The Soybean Rhg1 locus for resistance to the soybean cyst nematode Heterodera glycines regulates the expression of a large number of stress- and defense-related genes in degenerating feeding cells

Signal Transduction in Plant⁻Nematode Interactions

To successfully invade and infect their host plants, plant parasitic nematodes (PPNs) need to evolve molecular mechanisms to overcome the defense responses from the plants. Nematode-associated molecular patterns (NAMPs), including ascarosides and certain proteins, while instrumental in enabling the infection, can be perceived by the host plants, which then initiate a signaling cascade leading to the induction of basal defense responses. To combat host resistance, some nematodes can inject effectors into the cells of susceptible hosts to reprogram the basal resistance signaling and also modulate the hosts’ gene expression patterns to facilitate the establishment of nematode feeding sites (NFSs). In this review, we summarized all the known signaling pathways involved in plant–nematode interactions. Specifically, we placed particular focus on the effector proteins from PPNs that mimic the signaling of the defense responses in host plants. Furthermore, we gave an updated overview of the regulation by PPNs of different host defense pathways such as salicylic acid (SA)/jasmonic acid (JA), auxin, and cytokinin and reactive oxygen species (ROS) signaling to facilitate their parasitic successes in plants. This review will enhance the understanding of the molecular signaling pathways involved in both compatible and incompatible plant–nematode interactions.

Plant elicitor peptides promote plant defences against nematodes in soybean

Plant elicitor peptides (Peps) are widely distributed among angiosperms, and have been shown to amplify immune responses in multiple plant families. Here, we characterize three Peps from soybean (Glycine max) and describe their effects on plant defences against two damaging agricultural pests, the root-knot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines). Seed treatments with exogenous GmPep1, GmPep2 or GmPep3 significantly reduced the reproduction of both nematodes. Pep treatment also protected plants from the inhibitory effects of root-knot nematodes on above-ground growth, and up-regulated basal expression levels of nematode-responsive defence genes. GmPep1 induced the expression of its propeptide precursor (GmPROPEP1), a nucleotide-binding site leucine-rich repeat protein (NBS-LRR), a pectin methylesterase inhibitor (PMEI), Respiratory Burst Oxidase Protein D (RBOHD) and the accumulation of reactive oxygen species (ROS) in leaves. In addition, GmPep2 and GmPep3 seed treatments up-regulated RBOHD expression and ROS accumulation in roots and leaves. These results suggest that GmPeps activate plant defences through systemic transcriptional reprogramming and ROS signalling, and that Pep seed treatments represent a potential strategy for nematode management.

RNA-seq data comparisons of wild soybean genotypes in response to soybean cyst nematode (Heterodera glycines)

Soybean [Glycine max (L.) Merr.] is an important crop rich in vegetable protein and oil, and is a staple food for human and animals worldwide. However, soybean plants have been challenged by soybean cyst nematode (SCN, Heterodera glycines), one of the most damaging pests found in soybean fields. Applying SCN-resistant cultivars is the most efficient and environmentally friendly strategy to manage SCN. Currently, soybean breeding and further improvement in soybean agriculture are hindered by severely limited genetic diversity in cultivated soybeans. G. soja is a soybean wild progenitor with much higher levels of genetic diversity compared to cultivated soybeans. In this study, transcriptomes of the resistant and susceptible genotypes of the wild soybean, Glycine soja Sieb & Zucc, were sequenced to examine the genetic basis of SCN resistance. Seedling roots were treated with infective second-stage juveniles (J2s) of the soybean cyst nematode (HG type 2.5.7) for 3, 5, 8 days and pooled for library construction and RNA sequencing. The transcriptome sequencing generated approximately 245 million (M) high quality (Q > 30) raw sequence reads (125 bp in length) for twelve libraries. The raw sequence reads were deposited in NCBI sequence read archive (SRA) database, with the accession numbers SRR5227314-25. Further analysis of this data would be helpful to improve our understanding of the molecular mechanisms of soybean-SCN interaction and facilitate the development of diverse SCN resistance cultivars.

Genetic architecture of wild soybean (Glycine soja) response to soybean cyst nematode (Heterodera glycines)

The soybean cyst nematode (SCN) is one of the most destructive pathogens of soybean plants worldwide. Host-plant resistance is an environmentally friendly method to mitigate SCN damage. To date, the resistant soybean cultivars harbor limited genetic variation, and some are losing resistance. Thus, a better understanding of the genetic mechanisms of the SCN resistance, as well as developing diverse resistant soybean cultivars, is urgently needed. In this study, a genome-wide association study (GWAS) was conducted using 1032 wild soybean (Glycine soja) accessions with over 42,000 single-nucleotide polymorphisms (SNPs) to understand the genetic architecture of G. soja resistance to SCN race 1. Ten SNPs were significantly associated with the response to race 1. Three SNPs on chromosome 18 were localized within the previously identified quantitative trait loci (QTLs), and two of which were localized within a strong linkage disequilibrium block encompassing a nucleotide-binding (NB)-ARC disease resistance gene (Glyma.18G102600). Genes encoding methyltransferases, the calcium-dependent signaling protein, the leucine-rich repeat kinase family protein, and the NB-ARC disease resistance protein, were identified as promising candidate genes. The identified SNPs and candidate genes can not only shed light on the molecular mechanisms underlying SCN resistance, but also can facilitate soybean improvement employing wild genetic resources.

Systematic Mutagenesis of Serine Hydroxymethyltransferase Reveals an Essential Role in Nematode Resistance

Rhg4 is a major genetic locus that contributes to soybean cyst nematode (SCN) resistance in the Peking-type resistance of soybean (Glycine max), which also requires the rhg1 gene. By map-based cloning and functional genomic approaches, we previously showed that the Rhg4 gene encodes a predicted cytosolic serine hydroxymethyltransferase (GmSHMT08); however, the novel gain of function of GmSHMT08 in SCN resistance remains to be characterized. Using a forward genetic screen, we identified an allelic series of GmSHMT08 mutants that shed new light on the mechanistic aspects of GmSHMT08-mediated resistance. The new mutants provide compelling genetic evidence that Peking-type rhg1 resistance in cv Forrest is fully dependent on the GmSHMT08 gene and demonstrates that this resistance is mechanistically different from the PI 88788-type of resistance that only requires rhg1 We also demonstrated that rhg1-a from cv Forrest, although required, does not exert selection pressure on the nematode to shift from HG type 7, which further validates the bigenic nature of this resistance. Mapping of the identified mutations onto the SHMT structural model uncovered key residues for structural stability, ligand binding, enzyme activity, and protein interactions, suggesting that GmSHMT08 has additional functions aside from its main enzymatic role in SCN resistance. Lastly, we demonstrate the functionality of the GmSHMT08 SCN resistance gene in a transgenic soybean plant.

