Gardening the genome: DNA methylation in Arabidopsis thaliana

Cause or effect: Probing the roles of epigenetics in plant development and environmental responses

Epigenetic modifications are inheritable, reversible changes that control gene expression without altering the DNA sequence itself. Recent advances in epigenetic and sequencing technologies have revealed key regulatory regions in genes with multiple epigenetic changes. However, causal associations between epigenetic changes and physiological events have rarely been examined. Epigenome editing enables alterations to the epigenome without changing the underlying DNA sequence. Modifying epigenetic information in plants has important implications for causality assessment of the epigenome. Here, we briefly review tools for selectively interrogating the epigenome. We highlight promising research on site-specific DNA methylation and histone modifications and propose future research directions to more deeply investigate epigenetic regulation, including cause-and-effect relationships between epigenetic modifications and the development/environmental responses of Arabidopsis thaliana.

Uncovering DNA methylation landscapes to decipher evolutionary footprints of phenotypic diversity in chickpea

Genetic diversity and environmental factors are long believed to be the dominant contributors to phenotypic diversity in crop plants. However, it has been recently established that, besides genetic variation, epigenetic variation, especially variation in DNA methylation, plays a significant role in determining phenotypic diversity in crop plants. Therefore, assessing DNA methylation diversity in crop plants becomes vital, especially in the case of crops like chickpea, which has a narrow genetic base. Thus, in the present study, we employed whole-genome bisulfite sequencing to assess DNA methylation diversity in wild and cultivated (desi and kabuli) chickpea. This revealed extensive DNA methylation diversity in both wild and cultivated chickpea. Interestingly, the methylation diversity was found to be significantly higher than genetic diversity, suggesting its potential role in providing vital phenotypic diversity for the evolution and domestication of the Cicer gene pool. The phylogeny based on DNA methylation variation also indicates a potential complementary role of DNA methylation variation in addition to DNA sequence variation in shaping chickpea evolution. Besides, the study also identified diverse epi-alleles of many previously known genes of agronomic importance. The Cicer MethVarMap database developed in this study enables researchers to readily visualize methylation variation within the genes and genomic regions of their interest (http://223.31.159.7/cicer/public/). Therefore, epigenetic variation like DNA methylation variation can potentially explain the paradox of high phenotypic diversity despite the narrow genetic base in chickpea and can potentially be employed for crop improvement.

DNA methylation analysis of floral parts revealed dynamic changes during the development of homostylous Fagopyrum tataricum and heterostylous F. esculentum flowers

BACKGROUND

Proper flower development is essential for plant reproduction, a crucial aspect of the plant life cycle. This process involves precisely coordinating transcription factors, enzymes, and epigenetic modifications. DNA methylation, a ubiquitous and heritable epigenetic mechanism, is pivotal in regulating gene expression and shaping chromatin structure. Fagopyrum esculentum demonstrates anti-hypertensive, anti-diabetic, anti-inflammatory, cardio-protective, hepato-protective, and neuroprotective properties. However, the heteromorphic heterostyly observed in F. esculentum poses a significant challenge in breeding efforts. F. tataricum has better resistance to high altitudes and harsh weather conditions such as drought, frost, UV-B radiation damage, and pests. Moreover, F. tataricum contains significantly higher levels of rutin and other phenolics, more flavonoids, and a balanced amino acid profile compared to common buckwheat, being recognised as functional food, rendering it an excellent candidate for functional food applications.

RESULTS

This study aimed to compare the DNA methylation profiles between the Pin and Thrum flower components of F. esculentum, with those of self-fertile species of F. tataricum, to understand the potential role of this epigenetic mechanism in Fagopyrum floral development. Notably, F. tataricum flowers are smaller than those of F. esculentum (Pin and Thrum morphs). The decline in DNA methylation levels in the developed open flower components, such as petals, stigmas and ovules, was consistent across both species, except for the ovule in the Thrum morph. Conversely, Pin and Tartary ovules exhibited a minor decrease in DNA methylation levels. The highest DNA methylation level was observed in Pin stigma from closed flowers, and the most significant decrease was in Pin stigma from open flowers. In opposition, the nectaries of open flowers exhibited higher levels of DNA methylation than those of closed flowers. The decrease in DNA methylation might correspond with the downregulation of genes encoding methyltransferases.

CONCLUSIONS

Reduced overall DNA methylation and the expression of genes associated with these epigenetic markers in fully opened flowers of both species may indicate that demethylation is necessary to activate the expression of genes involved in floral development.

Genome-wide methylation, transcriptome and characteristic metabolites reveal the balance between diosgenin and brassinosteroids in Dioscorea zingiberensis

Diosgenin (DG) is a bioactive metabolite isolated from Dioscorea species, renowned for its medicinal properties. Brassinosteroids (BRs) are a class of crucial plant steroidal hormones. Cholesterol and campesterol are important intermediates of DG and BR biosynthesis, respectively. DG and BRs are structurally similar components; however, the regulatory network and metabolic interplays have not been fully elucidated. In an effort to decode these complex networks, we conducted a comprehensive study integrating genome-wide methylation, transcriptome and characteristic metabolite data from Dioscorea zingiberensis. Leveraging these data, we were able to construct a comprehensive regulatory network linking DG and BRs. Mass spectrometry results enabled us to clarify the alterations in cholesterol, campesterol, diosgenin, and castasterone (one of the major active BRs). The DG content decreased by 27.72% at 6 h after brassinolide treatment, whereas the content increased by 85.34% at 6 h after brassinazole treatment. Moreover, we pinpointed DG/BR-related genes, such as CASs, CYP90s, and B3-ARFs, implicated in the metabolic pathways of DG and BRs. Moreover, CASs and CYP90s exhibit hypomethylation, which is closely related to their high transcription. These findings provide robust evidence for the homeostasis between DG and BRs. In conclusion, our research revealed the existence of a balance between DG and BRs in D. zingiberensis. Furthermore, our work not only provides new insights into the relationship between the two pathways but also offers a fresh perspective on the functions of secondary metabolites.

Methylation of microRNA genes and its effect on secondary xylem development of stem in poplar

MicroRNAs (miRNAs) and DNA methylation are both vital regulators of gene expression. DNA methylation can affect the transcription of miRNAs, just like coding genes, through methylating the CpG islands in the gene regions of miRNAs. Although previous studies have shown that DNA methylation and miRNAs can each be involved in the process of wood formation, the relationship between the two has been relatively little studied in plant wood formation. Studies have shown that the second internode (IN2) (from top to bottom) of 3-month-old poplar trees can represent the primary stage of poplar stem development and IN8 can represent the secondary stage. There were also significant differences in DNA methylation patterns and miRNA expression patterns obtained from PS and SS. In this study, we first interactively analyzed methylation and miRNA sequencing data to identify 43 differentially expressed miRNAs regulated by differential methylation from the primary stage and secondary stage, which were found to be involved in multiple biological processes related to wood formation by enrichment analysis. In addition, six miRNA/target gene modules were finally identified as potentially involved in secondary xylem development of poplar stems through degradome sequencing and functional analysis. In conclusion, this study provides important reference information on the mechanism of interaction between different regulatory pathways of wood formation.

