A 5-methylcytosine DNA glycosylase/lyase demethylates the retrotransposon Tos17 and promotes its transposition in rice

Inheritance of acquired adaptive cold tolerance in rice through DNA methylation

Epigenetic pathways could provide a mechanistic explanation for the inheritance of acquired characteristics, as proposed by Lamarck in 1802, but epigenetic alterations that endow adaptive hereditary traits have rarely been observed. Here, in cultivated Asian rice (Oryzasativa L.), we identified an epiallele conferring acquired and heritable cold tolerance, an adaptive trait enabling northward spread from its tropical origins. We subjected cold-sensitive rice to multigenerational cold stress and identified a line with acquired stable inheritance of cold tolerance. DNA-hypomethylation variation in the acquiredcoldtolerance 1 (ACT1) promoter region rendered its expression insensitive to cold. This change is, in large part, responsible for the acquired cold tolerance, as confirmed by DNA-methylation editing. Natural variation in ACT1 DNA hypomethylation is associated with cold tolerance and rice geographic distribution. Hypomethylation at ACT1 triggers adaptive cold tolerance, presenting a route to epigenetic-variation-driven inheritance of acquired characteristics.

DNA methylation dynamics play crucial roles in shaping the distinct transcriptomic profiles for different root-type initiation in rice

BACKGROUND

Monocots possess a fibrous root system comprising an embryonic root, crown roots, and lateral roots. The distinct cellular origins highlight the diversity of the initiation mechanism. To date, the distinct initiation mechanisms have been poorly studied. In this study, we conduct a comprehensive transcriptome and DNA methylome assay of these root types during their initiation.

RESULTS

Our findings indicate significant divergence in transcriptome regulation trajectories with apparent transcriptional activation in post-embryonic root initials (crown root and lateral root) contrasted by suppression in embryonic root generation. Additionally, CHH methylation is dynamically and differentially regulated across the initiation stages of the various root types, and is significantly associated with the short transposon element within the promoter regions of functional genes, which plays crucial roles in determining the genes' spatiotemporal transcription. Moreover, our work reveals that the activation of DNA glycosylase 702 (DNG702) and repression of Domains Rearranged Methyltransferase 2 (DRM2) play important roles in the erasure of CHH methylation and activation of functional genes during the processes, such as a novel identified key regulatory bZip65, thus directly impacting the initiation of post-embryonic roots in rice.

CONCLUSIONS

Our extensive analysis delineates the landscapes of spatiotemporal transcriptomes and DNA methylomes during the initiation of the three root types in rice, shedding light on the pivotal role of CHH methylation in the spatiotemporal regulation of various key genes, ensuring the successful initiation of distinct root types in rice.

ARGONAUTE4 and the DNA demethylase REPRESSOR OF SILENCING 1C mediate dehydroascorbate-induced intergenerational nematode resistance in rice

Plants can transmit information to the next generation and modulate the phenotype of their offspring through epigenetic mechanisms. In this study, we demonstrate the activation of "intergenerational acquired resistance" (IAR) in the progeny of rice (Oryza sativa) plants exogenously treated with dehydroascorbate (DHA). The offspring of lifelong DHA-treated plants (DHA-IAR) were significantly less susceptible to the root-knot nematode Meloidogyne graminicola and partially inherited the DHA-induced transcriptional response found in the parental plants. Phytohormone analyses on the DHA-IAR plants unveiled higher basal abscisic acid levels and a primed induction of the jasmonic acid pathway. RNA-seq analysis on the embryonic tissues of immature seeds of DHA-treated plants revealed major shifts in the expression of genes associated with epigenetic pathways. We confirmed that DHA treatment leads to a significant but transient pattern of global DNA hypomethylation in the parental plants 12 to 24 h after treatment. The induction of resistance in the parental plants requires the DNA demethylase REPRESSOR OF SILENCING 1C (ROS1c) and ARGONAUTE 4, suggesting a role for DNA demethylation and subsequent remethylation in establishment of this phenotype. Confirming the transience of global hypomethylation upon DHA treatment, no significant change in global DNA methylation levels was observed in DHA-IAR versus naïve plants. Finally, DHA could not induce IAR in the ros1c mutant line and the ARGONAUTE 4 (ago4ab)-RNAi line. These data indicate that a controlled collaboration between transient DNA demethylation and remethylation underlies the induced resistance and IAR phenotypes upon DHA treatment.

Impact of Abiotic Stress on Rice and the Role of DNA Methylation in Stress Response Mechanisms

With the intensification of global climate change and the increasing complexity of agricultural environments, the improvement of rice stress tolerance is an important focus of current breeding research. This review summarizes the current knowledge on the impact of various abiotic stresses on rice and the associated epigenetic responses (DNA methylation). Abiotic stress factors, including high temperature, drought, cold, heavy metal pollution, and high salinity, have a negative impact on crop productivity. Epigenetic changes are key regulatory factors in plant stress responses, and DNA methylation is one of the earliest discovered and thoroughly studied mechanisms in these epigenetic regulatory mechanisms. The normal growth of rice is highly dependent on the environment, and changes in the environment can lead to rice sterility and severe yield loss. Changes in the regulation of the DNA methylation pathway are involved in rice's response to stress. Various DNA methylation-regulating protein complexes that function during rice development have been identified. Significant changes in DNA methylation occur in numerous stress-responsive genes, particularly those in the abscisic acid signaling pathway. These findings underscore the complex mechanisms of the abiotic stress response in rice. We propose the effective improvement of tolerance traits by regulating the epigenetic status of rice and emphasize the role of DNA methylation in abiotic stress tolerance, thereby addressing global climate change and ensuring food security.

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.

Genome-Wide Analysis of DNA Demethylases in Land Plants and Their Expression Pattern in Rice

DNA demethylation is a very important biochemical pathway regulating a group of biological processes, such as embryo development, fruit ripening, and response to stress. Despite the essential role of DNA demethylases, their evolutionary relationship and detailed biological functions in different land plants remain unclear. In this study, 48 DNA demethylases in 12 land plants were identified and classified. A phylogenetic tree was constructed to demonstrate the evolutionary relationships among these DNA demethylases, indicating how they are related across different species. Conserved domain, protein motif, and gene structure analysis showed that these 48 DNA demethylases fell into the presently identified four classes of DNA demethylases. Amino acid alignment revealed conserved catalytic sites and a previously less-studied protein region (referred to as domain A) within the DNA demethylases. An analysis showed a conserved pattern of gene duplication for DNA demethylases throughout their evolutionary history, suggesting that these genes had been maintained due to their importance. The examination of promoter cis-elements displayed potential signaling and regulating pathways of DNA demethylases. Furthermore, the expression profile was analyzed to investigate the physiological role of rice DNA demethylase in different developmental stages, in tissues, and in response to stress and various phytohormone signals. The findings offer a deeper insight into the functional regions of DNA demethylases and their evolutionary relationships, which can guide future research directions. Understanding the role of DNA demethylases can lead to improved plant stress resistance and contribute to the development of better crop and fruit varieties.

