes have enlarged leaf angles and are hypersensitive to brassinolide treatment compared to wild type rice. Transgenic plants silenced for OsRLCK57 had significantly fewer tillers and reduced panicle secondary branching, and lines silenced for OsRLCK107 and OsRLCK118 produce fewer seeds. Silencing of these genes decreased Xa21 gene expression and compromised XA21-mediated immunity to Xanthomonas oryzae pv. oryzae. Our study demonstrates that these OsRLCKs negatively regulate BR signalling, while positively regulating immune responses by contributing to the expression of the immune receptor XA21.

Detection of an inversion in the Ty-2 region between S. lycopersicum and S. habrochaites by a combination of de novo genome assembly and BAC cloning

A chromosomal inversion associated with the tomato Ty - 2 gene for TYLCV resistance is the cause of severe suppression of recombination in a tomato Ty - 2 introgression line. Among tomato and its wild relatives inversions are often observed, which result in suppression of recombination. Such inversions hamper the transfer of important traits from a related species to the crop by introgression breeding. Suppression of recombination was reported for the TYLCV resistance gene, Ty-2, which has been introgressed in cultivated tomato (Solanum lycopersicum) from the wild relative S. habrochaites accession B6013. Ty-2 was mapped to a 300-kb region on the long arm of chromosome 11. The suppression of recombination in the Ty-2 region could be caused by chromosomal rearrangements in S. habrochaites compared with S. lycopersicum. With the aim of visualizing the genome structure of the Ty-2 region, we compared the draft de novo assembly of S. habrochaites accession LYC4 with the sequence of cultivated tomato ('Heinz'). Furthermore, using populations derived from intraspecific crosses of S. habrochaites accessions, the order of markers in the Ty-2 region was studied. Results showed the presence of an inversion of approximately 200 kb in the Ty-2 region when comparing S. lycopersicum and S. habrochaites. By sequencing a BAC clone from the Ty-2 introgression line, one inversion breakpoint was identified. Finally, the obtained results are discussed with respect to introgression breeding and the importance of a priori de novo sequencing of the species involved.

Genetic characteristics of soybean resistance to HG type 0 and HG type 1.2.3.5.7 of the cyst nematode analyzed by genome-wide association mapping

BACKGROUND

Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is one of the most fatal pests of soybean (Glycine max (L.) Merr.) worldwide and causes huge loss of soybean yield each year. Multiple sources of resistance are urgently needed for effective management of SCN via the development of resistant cultivars. The aim of the present study was to investigate the genetic architecture of resistance to SCN HG Type 0 (race 3) and HG Type 1.2.3.5.7 (race 4) in landraces and released elite soybean cultivars mostly from China.

RESULTS

A total of 440 diverse soybean landraces and elite cultivars were screened for resistance to SCN HG Type 0 and HG Type 1.2.3.5.7. Exactly 131 new sources of SCN resistance were identified. Lines were genotyped by SNP markers detected by the Specific Locus Amplified Fragment Sequencing (SLAF-seq) approach. A total of 36,976 SNPs were identified with minor allele frequencies (MAF) > 4% that were present in 97% of all the genotypes. Genome-wide association mapping showed that a total of 19 association signals were significantly related to the resistance for the two HG Types. Of the 19 association signals, eight signals overlapped with reported QTL including Rhg1 and Rhg4 genes. Another eight were located in the linked regions encompassing known QTL. Three QTL were found that were not previously reported. The average value of female index (FI) of soybean accessions with resistant alleles was significantly lower than those with susceptible alleles for each peak SNP. Disease resistance proteins with leucine rich regions, cytochrome P450s, protein kinases, zinc finger domain proteins, RING domain proteins, MYB and WRKY transcription activation families were identified. Such proteins may participate in the resistant reaction to SCN and were frequently found in the tightly linked genomic regions of the peak SNPs.

CONCLUSIONS

GWAS extended understanding of the genetic architecture of SCN resistance in multiple genetic backgrounds. Nineteen association signals were obtained for the resistance to the two Hg Types of SCN. The multiple beneficial alleles from resistant germplasm sources will be useful for the breeding of cultivars with improved resistance to SCN. Analysis of genes near association signals may facilitate the recognition of the causal gene(s) underlying SCN resistances.

