Identification of introduced and stably inherited DNA methylation variants in soybean associated with soybean cyst nematode parasitism

Rice Serine Hydroxymethyltransferases: Evolution, Subcellular Localization, Function and Perspectives

In rice, there is a lack of comprehensive research on the functional aspects of the members of the serine hydroxymethyltransferase (SHMT) gene family. This study provides a comprehensive investigation of the SHMT gene family, covering phylogeny, gene structure, promoter analysis, expression analysis, subcellular localization, and protein interaction. Remarkably, we discovered a specific gene loss event occurred in the chloroplast-localized group IIa SHMTs in monocotyledons. However, OsSHMT3, which originally classified within cytoplasmic-localized group Ib, was found to be situated within chloroplasts in rice protoplasts. All five OsSHMTs are capable of forming homodimers, with OsSHMT3 being the only one able to form dimers with other OsSHMTs, except for OsSHMT1. It is proposed that OsSHMT3 functions as a mobile protein, collaborating with other OsSHMT proteins. Furthermore, the results of cis-acting element prediction and expression analysis suggested that members of the OsSHMT family could be involved in diverse stress responses and hormone regulation. Our study aims to provide novel insights for the future exploration of SHMTs.

Manipulating epigenetic diversity in crop plants: Techniques, challenges and opportunities

Epigenetic modifications act as conductors of inheritable alterations in gene expression, all while keeping the DNA sequence intact, thereby playing a pivotal role in shaping plant growth and development. This review article presents an overview of techniques employed to investigate and manipulate epigenetic diversity in crop plants, focusing on both naturally occurring and artificially induced epialleles. The significance of epigenetic modifications in facilitating adaptive responses is explored through the examination of how various biotic and abiotic stresses impact them. Further, environmental chemicals are explored for their role in inducing epigenetic changes, particularly focusing on inhibitors of DNA methylation like 5-AzaC and zebularine, as well as inhibitors of histone deacetylation including trichostatin A and sodium butyrate. The review delves into various approaches for generating epialleles, including tissue culture techniques, mutagenesis, and grafting, elucidating their potential to induce heritable epigenetic modifications in plants. In addition, the ground breaking CRISPR/Cas is emphasized for its accuracy in targeting specific epigenetic changes. This presents a potent tools for deciphering the intricacies of epigenetic mechanisms. Furthermore, the intricate relationship between epigenetic modifications and non-coding RNA expression, including siRNAs and miRNAs, is investigated. The emerging role of exo-RNAi in epigenetic regulation is also introduced, unveiling its promising potential for future applications. The article concludes by addressing the opportunities and challenges presented by these techniques, emphasizing their implications for crop improvement. Conclusively, this extensive review provides valuable insights into the intricate realm of epigenetic changes, illuminating their significance in phenotypic plasticity and their potential in advancing crop improvement.

Conceptual Framework of Epigenetic Analyses of Plant Responses to Sedentary Endoparasitic Nematodes

Epigenetic modifications including miRNA regulation, DNA methylation, and histone modifications play fundamental roles in establishing the interactions between host plants and parasitic nematodes. Over the past decade, an increasing number of studies revealed the key functions of various components of the plant epigenome in the regulation of gene expression and shaping plant responses to nematode infection. In this chapter, we provide a conceptual framework for methods used to investigate epigenetic regulation during plant-nematode interactions. We focus specifically on current and emerging methods used to study miRNA regulation and function. We also highlight various methods and analytical tools used to profile DNA methylation patterns and histone modification marks at the genome level. Our intention is simply to explain the advantages of various methods and how to overcome some limitations. With rapid development of single-cell sequencing technology and genome editing, advanced and new methodologies are expected to emerge in the near future to further improve our understanding of epigenetic regulation and function during plant-nematode interactions.