Smart Parasitic Nematodes Use Multifaceted Strategies to Parasitize Plants

Nematodes are omnipresent in nature including many species which are parasitic to plants and cause enormous economic losses in various crops. During the process of parasitism, sedentary phytonematodes use their stylet to secrete effector proteins into the plant cells to induce the development of specialized feeding structures. These effectors are used by the nematodes to develop compatible interactions with plants, partly by mimicking the expression of host genes. Intensive research is going on to investigate the molecular function of these effector proteins in the plants. In this review, we have summarized which physiological and molecular changes occur when endoparasitic nematodes invade the plant roots and how they develop a successful interaction with plants using the effector proteins. We have also mentioned the host genes which are induced by the nematodes for a compatible interaction. Additionally, we discuss how nematodes modulate the reactive oxygen species (ROS) and RNA silencing pathways in addition to post-translational modifications in their own favor for successful parasitism in plants.

Comparative RNA-Seq Analysis Uncovers a Complex Regulatory Network for Soybean Cyst Nematode Resistance in Wild Soybean (Glycine soja)

Soybean cyst nematode (SCN) is the most damaging pest of soybean worldwide. The molecular mechanism of SCN resistance remains largely unknown. We conducted a global RNA-seq comparison between a resistant genotype (S54) and a susceptible genotype (S67) of Glycine soja, the wild progenitor of soybean, to understand its regulatory network in SCN defense. The number of differentially expressed genes (DEGs) in S54 (2,290) was much larger than that in S67 (555). A number of defense-related genes/pathways were significantly induced only in S54, while photosynthesis and several metabolic pathways were affected in both genotypes with SCN infection. These defense-associated DEGs were involved in pathogen recognition, calcium/calmodulin-mediated defense signaling, jasmonic acid (JA)/ethylene (ET) and sialic acid (SA)-involved signaling, the MAPK signaling cascade, and WRKY-involved transcriptional regulation. Our results revealed a comprehensive regulatory network involved in SCN resistance and provided insights into the complex molecular mechanisms of SCN resistance in wild soybean.

Genome Evolution of Plant-Parasitic Nematodes

Plant parasitism has evolved independently on at least four separate occasions in the phylum Nematoda. The application of next-generation sequencing (NGS) to plant-parasitic nematodes has allowed a wide range of genome- or transcriptome-level comparisons, and these have identified genome adaptations that enable parasitism of plants. Current genome data suggest that horizontal gene transfer, gene family expansions, evolution of new genes that mediate interactions with the host, and parasitism-specific gene regulation are important adaptations that allow nematodes to parasitize plants. Sequencing of a larger number of nematode genomes, including plant parasites that show different modes of parasitism or that have evolved in currently unsampled clades, and using free-living taxa as comparators would allow more detailed analysis and a better understanding of the organization of key genes within the genomes. This would facilitate a more complete understanding of the way in which parasitism has shaped the genomes of plant-parasitic nematodes.

Cooperative Regulatory Functions of miR858 and MYB83 during Cyst Nematode Parasitism

MicroRNAs (miRNAs) recently have been established as key regulators of transcriptome reprogramming that define cell function and identity. Nevertheless, the molecular functions of the greatest number of miRNA genes remain to be determined. Here, we report cooperative regulatory functions of miR858 and its MYB83 transcription factor target gene in transcriptome reprogramming during Heterodera cyst nematode parasitism of Arabidopsis (Arabidopsis thaliana). Gene expression analyses and promoter-GUS fusion assays documented a role of miR858 in posttranscriptional regulation of MYB83 in the Heterodera schachtii-induced feeding sites, the syncytia. Constitutive overexpression of miR858 interfered with H. schachtii parasitism of Arabidopsis, leading to reduced susceptibility, while reduced miR858 abundance enhanced plant susceptibility. Similarly, MYB83 expression increases were conducive to nematode infection because overexpression of a noncleavable coding sequence of MYB83 significantly increased plant susceptibility, whereas a myb83 mutation rendered the plants less susceptible. In addition, RNA-seq analysis revealed that genes involved in hormone signaling pathways, defense response, glucosinolate biosynthesis, cell wall modification, sugar transport, and transcriptional control are the key etiological factors by which MYB83 facilitates nematode parasitism of Arabidopsis. Furthermore, we discovered that miR858-mediated silencing of MYB83 is tightly regulated through a feedback loop that might contribute to fine-tuning the expression of more than a thousand of MYB83-regulated genes in the H. schachtii-induced syncytium. Together, our results suggest a role of the miR858-MYB83 regulatory system in finely balancing gene expression patterns during H. schachtii parasitism of Arabidopsis to ensure optimal cellular function.

Transgenic Strategies for Enhancement of Nematode Resistance in Plants

Plant parasitic nematodes (PPNs) are obligate biotrophic parasites causing serious damage and reduction in crop yields. Several economically important genera parasitize various crop plants. The root-knot, root lesion, and cyst nematodes are the three most economically damaging genera of PPNs on crops within the family Heteroderidae. It is very important to devise various management strategies against PPNs in economically important crop plants. Genetic engineering has proven a promising tool for the development of biotic and abiotic stress tolerance in crop plants. Additionally, the genetic engineering leading to transgenic plants harboring nematode resistance genes has demonstrated its significance in the field of plant nematology. Here, we have discussed the use of genetic engineering for the development of nematode resistance in plants. This review article also provides a detailed account of transgenic strategies for the resistance against PPNs. The strategies include natural resistance genes, cloning of proteinase inhibitor coding genes, anti-nematodal proteins and use of RNA interference to suppress nematode effectors. Furthermore, the manipulation of expression levels of genes induced and suppressed by nematodes has also been suggested as an innovative approach for inducing nematode resistance in plants. The information in this article will provide an array of possibilities to engineer resistance against PPNs in different crop plants.