An RRM domain protein SOE suppresses transgene silencing in rice

Gene expression is regulated at multiple levels, including RNA processing and DNA methylation/demethylation. How these regulations are controlled remains unclear. Here, through analysis of a suppressor for the OsEIN2 over-expressor, we identified an RNA recognition motif protein SUPPRESSOR OF EIN2 (SOE). SOE is localized in nuclear speckles and interacts with several components of the spliceosome. We find SOE associates with hundreds of targets and directly binds to a DNA glycosylase gene DNG701 pre-mRNA for efficient splicing and stabilization, allowing for subsequent DNG701-mediated DNA demethylation of the transgene promoter for proper gene expression. The V81M substitution in the suppressor mutant protein mSOE impaired its protein stability and binding activity to DNG701 pre-mRNA, leading to transgene silencing. SOE mutation enhances grain size and yield. Haplotype analysis in c. 3000 rice accessions reveals that the haplotype 1 (Hap 1) promoter is associated with high 1000-grain weight, and most of the japonica accessions, but not indica ones, have the Hap 1 elite allele. Our study discovers a novel mechanism for the regulation of gene expression and provides an elite allele for the promotion of yield potentials in rice.

Arabidopsis thaliana: a powerful model organism to explore histone modifications and their upstream regulations

Histones are subjected to extensive covalent modifications that affect inter-nucleosomal interactions as well as alter chromatin structure and DNA accessibility. Through switching the corresponding histone modifications, the level of transcription and diverse downstream biological processes can be regulated. Although animal systems are widely used in studying histone modifications, the signalling processes that occur outside the nucleus prior to histone modifications have not been well understood due to the limitations including non viable mutants, partial lethality, and infertility of survivors. Here, we review the benefits of using Arabidopsis thaliana as the model organism to study histone modifications and their upstream regulations. Similarities among histones and key histone modifiers such as the Polycomb group (PcG) and Trithorax group (TrxG) in Drosophila, Human, and Arabidopsis are examined. Furthermore, prolonged cold-induced vernalization system has been well-studied and revealed the relationship between the controllable environment input (duration of vernalization), its chromatin modifications of FLOWERING LOCUS C (FLC), following gene expression, and the corresponding phenotypes. Such evidence suggests that research on Arabidopsis can bring insights into incomplete signalling pathways outside of the histone box, which can be achieved through viable reverse genetic screenings based on the phenotypes instead of direct monitoring of histone modifications among individual mutants. The potential upstream regulators in Arabidopsis can provide cues or directions for animal research based on the similarities between them.

DNA cytosine methylation dynamics and functional roles in horticultural crops

Methylation of cytosine is a conserved epigenetic modification that maintains the dynamic balance of methylation in plants under the regulation of methyltransferases and demethylases. In recent years, the study of DNA methylation in regulating the growth and development of plants and animals has become a key area of research. This review describes the regulatory mechanisms of DNA cytosine methylation in plants. It summarizes studies on epigenetic modifications of DNA methylation in fruit ripening, development, senescence, plant height, organ size, and under biotic and abiotic stresses in horticultural crops. The review provides a theoretical basis for understanding the mechanisms of DNA methylation and their relevance to breeding, genetic improvement, research, innovation, and exploitation of new cultivars of horticultural crops.

Coevolution of the CDCA7-HELLS ICF-related nucleosome remodeling complex and DNA methyltransferases

5-Methylcytosine (5mC) and DNA methyltransferases (DNMTs) are broadly conserved in eukaryotes but are also frequently lost during evolution. The mammalian SNF2 family ATPase HELLS and its plant ortholog DDM1 are critical for maintaining 5mC. Mutations in HELLS, its activator CDCA7, and the de novo DNA methyltransferase DNMT3B, cause immunodeficiency-centromeric instability-facial anomalies (ICF) syndrome, a genetic disorder associated with the loss of DNA methylation. We here examine the coevolution of CDCA7, HELLS and DNMTs. While DNMT3, the maintenance DNA methyltransferase DNMT1, HELLS, and CDCA7 are all highly conserved in vertebrates and green plants, they are frequently co-lost in other evolutionary clades. The presence-absence patterns of these genes are not random; almost all CDCA7 harboring eukaryote species also have HELLS and DNMT1 (or another maintenance methyltransferase, DNMT5). Coevolution of presence-absence patterns (CoPAP) analysis in Ecdysozoa further indicates coevolutionary linkages among CDCA7, HELLS, DNMT1 and its activator UHRF1. We hypothesize that CDCA7 becomes dispensable in species that lost HELLS or DNA methylation, and/or the loss of CDCA7 triggers the replacement of DNA methylation by other chromatin regulation mechanisms. Our study suggests that a unique specialized role of CDCA7 in HELLS-dependent DNA methylation maintenance is broadly inherited from the last eukaryotic common ancestor.

A near-complete genome assembly of the allotetrapolyploid Cenchrus fungigraminus (JUJUNCAO) provides insights into its evolution and C4 photosynthesis

JUJUNCAO (Cenchrus fungigraminus; 2n = 4x = 28) is a Cenchrus grass with the highest biomass production among cultivated plants, and it can be used for mushroom cultivation, animal feed, and biofuel production. Here, we report a nearly complete genome assembly of JUJUNCAO and reveal that JUJUNCAO is an allopolyploid that originated ∼2.7 million years ago (mya). Its genome consists of two subgenomes, and subgenome A shares high collinear synteny with pearl millet. We also investigated the genome evolution of JUJUNCAO and suggest that the ancestral karyotype of Cenchrus split into the A and B ancestral karyotypes of JUJUNCAO. Comparative transcriptome and DNA methylome analyses revealed functional divergence of homeologous gene pairs between the two subgenomes, which was a further indication of asymmetric DNA methylation. The three types of centromeric repeat in the JUJUNCAO genome (CEN137, CEN148, and CEN156) may have evolved independently within each subgenome, with some introgressions of CEN156 from the B to the A subgenome. We investigated the photosynthetic characteristics of JUJUNCAO, revealing its typical C4 Kranz anatomy and high photosynthetic efficiency. NADP-ME and PEPCK appear to cooperate in the major C4 decarboxylation reaction of JUJUNCAO, which is different from other C4 photosynthetic subtypes and may contribute to its high photosynthetic efficiency and biomass yield. Taken together, our results provide insights into the highly efficient photosynthetic mechanism of JUJUNCAO and provide a valuable reference genome for future genetic and evolutionary studies, as well as genetic improvement of Cenchrus grasses.

Unveiling the biosynthesis, mechanisms, and impacts of miRNAs in drought stress resilience in plants

Drought stress is one of the most serious threats to sustainable agriculture and is predicted to be further intensified in the coming decades. Therefore, understanding the mechanism of drought stress tolerance and the development of drought-resilient crops are the major goals at present. In recent years, noncoding microRNAs (miRNAs) have emerged as key regulators of gene expressions under drought stress conditions and are turning out to be the potential candidates that can be targeted to develop drought-resilient crops in the future. miRNAs are known to target and decrease the expression of various genes to govern the drought stress response in plants. In addition, emerging evidence also suggests a regulatory role of long non-coding RNAs (lncRNAs) in the regulation of miRNAs and the expression of their target genes by a process referred as miRNA sponging. In this review, we present the regulatory roles of miRNAs in the modulation of drought-responsive genes along with discussing their biosynthesis and action mechanisms. Additionally, the interactive roles of miRNAs with phytohormone signaling components have also been highlighted to present the global view of miRNA functioning under drought-stress conditions.