The soybean immune receptor GmBIR1 regulates host transcriptome, spliceome, and immunity during cyst nematode infection

BAK1-INTERACTING RECEPTOR LIKE KINASE1 (BIR1) is a negative regulator of various aspects of disease resistance and immune responses. Here, we investigated the functional role of soybean (Glycine max) BIR1 (GmBIR1) during soybean interaction with soybean cyst nematode (SCN, Heterodera glycines) and the molecular mechanism through which GmBIR1 regulates plant immunity. Overexpression of wild-type variant of GmBIR1 (WT-GmBIR1) using transgenic soybean hairy roots significantly increased soybean susceptibility to SCN, whereas overexpression of kinase-dead variant (KD-GmBIR1) significantly increased plant resistance. Transcriptome analysis revealed that genes oppositely regulated in WT-GmBIR1 and KD-GmBIR1 upon SCN infection were enriched primarily in defense and immunity-related functions. Quantitative phosphoproteomic analysis identified 208 proteins as putative substrates of the GmBIR1 signaling pathway, 114 of which were differentially phosphorylated upon SCN infection. In addition, the phosphoproteomic data pointed to a role of the GmBIR1 signaling pathway in regulating alternative pre-mRNA splicing. Genome-wide analysis of splicing events provided compelling evidence supporting a role of the GmBIR1 signaling pathway in establishing alternative splicing during SCN infection. Our results provide novel mechanistic insights into the function of the GmBIR1 signaling pathway in regulating soybean transcriptome and spliceome via differential phosphorylation of splicing factors and regulation of splicing events of pre-mRNA decay- and spliceosome-related genes.

Overexpression of soybean GmNAC19 and GmGRAB1 enhances root growth and water-deficit stress tolerance in soybean

Soybean (Glycine max) is an important crop in agricultural production where water shortage limits yields in soybean. Root system plays important roles in water-limited environments, but the underlying mechanisms are largely unknown. In our previous study, we produced a RNA-seq dataset generated from roots of soybean at three different growth stages (20-, 30-, and 44-day-old plants). In the present study, we performed a transcriptome analysis of the RNA-seq data to select candidate genes with probable association with root growth and development. Candidate genes were functionally examined in soybean by overexpression of individual genes using intact soybean composite plants with transgenic hairy roots. Root growth and biomass in the transgenic composite plants were significantly increased by overexpression of the GmNAC19 and GmGRAB1 transcriptional factors, showing up to 1.8-fold increase in root length and/or 1.7-fold increase in root fresh/dry weight. Furthermore, greenhouse-grown transgenic composite plants had significantly higher seed yield by about 2-fold than control plants. Expression profiling in different developmental stages and tissues showed that GmNAC19 and GmGRAB1 were most highly expressed in roots, displaying a distinct root-preferential expression. Moreover, we found that under water-deficit conditions, overexpression of GmNAC19 enhanced water stress tolerance in transgenic composite plants. Taken together, these results provide further insights into the agricultural potential of these genes for development of soybean cultivars with improved root growth and enhanced tolerance to water-deficit conditions.

Establishment and maintenance of DNA methylation in nematode feeding sites

A growing body of evidence indicates that epigenetic mechanisms, particularly DNA methylation, play key regulatory roles in plant-nematode interactions. Nevertheless, the transcriptional activity of key genes mediating DNA methylation and active demethylation in the nematode feeding sites remains largely unknown. Here, we profiled the promoter activity of 12 genes involved in maintenance and de novo establishment of DNA methylation and active demethylation in the syncytia and galls induced respectively by the cyst nematode Heterodera schachtii and the root-knot nematode Meloidogyne incognita in Arabidopsis roots. The promoter activity assays revealed that expression of the CG-context methyltransferases is restricted to feeding site formation and development stages. Chromomethylase1 (CMT1), CMT2, and CMT3 and Domains Rearranged Methyltransferase2 (DRM2) and DRM3, which mediate non-CG methylation, showed similar and distinct expression patterns in the syncytia and galls at various time points. Notably, the promoters of various DNA demethylases were more active in galls as compared with the syncytia, particularly during the early stage of infection. Mutants impaired in CG or CHH methylation similarly enhanced plant susceptibility to H. schachtii and M. incognita, whereas mutants impaired in CHG methylation reduced plant susceptibility only to M. incognita. Interestingly, hypermethylated mutants defective in active DNA demethylation exhibited contrasting responses to infection by H. schachtii and M. incognita, a finding most likely associated with differential regulation of defense-related genes in these mutants upon nematode infection. Our results point to methylation-dependent mechanisms regulating plant responses to infection by cyst and root-knot nematodes.