Gene expression profiling describes the genetic regulation of Meloidogyne arenaria resistance in Arachis hypogaea and reveals a candidate gene for resistance

Resistance to root-knot nematode was introgressed into cultivated peanut Arachis hypogaea from a wild peanut relative, A. cardenasii and previously mapped to chromosome A09. The highly resistant recombinant inbred RIL 46 and moderately resistant RIL 48 were selected from a population with cv. Gregory (susceptible) and Tifguard (resistant) as female and male parents, respectively. RNA-seq analysis was performed on these four genotypes using root tissue harvested from root-knot nematode infected plants at 0, 3, 7 days after inoculation. Differential gene expression analysis provides evidence that root-knot nematodes modulate biological pathways involved in plant hormone, defense, cell signaling, cytoskeleton and cell wall metabolism in a susceptible reaction. Corresponding to resistance reaction, an effector-induced-immune response mediated by an R-gene was identified in Tifguard. Mapping of the introgressed region indicated that 92% of linkage group A09 was of A. cardenasii origin in Tifguard. RIL46 and RIL 48 possessed 3.6% and 83.5% of the introgression on A09, respectively. Within the small introgressed region carried by RIL 46, a constitutively expressed TIR-NBS-LRR gene was identified as the candidate for nematode resistance. Potential defense responsive pathways include effector endocytosis through clathrin-coated vesicle trafficking, defense signaling through membrane lipid metabolism and mucilage production.

The soybean GmSNAP18 gene underlies two types of resistance to soybean cyst nematode

Two types of resistant soybean (Glycine max (L.) Merr.) sources are widely used against soybean cyst nematode (SCN, Heterodera glycines Ichinohe). These include Peking-type soybean, whose resistance requires both the rhg1-a and Rhg4 alleles, and PI 88788-type soybean, whose resistance requires only the rhg1-b allele. Multiple copy number of PI 88788-type GmSNAP18, GmAAT, and GmWI12 in one genomic segment simultaneously contribute to rhg1-b resistance. Using an integrated set of genetic and genomic approaches, we demonstrate that the rhg1-a Peking-type GmSNAP18 is sufficient for resistance to SCN in combination with Rhg4. The two SNAPs (soluble NSF attachment proteins) differ by only five amino acids. Our findings suggest that Peking-type GmSNAP18 is performing a different role in SCN resistance than PI 88788-type GmSNAP18. As such, this is an example of a pathogen resistance gene that has evolved to underlie two types of resistance, yet ensure the same function within a single plant species.

Transcriptome analysis of resistant and susceptible tobacco (Nicotiana tabacum) in response to root-knot nematode Meloidogyne incognita infection

The root-knot nematode (RKN) Meloidogyne incognita reproduces on the roots of tobacco (Nicotiana tabacum), damaging crops, reducing crop yield, and causing economic losses annually. The development of resistant genotypes is an alternative strategy to effectively control these losses. However, the molecular mechanism responsible for host pathogenesis and defense responses in tobacco specifically against RKNs remain poorly understood. Here, root transcriptome analysis of resistant (Yuyan12) and susceptible (Changbohuang) tobacco varieties infected with RKNs was performed. Moreover, 2623 and 545 differentially expressed genes (DEGs) in RKN-infected roots were observed in Yuyan12 and Changbohuang, respectively, compared to those in non-infected roots, including 289 DEGs commonly expressed in the two genotypes. Among these DEGs, genes encoding cell wall modifying proteins, auxin-related proteins, the ROS scavenging system, and transcription factors involved in various biological and physiochemical processes were significantly expressed in both the resistant and susceptible genotypes. This work is thus the first report on the relationships in the RKN-tobacco interaction using transcriptome analysis, and the results provide important information on the mechanism of RKN resistance in tobacco.

Soybean Resistance to the Soybean Cyst Nematode Heterodera glycines: An Update

The soybean cyst nematode (SCN), Heterodera glycines, remains a serious threat to soybean production throughout the world. A lack of genetic diversity in resistant soybean cultivars has led to a widespread shift toward virulence in SCN populations, leaving farmers with few proven options other than nonhost rotation to manage this nematode. Recent advances in our understanding of the genes controlling resistance to the nematode have led to improved molecular markers, which are, in turn, increasing the efficiency and precision of the breeding pipeline. A better understanding of the molecular and biochemical basis of SCN resistance and nematode virulence will provide information useful for the development of a long-term strategic plan for diversification and the deployment of cultivars that protect current sources of natural resistance while identifying new targets for engineering novel resistance.

Disease resistance through impairment of α-SNAP-NSF interaction and vesicular trafficking by soybean Rhg1

α-SNAP [soluble NSF (N-ethylmaleimide-sensitive factor) attachment protein] and NSF proteins are conserved across eukaryotes and sustain cellular vesicle trafficking by mediating disassembly and reuse of SNARE protein complexes, which facilitate fusion of vesicles to target membranes. However, certain haplotypes of the Rhg1 (resistance to Heterodera glycines 1) locus of soybean possess multiple repeat copies of an α-SNAP gene (Glyma.18G022500) that encodes atypical amino acids at a highly conserved functional site. These Rhg1 loci mediate resistance to soybean cyst nematode (SCN; H. glycines), the most economically damaging pathogen of soybeans worldwide. Rhg1 is widely used in agriculture, but the mechanisms of Rhg1 disease resistance have remained unclear. In the present study, we found that the resistance-type Rhg1 α-SNAP is defective in interaction with NSF. Elevated in planta expression of resistance-type Rhg1 α-SNAPs depleted the abundance of SNARE-recycling 20S complexes, disrupted vesicle trafficking, induced elevated abundance of NSF, and caused cytotoxicity. Soybean, due to ancient genome duplication events, carries other loci that encode canonical (wild-type) α-SNAPs. Expression of these α-SNAPs counteracted the cytotoxicity of resistance-type Rhg1 α-SNAPs. For successful growth and reproduction, SCN dramatically reprograms a set of plant root cells and must sustain this sedentary feeding site for 2-4 weeks. Immunoblots and electron microscopy immunolocalization revealed that resistance-type α-SNAPs specifically hyperaccumulate relative to wild-type α-SNAPs at the nematode feeding site, promoting the demise of this biotrophic interface. The paradigm of disease resistance through a dysfunctional variant of an essential gene may be applicable to other plant-pathogen interactions.