Base Excision DNA Repair in Plants: Arabidopsis and Beyond

Base excision DNA repair (BER) is a key pathway safeguarding the genome of all living organisms from damage caused by both intrinsic and environmental factors. Most present knowledge about BER comes from studies of human cells, E. coli, and yeast. Plants may be under an even heavier DNA damage threat from abiotic stress, reactive oxygen species leaking from the photosynthetic system, and reactive secondary metabolites. In general, BER in plant species is similar to that in humans and model organisms, but several important details are specific to plants. Here, we review the current state of knowledge about BER in plants, with special attention paid to its unique features, such as the existence of active epigenetic demethylation based on the BER machinery, the unexplained diversity of alkylation damage repair enzymes, and the differences in the processing of abasic sites that appear either spontaneously or are generated as BER intermediates. Understanding the biochemistry of plant DNA repair, especially in species other than the Arabidopsis model, is important for future efforts to develop new crop varieties.

Genome-wide identification and comparative analysis of DNA methyltransferase and demethylase gene families in two ploidy Cyclocarya paliurus and their potential function in heterodichogamy

BACKGROUND

DNA methylation is one of the most abundant epigenetic modifications, which plays important roles in flower development, sex differentiation, and regulation of flowering time. Its pattern is affected by cytosine-5 DNA methyltransferase (C5-MTase) and DNA demethylase (dMTase). At present, there are no reports on C5-MTase and dMTase genes in heterodichogamous Cyclocarya paliurus.

RESULTS

In this study, 6 CpC5-MTase and 3 CpdMTase genes were identified in diploid (2n = 2 ×  = 32) C. paliurus, while 20 CpC5-MTase and 13 CpdMTase genes were identified in autotetraploid (2n = 4 ×  = 64). 80% of identified genes maintained relatively fixed positions on chromosomes during polyploidization. In addition, we found that some DRM subfamily members didn't contain the UBA domain. The transcript abundance of CpC5-MTase and CpdMTase in male and female flowers of two morphs (protandry and protogyny) from diploidy was analyzed. Results showed that all genes were significantly up-regulated at the stage of floral bud break (S2), but significantly down-regulated at the stage of flower maturation (S4). At S2, some CpC5-MTase genes showed higher expression levels in PG-M than in PG-F, whereas some CpdMTase genes showed higher expression levels in PA-M than in PA-F. In addition, these genes were significantly associated with gibberellin synthesis-related genes (e.g. DELLA and GID1), suggesting that DNA methylation may play a role in the asynchronous floral development process through gibberellin signal.

CONCLUSIONS

These results broaden our understanding of the CpC5-MTase and CpdMTase genes in diploid and autotetraploid C. paliurus, and provide a novel insight into regulatory mechanisms of DNA methylation in heterodichogamy.

Interplay of phytohormones and epigenetic regulation: A recipe for plant development and plasticity

Both phytohormone signaling and epigenetic mechanisms have long been known to play crucial roles in plant development and plasticity in response to ambient stimuli. Indeed, diverse signaling pathways mediated by phytohormones and epigenetic processes integrate multiple upstream signals to regulate various plant traits. Emerging evidence indicates that phytohormones and epigenetic processes interact at multiple levels. In this review, we summarize the current knowledge of the interplay between phytohormones and epigenetic processes from the perspective of phytohormone biology. We also review chemical regulators used in epigenetic studies and propose strategies for developing novel regulators using multidisciplinary approaches.

A new demethylase gene, OsDML4, is involved in high temperature-increased grain chalkiness in rice

High temperature (HT) can affect the accumulation of seed storage materials and cause adverse effects on the yield and quality of rice. DNA methylation plays an important role in plant growth and development. Here, we identified a new demethylase gene OsDML4 and discovered its function in cytosine demethylation to affect endosperm formation. Loss of function of OsDML4 induced chalky endosperm only under HT and dramatically reduced the transcription and accumulation of glutelins and 16 kDa prolamin. The expression of two transcription factor genes RISBZ1 and RPBF was significantly decreased in the osdml4 mutants, which caused adverse effects on the formation of protein bodies (PBs) with greatly decreased PB-II number, and incomplete and abnormally shaped PB-IIs. Whole-genome bisulfite sequencing analysis of seeds at 15 d after pollination revealed much higher global methylation levels of CG, CHG, and CHH contexts in the osdml4 mutants compared with the wild type. Moreover, the RISBZ1 promoter was hypermethylated but the RPBF promoter was almost unchanged under HT. No significant difference was detected between the wild type and osdml4 mutants under normal temperature. Our study demonstrated a novel OsDML4-mediated DNA methylation involved in the formation of chalky endosperm only under HT and provided a new perspective in regulating endosperm development and the accumulation of seed storage proteins in rice.

Loss of Function of the RRMF Domain in OsROS1a Causes Sterility in Rice (Oryza sativa L.)

For crop seed production, the development of anthers and male fertility are the main agronomic traits and key biological processes for flowering plants. Active DNA demethylation regulates many plant developmental processes and is ensured by 5-meC DNA glycosylase enzymes. To find out the role of OsROS1a, OsROS1a gene editing mutants were generated using the CRISPR/Cas9 system. The osros1a mutants had shrink spikelets, smaller anthers and pollen grains, and were not stained by iodine staining showing a significant reduction in total soluble sugar and starch contents as compared to wildtype (WT), which caused complete male sterility. Similarly, the expression of genes involved in pollen and anther development was decreased in osros1a mutants as compared to WT. Furthermore, bisulfite sequencing showed that the CG and CHG methylation of the OsPKS2 gene promoter was significantly increased in the osros1a mutant, which caused a reduced expression of OsPKS2 in osros1a mutants. DNA methylation of the TDR gene promoter was similar between WT and osros1a mutants, indicating that the DNA methylation effect by OsROS1a was gene specific. The expression of OsROS1a in the mutants was not changed, but it produced a frame-shift mutation to truncate the Pem-CXXC and RRMF domains. Combined with previous studies, our findings suggested that the RRMF domain in OsROS1a is the functional domain and loss of RRMF for OsROS1a causes sterility in rice.

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.