Integrated Analysis of DNA Methylome and Transcriptome Reveals Epigenetic Regulation of Cold Tolerance in Litopenaeus vannamei

DNA methylation is an important epigenetic modification that has been shown to be associated with responses to non-biological stressors. However, there is currently no research on DNA methylation in response to environmental signals in shrimp. In this study, we conducted a comprehensive comparative analysis of DNA methylation profiles and differentially expressed genes between two strains of Litopenaeus vannamei with significantly different cold tolerance through whole genome bisulfite sequencing (WGBS) and transcriptome sequencing. Between Lv-C and Lv-T (constant temperature of 28 °C and low temperatures of 18 °C and 10 °C) under cytosine-guanine (CG) environments, 39,100 differentially methylated regions (DMRs) were identified, corresponding to 9302 DMR-related genes (DMRGs). The DMRs were mainly located in the gene body (exons and introns). Gene Ontology (GO) analysis showed that these DMRGs were significantly enriched in cell parts, catalytic activity, and metabolic processes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed significant enrichment of these DMRGs in pathways such as proteasome (ko03050), oxidative phosphorylation (ko00190), mTOR signaling pathway (ko04150), fatty acid metabolism (ko01212), and fatty acid degradation (ko00071). The comprehensive results suggested that L. vannamei mainly regulates gene expression in response to low temperatures through hypermethylation or demethylation of some genes involved in thermogenesis, glycolysis, the autophagy pathway, the peroxisome, and drug metabolism pathways. These results provide important clues for studying DNA methylation patterns and identifying cold tolerance genes in shrimp.

Multiomics Reveals the Regulatory Mechanisms of Arabidopsis Tissues under Heat Stress

Understanding the mechanisms of responses to high temperatures in Arabidopsis will provide insights into how plants may mitigate heat stress under global climate change. And exploring the interconnections of different modification levels in heat stress response could help us to understand the molecular mechanism of heat stress response in Arabidopsis more comprehensively and precisely. In this paper, we combined multiomics analyses to explore the common heat stress-responsive genes and specific heat-responsive metabolic pathways in Arabidopsis leaf, seedling, and seed tissues. We found that genes such as AT1G54050 play a role in promoting proper protein folding in response to HS (Heat stress). In addition, it was revealed that the binding profile of A1B is altered under elevated temperature conditions. Finally, we also show that two microRNAs, ath-mir156h and ath-mir166b-5p, may be core regulatory molecules in HS. Also elucidated that under HS, plants can regulate specific regulatory mechanisms, such as oxygen levels, by altering the degree of CHH methylation.

Characteristics of Salvia miltiorrhiza methylome and the regulatory mechanism of DNA methylation in tanshinone biosynthesis

Salvia miltiorrhiza is a model medicinal plant with significant economic and medicinal value. Its roots produce a group of diterpenoid lipophilic bioactive components, termed tanshinones. Biosynthesis and regulation of tanshinones has attracted widespread interest. However, the methylome of S. miltiorrhiza has not been analysed and the regulatory mechanism of DNA methylation in tanshinone production is largely unknown. Here we report single-base resolution DNA methylomes from roots and leaves. Comparative analysis revealed differential methylation patterns for CG, CHG, and CHH contexts and the association between DNA methylation and the expression of genes and small RNAs. Lowly methylated genes always had higher expression levels and 24-nucleotide sRNAs could be key players in the RdDM pathway in S. miltiorrhiza. DNA methylation variation analysis showed that CHH methylation contributed mostly to the difference. Go enrichment analysis showed that diterpenoid biosynthetic process was significantly enriched for genes with downstream overlapping with hypoCHHDMR in July_root when comparing with those in March_root. Tanshinone biosynthesis-related enzyme genes, such as DXS2, CMK, IDI1, HMGR2, DXR, MDS, CYP76AH1, 2OGD25, and CYP71D373, were less CHH methylated in gene promoters or downstream regions in roots collected in July than those collected in March. Consistently, gene expression was up-regulated in S. miltiorrhiza roots collected in July compared with March and the treatment of DNA methylation inhibitor 5-azacytidine significantly promoted tanshinone production. It suggests that DNA methylation plays a significant regulatory role in tanshinone biosynthesis in S. miltiorrhiza through changing the levels of CHH methylation in promoters or downstreams of key enzyme genes.

Light on perenniality: Para-dormancy is based on ABA-GA antagonism and endo-dormancy on the shutdown of GA biosynthesis

Perennial para- and endo-dormancy are seasonally separate phenomena. Whereas para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, endo-dormancy is the short-day elicited arrest of terminal and AXBs. In hybrid aspen (Populus tremula x P. tremuloides) compromising the apex releases para-dormancy, whereas endo-dormancy requires chilling. ABA and GA are implicated in both phenomena. To untangle their roles, we blocked ABA biosynthesis with fluridone (FD), which significantly reduced ABA levels, downregulated GA-deactivation genes, upregulated the major GA3ox-biosynthetic genes, and initiated branching. Comprehensive GA-metabolite analyses suggested that FD treatment shifted GA production to the non-13-hydroxylation pathway, enhancing GA₄ function. Applied ABA counteracted FD effects on GA metabolism and downregulated several GA_3/4 -inducible α- and γ-clade 1,3-β-glucanases that hydrolyze callose at plasmodesmata (PD), thereby enhancing PD-callose accumulation. Remarkably, ABA-deficient plants repressed GA₄ biosynthesis and established endo-dormancy like controls but showed increased stress sensitivity. Repression of GA₄ biosynthesis involved short-day induced DNA methylation events within the GA3ox2 promoter. In conclusion, the results cast new light on the roles of ABA and GA in dormancy cycling. In para-dormancy, PD-callose turnover is antagonized by ABA, whereas in short-day conditions, lack of GA₄ biosynthesis promotes callose deposition that is structurally persistent throughout endo-dormancy.

Impacts of DNA methylases and demethylases on the methylation and expression of Arabidopsis ethylene signal pathway genes

Arabidopsis ethylene (ET) signal pathway plays important roles in various aspects. Cytosine DNA methylation is significant in controlling gene expression in plants. Here, we analyzed the bisulfite sequencing and mRNA sequencing data from Arabidopsis (de)methylase mutants met1, cmt3, drm1/2, ddm1, ros1-4, and rdd to investigate how DNA (de)methylases influence the DNA methylation and expression of Arabidopsis ET pathway genes. At least 32 genes are found to involved in Arabidopsis ET pathway by text mining. Among them, 14 genes are unmethylated or methylated with very low levels. ACS6 and ACS9 are conspicuously methylated within their upstream regions. The other 16 genes are predominantly methylated at the CG sites within gene body regions in wild-type plants, and mutation of MET1 resulted in almost entire elimination of the CG methylations. In addition, CG methylations within some genes are jointly maintained by MET1 and other (de)methylases. Analyses of mRNA-seq data indicated that some ET pathway genes were differentially expressed between wild-type and diverse mutants. PDF1.2, the marker gene of ET signal pathway, was found being regulated indirectly by the methylases. Eighty-two transposable elements (TEs) were identified to be associated to 15 ET pathway genes. ACS11 is found located in a heterochromatin region that contains 57 TEs, indicating its specific expression and regulation. Together, our results suggest that DNA (de)methylases are implicated in the regulation of CG methylation within gene body regions and transcriptional activity of some ET pathway genes and that maintenance of normal CG methylation is essential for ET pathway in Arabidopsis.

Transcriptome and DNA methylome divergence of inflorescence development between two ecotypes in Panicum hallii

The morphological diversity of the inflorescence determines flower and seed production, which is critical for plant adaptation. Hall's panicgrass (Panicum hallii, P. hallii) is a wild perennial grass that has been developed as a model to study perennial grass biology and adaptive evolution. Highly divergent inflorescences have evolved between the two major ecotypes in P. hallii, the upland ecotype (P. hallii var hallii, HAL2 genotype) with compact inflorescence and large seed and the lowland ecotype (P. hallii var filipes, FIL2 genotype) with an open inflorescence and small seed. Here we conducted a comparative analysis of the transcriptome and DNA methylome, an epigenetic mark that influences gene expression regulation, across different stages of inflorescence development using genomic references for each ecotype. Global transcriptome analysis of differentially expressed genes (DEGs) and co-expression modules underlying the inflorescence divergence revealed the potential role of cytokinin signaling in heterochronic changes. Comparing DNA methylome profiles revealed a remarkable level of differential DNA methylation associated with the evolution of P. hallii inflorescence. We found that a large proportion of differentially methylated regions (DMRs) were located in the flanking regulatory regions of genes. Intriguingly, we observed a substantial bias of CHH hypermethylation in the promoters of FIL2 genes. The integration of DEGs, DMRs, and Ka/Ks ratio results characterized the evolutionary features of DMRs-associated DEGs that contribute to the divergence of the P. hallii inflorescence. This study provides insights into the transcriptome and epigenetic landscape of inflorescence divergence in P. hallii and a genomic resource for perennial grass biology.