Molecular Soybean-Pathogen Interactions

Soybean hosts a wide variety of pathogens that cause significant yield losses. The importance of soybean as a major oilseed crop has led to research focused on its interactions with pathogens, such as Soybean mosaic virus, Pseudomonas syringae, Phytophthora sojae, Phakopsora pachyrhizi, and Heterodera glycines. Pioneering work on soybean's interactions with these organisms, which represent the five major pathogen groups (viruses, bacteria, oomycetes, fungi, and nematodes), has contributed to our understanding of the molecular mechanisms underlying virulence and immunity. These mechanisms involve conserved and unique features that validate the need for research in both soybean and homologous model systems. In this review, we discuss identification of effectors and their functions as well as resistance gene-mediated recognition and signaling. We also point out areas in which model systems and recent advances in resources and tools have provided opportunities to gain deeper insights into soybean-pathogen interactions.

Comparative Transcriptome Analysis of Resistant and Susceptible Common Bean Genotypes in Response to Soybean Cyst Nematode Infection

Soybean cyst nematode (SCN; Heterodera glycines Ichinohe) reproduces on the roots of common bean (Phaseolus vulgaris L.) and can cause reductions in plant growth and seed yield. The molecular changes in common bean roots caused by SCN infection are unknown. Identification of genetic factors associated with SCN resistance could help in development of improved bean varieties with high SCN resistance. Gene expression profiling was conducted on common bean roots infected by SCN HG type 0 using next generation RNA sequencing technology. Two pinto bean genotypes, PI533561 and GTS-900, resistant and susceptible to SCN infection, respectively, were used as RNA sources eight days post inoculation. Total reads generated ranged between ~ 3.2 and 5.7 million per library and were mapped to the common bean reference genome. Approximately 70-90% of filtered RNA-seq reads uniquely mapped to the reference genome. In the inoculated roots of resistant genotype PI533561, a total of 353 genes were differentially expressed with 154 up-regulated genes and 199 down-regulated genes when compared to the transcriptome of non- inoculated roots. On the other hand, 990 genes were differentially expressed in SCN-inoculated roots of susceptible genotype GTS-900 with 406 up-regulated and 584 down-regulated genes when compared to non-inoculated roots. Genes encoding nucleotide-binding site leucine-rich repeat resistance (NLR) proteins, WRKY transcription factors, pathogenesis-related (PR) proteins and heat shock proteins involved in diverse biological processes were differentially expressed in both resistant and susceptible genotypes. Overall, suppression of the photosystem was observed in both the responses. Furthermore, RNA-seq results were validated through quantitative real time PCR. This is the first report describing genes/transcripts involved in SCN-common bean interaction and the results will have important implications for further characterization of SCN resistance genes in common bean.

The Role of Cytokinin During Infection of Arabidopsis thaliana by the Cyst Nematode Heterodera schachtii

Plant-parasitic cyst nematodes induce the formation of hypermetabolic feeding sites, termed syncytia, as their sole source of nutrients. The formation of the syncytium is orchestrated by the nematode, in part, by modulation of phytohormone responses, including cytokinin. In response to infection by the nematode Heterodera schachtii, cytokinin signaling is transiently induced at the site of infection and in the developing syncytium. Arabidopsis lines with reduced cytokinin sensitivity show reduced susceptibility to nematode infection, indicating that cytokinin signaling is required for optimal nematode development. Furthermore, lines with increased cytokinin sensitivity also exhibit reduced nematode susceptibility. To ascertain why cytokinin hypersensitivity reduces nematode parasitism, we examined the transcriptomes in wild type and a cytokinin-hypersensitive type-A arr Arabidopsis mutant in response to H. schachtii infection. Genes involved in the response to biotic stress and defense response were elevated in the type-A arr mutant in the absence of nematodes and were hyperinduced following H. schachtii infection, which suggests that the Arabidopsis type-A arr mutants impede nematode development because they are primed to respond to pathogen infection. These results suggest that cytokinin signaling is required for optimal H. schachtii parasitism of Arabidopsis but that elevated cytokinin signaling triggers a heightened immune response to nematode infection.

A high efficient protocol for soybean root transformation by Agrobacterium rhizogenes and most stable reference genes for RT-qPCR analysis

KEY MESSAGE

A 55% transformation efficiency was obtained by our optimized protocol; and we showed that GmELF1 - β and GmELF1 - α are the most stable reference genes for expression analyses under this specific condition. Gene functional analyses are essential to the validation of results obtained from in silico and/or gene-prospecting studies. Genetic transformation methods that yield tissues of transient expression quickly have been of considerable interest to researchers. Agrobacterium rhizogenes-mediated transformation methods, which are employed to generate plants with transformed roots, have proven useful for the study of stress caused by root phytopathogens via gene overexpression and/or silencing. While some protocols have been adapted to soybean plants, transformation efficiencies remain limited; thus, few viable plants are available for performing bioassays. Furthermore, mRNA analyses that employ reverse transcription quantitative polymerase chain reactions (RT-qPCR) require the use of reference genes with stable expression levels across different organs, development steps and treatments. In the present study, an A. rhizogenes-mediated soybean root transformation approach was optimized. The method delivers significantly higher transformation efficiency levels and rates of transformed plant recovery, thus enhancing studies of soybean abiotic conditions or interactions between phytopathogens, such as nematodes. A 55% transformation efficiency was obtained following the addition of an acclimation step that involves hydroponics and different selection processes. The present study also validated the reference genes GmELF1-β and GmELF1-α as the most stable to be used in RT-qPCR analysis in composite plants, mainly under nematode infection.