Exploring Soybean Resistance to Soybean Cyst Nematode

Resistance to the soybean cyst nematode (SCN) is a topic incorporating multiple mechanisms and multiple types of science. It is also a topic of substantial agricultural importance, as SCN is estimated to cause more yield damage than any other pathogen of soybean, one of the world's main food crops. Both soybean and SCN have experienced jumps in experimental tractability in the past decade, and significant advances have been made. The rhg1-b locus, deployed on millions of farm acres, has been durable and will remain important, but local SCN populations are gradually evolving to overcome rhg1-b. Multiple other SCN resistance quantitative trait loci (QTL) of proven value are now in play with soybean breeders. QTL causal gene discovery and mechanistic insights into SCN resistance are contributing to both basic and applied disciplines. Additional understanding of SCN and other cyst nematodes will also grow in importance and lead to novel disease control strategies.

miR778 mediates gene expression, histone modification, and DNA methylation during cyst nematode parasitism

Despite the known critical regulatory functions of microRNAs, histone modifications, and DNA methylation in reprograming plant epigenomes in response to pathogen infection, the molecular mechanisms underlying the tight coordination of these components remain poorly understood. Here, we show how Arabidopsis (Arabidopsis thaliana) miR778 coordinately modulates the root transcriptome, histone methylation, and DNA methylation via post-transcriptional regulation of the H3K9 methyltransferases SU(var)3-9 homolog 5 (SUVH5) and SUVH6 upon infection by the beet cyst nematode Heterodera schachtii. miR778 post-transcriptionally silences SUVH5 and SUVH6 upon nematode infection. Manipulation of the expression of miR778 and its two target genes significantly altered plant susceptibility to H. schachtii. RNA-seq analysis revealed a key role of SUVH5 and SUVH6 in reprograming the transcriptome of Arabidopsis roots upon H. schachtii infection. In addition, chromatin immunoprecipitation (ChIP)-seq analysis established SUVH5 and SUVH6 as the main enzymes mediating H3K9me2 deposition in Arabidopsis roots in response to nematode infection. ChIP-seq analysis also showed that these methyltransferases possess distinct DNA binding preferences in that they are targeting transposable elements under noninfected conditions and protein-coding genes in infected plants. Further analyses indicated that H3K9me2 deposition directed by SUVH5 and SUVH6 contributes to gene expression changes both in roots and in nematode feeding sites and preferentially associates with CG DNA methylation. Together, our results uncovered multi-layered epigenetic regulatory mechanisms coordinated by miR778 during Arabidopsis-H. schachtii interactions.

Role of Epigenetics in Modulating Phenotypic Plasticity against Abiotic Stresses in Plants

Plants being sessile are always exposed to various environmental stresses, and to overcome these stresses, modifications at the epigenetic level can prove vital for their long-term survival. Epigenomics refers to the large-scale study of epigenetic marks on the genome, which include covalent modifications of histone tails (acetylation, methylation, phosphorylation, ubiquitination, and the small RNA machinery). Studies based on epigenetics have evolved over the years especially in understanding the mechanisms at transcriptional and posttranscriptional levels in plants against various environmental stimuli. Epigenomic changes in plants through induced methylation of specific genes that lead to changes in their expression can help to overcome various stress conditions. Recent studies suggested that epigenomics has a significant potential for crop improvement in plants. By the induction and modulation of various cellular processes like DNA methylation, histone modification, and biogenesis of noncoding RNAs, the plant genome can be activated which can help in achieving a quicker response against various plant stresses. Epigenetic modifications in plants allow them to adjust under varied environmental stresses by modulating their phenotypic plasticity and at the same time ensure the quality and yield of crops. The plasticity of the epigenome helps to adapt the plants during pre- and postdevelopmental processes. The variation in DNA methylation in different organisms exhibits variable phenotypic responses. The epigenetic changes also occur sequentially in the genome. Various studies indicated that environmentally stimulated epimutations produce variable responses especially in differentially methylated regions (DMR) that play a major role in the management of stress conditions in plants. Besides, it has been observed that environmental stresses cause specific changes in the epigenome that are closely associated with phenotypic modifications. However, the relationship between epigenetic modifications and phenotypic plasticity is still debatable. In this review, we will be discussing the role of various factors that allow epigenetic changes to modulate phenotypic plasticity against various abiotic stress in plants.