Usefulness of 10 genomic regions in soybean associated with sudden death syndrome resistance

Sudden death syndrome (SDS) is an important soybean [Glycine max (L) Merrill] disease caused by the soilborne fungus Fusarium virguliforme. Currently, 14 quantitative trait loci (QTL) had been confirmed associated with resistance or tolerance to SDS. The objective of the study was to evaluate usefulness of 10 of these QTL in controlling disease expression. Six populations were developed providing a total of 321 F2-derived lines for the study. Recombinant inbred lines (RIL) used as parents were obtained from populations of 'Essex' × 'Forrest' (EF), 'Flyer' × 'Hartwig' (FH), and 'Pyramid' × 'Douglas' (PD). Disease resistance was evaluated in the greenhouse at three different planting times, each with four replications, using sorghum infested with F. virguliforme homogeneously mixed in the soil (Luckew et al., Crop Sci 52:2215-2223, 2012). Four disease assessment criteria-foliar disease incidence (DI), foliar leaf scorch disease severity (DS), area under the disease progress curve (AUDPC), and root rot severity-were used. QTL were identified in more than one of the disease assessment criteria, mainly associated with lines in the most resistant categories. Five QTL (qRfs4, qRfs5, qRfs7, qRfs12, and Rfs16) were associated with at least one of the disease assessments across multiple populations. Of the five, qRfs4 was associated with DI, AUDPC, and root rot severity, and Rfs16 with AUDPC and root rot severity. The findings suggest it may be possible for plant breeders to focus on stacking a subset of the previously identified QTL to improve resistance to SDS in soybean.

Homo-dimerization and ligand binding by the leucine-rich repeat domain at RHG1/RFS2 underlying resistance to two soybean pathogens

BACKGROUND

The protein encoded by GmRLK18-1 (Glyma_18_02680 on chromosome 18) was a receptor like kinase (RLK) encoded within the soybean (Glycine max L. Merr.) Rhg1/Rfs2 locus. The locus underlies resistance to the soybean cyst nematode (SCN) Heterodera glycines (I.) and causal agent of sudden death syndrome (SDS) Fusarium virguliforme (Aoki). Previously the leucine rich repeat (LRR) domain was expressed in Escherichia coli.

RESULTS

The aims here were to evaluate the LRRs ability to; homo-dimerize; bind larger proteins; and bind to small peptides. Western analysis suggested homo-dimers could form after protein extraction from roots. The purified LRR domain, from residue 131-485, was seen to form a mixture of monomers and homo-dimers in vitro. Cross-linking experiments in vitro showed the H274N region was close (<11.1 A) to the highly conserved cysteine residue C196 on the second homo-dimer subunit. Binding constants of 20-142 nM for peptides found in plant and nematode secretions were found. Effects on plant phenotypes including wilting, stem bending and resistance to infection by SCN were observed when roots were treated with 50 pM of the peptides. Far-Western analyses followed by MS showed methionine synthase and cyclophilin bound strongly to the LRR domain. A second LRR from GmRLK08-1 (Glyma_08_g11350) did not show these strong interactions.

CONCLUSIONS

The LRR domain of the GmRLK18-1 protein formed both a monomer and a homo-dimer. The LRR domain bound avidly to 4 different CLE peptides, a cyclophilin and a methionine synthase. The CLE peptides GmTGIF, GmCLE34, GmCLE3 and HgCLE were previously reported to be involved in root growth inhibition but here GmTGIF and HgCLE were shown to alter stem morphology and resistance to SCN. One of several models from homology and ab-initio modeling was partially validated by cross-linking. The effect of the 3 amino acid replacements present among RLK allotypes, A87V, Q115K and H274N were predicted to alter domain stability and function. Therefore, the LRR domain of GmRLK18-1 might underlie both root development and disease resistance in soybean and provide an avenue to develop new variants and ligands that might promote reduced losses to SCN.