The Methylome of Soybean Roots during the Compatible Interaction with the Soybean Cyst Nematode

The soybean cyst nematode (SCN; Heterodera glycines) induces the formation of a multinucleated feeding site, or syncytium, whose etiology includes massive gene expression changes. Nevertheless, the genetic networks underlying gene expression control in the syncytium are poorly understood. DNA methylation is a critical epigenetic mark that plays a key role in regulating gene expression. To determine the extent to which DNA methylation is altered in soybean (Glycine max) roots during the susceptible interaction with SCN, we generated whole-genome cytosine methylation maps at single-nucleotide resolution. The methylome analysis revealed that SCN induces hypomethylation to a much higher extent than hypermethylation. We identified 2,465 differentially hypermethylated regions and 4,692 hypomethylated regions in the infected roots compared with the noninfected control. In addition, 703 and 1,346 unique genes were identified as overlapping with hyper- or hypomethylated regions, respectively. The differential methylation in genes apparently occurs independently of gene size and GC content but exhibits strong preference for recently duplicated paralogs. Furthermore, a set of 278 genes was identified as specifically syncytium differentially methylated genes. Of these, we found genes associated with epigenetic regulation, phytohormone signaling, cell wall architecture, signal transduction, and ubiquitination. This study provides, to our knowledge, new evidence that differential methylation is part of the regulatory mechanisms controlling gene expression changes in the nematode-induced syncytium.

Enhanced resistance to soybean cyst nematode Heterodera glycines in transgenic soybean by silencing putative CLE receptors

CLE peptides are small extracellular proteins important in regulating plant meristematic activity through the CLE-receptor kinase-WOX signalling module. Stem cell pools in the SAM (shoot apical meristem), RAM (root apical meristem) and vascular cambium are controlled by CLE signalling pathways. Interestingly, plant-parasitic cyst nematodes secrete CLE-like effector proteins, which act as ligand mimics of plant CLE peptides and are required for successful parasitism. Recently, we demonstrated that Arabidopsis CLE receptors CLAVATA1 (CLV1), the CLAVATA2 (CLV2)/CORYNE (CRN) heterodimer receptor complex and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), which transmit the CLV3 signal in the SAM, are required for perception of beet cyst nematode Heterodera schachtii CLEs. Reduction in nematode infection was observed in clv1, clv2, crn, rpk2 and combined double and triple mutants. In an effort to develop nematode resistance in an agriculturally important crop, orthologues of Arabidopsis receptors including CLV1, CLV2, CRN and RPK2 were identified from soybean, a host for the soybean cyst nematode Heterodera glycines. For each of the receptors, there are at least two paralogues in the soybean genome. Localization studies showed that most receptors are expressed in the root, but vary in their level of expression and spatial expression patterns. Expression in nematode-induced feeding cells was also confirmed. In vitro direct binding of the soybean receptors with the HgCLE peptide was analysed. Knock-down of the receptors in soybean hairy roots showed enhanced resistance to SCN. Our findings suggest that targeted disruption of nematode CLE signalling may be a potential means to engineer nematode resistance in crop plants.

The Medicago sativa gene index 1.2: a web-accessible gene expression atlas for investigating expression differences between Medicago sativa subspecies

Background

Alfalfa (Medicago sativa L.) is the primary forage legume crop species in the United States and plays essential economic and ecological roles in agricultural systems across the country. Modern alfalfa is the result of hybridization between tetraploid M. sativa ssp. sativa and M. sativa ssp. falcata. Due to its large and complex genome, there are few genomic resources available for alfalfa improvement.

Results

A de novo transcriptome assembly from two alfalfa subspecies, M. sativa ssp. sativa (B47) and M. sativa ssp. falcata (F56) was developed using Illumina RNA-seq technology. Transcripts from roots, nitrogen-fixing root nodules, leaves, flowers, elongating stem internodes, and post-elongation stem internodes were assembled into the Medicago sativa Gene Index 1.2 (MSGI 1.2) representing 112,626 unique transcript sequences. Nodule-specific and transcripts involved in cell wall biosynthesis were identified. Statistical analyses identified 20,447 transcripts differentially expressed between the two subspecies. Pair-wise comparisons of each tissue combination identified 58,932 sequences differentially expressed in B47 and 69,143 sequences differentially expressed in F56. Comparing transcript abundance in floral tissues of B47 and F56 identified expression differences in sequences involved in anthocyanin and carotenoid synthesis, which determine flower pigmentation. Single nucleotide polymorphisms (SNPs) unique to each M. sativa subspecies (110,241) were identified.

Conclusions

The Medicago sativa Gene Index 1.2 increases the expressed sequence data available for alfalfa by ninefold and can be expanded as additional experiments are performed. The MSGI 1.2 transcriptome sequences, annotations, expression profiles, and SNPs were assembled into the Alfalfa Gene Index and Expression Database (AGED) at http://plantgrn.noble.org/AGED/, a publicly available genomic resource for alfalfa improvement and legume research.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1718-7) contains supplementary material, which is available to authorized users.

The dynamics of cereal cyst nematode infection differ between susceptible and resistant barley cultivars and lead to changes in (1,3;1,4)-β-glucan levels and HvCslF gene transcript abundance

Heterodera avenae (cereal cyst nematode, CCN) infects the roots of barley (Hordeum vulgare) forming syncytial feeding sites. In resistant host plants, relatively few females develop to maturity. Little is known about the physiological and biochemical changes induced during CCN infection. Responses to CCN infection were investigated in resistant (Rha2) and susceptible barley cultivars through histological, compositional and transcriptional analysis. Two phases were identified that influence CCN viability, including feeding site establishment and subsequent cyst maturation. Syncytial development progressed faster in the resistant cultivar Chebec than in the susceptible cultivar Skiff, and was accompanied by changes in cell wall polysaccharide abundance, particularly (1,3;1,4)-β-glucan. Transcriptional profiling identified several glycosyl transferase genes, including CELLULOSE SYNTHASE-LIKE F10 (HvCslF10), which may contribute to differences in polysaccharide abundance between resistant and susceptible cultivars. In barley, Rha2-mediated CCN resistance drives rapid deterioration of CCN feeding sites, specific changes in cell wall-related transcript abundance and changes in cell wall composition. During H. avenae infection, (1,3;1,4)-β-glucan may influence CCN feeding site development by limiting solute flow, similar to (1,3)-β-glucan during dicot cyst nematode infections. Dynamic transcriptional changes in uncharacterized HvCslF genes, possibly involved in (1,3;1,4)-β-glucan synthesis, suggest a role for these genes in the CCN infection process.