Plant Responses to Herbivory, Wounding, and Infection

Plants have various self-defense mechanisms against biotic attacks, involving both physical and chemical barriers. Physical barriers include spines, trichomes, and cuticle layers, whereas chemical barriers include secondary metabolites (SMs) and volatile organic compounds (VOCs). Complex interactions between plants and herbivores occur. Plant responses to insect herbivory begin with the perception of physical stimuli, chemical compounds (orally secreted by insects and herbivore-induced VOCs) during feeding. Plant cell membranes then generate ion fluxes that create differences in plasma membrane potential (Vm), which provokes the initiation of signal transduction, the activation of various hormones (e.g., jasmonic acid, salicylic acid, and ethylene), and the release of VOCs and SMs. This review of recent studies of plant–herbivore–infection interactions focuses on early and late plant responses, including physical barriers, signal transduction, SM production as well as epigenetic regulation, and phytohormone responses.

Phenotypic and Methylome Responses to Salt Stress in Arabidopsis thaliana Natural Accessions

Salt stress threatens plant growth, development and crop yields, and has become a critical global environmental issue. Increasing evidence has suggested that the epigenetic mechanism such as DNA methylation can mediate plant response to salt stress through transcriptional regulation and transposable element (TE) silencing. However, studies exploring genome-wide methylation dynamics under salt stress remain limited, in particular, for studies on multiple genotypes. Here, we adopted four natural accessions of the model species Arabidopsis thaliana and investigated the phenotypic and genome-wide methylation responses to salt stress through whole-genome bisulfite sequencing (WGBS). We found that salt stress significantly changed plant phenotypes, including plant height, rosette diameter, fruit number, and aboveground biomass, and the change in biomass tended to depend on accessions. Methylation analysis revealed that genome-wide methylation patterns depended primarily on accessions, and salt stress caused significant methylation changes in ∼ 0.1% cytosines over the genomes. About 33.5% of these salt-induced differential methylated cytosines (DMCs) were located to transposable elements (TEs). These salt-induced DMCs were mainly hypermethylated and accession-specific. TEs annotated to have DMCs (DMC-TEs) across accessions were found mostly belonged to the superfamily of Gypsy, a type II transposon, indicating a convergent DMC dynamic on TEs across different genetic backgrounds. Moreover, 8.0% of salt-induced DMCs were located in gene bodies and their proximal regulatory regions. These DMCs were also accession-specific, and genes annotated to have DMCs (DMC-genes) appeared to be more accession-specific than DMC-TEs. Intriguingly, both accession-specific DMC-genes and DMC-genes shared by multiple accessions were enriched in similar functions, including methylation, gene silencing, chemical homeostasis, polysaccharide catabolic process, and pathways relating to shifts between vegetative growth and reproduction. These results indicate that, across different genetic backgrounds, methylation changes may have convergent functions in post-transcriptional, physiological, and phenotypic modulation under salt stress. These convergent methylation dynamics across accession may be autonomous from genetic variation or due to convergent genetic changes, which requires further exploration. Our study provides a more comprehensive picture of genome-wide methylation dynamics under salt stress, and highlights the importance of exploring stress response mechanisms from diverse genetic backgrounds.

Kinase-dead mutation: A novel strategy for improving soybean resistance to soybean cyst nematode Heterodera glycines

Protein kinases phosphorylate proteins for functional changes and are involved in nearly all cellular processes, thereby regulating almost all aspects of plant growth and development, and responses to biotic and abiotic stresses. We generated two independent co-expression networks of soybean genes using control and stress response gene expression data and identified 392 differentially highly interconnected kinase hub genes among the two networks. Of these 392 kinases, 90 genes were identified as "syncytium highly connected hubs", potentially essential for activating kinase signalling pathways in the nematode feeding site. Overexpression of wild-type coding sequences of five syncytium highly connected kinase hub genes using transgenic soybean hairy roots enhanced plant susceptibility to soybean cyst nematode (SCN; Heterodera glycines) Hg Type 0 (race 3). In contrast, overexpression of kinase-dead variants of these five syncytium kinase hub genes significantly enhanced soybean resistance to SCN. Additionally, three of the five tested kinase hub genes enhanced soybean resistance to SCN Hg Type 1.2.5.7 (race 2), highlighting the potential of the kinase-dead approach to generate effective and durable resistance against a wide range of SCN Hg types. Subcellular localization analysis revealed that kinase-dead mutations do not alter protein cellular localization, confirming the structure-function of the kinase-inactive variants in producing loss-of-function phenotypes causing significant decrease in nematode susceptibility. Because many protein kinases are highly conserved and are involved in plant responses to various biotic and abiotic stresses, our approach of identifying kinase hub genes and their inactivation using kinase-dead mutation could be translated for biotic and abiotic stress tolerance.