Genetic and epigenetic control of the plant metabolome

Plant metabolites are mainly produced through chemical reactions catalysed by enzymes encoded in the genome. Mutations in enzyme-encoding or transcription factor-encoding genes can alter the metabolome by changing the enzyme's catalytic activity or abundance, respectively. Insertion of transposable elements into non-coding regions has also been reported to affect transcription and ultimately metabolite content. In addition to genetic mutations, transgenerational epigenetic variations have also been found to affect metabolic content by controlling the transcription of metabolism-related genes. However, the majority of cases reported so far, in which epigenetic mechanisms are associated with metabolism, are non-transgenerational, and are triggered by developmental signals or environmental stress. Although, accumulating research has provided evidence of strong genetic control of the metabolome, epigenetic control has been largely untouched. Here, we provide a review of the genetic and epigenetic control of metabolism with a focus on epigenetics. We discuss both transgenerational and non-transgenerational epigenetic marks regulating metabolism as well as prospects of the field of metabolic control where intricate interactions between genetics and epigenetics are involved.

Mechanisms of DNA methylation and histone modifications

The field of genetics has expanded a lot in the past few decades due to the accessibility of human genome sequences, but still, the regulation of transcription cannot be explicated exclusively by the sequence of DNA of an individual. The coordination and crosstalk between chromatin factors which are conserved is indispensable for all living creatures. The regulation of gene expression has been dependent on the methylation of DNA, post-translational modifications of histones, effector proteins, chromatin remodeler enzymes that affect the chromatin structure and function, and other cellular activities such as DNA replication, DNA repair, proliferation and growth. The mutation and deletion of these factors can lead to human diseases. Various studies are being performed to identify and understand the gene regulatory mechanisms in the diseased state. The information from these high throughput screening studies is able to aid the treatment developments based on the epigenetics regulatory mechanisms. This book chapter will discourse on various modifications and their mechanisms that take place on histones and DNA that regulate the transcription of genes.

DNA methylation in clonal duckweed (Lemna minor L.) lineages reflects current and historical environmental exposures

Environmentally induced DNA methylation variants may mediate gene expression responses to environmental changes. If such induced variants are transgenerationally stable, there is potential for expression responses to persist over multiple generations. Our current knowledge in plants, however, is almost exclusively based on studies conducted in sexually reproducing species where the majority of DNA methylation changes are subject to resetting in germlines, limiting the potential for transgenerational epigenetics stress memory. Asexual reproduction circumvents germlines, and may therefore be more conducive to long-term inheritance of epigenetic marks. Taking advantage of the rapid clonal reproduction of the common duckweed Lemna minor, we hypothesize that long-term, transgenerational stress memory from exposure to high temperature can be detected in DNA methylation profiles. Using a reduced representation bisulphite sequencing approach (epiGBS), we show that temperature stress induces DNA hypermethylation at many CG and CHG cytosine contexts but not CHH. Additionally, differential methylation in CHG context that was observed was still detected in a subset of cytosines, even after 3-12 generations of culturing in a common environment. This demonstrates a memory effect of stress reflected in the methylome and that persists over multiple clonal generations. Structural annotation revealed that this memory effect in CHG methylation was enriched in transposable elements. The observed epigenetic stress memory is probably caused by stable transgenerational persistence of temperature-induced DNA methylation variants across clonal generations. To the extent that such epigenetic memory has functional consequences for gene expression and phenotypes, this result suggests potential for long-term modulation of stress responses in asexual plants.

BPM1 regulates RdDM-mediated DNA methylation via a cullin 3 independent mechanism

BPM1 interacts with components of the DDR complex and stimulates DNA methylation at CHH sites, suggesting its involvement in the RdDM methylation pathway. The best-known function of MATH-BTB proteins, including Arabidopsis BPM proteins, is their role as substrate-specific adaptors of CUL3-based E3 ligases in the ubiquitin-proteasome pathway. This paper reports a new CUL3-independent role of BPM1 in RNA-directed DNA methylation (RdDM). Using quantitative and qualitative Y2H, pull down, microscale thermophoresis and FRET-FLIM, we demonstrate that BPM1 interacts with DMS3 and RDM1, components of the chromatin remodeling DDR complex involved in the recruitment of the RdDM methylation machinery. All three proteins colocalized predominantly in the nucleus. The MATH domain, which specifically binds proteins destined for degradation, was not essential for interactions with DMS3 and RDM1. In plants overexpressing BPM1, endogenous DMS3 protein levels were stable, indicating that BPM1 does not induce proteasomal degradation. In RDM1-overexpressing plants, RDM1 was not ubiquitinated. Together, these results suggest that BPM1 does not mediate the degradation of DMS3 and RDM1. Additionally, overexpression of BPM1 caused increased global methylation levels as well as CHH methylation in promoters of two RdDM-regulated genes, FWA and CML41. Overall, BPM1 seems to have a stimulating effect on RdDM activity, and this role appears to be unrelated to its known function as a Cul3-based E3 ligase adaptor.

Comparative analysis of sperm DNA methylation supports evolutionary acquired epigenetic plasticity for organ speciation

Aim: To perform a comparative epigenomic analysis of DNA methylation in spermatozoa from humans, mice, rats and mini-pigs. Materials & methods: Genome-wide DNA methylation analysis was used to compare the methylation profiles of orthologous CpG sites. Transcription profiles of early embryo development were analyzed to provide insight into the association between sperm methylation and gene expression programming. Results: We identified DNA methylation variation near genes related to the central nervous system and signal transduction. Gene expression dynamics at different time points of preimplantation stages were modestly associated with spermatozoal DNA methylation at the nearest promoters. Conclusion: Conserved genomic regions subject to epigenetic variation across different species were associated with specific organ functions, suggesting their potential contribution to organ speciation and long-term adaptation to the environment.

Molecular insights into mechanisms underlying thermo-tolerance in tomato

Plant productivity is being seriously compromised by climate-change-induced temperature extremities. Agriculture and food safety are threatened due to global warming, and in many cases the negative impacts have already begun. Heat stress leads to significant losses in yield due to changes in growth pattern, plant phonologies, sensitivity to pests, flowering, grain filling, maturity period shrinkage, and senescence. Tomato is the second most important vegetable crop. It is very sensitive to heat stress and thus, yield losses in tomato due to heat stress could affect food and nutritional security. Tomato plants respond to heat stress with a variety of cellular, physiological, and molecular responses, beginning with the early heat sensing, followed by signal transduction, antioxidant defense, osmolyte synthesis and regulated gene expression. Recent findings suggest that specific plant organs are extremely sensitive to heat compared to the entire plant, redirecting the research more towards generative tissues. This is because, during sexual reproduction, developing pollens are the most sensitive to heat. Often, just a few degrees of temperature elevation during pollen development can have a negative effect on crop production. Furthermore, recent research has discovered certain genetic and epigenetic mechanisms playing key role in thermo-tolerance and have defined new directions for tomato heat stress response (HSR). Present challenges are to increase the understanding of molecular mechanisms underlying HS, and to identify superior genotypes with more tolerance to extreme temperatures. Several metabolites, genes, heat shock factors (HSFs) and microRNAs work together to regulate the plant HSR. The present review provides an insight into molecular mechanisms of heat tolerance and current knowledge of genetic and epigenetic control of heat-tolerance in tomato for sustainable agriculture in the future. The information will significantly contribute to improve breeding programs for development of heat tolerant cultivars.