Suppression of NGB and NAB/ERabp1 in tomato modifies root responses to potato cyst nematode infestation

Plant-parasitic nematodes cause significant damage to major crops throughout the world. The small number of genes conferring natural plant resistance and the limitations of chemical control require the development of new protective strategies. RNA interference or the inducible over-expression of nematicidal genes provides an environment-friendly approach to this problem. Candidate genes include NGB, which encodes a small GTP-binding protein, and NAB/ERabp1, which encodes an auxin-binding protein, which were identified as being up-regulated in tomato roots in a transcriptome screen of potato cyst nematode (Globodera rostochiensis) feeding sites. Real-time reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization confirmed the localized up-regulation of these genes in syncytia and surrounding cells following nematode infection. Gene-silencing constructs were introduced into tomato, resulting in a 20%-98% decrease in transcription levels. Nematode infection tests conducted on transgenic plants showed 57%-82% reduction in the number of G. rostochiensis females in vitro and 30%-46% reduction in pot trials. Transmission electron microscopy revealed a deterioration of cytoplasm, and degraded mitochondria and plastids, in syncytia induced in plants with reduced NAB/ERabp1 expression. Cytoplasm in syncytia induced in plants with low NGB expression was strongly electron translucent and contained very few ribosomes; however, mitochondria and plastids remained intact. Functional impairments in syncytial cytoplasm of silenced plants may result from NGB's role in ribosome biogenesis; this was confirmed by localization of yellow fluorescent protein (YFP)-labelled NGB protein in nucleoli and co-repression of NGB in plants with reduced NAB/ERabp1 expression. These results demonstrate that NGB and NAB/ERabp1 play important roles in the development of nematode-induced syncytia.

Whole-genome gene expression profiling revealed genes and pathways potentially involved in regulating interactions of soybean with cyst nematode (Heterodera glycines Ichinohe)

BACKGROUND

Soybean cyst nematode (SCN, Heterodera glycines Ichinohe) is the most devastating pathogen of soybean. Many gene expression profiling studies have been conducted to investigate the responses of soybean to the infection by this pathogen using primarily the first-generation soybean genome array that covered approximately 37,500 soybean transcripts. However, no study has been reported yet using the second-generation Affymetrix soybean whole-genome transcript array (Soybean WT array) that represents approximately 66,000 predicted soybean transcripts.

RESULTS

In the present work, the gene expression profiles of two soybean plant introductions (PIs) PI 437654 and PI 567516C (both resistant to multiple SCN HG Types) and cultivar Magellan (susceptible to SCN) were compared in the presence or absence of the SCN inoculum at 3 and 8 days post-inoculation using the Soybean WT array. Data analysis revealed that the two resistant soybean lines showed distinctive gene expression profiles from each other and from Magellan not only in response to the SCN inoculation, but also in the absence of SCN. Overall, 1,413 genes and many pathways were revealed to be differentially regulated. Among them, 297 genes were constitutively regulated in the two resistant lines (compared with Magellan) and 1,146 genes were responsive to the SCN inoculation in the three lines, with 30 genes regulated both constitutively and by SCN. In addition to the findings similar to those in the published work, many genes involved in ethylene, protein degradation, and phenylpropanoid pathways were also revealed differentially regulated in the present study. GC-rich elements (e.g., GCATGC) were found over-represented in the promoter regions of certain groups of genes. These have not been observed before, and could be new defense-responsive regulatory elements.

CONCLUSIONS

Different soybean lines showed different gene expression profiles in the presence and absence of the SCN inoculum. Both inducible and constitutive gene expression may contribute to resistance to multiple SCN HG Types in the resistant soybean PI lines. Ethylene, protein degradation, and phenylpropanoid pathways, as well as many other pathways reported previously, may play important roles in mediating the soybean-SCN interactions. The revealed genes, pathways, and promoter elements can be further explored to regulate or engineer soybean for resistance to SCN.

Integrated signaling networks in plant responses to sedentary endoparasitic nematodes: a perspective

Sedentary plant endoparasitic nematodes can cause detrimental yield losses in crop plants making the study of detailed cellular, molecular, and whole plant responses to them a subject of importance. In response to invading nematodes and nematode-secreted effectors, plant susceptibility/resistance is mainly determined by the coordination of different signaling pathways including specific plant resistance genes or proteins, plant hormone synthesis and signaling pathways, as well as reactive oxygen signals that are generated in response to nematode attack. Crosstalk between various nematode resistance-related elements can be seen as an integrated signaling network regulated by transcription factors and small RNAs at the transcriptional, posttranscriptional, and/or translational levels. Ultimately, the outcome of this highly controlled signaling network determines the host plant susceptibility/resistance to nematodes.

Genomic and transcriptomic characterization of the transcription factor family R2R3-MYB in soybean and its involvement in the resistance responses to Phakopsora pachyrhizi

Myb genes constitute one of the largest transcription factor families in the plant kingdom. Soybean MYB transcription factors have been related to the plant response to biotic stresses. Their involvement in response to Phakopsora pachyrhizi infection has been reported by several transcriptional studies. Due to their apparently highly diverse functions, these genes are promising targets for developing crop varieties resistant to diseases. In the present study, the identification and phylogenetic analysis of the soybean R2R3-MYB (GmMYB) transcription factor family was performed and the expression profiles of these genes under biotic stress were determined. GmMYBs were identified from the soybean genome using bioinformatic tools, and their putative functions were determined based on the phylogenetic tree and classified into subfamilies using guides AtMYBs describing known functions. The transcriptional profiles of GmMYBs upon infection with different pathogen were revealed by in vivo and in silico analyses. Selected target genes potentially involved in disease responses were assessed by RT-qPCR after different times of inoculation with P. pachyrhizi using different genetic backgrounds related to resistance genes (Rpp2 and Rpp5). R2R3-MYB transcription factors related to lignin synthesis and genes responsive to chitin were significantly induced in the resistant genotypes.

Regulatory interplay between soybean root and soybean cyst nematode during a resistant and susceptible reaction

BACKGROUND

Plant-parasitic nematodes (PPNs) are obligate parasites that feed on the roots of living host plants. Often, these nematodes can lay hundreds of eggs, each capable of surviving without a host for as long as 12 years. When it comes to wreaking havoc on agricultural yield, few nematodes can compare to the soybean cyst nematode (SCN). Quantifying soybean (Glycine max) transcription factor binding sites (TFBSs) during a late-stage SCN resistant and susceptible reaction can shed light onto the systematic interplay between host and pathogen, thereby elucidating underlying cis-regulatory mechanisms.