Progress in soybean functional genomics over the past decade

Soybean is one of the most important oilseed and fodder crops. Benefiting from the efforts of soybean breeders and the development of breeding technology, large number of germplasm has been generated over the last 100 years. Nevertheless, soybean breeding needs to be accelerated to meet the needs of a growing world population, to promote sustainable agriculture and to address future environmental changes. The acceleration is highly reliant on the discoveries in gene functional studies. The release of the reference soybean genome in 2010 has significantly facilitated the advance in soybean functional genomics. Here, we review the research progress in soybean omics (genomics, transcriptomics, epigenomics and proteomics), germplasm development (germplasm resources and databases), gene discovery (genes that are responsible for important soybean traits including yield, flowering and maturity, seed quality, stress resistance, nodulation and domestication) and transformation technology during the past decade. At the end, we also briefly discuss current challenges and future directions.

A Broad Review of Soybean Research on the Ongoing Race to Overcome Soybean Cyst Nematode

Plant pathogens greatly impact food security of the ever-growing human population. Breeding resistant crops is one of the most sustainable strategies to overcome the negative effects of these biotic stressors. In order to efficiently breed for resistant plants, the specific plant-pathogen interactions should be understood. Soybean is a short-day legume that is a staple in human food and animal feed due to its high nutritional content. Soybean cyst nematode (SCN) is a major soybean stressor infecting soybean worldwide including in China, Brazil, Argentina, USA and Canada. There are many Quantitative Trait Loci (QTLs) conferring resistance to SCN that have been identified; however, only two are widely used: rhg1 and Rhg4. Overuse of cultivars containing these QTLs/genes can lead to SCN resistance breakdown, necessitating the use of additional strategies. In this manuscript, a literature review is conducted on research related to soybean resistance to SCN. The main goal is to provide a current understanding of the mechanisms of SCN resistance and list the areas of research that could be further explored.

Crosstalk of DNA Methylation Triggered by Pathogen in Poplars With Different Resistances

DNA methylation plays crucial roles in responses to environmental stimuli. Modification of DNA methylation during development and abiotic stress responses has been confirmed in increasing numbers of plants, mainly annual plants. However, the epigenetic regulation mechanism underlying the immune response to pathogens remains largely unknown in plants, especially trees. To investigate whether DNA methylation is involved in the response to infection process or is related to the resistance differences among poplars, we performed comprehensive whole-genome bisulfite sequencing of the infected stem of the susceptible type Populus × euramerican ‘74/76’ and resistant type Populus tomentosa ‘henan’ upon Lonsdalea populi infection. The results revealed that DNA methylation changed dynamically in poplars during the infection process with a remarkable decrease seen in the DNA methylation ratio. Intriguingly, the resistant P. tomentosa ‘henan’ had a much lower basal DNA methylation ratio than the susceptible P. × euramerican ‘74/76’. Compared to mock-inoculation, both poplar types underwent post-inoculation CHH hypomethylation; however, significant decreases in mC and mCHH proportions were found in resistant poplar. In addition, most differentially CHH-hypomethylated regions were distributed in repeat and promoter regions. Based on comparison of DNA methylation modification with the expression profiles of genes, DNA methylation occurred in resistance genes, pathogenesis-related genes, and phytohormone genes in poplars during pathogen infection. Additionally, transcript levels of genes encoding methylation-related enzymes changed during pathogen infection. Interestingly, small-regulator miRNAs were subject to DNA methylation in poplars experiencing pathogen infection. This investigation highlights the critical role of DNA methylation in the poplar immune response to pathogen infection and provides new insights into epigenetic regulation in perennial plants in response to biotic stress.