Integrated analysis of DNA methylation, transcriptome, and global metabolites in interspecific heterotic Capsicum F1 hybrid

Hybrid breeding is one of the efficacious methods of crop improvement. Here, we report our work towards understanding the molecular basis of F1 hybrid heterosis from Capsicum chinense and C. frutescens cross. Bisulfite sequencing identified a total of 70597 CG, 108797 CHG, and 38418 CHH differentially methylated regions (DMRs) across F₁ hybrid and parents, and of these, 4891 DMRs showed higher methylation in F₁ compared to the mid-parental methylation values (MPMV). Transcriptome analysis showed higher expression of 46–55% differentially expressed genes (DE-Gs) in the F₁ hybrid. The qRT-PCR analysis of 24 DE-Gs with negative promoter methylation revealed 91.66% expression similarity with the transcriptome data. A few metabolites and 65–72% enriched genes in metabolite biosynthetic pathways showed overall increased expression in the F₁ hybrid compared to parents. These findings, taken together, provided insights into the integrated role of DNA methylation, and genes and metabolites expression in the manifestation of heterosis in Capsicum.

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Global methylation identified significantly different proportions of mCs in hybrid

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Of common DMRs, 33.08% showed different methylation in hybrid from the mid-parental value

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Negatively correlated DEG pDMR-genes were enriched in metabolic pathways

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Significant higher expression of metabolites and DE-Gs were identified in the F₁ hybrid

DNA hypermethylation promotes the flowering of orchardgrass during vernalization

Vernalization, influenced by environmental factors, is an essential process associated with the productivity of temperate crops, during which epigenetic regulation of gene expression plays an important role. Although DNA methylation is one of the major epigenetic mechanisms associated with the control of gene expression, global changes in DNA methylation in the regulation of gene expression during vernalization-induced flowering of temperate plants remain largely undetermined. To characterize vernalization-associated DNA methylation dynamics, we performed whole-genome bisulfite-treated sequencing and transcriptome sequencing in orchardgrass (Dactylis glomerata) during vernalization. The results revealed that increased levels of genome DNA methylation during the early vernalization of orchardgrass were associated with transcriptional changes in DNA methyltransferase and demethylase genes. Upregulated expression of vernalization-related genes during early vernalization was attributable to an increase in mCHH in the promoter regions of these genes. Application of an exogenous DNA methylation accelerator or overexpression of orchardgrass NUCLEAR POLY(A) POLYMERASE (DgPAPS4) promoted earlier flowering, indicating that DNA hypermethylation plays an important role in vernalization-induced flowering. Collectively, our findings revealed that vernalization-induced hypermethylation is responsible for floral primordium initiation and development. These observations provide a theoretical foundation for further studies on the molecular mechanisms underlying the control of vernalization in temperate grasses.

Demethylation of the 3' LTR region of retrotransposon in VvMYBA1BEN allele enhances anthocyanin biosynthesis in berry skin and flesh in 'Brazil' grape

Black-skinned and red-fleshed grape 'Brazil' is a bud sport of rosy-skinned 'Benitaka'. 'Brazil' has a much higher anthocyanin content in the skin than that of 'Benitaka' and is characterized by the accumulation of anthocyanins in the flesh. Our genomic analysis of the VvMYBA loci, which regulate anthocyanin biosynthesis, suggested that the difference in skin and flesh color between 'Brazil' and 'Benitaka' cannot be explained by genomic alteration at the loci. Expression levels of VvMYBA1 and anthocyanin biosynthesis-related genes in skin and flesh were significantly higher in 'Brazil' than in 'Benitaka' throughout berry development. DNA methylation levels in the 3' long terminal repeat (LTR) of a retrotransposon in the upstream region of VvMYBA1^BEN allele were clearly higher in the skin and flesh of 'Benitaka' than in those of 'Brazil' throughout berry development. These findings suggest that a dramatic decrease in DNA methylation level in the 3' LTR of the retrotransposon in the VvMYBA1^BEN allele in 'Brazil' increases the expression levels of VvMYBA1 and anthocyanin accumulation in skin and flesh. Our findings also suggest that skin and flesh colors are inherited together and vary depending on the presence or absence of the VvMYBA1^BEN allele.

Evolutionary fates of gene-body methylation and its divergent association with gene expression in pigeonpea

Pigeonpea (Cajanus cajan L. Huth) is an agronomically important legume cultivated worldwide. In this study, we extensively analyzed gene-body methylation (GbM) patterns in pigeonpea. We found a bimodal distribution of CG and CHG methylation patterns. GbM features- slow evolution rate and increased length remained conserved. Genes with moderate CG body methylation showed highest expression where as highly-methylated genes showed lowest expression. Transposable element (TE)-related genes were methylated in multiple contexts and hence classified as C-methylated genes. A low expression among C-methylated genes was associated with transposons insertion in gene-body and upstream regulatory regions. The CG methylation patterns were found to be conserved in orthologs compared with non-CG methylation. By comparing methylation patterns between differentially methylated regions (DMRs) of the three genotypes, we found that variably methylated marks are less likely to target evolutionary conserved sequences. Finally, our analysis showed enrichment of nitrogen-related genes in GbM orthologs of legumes, which could be promising candidates for generating epialleles for crop improvement.

Epigenetic Mutation in a Tubulin-Folding Cofactor B (ZmTFCB) Gene Arrests Kernel Development in Maize

Epialleles, the heritable epigenetic variants that are not caused by changes in DNA sequences, can broaden genetic and phenotypic diversity and benefit to crop breeding, but very few epialleles related to agricultural traits have been identified in maize. Here, we cloned a small kernel mutant, smk-wl10, from maize, which encoded a tubulin-folding cofactor B (ZmTFCB) protein. Expression of the ZmTFCB gene decreased in the smk-wl10 mutant, which arrested embryo, endosperm and basal endosperm transfer layer developments. Overexpression of ZmTFCB could complement the defective phenotype of smk-wl10. No nucleotide sequence variation in ZmTFCB could be found between smk-wl10 and wild type (WT). Instead, we detected hypermethylation of nucleotide CHG (where H is A, C or T nucleotide) sequence contexts and increased level of histone H3K9me2 methylation in the upstream sequence of ZmTFCB in smk-wl10 compared with WT, which might respond to the attenuating transcription of ZmTFCB. In addition, yeast two-hybrid and bimolecular fluorescence complementation assays identified a strong interaction between ZmTFCB and its homolog ZmTFCE. Thus, our work identifies a novel epiallele of the maize ZmTFCB gene, which might represent a common phenomenon in the epigenetic regulation of important traits such as kernel development in maize.

Identification and Characterization of DNA Demethylase Genes and Their Association With Thermal Stress in Wheat (Triticum aestivum L.)

DNA demethylases (dMTases) are essential proteins in plants that regulate DNA methylation levels. The dMTase genes have been explored in a number of plant species, however, members of this family have not been reported in wheat. We identified 12 wheat dMTase genes divided into two subfamilies: repressor of silencing 1 (ROS1) and DEMETER-Like (DML). The TadMTases in the same subfamily or clade in the phylogenetic tree have similar gene structures, protein motifs, and domains. The promoter sequence contains multiple cis-regulatory elements (CREs) that respond to abiotic stress, hormones, and light, suggesting that the majority of TadMTase genes play a role in wheat growth, development, and stress response. The nuclear localization signals (NLSs), subcellular localization, and SRR motifs were also analyzed. The expression profile analyses revealed that TadMTase genes showed differential gene expression patterns in distinct developmental stages and tissues as well as under heat stress (HS). Furthermore, the qRT-PCR analysis revealed that TadMTase gene expression differed amongst wheat cultivars with varying degrees of HS tolerance. Overall, this work contributes to the understanding of the biological function of wheat dMTases and lays the foundation for future investigations.