RESULTS

We sequenced the soybean root transcriptome at 6 and 8 days upon independent inoculation with a virulent and avirulent SCN population. Genes such as β-1,4 glucanase, chalcone synthase, superoxide dismutase and various heat shock proteins (HSPs) exhibited reaction-specific expression profiles. Several likely defense-response genes candidates were also identified which are believed to confer SCN resistance. To explore magnitude of TFBS representation during SCN pathogenesis, a multivariate statistical software identified 46 over-represented TFBSs which capture soybean regulatory dynamics across both reactions.

CONCLUSIONS

Our results reveal a set of soybean TFBSs which are over-represented solely throughout a resistant and susceptible SCN reaction. This set furthers our understanding of soybean cis-regulatory dynamics by providing reaction-specific levels of over-representation at 6 and 8 days after inoculation (dai) with SCN.

The activation and suppression of plant innate immunity by parasitic nematodes

Plant-parasitic nematodes engage in prolonged and intimate relationships with their host plants, often involving complex alterations in host cell morphology and function. It is puzzling how nematodes can achieve this, seemingly without activating the innate immune system of their hosts. Secretions released by infective juvenile nematodes are thought to be crucial for host invasion, for nematode migration inside plants, and for feeding on host cells. In the past, much of the research focused on the manipulation of developmental pathways in host plants by plant-parasitic nematodes. However, recent findings demonstrate that plant-parasitic nematodes also deliver effectors into the apoplast and cytoplasm of host cells to suppress plant defense responses. In this review, we describe the current insights in the molecular and cellular mechanisms underlying the activation and suppression of host innate immunity by plant-parasitic nematodes along seven critical evolutionary and developmental transitions in plant parasitism.

Overexpression of a soybean salicylic acid methyltransferase gene confers resistance to soybean cyst nematode

Salicylic acid plays a critical role in activating plant defence responses after pathogen attack. Salicylic acid methyltransferase (SAMT) modulates the level of salicylic acid by converting salicylic acid to methyl salicylate. Here, we report that a SAMT gene from soybean (GmSAMT1) plays a role in soybean defence against soybean cyst nematode (Heterodera glycines Ichinohe, SCN). GmSAMT1 was identified as a candidate SCN defence-related gene in our previous analysis of soybean defence against SCN using GeneChip microarray experiments. The current study started with the isolation of the full-length cDNAs of GmSAMT1 from a SCN-resistant soybean line and from a SCN-susceptible soybean line. The two cDNAs encode proteins of identical sequences. The GmSAMT1 cDNA was expressed in Escherichia coli. Using in vitro enzyme assays, E. coli-expressed GmSAMT1 was confirmed to function as salicylic acid methyltransferase. The apparent Km value of GmSAMT1 for salicylic acid was approximately 46 μM. To determine the role of GmSAMT1 in soybean defence against SCN, transgenic hairy roots overexpressing GmSAMT1 were produced and tested for SCN resistance. Overexpression of GmSAMT1 in SCN-susceptible backgrounds significantly reduced the development of SCN, indicating that overexpression of GmSAMT1 in the transgenic hairy root system could confer resistance to SCN. Overexpression of GmSAMT1 in transgenic hairy roots was also found to affect the expression of selected genes involved in salicylic acid biosynthesis and salicylic acid signal transduction.

Nematode effector proteins: an emerging paradigm of parasitism

Phytonematodes use a stylet and secreted effectors to modify host cells and ingest nutrients to support their growth and development. The molecular function of nematode effectors is currently the subject of intense investigation. In this review, we summarize our current understanding of nematode effectors, with a particular focus on proteinaceous stylet-secreted effectors of sedentary endoparasitic phytonematodes, for which a wealth of information has surfaced in the past 10 yr. We provide an update on the effector repertoires of several of the most economically important genera of phytonematodes and discuss current approaches to dissecting their function. Lastly, we highlight the latest breakthroughs in effector discovery that promise to shed new light on effector diversity and function across the phylum Nematoda.

Genome-wide analysis of the Hsp20 gene family in soybean: comprehensive sequence, genomic organization and expression profile analysis under abiotic and biotic stresses

BACKGROUND

The Hsp20 genes are associated with stress caused by HS and other abiotic factors, but have recently been found to be associated with the response to biotic stresses. These genes represent the most abundant class among the HSPs in plants, but little is known about this gene family in soybean. Because of their apparent multifunctionality, these proteins are promising targets for developing crop varieties that are better adapted to biotic and abiotic stresses. Thus, in the present study an in silico identification of GmHsp20 gene family members was performed, and the genes were characterized and subjected to in vivo expression analysis under biotic and abiotic stresses.

RESULTS

A search of the available soybean genome databases revealed 51 gene models as potential GmHsp20 candidates. The 51 GmHsp20 genes were distributed across a total of 15 subfamilies where a specific predicted secondary structure was identified. Based on in vivo analysis, only 47 soybean Hsp20 genes were responsive to heat shock stress. Among the GmHsp20 genes that were potentials HSR, five were also cold-induced, and another five, in addition to one GmAcd gene, were responsive to Meloidogyne javanica infection. Furthermore, one predicted GmHsp20 was shown to be responsive only to nematode infection; no expression change was detected under other stress conditions. Some of the biotic stress-responsive GmHsp20 genes exhibited a divergent expression pattern between resistant and susceptible soybean genotypes under M. javanica infection. The putative regulatory elements presenting some conservation level in the GmHsp20 promoters included HSE, W-box, CAAT box, and TA-rich elements. Some of these putative elements showed a unique occurrence pattern among genes responsive to nematode infection.

CONCLUSIONS

The evolution of Hsp20 family in soybean genome has most likely involved a total of 23 gene duplications. The obtained expression profiles revealed that the majority of the 51 GmHsp20 candidates are induced under HT, but other members of this family could also be involved in normal cellular functions, unrelated to HT. Some of the GmHsp20 genes might be specialized to respond to nematode stress, and the predicted promoter structure of these genes seems to have a particular conserved pattern related to their biological function.