The Function of DNA Demethylase Gene ROS1a Null Mutant on Seed Development in Rice (Oryza Sativa) Using the CRISPR/CAS9 System

The endosperm is the main nutrient source in cereals for humans, as it is a highly specialized storage organ for starch, lipids, and proteins, and plays an essential role in seed growth and development. Active DNA demethylation regulates plant developmental processes and is ensured by cytosine methylation (5-meC) DNA glycosylase enzymes. To find out the role of OsROS1a in seed development, the null mutant of OsROS1a was generated using the CRISPR/Cas9 system. The null mutant of OsROS1a was stable and heritable, which affects the major agronomic traits, particularly in rice seeds. The null mutant of OsROS1a showed longer and narrower grains, and seeds were deformed containing an underdeveloped and less-starch-producing endosperm with slightly irregularly shaped embryos. In contrast to the transparent grains of the wild type, the grains of the null mutant of OsROS1a were slightly opaque and rounded starch granules, with uneven shapes, sizes, and surfaces. A total of 723 differential expression genes (DEGs) were detected in the null mutant of OsROS1a by RNA-Seq, of which 290 were downregulated and 433 were upregulated. The gene ontology (GO) terms with the top 20 enrichment factors were visualized for cellular components, biological processes, and molecular functions. The key genes that are enriched for these GO terms include starch synthesis genes (OsSSIIa and OsSSIIIa) and cellulose synthesis genes (CESA2, CESA3, CESA6, and CESA8). Genes encoding polysaccharides and glutelin were found to be downregulated in the mutant endosperm. The glutelins were further verified by SDS-PAGE, suggesting that glutelin genes could be involved in the null mutant of OsROS1a seed phenotype and OsROS1a could have the key role in the regulation of glutelins. Furthermore, 378 differentially alternative splicing (AS) genes were identified in the null mutant of OsROS1a, suggesting that the OsROS1a gene has an impact on AS events. Our findings indicated that the function on rice endosperm development in the null mutant of OsROS1a could be influenced through regulating gene expression and AS, which could provide the base to properly understand the molecular mechanism related to the OsROS1a gene in the regulation of rice seed development.

Decoding the sorghum methylome: understanding epigenetic contributions to agronomic traits

DNA methylation is a chromatin modification that plays an essential role in regulating gene expression and genome stability and it is typically associated with gene silencing and heterochromatin. Owing to its heritability, alterations in the patterns of DNA methylation have the potential to provide for epigenetic inheritance of traits. Contemporary epigenomic technologies provide information beyond sequence variation and could supply alternative sources of trait variation for improvement in crops such as sorghum. Yet, compared with other species such as maize and rice, the sorghum DNA methylome is far less well understood. The distribution of CG, CHG, and CHH methylation in the genome is different compared with other species. CG and CHG methylation levels peak around centromeric segments in the sorghum genome and are far more depleted in the gene dense chromosome arms. The genes regulating DNA methylation in sorghum are also yet to be functionally characterised; better understanding of their identity and functional analysis of DNA methylation machinery mutants in diverse genotypes will be important to better characterise the sorghum methylome. Here, we catalogue homologous genes encoding methylation regulatory enzymes in sorghum based on genes in Arabidopsis, maize, and rice. Discovering variation in the methylome may uncover epialleles that provide extra information to explain trait variation and has the potential to be applied in epigenome-wide association studies or genomic prediction. DNA methylation can also improve genome annotations and discover regulatory elements underlying traits. Thus, improving our knowledge of the sorghum methylome can enhance our understanding of the molecular basis of traits and may be useful to improve sorghum performance.

Roles of IDM3 and SDJ1/2/3 in Establishment and/or Maintenance of DNA Methylation in Arabidopsis

Previous studies had demonstrated that in Arabidopsis, IDM3 is involved in ROS1-mediated DNA demethylation pathway, and SUVH-SDJ complex functions as a DNA methylation reader complex for enhancing gene transcription, which presumably recruits ROS1 to the promoters of target genes for DNA demethylation. Here, our analyses, however, showed that the IDM3 and SDJ1/2/3, the components of the SUVH-SDJ complex, are implicated in establishing and/or maintaining DNA methylation as well through DDR (DRD1-DMS3-RDM1) complex. idm3-3 or sdj1/2/3 mutations led to genome-wide DNA hypomethylation, and both mutants shared a large number of common hypo-DMRs (Differentially Methylated Regions) with rdm1-4 and dms3-4, suggesting that IDM3 and SDJ1/2/3 help establish and/or maintain DNA methylation, mediated by RdDM pathway, at a subset of genomic regions largely through DDR complex. IDM3 is able to strongly interact with RDM1 and DMS3, but weakly with SDJ1 and SDJ3; SDJ1 and SDJ3 is capable of interacting separately with RDM1 and DMS3. Furthermore, comparisons of DNA methylation features in idm3-3 and sdj1/2/3 indicated that idm3-3 and sdj1/2/3 mutations make differential impacts on DNA methylation levels and patterns on a genome-wide scale, indicating that they are targeted to quite distinct genomic regions for aiding in DNA methylation. Further analyses on ChIP-seq data demonstrated that RDM1, DMS3 and NRPE1 are enriched in IDM3- and SDJ1/2/3-targted regions. Altogether, our results provide clear demonstration that IDM3 and SDJ1/2/3 play a part in establishing and/or maintaining DNA methylation of a group of genomic regions, through the DDR complex.

DNA demethylases remodel DNA methylation in rice gametes and zygote and are required for reproduction

Fertilization constitutes a critical step in the plant life cycle during which the gamete genomes undergo chromatin dynamics in preparation for embryogenesis. In mammals, parental chromatin is extensively reprogrammed through the global erasure of DNA methylation. However, in flowering plants it remains unclear whether and how DNA methylation is remodeled in gametes and after fertilization in the zygote. In this study, we characterize DNA methylation patterns and investigate the function of DNA glycosylases in rice eggs, sperm, and unicellular zygotes and during embryogenesis. We found that DNA methylation is locally reconfigured after fertilization and is intensified during embryogenesis. Genetic, epigenomic, and transcriptomic analysis revealed that three rice DNA glycosylases, DNG702, DNG701, and DNG704, demethylate DNA at distinct genomic regions in the gametes and the zygote, and are required for zygotic gene expression and development. Collectively, these results indicate that active DNA demethylation takes place in the gametes and the zygote to locally remodel DNA methylation, which is critical for egg and zygote gene expression and reproduction in rice.

Androgenic-Induced Transposable Elements Dependent Sequence Variation in Barley

A plant genome usually encompasses different families of transposable elements (TEs) that may constitute up to 85% of nuclear DNA. Under stressful conditions, some of them may activate, leading to sequence variation. In vitro plant regeneration may induce either phenotypic or genetic and epigenetic changes. While DNA methylation alternations might be related, i.e., to the Yang cycle problems, DNA pattern changes, especially DNA demethylation, may activate TEs that could result in point mutations in DNA sequence changes. Thus, TEs have the highest input into sequence variation (SV). A set of barley regenerants were derived via in vitro anther culture. High Performance Liquid Chromatography (RP-HPLC), used to study the global DNA methylation of donor plants and their regenerants, showed that the level of DNA methylation increased in regenerants by 1.45% compared to the donors. The Methyl-Sensitive Transposon Display (MSTD) based on methylation-sensitive Amplified Fragment Length Polymorphism (metAFLP) approach demonstrated that, depending on the selected elements belonging to the TEs family analyzed, varying levels of sequence variation were evaluated. DNA sequence contexts may have a different impact on SV generated by distinct mobile elements belonged to various TE families. Based on the presented study, some of the selected mobile elements contribute differently to TE-related SV. The surrounding context of the TEs DNA sequence is possibly important here, and the study explained some part of SV related to those contexts.