Genome-wide high-resolution mapping of DNA methylation reveals epigenetic variation in the offspring of sexual and asexual propagation in Robinia pseudoacacia

We detected the genome-wide pattern of DNA methylation and its association with gene expression in sexual and asexual progenies of mature Robinia pseudoacacia trees. DNA methylation plays an important role in plant reproduction and development. Although some studies on sexual reproduction have been carried out in model plants, little is known about the dynamic changes in DNA methylation and their effect on gene expression in sexual and asexual progeny of woody plants. Here, through whole-genome bisulfite sequencing, we revealed DNA methylation patterns in the sexual and asexual progenies of mature Robinia pseudoacacia to understand the regulation of gene expression by DNA methylation in juvenile seedlings. An average of 53% CG, 34% CHG and 5% CHH contexts was methylated in the leaves of mature and juvenile individuals. The CHH methylation level of asexually propagated seedlings was significantly lower than that of seed-derived seedlings and mature trees. The intergenic regions had the highest methylation level. Analysis of differentially methylated regions (DMRs) showed that most of them were hypermethylated and located in the gene upstream and introns. A total of 24, 108 and 162 differentially expressed genes containing DMRs were identified in root sprouts (RSs), root cuttings (RCs) and seed-derived seedlings (SSs), respectively, and a large proportion of them showed hypermethylation. In addition, DMRs were enriched within GO subcategories including catalytic activity, metabolic process and cellular process. The results reveal widespread DNA methylation changes between mature plants and their progenies through sexual/asexual reproduction, which provides novel insights into DNA methylation reprogramming and the regulation of gene expression in woody plants.

Expression Patterns of DNA Methylation and Demethylation Genes during Plant Development and in Response to Phytohormones

DNA methylation and demethylation precisely and effectively modulate gene expression during plant growth and development and in response to stress. However, expression profiles of genes involved in DNA methylation and demethylation during plant development and their responses to phytohormone treatments remain largely unknown. We characterized the spatiotemporal expression patterns of genes involved in de novo methylation, methyl maintenance, and active demethylation in roots, shoots, and reproductive organs using β-glucuronidase (GUS) reporter lines. Promoters of DNA demethylases were generally more highly active at the mature root tissues, whereas the promoters of genes involved in DNA methylation were more highly active at fast-growing root tissues. The promoter activity also implies that methylation status in shoot apex, leaf primordia, floral organs, and developing embryos is under tight equilibrium through the activity of genes involved in DNA methylation and demethylation. The promoter activity of DNA methylation and demethylation-related genes in response to various phytohormone treatments revealed that phytohormones can alter DNA methylation status in specific and redundant ways. Overall, our results illustrate that DNA methylation and demethylation pathways act synergistically and antagonistically in various tissues and in response to phytohormone treatments and point to the existence of hormone-linked methylome regulation mechanisms that may contribute to tissue differentiation and development.

The Underlying Nature of Epigenetic Variation: Origin, Establishment, and Regulatory Function of Plant Epialleles

In plants, the gene expression and associated phenotypes can be modulated by dynamic changes in DNA methylation, occasionally being fixed in certain genomic loci and inherited stably as epialleles. Epiallelic variations in a population can occur as methylation changes at an individual cytosine position, methylation changes within a stretch of genomic regions, and chromatin changes in certain loci. Here, we focus on methylated regions, since it is unclear whether variations at individual methylated cytosines can serve any regulatory function, and the evidence for heritable chromatin changes independent of genetic changes is limited. While DNA methylation is known to affect and regulate wide arrays of plant phenotypes, most epialleles in the form of methylated regions have not been assigned any biological function. Here, we review how epialleles can be established in plants, serve a regulatory function, and are involved in adaptive processes. Recent studies suggest that most epialleles occur as byproducts of genetic variations, mainly from structural variants and Transposable Element (TE) activation. Nevertheless, epialleles that occur spontaneously independent of any genetic variations have also been described across different plant species. Here, we discuss how epialleles that are dependent and independent of genetic architecture are stabilized in the plant genome and how methylation can regulate a transcription relative to its genomic location.