An iron-chelating sulfonamide identified from Drosophila-based screening for antipathogenic discovery

We exploited bacterial infection assays using the fruit fly Drosophila melanogaster to identify anti-infective compounds that abrogate the pathological consequences in the infected hosts. Here, we demonstrated that a pyridine-3-N-sulfonylpiperidine derivative (4a) protects Drosophila from the acute infections caused by bacterial pathogens including Pseudomonas aeruginosa. 4a did not inhibit the growth of P. aeruginosa in vitro, but inhibited the production of secreted toxins such as pyocyanin and hydrogen cyanide, while enhancing the production of pyoverdine and pyochelin, indicative of iron deprivation. Based on its catechol moiety, 4a displayed iron-chelating activity in vitro toward both iron (II) and iron (III), more efficiently than the approved iron-chelating drugs such as deferoxamine and deferiprone, concomitant with more potent antibacterial efficacy in Drosophila infections and unique transcriptome profile. Taken together, these results delineate a Drosophila-based strategy to screen for antipathogenic compounds, which interfere with iron uptake crucial for bacterial virulence and survival in host tissues.

High mobility group A3 enhances transcription of the DNA demethylase gene SlDML2 to promote tomato fruit ripening

DNA methylation plays an important role in regulating tomato (Solanum lycopersicum) fruit ripening. Although SlDML2, a DNA demethylase (DML) gene, is critically involved in tomato fruit ripening, little is known about genes that regulate its expression. Using yeast one-hybrid screening, we identified a High Mobility Group A protein, named SlHMGA3, and demonstrated its binding activity to the AT-rich region of the SlDML2 promoter. We produced slhmga3 tomato mutants using CRISPR/Cas9 and observed that slhmga3 fruit reached the breaker stage much later than fruit from the wild-type. We further demonstrated that at the initiation stage of fruit ripening, the increased expression of SlDML2 and ethylene biosynthetic and signaling genes was significantly delayed in slhmga3 fruit, along with delays in ethylene production and demethylation and activation of ripening-associated transcription factor genes. Our results demonstrate that SlHMGA3 plays a role in enhancing SlDML2 expression, and its effects on tomato fruit ripening are largely through DNA demethylation of ripening-associated transcription factor genes.

The Multiverse of Plant Small RNAs: How Can We Explore It?

Plant small RNAs (sRNAs) are a heterogeneous group of noncoding RNAs with a length of 20–24 nucleotides that are widely studied due to their importance as major regulators in various biological processes. sRNAs are divided into two main classes—microRNAs (miRNAs) and small interfering RNAs (siRNAs)—which differ in their biogenesis and functional pathways. Their identification and enrichment with new structural variants would not be possible without the use of various high-throughput sequencing (NGS) techniques, allowing for the detection of the total population of sRNAs in plants. Classifying sRNAs and predicting their functional role based on such high-performance datasets is a nontrivial bioinformatics task, as plants can generate millions of sRNAs from a variety of biosynthetic pathways. Over the years, many computing tools have been developed to meet this challenge. Here, we review more than 35 tools developed specifically for plant sRNAs over the past few years and explore some of their basic algorithms for performing tasks related to predicting, identifying, categorizing, and quantifying individual sRNAs in plant samples, as well as visualizing the results of these analyzes. We believe that this review will be practical for biologists who want to analyze their plant sRNA datasets but are overwhelmed by the number of tools available, thus answering the basic question of how to choose the right one for a particular study.

Arabidopsis AGO4 loses its Cajal body localization when heterologously expressed in Nicotiana benthamiana

In plants, the RNA-directed DNA methylation (RdDM) pathway plays a major role in establishing DNA methylation. At least some components of the RdDM machinery, including the central component AGO4, are known to concentrate in a subnuclear compartment called the Cajal body in the model plant Arabidopsis thaliana. The molecular underpinnings of Cajal body localization, however, have remained elusive so far. Here, we found that Arabidopsis AGO4 (AtAGO4) fused to GFP does not present its typical Cajal body localization, when transiently expressed in Nicotiana benthamiana. Nevertheless, the endogenous AGO4 protein from N. benthamiana shows a clear accumulation in the Cajal body. Thus, our results suggest that the Cajal body localization of AtAGO4 requires specific molecular machinery that cannot be replaced by orthologues in N. benthamiana. This study presents an experimental system that could lead to mechanistic insights into the targeting of proteins to and localization in the Cajal body in plants.

Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation

Heavy metal pollution in soil is a global problem with serious impacts on human health and ecological security. Phytoextraction in phytoremediation, in which plants uptake and transport heavy metals (HMs) to the tissues of aerial parts, is the most environmentally friendly method to reduce the total amount of HMs in soil and has wide application prospects. However, the molecular mechanism of phytoextraction is still under investigation. The uptake, translocation, and retention of HMs in plants are mainly mediated by a variety of transporter proteins. A better understanding of the accumulation strategy of HMs via transporters in plants is a prerequisite for the improvement of phytoextraction. In this review, the biochemical structure and functions of HM transporter families in plants are systematically summarized, with emphasis on their roles in phytoremediation. The accumulation mechanism and regulatory pathways related to hormones, regulators, and reactive oxygen species (ROS) of HMs concerning these transporters are described in detail. Scientific efforts and practices for phytoremediation carried out in recent years suggest that creation of hyperaccumulators by transgenic or gene editing techniques targeted to these transporters and their regulators is the ultimate powerful path for the phytoremediation of HM contaminated soils.

Potential Role of Domains Rearranged Methyltransferase7 in Starch and Chlorophyll Metabolism to Regulate Leaf Senescence in Tomato

Deoxyribonucleic acid (DNA) methylation is an important epigenetic mark involved in diverse biological processes. Here, we report the critical function of tomato (Solanum lycopersicum) Domains Rearranged Methyltransferase7 (SlDRM7) in plant growth and development, especially in leaf interveinal chlorosis and senescence. Using a hairpin RNA-mediated RNA interference (RNAi), we generated SlDRM7-RNAi lines and observed pleiotropic developmental defects including small and interveinal chlorosis leaves. Combined analyses of whole genome bisulfite sequence (WGBS) and RNA-seq revealed that silencing of SlDRM7 caused alterations in both methylation levels and transcript levels of 289 genes, which are involved in chlorophyll synthesis, photosynthesis, and starch degradation. Furthermore, the photosynthetic capacity decreased in SlDRM7-RNAi lines, consistent with the reduced chlorophyll content and repression of genes involved in chlorophyll biosynthesis, photosystem, and photosynthesis. In contrast, starch granules were highly accumulated in chloroplasts of SlDRM7-RNAi lines and associated with lowered expression of genes in the starch degradation pathway. In addition, SlDRM7 was activated by aging- and dark-induced senescence. Collectively, these results demonstrate that SlDRM7 acts as an epi-regulator to modulate the expression of genes related to starch and chlorophyll metabolism, thereby affecting leaf chlorosis and senescence in tomatoes.