From Trash to Luxury: The Potential Role of Plant LncRNA in DNA Methylation During Abiotic Stress

Remarkable progress has been made in elucidating important roles of plant non-coding RNAs. Among these RNAs, long noncoding RNAs (lncRNAs) have gained widespread attention, especially their role in plant environmental stress responses. LncRNAs act at different levels of gene expression regulation, and one of these mechanisms is by recruitment of DNA methyltransferases or demethylases to regulate the target gene transcription. In this mini-review, we highlight the function of lncRNAs, including their potential role in RNA-directed DNA Methylation (RdDM) silencing pathway and their potential function under abiotic stresses conditions. Moreover, we also present and discuss studies of lncRNAs in crops. Finally, we propose a path outlook for future research that may be important for plant breeding.

Genome-wide identification and analysis of DNA methyltransferase and demethylase gene families in Dendrobium officinale reveal their potential functions in polysaccharide accumulation

BACKGROUND

DNA methylation is a conserved and important epigenetic modification involved in the regulation of numerous biological processes, including plant development, secondary metabolism, and response to stresses. However, no information is available regarding the identification of cytosine-5 DNA methyltransferase (C5-MTase) and DNA demethylase (dMTase) genes in the orchid Dendrobium officinale.

RESULTS

In this study, we performed a genome-wide analysis of DoC5-MTase and DodMTase gene families in D. officinale. Integrated analysis of conserved motifs, gene structures and phylogenetic analysis showed that eight DoC5-MTases were divided into four subfamilies (DoCMT, DoDNMT, DoDRM, DoMET) while three DodMTases were divided into two subfamilies (DoDML3, DoROS1). Multiple cis-acting elements, especially stress-responsive and hormone-responsive ones, were found in the promoter region of DoC5-MTase and DodMTase genes. Furthermore, we investigated the expression profiles of DoC5-MTase and DodMTase in 10 different tissues, as well as their transcript abundance under abiotic stresses (cold and drought) and at the seedling stage, in protocorm-like bodies, shoots, and plantlets. Interestingly, most DoC5-MTases were downregulated whereas DodMTases were upregulated by cold stress. At the seedling stage, DoC5-MTase expression decreased as growth proceeded, but DodMTase expression increased.

CONCLUSIONS

These results provide a basis for elucidating the role of DoC5-MTase and DodMTase in secondary metabolite production and responses to abiotic stresses in D. officinale.

Dynamics of DNA Methylation and Its Functions in Plant Growth and Development

Epigenetic modifications in DNA bases and histone proteins play important roles in the regulation of gene expression and genome stability. Chemical modification of DNA base (e.g., addition of a methyl group at the fifth carbon of cytosine residue) switches on/off the gene expression during developmental process and environmental stresses. The dynamics of DNA base methylation depends mainly on the activities of the writer/eraser guided by non-coding RNA (ncRNA) and regulated by the developmental/environmental cues. De novo DNA methylation and active demethylation activities control the methylation level and regulate the gene expression. Identification of ncRNA involved in de novo DNA methylation, increased DNA methylation proteins guiding DNA demethylase, and methylation monitoring sequence that helps maintaining a balance between DNA methylation and demethylation is the recent developments that may resolve some of the enigmas. Such discoveries provide a better understanding of the dynamics/functions of DNA base methylation and epigenetic regulation of growth, development, and stress tolerance in crop plants. Identification of epigenetic pathways in animals, their existence/orthologs in plants, and functional validation might improve future strategies for epigenome editing toward climate-resilient, sustainable agriculture in this era of global climate change. The present review discusses the dynamics of DNA methylation (cytosine/adenine) in plants, its functions in regulating gene expression under abiotic/biotic stresses, developmental processes, and genome stability.

Understanding epigenomics based on the rice model

The purpose of this paper provides a comprehensive overview of the recent researches on rice epigenomics, including DNA methylation, histone modifications, noncoding RNAs, and three-dimensional genomics. The challenges and perspectives for future research in rice are discussed. Rice as a model plant for epigenomic studies has much progressed current understanding of epigenetics in plants. Recent results on rice epigenome profiling and three-dimensional chromatin structure studies reveal specific features and implication in gene regulation during rice plant development and adaptation to environmental changes. Results on rice chromatin regulator functions shed light on mechanisms of establishment, recognition, and resetting of epigenomic information in plants. Cloning of several rice epialleles associated with important agronomic traits highlights importance of epigenomic variation in rice plant growth, fitness, and yield. In this review, we summarize and analyze recent advances in rice epigenomics and discuss challenges and directions for future research in the field.

Inactivation of retrotransposon Tos17 Chr.7 in rice cultivar Nipponbare through CRISPR/Cas9-mediated gene editing

Retrotransposons are mobile genetic elements capable of transposition via reverse transcription of RNA intermediates. Rice cultivar Nipponbare contains two nearly identical genomic copies of Tos17, an endogenous copia-like LTR retrotransposon, on chromosomes 7 (Tos17 ^Chr.7) and 10 (Tos17 ^Chr.10), respectively. Previous studies demonstrated that only Tos17 ^Chr.7 is active in transposition during tissue culture. Tos17 ^Chr.7 has been extensively used for insertional mutagenesis as a tool for functional analysis of rice genes. However, Tos17 ^Chr.7 transposition might generate somaclonal mutagenesis with undesirable traits during rice transformation, which would affect the evaluation or application of transgenes. In this study, we generated a Tos17 ^Chr.7 knockout mutant D873 by using CRISPR/Cas9 gene editing system. The gene-edited allele of Tos17 ^Chr.7 in D873, designated as Tos17 ^D873, has an 873-bp DNA deletion in the pol gene of Tos17 ^Chr.7, which caused the deletion of the GAG-pre-integrase domain and the integrase core domain. Although the transcription of Tos17 ^D873 was activated in D873 calli, no transposition of Tos17 ^D873 was detected in the regenerated D873 plants. The results demonstrate that the GAG-pre-integrase domain and the integrase core domain are essential for Tos17 ^Chr.7 transposition and the deletion of the two domains could be not complemented by other LTR retrotransposons in rice genome. As the Tos17 ^Chr.7-derived somaclonal mutagenesis is blocked in the D873 plants, the generation of the Tos17 ^D873 allele will be helpful in production of transgenic rice plants for gene function study and genetic engineering. Similar approach can be used to inactivate other retrotransposons in crop breeding.