War of the worms: how plants fight underground attacks

Sedentary plant-parasitic nematodes (PPNs) establish specialized feeding cells within roots to maintain long-term relationships with their hosts. However, feeding cells degenerate prematurely in plants that harbor resistance (R) genes against these parasites reducing their life span and ability to reproduce. Recognition of the nematode, mediated directly or indirectly by plant R proteins, occurs via nematode secreted effectors and evokes a resistance response, which is referred to as effector-triggered immunity (ETI). Recent breakthroughs in nematode effector biology shed new light on key players mediating ETI and have identified those involved in plant defense suppression as novel targets for engineering resistance in transgenic plants. Advances in plant genetics and genomics has facilitated the discovery of R genes to nematodes. Nevertheless, underlying resistance mechanisms remain poorly understood and are confounded by recently identified R genes that do not fit previously proposed paradigms. Thus, there is still much to be learned about how plants fight off underground attacks from PPNs. In coming years, we can expect breakthroughs in our understanding of the nature and mechanisms of plant resistance and nematode virulence as we explore these novel R genes.

A ubiquitin carboxyl extension protein secreted from a plant-parasitic nematode Globodera rostochiensis is cleaved in planta to promote plant parasitism

Nematode effector proteins originating from esophageal gland cells play central roles in suppressing plant defenses and in formation of the plant feeding cells that are required for growth and development of cyst nematodes. A gene (GrUBCEP12) encoding a unique ubiquitin carboxyl extension protein (UBCEP) that consists of a signal peptide for secretion, a mono-ubiquitin domain, and a 12 amino acid carboxyl extension protein (CEP12) domain was cloned from the potato cyst nematode Globodera rostochiensis. This GrUBCEP12 gene was expressed exclusively within the nematode's dorsal esophageal gland cell, and was up-regulated in the parasitic second-stage juvenile, correlating with the time when feeding cell formation is initiated. We showed that specific GrUBCEP12 knockdown via RNA interference reduced nematode parasitic success, and that over-expression of the secreted Gr(Δ) (SP) UBCEP12 protein in potato resulted in increased nematode susceptibility, providing direct evidence that this secreted effector is involved in plant parasitism. Using transient expression assays in Nicotiana benthamiana, we found that Gr(Δ) (SP) UBCEP12 is processed into free ubiquitin and a CEP12 peptide (GrCEP12) in planta, and that GrCEP12 suppresses resistance gene-mediated cell death. A target search showed that expression of RPN2a, a gene encoding a subunit of the 26S proteasome, was dramatically suppressed in Gr(Δ) (SP) UBCEP12 but not GrCEP12 over-expression plants when compared with control plants. Together, these results suggest that, when delivered into host plant cells, Gr(Δ) (SP) UBCEP12 becomes two functional units, one acting to suppress plant immunity and the other potentially affecting the host 26S proteasome, to promote feeding cell formation.

A soybean cyst nematode resistance gene points to a new mechanism of plant resistance to pathogens

Soybean (Glycine max (L.) Merr.) is an important crop that provides a sustainable source of protein and oil worldwide. Soybean cyst nematode (Heterodera glycines Ichinohe) is a microscopic roundworm that feeds on the roots of soybean and is a major constraint to soybean production. This nematode causes more than US$1 billion in yield losses annually in the United States alone, making it the most economically important pathogen on soybean. Although planting of resistant cultivars forms the core management strategy for this pathogen, nothing is known about the nature of resistance. Moreover, the increase in virulent populations of this parasite on most known resistance sources necessitates the development of novel approaches for control. Here we report the map-based cloning of a gene at the Rhg4 (for resistance to Heterodera glycines 4) locus, a major quantitative trait locus contributing to resistance to this pathogen. Mutation analysis, gene silencing and transgenic complementation confirm that the gene confers resistance. The gene encodes a serine hydroxymethyltransferase, an enzyme that is ubiquitous in nature and structurally conserved across kingdoms. The enzyme is responsible for interconversion of serine and glycine and is essential for cellular one-carbon metabolism. Alleles of Rhg4 conferring resistance or susceptibility differ by two genetic polymorphisms that alter a key regulatory property of the enzyme. Our discovery reveals an unprecedented plant resistance mechanism against a pathogen. The mechanistic knowledge of the resistance gene can be readily exploited to improve nematode resistance of soybean, an increasingly important global crop.

The expression of a naturally occurring, truncated allele of an α-SNAP gene suppresses plant parasitic nematode infection

Transcriptional mapping experiments of the major soybean cyst nematode resistance locus, rhg1, identified expression of the vesicular transport machinery component, α soluble NSF attachment protein (α-SNAP), occurring during defense. Sequencing the α-SNAP coding regions from the resistant genotypes G. max ([Peking/PI 548402]) and G. max ([PI 437654]) revealed they are identical, but differ from the susceptible G. max ([Williams 82/PI 518671]) by the presence of several single nucleotide polymorphisms. Using G. max ([Williams 82/PI 518671]) as a reference, a G → T(2,822) transversion in the genomic DNA sequence at a functional splice site of the α-SNAP([Peking/PI 548402]) allele produced an additional 17 nucleotides of mRNA sequence that contains an in-frame stop codon caused by a downstream G → A(2,832) transition. The G. max ([Peking/PI 548402]) genotype has cell wall appositions (CWAs), structures identified as forming as part of a defense response by the activity of the vesicular transport machinery. In contrast, the 17 nt α-SNAP([Peking/PI 548402]) mRNA motif is not found in G. max ([PI 88788]) that exhibits defense to H. glycines, but lack CWAs. The α-SNAP([PI 88788]) promoter contains sequence elements that are nearly identical to the α-SNAP([Peking/PI 548402]) allele, but differs from the G. max ([Williams 82/PI 518671]) ortholog. Overexpressing the α-SNAP([Peking/PI 548402]) allele in the susceptible G. max ([Williams 82/PI 518671]) genotype suppressed H. glycines infection. The experiments indicate a role for the vesicular transport machinery during infection of soybean by the soybean cyst nematode. However, increased GmEREBP1, PR1, PR2, PR5 gene activity but suppressed PR3 expression accompanied the overexpression of the α-SNAP([Peking/PI 548402]) allele prior to infection.