Molecular mapping and breeding with microsatellite markers

In genetics databases for crop plant species across the world, there are thousands of mapped loci that underlie quantitative traits, oligogenic traits, and simple traits recognized by association mapping in populations. The number of loci will increase as new phenotypes are measured in more diverse genotypes and genetic maps based on saturating numbers of markers are developed. A period of locus reevaluation will decrease the number of important loci as those underlying mega-environmental effects are recognized. A second wave of reevaluation of loci will follow from developmental series analysis, especially for harvest traits like seed yield and composition. Breeding methods to properly use the accurate maps of QTL are being developed. New methods to map, fine map, and isolate the genes underlying the loci will be critical to future advances in crop biotechnology. Microsatellite markers are the most useful tool for breeders. They are codominant, abundant in all genomes, highly polymorphic so useful in many populations, and both economical and technically easy to use. The selective genotyping approaches, including genotype ranking (indexing) based on partial phenotype data combined with favorable allele data and bulked segregation event (segregant) analysis (BSA), will be increasingly important uses for microsatellites. Examples of the methods for developing and using microsatellites derived from genomic sequences are presented for monogenic, oligogenic, and polygenic traits. Examples of successful mapping, fine mapping, and gene isolation are given. When combined with high-throughput methods for genotyping and a genome sequence, the use of association mapping with microsatellite markers will provide critical advances in the analysis of crop traits.

Control of meiotic recombination frequency in plant genomes

Sexual eukaryotes reproduce via the meiotic cell division, where ploidy is halved and homologous chromosomes undergo reciprocal genetic exchange, termed crossover (CO). CO frequency has a profound effect on patterns of genetic variation and species evolution. Relative CO rates vary extensively both within and between plant genomes. Plant genome size varies by over 1000-fold, largely due to differential expansion of repetitive sequences, and increased genome size is associated with reduced CO frequency. Gene versus repeat sequences associate with distinct chromatin modifications, and evidence from plant genomes indicates that this epigenetic information influences CO patterns. This is consistent with data from diverse eukaryotes that demonstrate the importance of chromatin structure for control of meiotic recombination. In this review I will discuss CO frequency patterns in plant genomes and recent advances in understanding recombination distributions.

The receptor like kinase at Rhg1-a/Rfs2 caused pleiotropic resistance to sudden death syndrome and soybean cyst nematode as a transgene by altering signaling responses

BACKGROUND

Soybean (Glycine max (L. Merr.)) resistance to any population of Heterodera glycines (I.), or Fusarium virguliforme (Akoi, O'Donnell, Homma & Lattanzi) required a functional allele at Rhg1/Rfs2. H. glycines, the soybean cyst nematode (SCN) was an ancient, endemic, pest of soybean whereas F. virguliforme causal agent of sudden death syndrome (SDS), was a recent, regional, pest. This study examined the role of a receptor like kinase (RLK) GmRLK18-1 (gene model Glyma_18_02680 at 1,071 kbp on chromosome 18 of the genome sequence) within the Rhg1/Rfs2 locus in causing resistance to SCN and SDS.

RESULTS

A BAC (B73p06) encompassing the Rhg1/Rfs2 locus was sequenced from a resistant cultivar and compared to the sequences of two susceptible cultivars from which 800 SNPs were found. Sequence alignments inferred that the resistance allele was an introgressed region of about 59 kbp at the center of which the GmRLK18-1 was the most polymorphic gene and encoded protein. Analyses were made of plants that were either heterozygous at, or transgenic (and so hemizygous at a new location) with, the resistance allele of GmRLK18-1. Those plants infested with either H. glycines or F. virguliforme showed that the allele for resistance was dominant. In the absence of Rhg4 the GmRLK18-1 was sufficient to confer nearly complete resistance to both root and leaf symptoms of SDS caused by F. virguliforme and provided partial resistance to three different populations of nematodes (mature female cysts were reduced by 30-50%). In the presence of Rhg4 the plants with the transgene were nearly classed as fully resistant to SCN (females reduced to 11% of the susceptible control) as well as SDS. A reduction in the rate of early seedling root development was also shown to be caused by the resistance allele of the GmRLK18-1. Field trials of transgenic plants showed an increase in foliar susceptibility to insect herbivory.

CONCLUSIONS

The inference that soybean has adapted part of an existing pathogen recognition and defense cascade (H.glycines; SCN and insect herbivory) to a new pathogen (F. virguliforme; SDS) has broad implications for crop improvement. Stable resistance to many pathogens might be achieved by manipulation the genes encoding a small number of pathogen recognition proteins.