Construction and characterization of a knock-down RNA interference line of OsNRPD1 in rice (Oryza sativa ssp japonica cv Nipponbare)

In plants, RNA-directed DNA methylation (RdDM) is a silencing mechanism relying on the production of 24-nt small interfering RNAs (siRNAs) by RNA POLYMERASE IV (Pol IV) to trigger methylation and inactivation of transposable elements (TEs). We present the construction and characterization of osnrpd1, a knock-down RNA interference line of OsNRPD1 gene that encodes the largest subunit of Pol IV in rice (Oryza sativa ssp japonica cv Nipponbare). We show that osnrpd1 displays a lower accumulation of OsNRPD1 transcripts, associated with an overall reduction of 24-nt siRNAs and DNA methylation level in all three contexts, CG, CHG and CHH. We uncovered new insertions of known active TEs, the LTR retrotransposons Tos17 and Lullaby and the long interspersed nuclear element-type retrotransposon Karma. However, we did not observe any clear developmental phenotype, contrary to what was expected for a mutant severely affected in RdDM. In addition, despite the presence of many putatively functional TEs in the rice genome, we found no evidence of in planta global reactivation of transposition. This knock-down of OsNRPD1 likely led to a weakly affected line, with no effect on development and a limited effect on transposition. We discuss the possibility that a knock-out mutation of OsNRPD1 would cause sterility in rice. This article is part of a discussion meeting issue 'Crossroads between transposons and gene regulation'.

Active DNA Demethylation in Plants

Methylation of cytosine (5-meC) is a critical epigenetic modification in many eukaryotes, and genomic DNA methylation landscapes are dynamically regulated by opposed methylation and demethylation processes. Plants are unique in possessing a mechanism for active DNA demethylation involving DNA glycosylases that excise 5-meC and initiate its replacement with unmodified C through a base excision repair (BER) pathway. Plant BER-mediated DNA demethylation is a complex process involving numerous proteins, as well as additional regulatory factors that avoid accumulation of potentially harmful intermediates and coordinate demethylation and methylation to maintain balanced yet flexible DNA methylation patterns. Active DNA demethylation counteracts excessive methylation at transposable elements (TEs), mainly in euchromatic regions, and one of its major functions is to avoid methylation spreading to nearby genes. It is also involved in transcriptional activation of TEs and TE-derived sequences in companion cells of male and female gametophytes, which reinforces transposon silencing in gametes and also contributes to gene imprinting in the endosperm. Plant 5-meC DNA glycosylases are additionally involved in many other physiological processes, including seed development and germination, fruit ripening, and plant responses to a variety of biotic and abiotic environmental stimuli.

Dynamics and function of DNA methylation in plants

DNA methylation is a conserved epigenetic modification that is important for gene regulation and genome stability. Aberrant patterns of DNA methylation can lead to plant developmental abnormalities. A specific DNA methylation state is an outcome of dynamic regulation by de novo methylation, maintenance of methylation and active demethylation, which are catalysed by various enzymes that are targeted by distinct regulatory pathways. In this Review, we discuss DNA methylation in plants, including methylating and demethylating enzymes and regulatory factors, and the coordination of methylation and demethylation activities by a so-called methylstat mechanism; the functions of DNA methylation in regulating transposon silencing, gene expression and chromosome interactions; the roles of DNA methylation in plant development; and the involvement of DNA methylation in plant responses to biotic and abiotic stress conditions.

Molecular insights into the regulation of rice kernel elongation

A large number of rice agronomic traits are complex, multi factorial and polygenic. As the mechanisms and genes determining grain size and yield are largely unknown, the identification of regulatory genes related to grain development remains a preeminent approach in rice genetic studies and breeding programs. Genes regulating cell proliferation and expansion in spikelet hulls and participating in endosperm development are the main controllers of rice kernel elongation and grain size. We review here and discuss recent findings on genes controlling rice grain size and the mechanisms, epialleles, epigenomic variation, and assessment of controlling genes using genome-editing tools relating to kernel elongation.

Diversity and dynamics of DNA methylation: epigenomic resources and tools for crop breeding

DNA methylation is an epigenetic modification that can affect gene expression and transposable element (TE) activities. Because cytosine DNA methylation patterns are inherited through both mitotic and meiotic cell divisions, differences in these patterns can contribute to phenotypic variability. Advances in high-throughput sequencing technologies have enabled the generation of abundant DNA sequence data. Integrated analyses of genome-wide gene expression patterns and DNA methylation patterns have revealed the underlying mechanisms and functions of DNA methylation. Moreover, associations between DNA methylation and agronomic traits have also been uncovered. The resulting information may be useful for future applications of natural epigenomic variation, for crop breeding. Additionally, artificial epigenome editing may be an attractive new plant breeding technique for generating novel varieties with improved agronomic traits.

Tiny Yet Indispensable Plant MicroRNAs Are Worth to Explore as Key Components for Combating Genotoxic Stresses

Plants being sessile are always exposed to various stresses including biotic and abiotic stresses. Some of these stresses are genotoxic to cells causing DNA damage by forming lesions which include altered bases, cross-links, and breaking of DNA strands, which in turn hamper the genomic integrity. In order to survive through all these adverse conditions, plants have evolved different DNA repair mechanisms. As seen from the mammalian system and different human diseases, various microRNAs (miRNAs) can target the 3'-untranslated region of mRNAs that code for the proteins involved in DNA repair pathways. Since miRNAs play an important role in plant cells by regulating various metabolic pathways, it can also be possible that miRNAs play an important role in DNA repair pathways too. However, till date, only a handful of plant miRNAs have been identified to play important role in combating genotoxic stresses in plants. Limitation of information regarding involvement of miRNAs in DNA repair as well as in ROS scavenging prompted us to gather information about plant miRNAs specific for these tasks. This mini-review aims to present pertinent literature dealing with different genotoxic stresses that cause genome instability as well as plant specific responses to survive the damage. This is intertwined with the involvement of miRNAs in genotoxic stress in plants, challenges of applying miRNAs as a tool to combat DNA damage along with ways to overcome these challenges, and finally, the future prospective of these understudied aspects.

Systematic analysis of DEMETER-like DNA glycosylase genes shows lineage-specific Smi-miR7972 involved in SmDML1 regulation in Salvia miltiorrhiza

DEMETER-like DNA glycosylases (DMLs) initiate the base excision repair-dependent DNA demethylation to regulate a wide range of biological processes in plants. Six putative SmDML genes, termed SmDML1-SmDML6, were identified from the genome of S. miltiorrhiza, an emerging model plant for Traditional Chinese Medicine (TCM) studies. Integrated analysis of gene structures, sequence features, conserved domains and motifs, phylogenetic analysis and differential expression showed the conservation and divergence of SmDMLs. SmDML1, SmDML2 and SmDML4 were significantly down-regulated by the treatment of 5Aza-dC, a general DNA methylation inhibitor, suggesting involvement of SmDMLs in genome DNA methylation change. SmDML1 was predicted and experimentally validated to be target of Smi-miR7972. Computational analysis of forty whole genome sequences and almost all of RNA-seq data from Lamiids revealed that MIR7972s were only distributed in some plants of the three orders, including Lamiales, Solanales and Boraginales, and the number of MIR7972 genes varied among species. It suggests that MIR7972 genes underwent expansion and loss during the evolution of some Lamiids species. Phylogenetic analysis of MIR7972s showed closer evolutionary relationships between MIR7972s in Boraginales and Solanales in comparison with Lamiales. These results provide a valuable resource for elucidating DNA demethylation mechanism in S. miltiorrhiza.