Title: Hypermethylation of miRNA Genes During Nodule Development

DNA methylation has recently emerged as a powerful regulatory mechanism controlling the expression of key regulators of various developmental processes, including nodulation. However, the functional role of DNA methylation in regulating the expression of microRNA (miRNA) genes during the formation and development of nitrogen-fixing nodules remains largely unknown. In this study, we profiled DNA methylation patterns of miRNA genes during nodule formation, development, and early senescence stages in soybean (Glycine max) through the analysis of methylC-seq data. Absolute DNA methylation levels in the CG, CHH, and CHH sequence contexts over the promoter and primary transcript regions of miRNA genes were significantly higher in the nodules compared with the corresponding root tissues at these three distinct nodule developmental stages. We identified a total of 82 differentially methylated miRNAs in the nodules compared with roots. Differential DNA methylation of these 82 miRNAs was detected only in the promoter (69), primary transcript region (3), and both in the promoter and primary transcript regions (10). The large majority of these differentially methylated miRNAs were hypermethylated in nodules compared with the corresponding root tissues and were found mainly in the CHH context and showed stage-specific methylation patterns. Differentially methylated regions in the promoters of 25 miRNAs overlapped with transposable elements, a finding that may explain the vulnerability of miRNAs to DNA methylation changes during nodule development. Gene expression analysis of a set of promoter-differentially methylated miRNAs pointed to a negative association between DNA methylation and miRNA expression. Gene Ontology and pathways analyses indicate that changes in DNA methylation of miRNA genes are reprogrammed and contribute to nodule development through indirect regulation of genes involved in cellular processes and pathways with well-established roles in nodulation.

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.

Identification of Differentially Methylated miRNA Genes During Compatible and Incompatible Interactions Between Soybean and Soybean Cyst Nematode

DNA methylation is a widespread epigenetic mark that affects gene expression and transposon mobility during plant development and stress responses. However, the role of DNA methylation in regulating the expression of microRNA (miRNA) genes remains largely unexplored. Here, we analyzed DNA methylation changes of miRNA genes using a pair of soybean (Glycine max) near-isogenic lines (NILs) differing in their response to soybean cyst nematode (SCN; Heterodera glycines). Differences in global DNA methylation levels over miRNA genes in response to SCN infection were observed between the isogenic lines. miRNA genes with significant changes in DNA methylation levels in the promoter and primary transcript-coding regions were detected in both lines. In the susceptible isogenic line (NIL-S), 82 differentially methylated miRNAs were identified in response to SCN infection whereas, in the resistant isogenic line (NIL-R), only 16 differentially methylated miRNAs were identified. Interestingly, gma-miR5032, gma-miR5043, gma-miR1520b, and gma-2107-ch16 showed opposite methylation patterns in the isogenic lines. In addition, the miRNA paralogs gma-miR5770a and gma-miR5770b showed hypermethylation and hypomethylation in NIL-S and NIL-R, respectively. Gene expression quantification of gma-miR5032, gma-miR5043, gma-miR1520b, and gma-miR5770a/b and their confirmed targets indicated a role of DNA methylation in regulating miRNA expression and, thus, their targets upon SCN infection. Furthermore, overexpression of these four miRNAs in NIL-S using transgenic hairy root system enhanced plant resistance to SCN to various degrees with a key role observed for miR5032. Together, our results provide new insights into the role of epigenetic mechanisms in controlling miRNA regulatory function during SCN-soybean interactions.[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.

Epigenetic Mechanisms of Plant Adaptation to Biotic and Abiotic Stresses

Unlike animals, plants are immobile and could not actively escape the effects of aggressive environmental factors, such as pathogenic microorganisms, insect pests, parasitic plants, extreme temperatures, drought, and many others. To counteract these unfavorable encounters, plants have evolved very high phenotypic plasticity. In a rapidly changing environment, adaptive phenotypic changes often occur in time frames that are too short for the natural selection of adaptive mutations. Probably, some kind of epigenetic variability underlines environmental adaptation in these cases. Indeed, isogenic plants often have quite variable phenotypes in different habitats. There are examples of successful “invasions” of relatively small and genetically homogenous plant populations into entirely new habitats. The unique capability of quick environmental adaptation appears to be due to a high tendency to transmit epigenetic changes between plant generations. Multiple studies show that epigenetic memory serves as a mechanism of plant adaptation to a rapidly changing environment and, in particular, to aggressive biotic and abiotic stresses. In wild nature, this mechanism underlies, to a very significant extent, plant capability to live in different habitats and endure drastic environmental changes. In agriculture, a deep understanding of this mechanism could serve to elaborate more effective and safe approaches to plant protection.