Epigenetics of wheat-rust interaction: an update

The outcome of different host-pathogen interactions is influenced by both genetic and epigenetic systems, which determine the response of plants to pathogens and vice versa. This review highlights key molecular mechanisms and conceptual advances involved in epigenetic research and the progress made in epigenetics of wheat-rust interactions. Epigenetics implies the heritable changes in the way of gene expression as a consequence of the modification of DNA bases, histone proteins, and/or non-coding-RNA biogenesis without disturbing the underlying nucleotide sequence. The changes occurring between DNA and its surrounding chromatin without altering its DNA sequence and leading to significant changes in the genome of any organism are called epigenetic changes. Epigenetics has already been used successfully to explain the mechanism of human pathogens and in the identification of pathogen-induced modifications within various host plants. Wheat rusts are one of the most vital fungal diseases throughout the major wheat-growing areas of the world. The epigenome in plant pathogens causing diseases such as wheat rusts is mysterious. The investigations of host and pathogen epigenetics in the wheat rusts system can offer a piece of suitable evidence for elucidation of the molecular basis of host-pathogen interaction. Besides, the information on the epigenetic regulation of the genes involved in resistance or pathogenicity will provide better insights into the complex resistance signaling pathways and could provide answers to certain key questions, such as whether epigenetic regulation of certain genes is imparting resistance to host in response of certain pathogen elicitors or not. In the last few years, there has been an upsurge in research on the host as well as pathogen epigenetics and its outcome in plant-pathogen interactions. This review summarizes the progress made in the areas related to the epigenetic control of host-pathogen interaction with particular emphasis on wheat rusts.

Epigenetic Mechanisms of Senescence in Plants

Senescence is a major developmental transition in plants that requires a massive reprogramming of gene expression and includes various layers of regulations. Senescence is either an age-dependent or a stress-induced process, and is under the control of complex regulatory networks that interact with each other. It has been shown that besides genetic reprogramming, which is an important aspect of plant senescence, transcription factors and higher-level mechanisms, such as epigenetic and small RNA-mediated regulators, are also key factors of senescence-related genes. Epigenetic mechanisms are an important layer of this multilevel regulatory system that change the activity of transcription factors (TFs) and play an important role in modulating the expression of senescence-related gene. They include chromatin remodeling, DNA methylation, histone modification, and the RNA-mediated control of transcription factors and genes. This review provides an overview of the known epigenetic regulation of plant senescence, which has mostly been studied in the form of leaf senescence, and it also covers what has been reported about whole-plant senescence.

DNA methylation across the tree of life, from micro to macro-organism

DNA methylation is a process in which methyl (CH3) groups are added to the DNA molecule. The DNA segment does not change in the sequence, but DNA methylation could alter the action of DNA. Different enzymes like DNA methyltransferases (DNMTs) take part in methylation of cytosine/adenine nucleosides in DNA. In prokaryotes, DNA methylation is performed to prevent the attack of phage and also plays a role in the chromosome replication and repair. In fungi, DNA methylation is studied to see the transcriptional changes, as in insects, the DNA methylation is not that well-known, it plays a different role like other organisms. In mammals, the DNA methylation is related to different types of cancers and plays the most important role in the placental development and abnormal DNA methylation connected with diseases like cancer, autoimmune diseases, and rheumatoid arthritis.

Integration of Transcriptome and Methylome Analyses Provides Insight Into the Pathway of Floral Scent Biosynthesis in Prunus mume

DNA methylation is a common epigenetic modification involved in regulating many biological processes. However, the epigenetic mechanisms involved in the formation of floral scent have rarely been reported within a famous traditional ornamental plant Prunus mume emitting pleasant fragrance in China. By combining whole-genome bisulfite sequencing and RNA-seq, we determined the global change in DNA methylation and expression levels of genes involved in the biosynthesis of floral scent in four different flowering stages of P. mume. During flowering, the methylation status in the “CHH” sequence context (with H representing A, T, or C) in the promoter regions of genes showed the most significant change. Enrichment analysis showed that the differentially methylated genes (DMGs) were widely involved in eight pathways known to be related to floral scent biosynthesis. As the key biosynthesis pathway of the dominant volatile fragrance of P. mume, the phenylpropane biosynthesis pathway contained the most differentially expressed genes (DEGs) and DMGs. We detected 97 DMGs participated in the most biosynthetic steps of the phenylpropane biosynthesis pathway. Furthermore, among the previously identified genes encoding key enzymes in the biosynthesis of the floral scent of P. mume, 47 candidate genes showed an expression pattern matching the release of floral fragrances and 22 of them were differentially methylated during flowering. Some of these DMGs may or have already been proven to play an important role in biosynthesis of the key floral scent components of P. mume, such as PmCFAT1a/1c, PmBEAT36/37, PmPAL2, PmPAAS3, PmBAR8/9/10, and PmCNL1/3/5/6/14/17/20. In conclusion, our results for the first time revealed that DNA methylation is widely involved in the biosynthesis of floral scent and may play critical roles in regulating the floral scent biosynthesis of P. mume. This study provided insights into floral scent metabolism for molecular breeding.

Pup1 QTL Regulates Gene Expression Through Epigenetic Modification of DNA Under Phosphate Starvation Stress in Rice

Cytosine methylation, epigenetic DNA modification, is well known to regulate gene expression. Among the epigenetic modifications, 5-methylcytosine (5-mC) has been one of the extensively studied epigenetic changes responsible for regulating gene expression in animals and plants. Though a dramatic change in 5-mC content is observed at the genome level, the variation in gene expression is generally less than that it is expected. Only less is understood about the significance of 5-mC in gene regulation under P-starvation stress in plants. Using whole-genome bisulfite sequencing of a pair of rice [Pusa-44 and its near-isogenic line (NIL)-23 harboring Pup1 QTL] genotypes, we could decipher the role of Pup1 on DNA (de)methylation-mediated regulation of gene expression under P-starvation stress. We observed 13-15% of total cytosines to be methylated in the rice genome, which increased significantly under the stress. The number of differentially methylated regions (DMRs) for hypomethylation (6,068) was higher than those (5,279) for hypermethylated DMRs under the stress, particularly in root of NIL-23. Hypomethylation in CHH context caused upregulated expression of 489 genes in shoot and 382 genes in root of NIL-23 under the stress, wherein 387 genes in shoot and 240 genes in root were upregulated exclusively in NIL-23. Many of the genes for DNA methylation, a few for DNA demethylation, and RNA-directed DNA methylation were upregulated in root of NIL-23 under the stress. Methylation or demethylation of DNA in genic regions differentially affected gene expression. Correlation analysis for the distribution of DMRs and gene expression indicated the regulation of gene mainly through (de)methylation of promoter. Many of the P-responsive genes were hypomethylated or upregulated in roots of NIL-23 under the stress. Hypermethylation of gene body in CG, CHG, and CHH contexts caused up- or downregulated expression of transcription factors (TFs), P transporters, phosphoesterases, retrotransposon proteins, and other proteins. Our integrated transcriptome and methylome analyses revealed an important role of the Pup1 QTL in epigenetic regulation of the genes for transporters, TFs, phosphatases, carbohydrate metabolism, hormone-signaling, and chromatin architecture or epigenetic modifications in P-starvation tolerance. This provides insights into the molecular function of Pup1 in modulating gene expression through DNA (de)methylation, which might be useful in improving P-use efficiency or productivity of rice in P-deficient soil.

Priming seeds for the future: Plant immune memory and application in crop protection

Plants have evolved adaptive strategies to cope with pathogen infections that seriously threaten plant viability and crop productivity. Upon the perception of invading pathogens, the plant immune system is primed, establishing an immune memory that allows primed plants to respond more efficiently to the upcoming pathogen attacks. Physiological, transcriptional, metabolic, and epigenetic changes are induced during defense priming, which is essential to the establishment and maintenance of plant immune memory. As an environmental-friendly technique in crop protection, seed priming could effectively induce plant immune memory. In this review, we highlighted the recent advances in the establishment and maintenance mechanisms of plant defense priming and the immune memory associated, and discussed strategies and challenges in exploiting seed priming on crops to enhance disease resistance.