A transposition-active Phyllostachys edulis long terminal repeat (LTR) retrotransposon

Due to infrequent sexual reproduction, moso bamboo breeding by hybridization is extremely technically difficult. Insertional mutagenesis based on endogenous active transposons may thus serve as an alternative method to create new germplasm of moso bamboo. In the present study, using LTR-STRUC, a full-length intact long terminal repeat (LTR) retrotransposon was identified in the moso bamboo genome and was named PHRE2 (Phyllostachys edulis retrotransposon 2). The 5' and 3' LTR sequences of PHRE2 were highly (98.39%) similar. PHRE2 contains all domains necessary for transposition such as gag, pr, rt, rh, and int. The coding frames of these essential domains were complete and had no apparent mutations. In addition, PHRE2 possessed a prime binding site (PBS), a polypurine tract (PPT), and two typical sequences of LTR retrotransposons. A genome-wide scan showed that the moso bamboo genome has only one full-length sequence of PHYRE2. After its transfer to Arabidopsis thaliana, an increase in PHRE2 copy number occurred in the T₃ plants compared to in the T₂ plants. After moso bamboo seedlings were grown in tissue culture or treated by irradiation or plant hormones, the copy number of PHRE2 significantly increased. These findings indicate that PHRE2 has the capacity for transposition, which can be induced by environmental conditions.

The gymnastics of epigenomics in rice

Epigenomics is represented by the high-throughput investigations of genome-wide epigenetic alterations, which ultimately dictate genomic, transcriptomic, proteomic and metabolomic dynamism. Rice has been accepted as the global staple crop. As a result, this model crop deserves significant importance in the rapidly emerging field of plant epigenomics. A large number of recently available data reveal the immense flexibility and potential of variable epigenomic landscapes. Such epigenomic impacts and variability are determined by a number of epigenetic regulators and several crucial inheritable epialleles, respectively. This article highlights the correlation of the epigenomic landscape with growth, flowering, reproduction, non-coding RNA-mediated post-transcriptional regulation, transposon mobility and even heterosis in rice. We have also discussed the drastic epigenetic alterations which are reported in rice plants grown from seeds exposed to the extraterrestrial environment. Such abiotic conditions impose stress on the plants leading to epigenomic modifications in a genotype-specific manner. Some significant bioinformatic databases and in silico approaches have also been explained in this article. These softwares provide important interfaces for comparative epigenomics. The discussion concludes with a unified goal of developing epigenome editing to promote biological hacking of the rice epigenome. Such a cutting-edge technology if properly standardized, can integrate genomics and epigenomics together with the generation of high-yielding trait in several cultivars of rice.

Laser Irradiation-Induced DNA Methylation Changes Are Heritable and Accompanied with Transpositional Activation of mPing in Rice

DNA methylation is an integral component of the epigenetic code in most higher eukaryotes. Exploring the extent to which DNA methylation can be altered under a specific condition and its heritability is important for elucidating the biological functions of this epigenetic modification. Here, we conducted MSAP analysis of rice plants with altered phenotypes subsequent to a low-dose Nd³⁺YAG laser irradiation. We found that all four methylation patterns at the 5'-CCGG sites that are analyzable by MSAP showed substantial changes in the immediately treated M0 plants. Interestingly, the frequencies of hypo- and hypermethylation were of similar extents, which largely offset each other and render the total methylation levels unchanged. Further analysis revealed that the altered methylation patterns were meiotically heritable to at least the M2 generation but accompanied with further changes in each generation. The methylation changes and their heritability of the metastable epigenetic state were verified by bisulfite sequencing of portion of the retrotranspon, Tos17, an established locus for assessing DNA methylation liability in rice. Real-time PCR assay indicated that the expression of various methylation-related chromatin genes was perturbed, and a Pearson correlation analysis showed that many of these genes, especially two AGOs (AGO4-1 and AGO4-2), were significantly correlated with the methylation pattern alterations. In addition, excisions of a MITE transposon, mPing, occurred rampantly in the laser irradiated plants and their progenies. Together, our results indicate that heritable DNA methylation changes can be readily induced by low-dose laser irradiation, and which can be accompanied by transpostional activation of transposable elements.

Epigenetic regulation and epigenomic landscape in rice

Epigenetic regulation has been implicated in the control of complex agronomic traits in rice (Oryza sativa), a staple food crop and model monocot plant. Recent advances in high-throughput sequencing and the moderately complex genome of rice have made it possible to study epigenetic regulation in rice on a genome-wide scale. This review discusses recent advances in our understanding of epigenetic regulation in rice, with an emphasis on the roles of key epigenetic regulators, the epigenomic landscape, epigenetic variation, transposon repression, and plant development.

Locus- and Site-Specific DNA Methylation of 19 kDa Zein Genes in Maize

An interesting question in maize development is why only a single zein gene is highly expressed in each of the 19-kDa zein gene clusters (A and B types), z1A2-1 and z1B4, in the immature endosperm. For instance, epigenetic marks could provide a structural difference. Therefore, we investigated the DNA methylation of the arrays of gene copies in both promoter and gene body regions of leaf (non-expressing tissue as a control), normal endosperm, and cultured endosperm. Although we could show that expressed genes have much lower methylation levels in promoter regions than silent ones in both leaf and normal endosperm, there was surprisingly also a difference in the pattern of the z1A and z1B gene clusters. The expression of z1B gene is suppressed by increased DNA methylation and activated with reduced DNA methylation, whereas z1A gene expression is not. DNA methylation in gene coding regions is higher in leaf than in endosperm, whereas no significant difference is observed in gene bodies between expressed and non-expressed gene copies. A median CHG methylation (25–30%) appears to be optimal for gene expression. Moreover, tissue-cultured endosperm can reset the DNA methylation pattern and tissue-specific gene expression. These results reveal that DNA methylation changes of the 19-kDa zein genes is subject to plant development and tissue culture treatment, but varies in different chromosomal locations, indicating that DNA methylation changes do not apply to gene expression in a uniform fashion. Because tissue culture is used to produce transgenic plants, these studies provide new insights into variation of gene expression of integrated sequences.