Increased epigenetic diversity and transient epigenetic memory in response to salinity stress in Thlaspi arvense

Epigenetic diversity could play an important role in adaptive evolution of organisms, especially for plant species occurring in new and stressful environments. Thlaspi arvense (field pennycress), a valuable oilseed crop, is widespread in temperate regions of the northern hemisphere. In this study, we investigated the effect of salinity stress on the epigenetic variation of DNA methylation and epigenetic stress memory in pennycress using methylation-sensitive amplification polymorphism (MSAP) markers. We examined how the status of DNA methylation changes across individuals in response to salinity stress and whether such an effect of maternal stress could be transferred to offspring for one or two generations in nonstressed environments. Our results based on 306 epiloci indicated no consistent change of DNA methylation status in specific epiloci across individuals within the same conditions. In contrast, we found that the epigenetic diversity at population level increased significantly in response to the stimulation of salinity stress; and this "stimulation effect" could be transferred partially in the form of stress memory to at least two generations of offspring in nonstressed environments. In addition, we observed a parallel change in functionally important traits, that is, phenotypic variation was significantly higher in plants grown under salinity stress compared with those of control groups. Taken together, our results provide novel clues for the increased spontaneous epimutation rate in response to stress in plants, of potential adaptive significance.

Interactions of gene expression, alternative splicing, and DNA methylation in determining nodule identity

Soybean nodulation is a highly controlled process that involves complex gene regulation at both transcriptional and post-transcriptional levels. In the present study, we profiled gene expression changes, alternative splicing events, and DNA methylation patterns during nodule formation, development, and senescence. The transcriptome data uncovered key transcription patterns of nodule development that included 9669 core genes and 7302 stage-specific genes. Alternative splicing analysis uncovered a total of 2323 genes that undergo alternative splicing events in at least one nodule developmental stage, with activation of exon skipping and repression of intron retention being the most common splicing events in nodules compared to roots. Approximately 40% of the differentially spliced genes were also differentially expressed at the same nodule developmental stage, implying a substantial association between gene expression and alternative splicing. Genome-wide-DNA methylation analysis revealed dynamic changes in nodule methylomes that were specific to each nodule stage, occurred in a sequence-specific manner, and impacted the expression of 1864 genes. An attractive hypothesis raised by our data is that increased DNA methylation may contribute to the efficiency of alternative splicing. Together, our results provide intriguing insights into the associations between gene expression, alternative splicing, and DNA methylation that may shape transcriptome complexity and proteome specificity in developing soybean nodules.

Mutations at the Serine Hydroxymethyltransferase Impact its Interaction with a Soluble NSF Attachment Protein and a Pathogenesis-Related Protein in Soybean

Resistance to soybean cyst nematodes (SCN) in “Peking-type” resistance is bigenic, requiring Rhg4-a and rhg1-a. Rhg4-a encodes a serine hydroxymethyltransferase (GmSHMT08) and rhg1-a encodes a soluble NSF attachment protein (GmSNAP18). Recently, it has been shown that a pathogenesis-related protein, GmPR08-Bet VI, potentiates the interaction between GmSHMT08 and GmSNAP18. Mutational analysis using spontaneously occurring and ethyl methanesulfonate (EMS)-induced mutations was carried out to increase our knowledge of the interacting GmSHMT08/GmSNAP18/GmPR08-Bet VI multi-protein complex. Mutations affecting the GmSHMT08 protein structure (dimerization and tetramerization) and interaction sites with GmSNAP18 and GmPR08-Bet VI proteins were found to impact the multi-protein complex. Interestingly, mutations affecting the PLP/THF substrate binding and catalysis did not affect the multi-protein complex, although they resulted in increased susceptibility to SCN. Most importantly, GmSHMT08 and GmSNAP18 from PI88788 were shown to interact within the cell, being potentiated in the presence of GmPR08-Bet VI. In addition, we have shown the presence of incompatibility between the GmSNAP18 (rhg1-b) of PI88788 and GmSHMT08 (Rhg4-a) from Peking. Components of the reactive oxygen species (ROS) pathway were shown to be induced in the SCN incompatible reaction and were mapped to QTLs for resistance to SCN using different mapping populations.