Integrated Transcriptome Analysis and Single-Base Resolution Methylomes of Watermelon (Citrullus lanatus) Reveal Epigenome Modifications in Response to Osmotic Stress

DNA methylation plays an important role against adverse environment by reshaping transcriptional profile in plants. To better understand the molecular mechanisms of watermelon response to osmotic stress, the suspension cultured watermelon cells were treated with 100mM mannitol, and then a methylated cytosines map was generated by whole genome bisulfite sequencing (WGBS). Combined with transcriptome sequencing, the effects of osmotic stress on differentially methylated expressed genes (DMEGs) were assessed. It was found that genes related to plant hormone synthesis, signal transduction, osmoregulatory substance-related and reactive oxygen species scavenging-related enzyme could rapidly respond to osmotic stress. The overall methylation level of watermelon decreased after osmotic stress treatment, and demethylation occurred in CG, CHG, and CHH contexts. Moreover, differentially methylated expressed genes (DMEGs) were significantly enriched in RNA transport, starch and sucrose metabolism, plant hormone signal transduction and biosynthesis of secondary metabolites, especially in biosynthesis of osmolytes synthase genes. Interestingly, demethylation of a key enzyme gene Cla014489 in biosynthesis of inositol upregulated its expression and promoted accumulation of inositol, which could alleviate the inhibition of cell growth caused by osmotic stress. Meanwhile, a recombinant plasmid pET28a-Cla014489 was constructed and transferred into Escherichia coli BL21 for prokaryotic expression and the expression of ClMIPS protein could improve the tolerance of E. coli to osmotic stress. The effect of methylation level on the expression properties of inositol and its related genes was further confirmed by application of DNA methylation inhibitor 5-azacytidine. These results provide a preliminary insight into the altered methylation levels of watermelon cells in response to osmotic stress and suggest a new mechanism that how watermelon cells adapt to osmotic stress.

DNA methylation and histone modifications induced by abiotic stressors in plants

BACKGROUND

A review of research shows that methylation in plants is more complex and sophisticated than in microorganisms and animals. Overall, studies on the effects of abiotic stress on epigenetic modifications in plants are still scarce and limited to few species. Epigenetic regulation of plant responses to environmental stresses has not been elucidated. This study summarizes key effects of abiotic stressors on DNA methylation and histone modifications in plants.

DISCUSSION

Plant DNA methylation and histone modifications in responses to abiotic stressors varied and depended on the type and level of stress, plant tissues, age, and species. A critical analysis of the literature available revealed that 44% of the epigenetic modifications induced by abiotic stressors in plants involved DNA hypomethylation, 40% DNA hypermethylation, and 16% histone modification. The epigenetic changes in plants might be underestimated since most authors used methods such as methylation-sensitive amplification polymorphism (MSAP), High performance liquid chromatography (HPLC), and immunolabeling that are less sensitive compared to bisulfite sequencing and single-base resolution methylome analyses. More over, mechanisms underlying epigenetic changes in plants have not yet been determined since most reports showed only the level or/and distribution of DNA methylation and histone modifications.

CONCLUSIONS

Various epigenetic mechanisms are involved in response to abiotic stressors, and several of them are still unknown. Integrated analysis of the changes in the genome by omic approaches should help to identify novel components underlying mechanisms involved in DNA methylation and histone modifications associated with plant response to environmental stressors.

The RNA Directed DNA Methylation (RdDM) Pathway Regulates Anthocyanin Biosynthesis in Crabapple (Malus cv. spp.) Leaves by Methylating the McCOP1 Promoter

The synthesis of anthocyanin pigments in plants is known to be regulated by multiple mechanisms, including epigenetic regulation; however, the contribution of the RNA-directed DNA methylation (RdDM) pathway is not well understood. Here, we used bisulfite sequencing and Real Time (RT)-quantitative (q) PCR to analyze the methylation level of the promoter of constitutively photomorphogenic 1 (McCOP1) from Malus cv. spp, a gene involved in regulating anthocyanin biosynthesis. The CHH methylation level of the McCOP1 promoter was negatively correlated with McCOP1 RNA expression, and inhibiting DNA methylation caused decreased methylation of the McCOP1 promoter and asymmetric cytosine CHH methylation. We observed that the McCOP1 promoter was a direct target of the RdDM pathway argonaute RISC component 4 (McAGO4) protein, which bound to a McCOP1 promoter GGTTCGG site. Bimolecular fluorescence complementation (BIFC) analysis showed that RNA-directed DNA methylation (McRDM1) interacted with McAGO4 and another RdDM protein, domains rearranged methyltransferase 2 (McDRM2), to regulate the CHH methylation of the McCOP1 promoter. Detection of CHH methylation and COP1 gene expression in the Arabidopsis thalianaatago4, atdrm2 and atrdm1 mutants showed that RDM1 is the effector of the RdDM pathway. This was confirmed by silencing McRDM1 in crabapple leaves or apple fruit, which resulted in a decrease in McCOP1 CHH methylation and an increase in McCOP1 transcript levels, as well as in anthocyanin accumulation. In conclusion, these results show that the RdDM pathway is involved in regulating anthocyanin accumulation through CHH methylation of the McCOP1 promoter.

Impact of transposable elements on the evolution of complex living systems and their epigenetic control

Transposable elements (TEs) contribute to genomic innovations, as well as genome instability, across a wide variety of species. Popular designations such as 'selfish DNA' and 'junk DNA,' common in the 1980s, may be either inaccurate or misleading, while a more enlightened view of the TE-host relationship covers a range from parasitism to mutualism. Both plant and animal hosts have evolved epigenetic mechanisms to reduce the impact of TEs, both by directly silencing them and by reducing their ability to transpose in the genome. However, TEs have also been co-opted by both plant and animal genomes to perform a variety of physiological functions, ranging from TE-derived proteins acting directly in normal biological functions to innovations in transcription factor activity and also influencing gene expression. Their presence, in fact, can affect a range of features at genome, phenotype, and population levels. The impact TEs have had on evolution is multifaceted, and many aspects still remain unexplored. In this review, the epigenetic control of TEs is contextualized according to the evolution of complex living systems.

Increased CG, CHG and CHH methylation at the cycloidea gene in the "Peloria" mutant of Linaria vulgaris

Heritable DNA methylation variation is frequently observed in natural populations of plants, but is thought mostly to be functionally inconsequential. An exception to this is the "Peloria" mutant of Linaria vulgaris, which was originally described by Carl von Linné in 1744. A study in 1999 found that the Peloria phenotype is caused by an epiallele of the L. vulgaris cycloidea homolog Lcyc that showed increased levels of DNA methylation compared to wild-type. The DNA methylation results in silencing of Lcyc, which causes radial flower symmetry in the peloric mutant, whereas wild-type plants have flowers with bilateral symmetry. However, a detailed view of DNA methylation at Lcyc at the single-nucleotide level has not been available. In this study, we investigated DNA methylation at Lcyc and, as a control, at the LvHIRZ gene in wild-type and peloric plants of L. vulgaris using DNA bisulfite treatment coupled to next-generation sequencing. We found strong increases in CHG and CHH methylation at Lcyc, but not LvHIRZ, in Peloria. CG methylation was also increased, but wild-type Lcyc also showed moderate levels of CG methylation. Our results suggest that DNA methylation in all three sequence contexts has been maintained, and potentially transgenerationally inherited, in the peloric L. vulgaris population over decades or even centuries.