Histone H3K4me3 and H3K27me3 regulatory genes control stable transmission of an epimutation in rice

DNA methylation loss can produce inheritable active epialleles in plants. The mechanism involved in the stable transmission of hypomethylated epimuations is presently not clear. Here we show that maintenance of a stably hypomethylated active epiallele in rice required a CHD3 protein (CHR729) and that over-expression of an H3K4me3 demethylase (JMJ703) or H3K27me3 methyltransferase (SDG711) could stably resilence the epiallele. CHR729 and JMJ703 have antagonistic function in H3K4me3 in maintaining the active state of the epiallele, whereas SDG711-mediated H3K27me3 was sufficient to stably repress the locus. The data suggest that H3K4me3 and H3K27me3 controlled by these chromatin regulators may be involved in stable transmission/resetting of epigenetic variation in rice.

Salt Tolerant and Sensitive Rice Varieties Display Differential Methylome Flexibility under Salt Stress

DNA methylation has been referred as an important player in plant genomic responses to environmental stresses but correlations between the methylome plasticity and specific traits of interest are still far from being understood. In this study, we inspected global DNA methylation levels in salt tolerant and sensitive rice varieties upon salt stress imposition. Global DNA methylation was quantified using the 5-methylcytosine (5mC) antibody and an ELISA-based technique, which is an affordable and quite pioneer assay in plants, and in situ imaging of methylation sites in interphase nuclei of tissue sections. Variations of global DNA methylation levels in response to salt stress were tissue- and genotype-dependent. We show a connection between a higher ability of DNA methylation adjustment levels and salt stress tolerance. The salt-tolerant rice variety Pokkali was remarkable in its ability to quickly relax DNA methylation in response to salt stress. In spite of the same tendency for reduction of global methylation under salinity, in the salt-sensitive rice variety IR29 such reduction was not statistically supported. In 'Pokkali', the salt stress-induced demethylation may be linked to active demethylation due to increased expression of DNA demethylases under salt stress. In 'IR29', the induction of both DNA demethylases and methyltransferases may explain the lower plasticity of DNA methylation. We further show that mutations for epigenetic regulators affected specific phenotypic parameters related to salinity tolerance, such as the root length and biomass. This work emphasizes the role of differential methylome flexibility between salt tolerant and salt sensitive rice varieties as an important player in salt stress tolerance, reinforcing the need to better understand the connection between epigenetic networks and plant responses to environmental stresses.

Epigenetic regulation and functional exaptation of transposable elements in higher plants

Transposable elements (TEs) are mobile genetic elements that can proliferate in their host genomes. Because of their robust amplification, TEs have long been considered 'selfish DNA', harmful insertions that can threaten host genome integrity. The idea of TEs as junk DNA comes from analysis of epigenetic silencing of their mobility in plants and animals. This idea contrasts with McClintock's characterization of TEs as 'controlling elements'. Emerging studies on the regulatory functions of TEs in plant genomes have updated McClintock's characterization, indicating exaptation of TEs for genetic regulation. In this review, we summarize recent progress in TE silencing, particularly in Arabidopsis and rice, and show that TEs provide an abundant, natural source of regulation for the host genome.

Mutation of a major CG methylase in rice causes genome-wide hypomethylation, dysregulated genome expression, and seedling lethality

Cytosine methylation at CG sites ((m)CG) plays critical roles in development, epigenetic inheritance, and genome stability in mammals and plants. In the dicot model plant Arabidopsis thaliana, methyltransferase 1 (MET1), a principal CG methylase, functions to maintain (m)CG during DNA replication, with its null mutation resulting in global hypomethylation and pleiotropic developmental defects. Null mutation of a critical CG methylase has not been characterized at a whole-genome level in other higher eukaryotes, leaving the generality of the Arabidopsis findings largely speculative. Rice is a model plant of monocots, to which many of our important crops belong. Here we have characterized a null mutant of OsMet1-2, the major CG methylase in rice. We found that seeds homozygous for OsMet1-2 gene mutation (OsMET1-2(-/-)), which directly segregated from normal heterozygote plants (OsMET1-2(+/-)), were seriously maldeveloped, and all germinated seedlings underwent swift necrotic death. Compared with wild type, genome-wide loss of (m)CG occurred in the mutant methylome, which was accompanied by a plethora of quantitative molecular phenotypes including dysregulated expression of diverse protein-coding genes, activation and repression of transposable elements, and altered small RNA profiles. Our results have revealed conservation but also distinct functional differences in CG methylases between rice and Arabidopsis.

Environmental stress activation of plant long-terminal repeat retrotransposons

Genomic retrotransposons (RTs) are major components of most plant genomes. They spread throughout the genomes by a process termed retrotransposition, which consists of reverse transcription and reinsertion of the copied element into a new genomic location (a copy-and-paste system). Abiotic and biotic stresses activate long-terminal repeat (LTR) RTs in photosynthetic eukaryotes from algae to angiosperms. LTR RTs could represent a threat to the integrity of host genomes because of their activity and mutagenic potential by epigenetic regulation. Host genomes have developed mechanisms to control the activity of the retroelements and their mutagenic potential. Some LTR RTs escape these defense mechanisms, and maintain their ability to be activated and transpose as a result of biotic or abiotic stress stimuli. These stimuli include pathogen infection, mechanical damage, in vitro tissue culturing, heat, drought and salt stress, generation of doubled haploids, X-ray irradiation and many others. Reactivation of LTR RTs differs between different plant genomes. The expression levels of reactivated RTs are influenced by the transcriptional and post-transcriptional gene silencing mechanisms (e.g. DNA methylation, heterochromatin formation and RNA interference). Moreover, the insertion of RTs (e.g. Triticum aestivum L. Wis2-1A) into or next to coding regions of the host genome can generate changes in the expression of adjacent host genes of the host. In this paper, we review the ways that plant genomic LTR RTs are activated by environmental stimuli to affect restructuring and diversification of the host genome.

Dicer-like 3 produces transposable element-associated 24-nt siRNAs that control agricultural traits in rice

Transposable elements (TEs) and repetitive sequences make up over 35% of the rice (Oryza sativa) genome. The host regulates the activity of different TEs by different epigenetic mechanisms, including DNA methylation, histone H3K9 methylation, and histone H3K4 demethylation. TEs can also affect the expression of host genes. For example, miniature inverted repeat TEs (MITEs), dispersed high copy-number DNA TEs, can influence the expression of nearby genes. In plants, 24-nt small interfering RNAs (siRNAs) are mainly derived from repeats and TEs. However, the extent to which TEs, particularly MITEs associated with 24-nt siRNAs, affect gene expression remains elusive. Here, we show that the rice Dicer-like 3 homolog OsDCL3a is primarily responsible for 24-nt siRNA processing. Impairing OsDCL3a expression by RNA interference caused phenotypes affecting important agricultural traits; these phenotypes include dwarfism, larger flag leaf angle, and fewer secondary branches. We used small RNA deep sequencing to identify 535,054 24-nt siRNA clusters. Of these clusters, ∼82% were OsDCL3a-dependent and showed significant enrichment of MITEs. Reduction of OsDCL3a function reduced the 24-nt siRNAs predominantly from MITEs and elevated expression of nearby genes. OsDCL3a directly targets genes involved in gibberellin and brassinosteroid homeostasis; OsDCL3a deficiency may affect these genes, thus causing the phenotypes of dwarfism and enlarged flag leaf angle. Our work identifies OsDCL3a-dependent 24-nt siRNAs derived from MITEs as broadly functioning regulators for fine-tuning gene expression, which may reflect a conserved epigenetic mechanism in higher plants with genomes rich in dispersed repeats or TEs.

MicroRNA mediates DNA methylation of target genes

Small RNAs represented by microRNA (miRNA) plays important roles in plant development and responds to biotic and abiotic stresses. Previous studies have placed special emphasis on gene-repression mediated by miRNA. In this work, the DNA methylation pattern of microRNA genes (MIRs) was interrogated. Full-length cDNA and EST were used to confirm the entity of pri-miRNA. In parallel, miRNA in 24 nucleotides (nt) was pooled to detect chromatin modification effect by using bisulfite sequencing data. 97 MIRs were supported by full-length cDNA and 30 more were hit by EST. Notably, methylation levels of conserved MIRs were significantly lower than the non-conserved at all contexts (CG, CHG, and CHH). Additionally, a substantial part of 24-nt miRNA was able to induce target site methylation, providing a broader perspective for researchers.

Epigenetic regulation of rice flowering and reproduction

Current understanding of the epigenetic regulator roles in plant growth and development has largely derived from studies in the dicotyledonous model plant Arabidopsis thaliana. Rice (Oryza sativa) is one of the most important food crops in the world and has more recently becoming a monocotyledonous model plant in functional genomics research. During the past few years, an increasing number of studies have reported the impact of DNA methylation, non-coding RNAs and histone modifications on transcription regulation, flowering time control, and reproduction in rice. Here, we review these studies to provide an updated complete view about chromatin modifiers characterized in rice and in particular on their roles in epigenetic regulation of flowering time, reproduction, and seed development.

The study of a barley epigenetic regulator, HvDME, in seed development and under drought

BACKGROUND

Epigenetic factors such as DNA methylation and histone modifications regulate a wide range of processes in plant development. Cytosine methylation and demethylation exist in a dynamic balance and have been associated with gene silencing or activation, respectively. In Arabidopsis, cytosine demethylation is achieved by specific DNA glycosylases, including AtDME (DEMETER) and AtROS1 (REPRESSOR OF SILENCING1), which have been shown to play important roles in seed development. Nevertheless, studies on monocot DNA glycosylases are limited. Here we present the study of a DME homologue from barley (HvDME), an agronomically important cereal crop, during seed development and in response to conditions of drought.

RESULTS

An HvDME gene, identified in GenBank, was found to encode a protein with all the characteristic modules of DME-family DNA glycosylase proteins. Phylogenetic analysis revealed a high degree of homology to other monocot DME glycosylases, and sequence divergence from the ROS1, DML2 and DML3 orthologues. The HvDME gene contains the 5' and 3' Long Terminal Repeats (LTR) of a Copia retrotransposon element within the 3' downstream region. HvDME transcripts were shown to be present both in vegetative and reproductive tissues and accumulated differentially in different seed developmental stages and in two different cultivars with varying seed size. Additionally, remarkable induction of HvDME was evidenced in response to drought treatment in a drought-tolerant barley cultivar. Moreover, variable degrees of DNA methylation in specific regions of the HvDME promoter and gene body were detected in two different cultivars.

CONCLUSION

A gene encoding a DNA glycosylase closely related to cereal DME glycosylases was characterized in barley. Expression analysis during seed development and under dehydration conditions suggested a role for HvDME in endosperm development, seed maturation, and in response to drought. Furthermore, differential DNA methylation patterns within the gene in two different cultivars suggested epigenetic regulation of HvDME. The study of a barley DME gene will contribute to our understanding of epigenetic mechanisms operating during seed development and stress response in agronomically important cereal crops.

Rice epigenomics and epigenetics: challenges and opportunities

During recent years rice genome-wide epigenomic information such as DNA methylation and histone modifications, which are important for genome activity has been accumulated. The function of a number of rice epigenetic regulators has been studied, many of which are found to be involved in a diverse range of developmental and stress-responsive pathways. Analysis of epigenetic variations among different rice varieties indicates that epigenetic modification may lead to inheritable phenotypic variation. Characterizing phenotypic consequences of rice epigenomic variations and the underlining chromatin mechanism and identifying epialleles related to important agronomic traits may provide novel strategies to enhance agronomically favorable traits and grain productivity in rice.

Mutant resources for the functional analysis of the rice genome

Rice is one of the most important crops worldwide, both as a staple food and as a model system for genomic research. In order to systematically assign functions to all predicted genes in the rice genome, a large number of rice mutant lines, including those created by T-DNA insertion, Ds/dSpm tagging, Tos17 tagging, and chemical/irradiation mutagenesis, have been generated by groups around the world. In this study, we have reviewed the current status of mutant resources for functional analysis of the rice genome. A total of 246 566 flanking sequence tags from rice mutant libraries with T-DNA, Ds/dSpm, or Tos17 insertion have been collected and analyzed. The results show that, among 211 470 unique hits, inserts located in the genic region account for 68.16%, and 60.49% of nuclear genes contain at least one insertion. Currently, 57% of non-transposable-element-related genes in rice have insertional tags. In addition, chemical/irradiation-induced rice mutant libraries have contributed a lot to both gene identification and new technology for the identification of mutant sites. In this review, we summarize how these tools have been used to generate a large collection of mutants. In addition, we discuss the merits of classic mutation strategies. In order to achieve saturation of mutagenesis in rice, DNA targeting, and new resources like RiceFox for gene functional identification are reviewed from a perspective of the future generation of rice mutant resources.

Control of transposon activity by a histone H3K4 demethylase in rice

Transposable elements (TEs) are ubiquitously present in plant genomes and often account for significant fractions of the nuclear DNA. For example, roughly 40% of the rice genome consists of TEs, many of which are retrotransposons, including 14% LTR- and ∼1% non-LTR retrotransposons. Despite their wide distribution and abundance, very few TEs have been found to be transpositional, indicating that TE activities may be tightly controlled by the host genome to minimize the potentially mutagenic effects associated with active transposition. Consistent with this notion, a growing body of evidence suggests that epigenetic silencing pathways such as DNA methylation, RNA interference, and H3K9me2 function collectively to repress TE activity at the transcriptional and posttranscriptional levels. It is not yet clear, however, whether the removal of histone modifications associated with active transcription is also involved in TE silencing. Here, we show that the rice protein JMJ703 is an active H3K4-specific demethylase required for TEs silencing. Impaired JMJ703 activity led to elevated levels of H3K4me3, the misregulation of numerous endogenous genes, and the transpositional reactivation of two families of non-LTR retrotransposons. Interestingly, loss of JMJ703 did not affect TEs (such as Tos17) previously found to be silenced by other epigenetic pathways. These results indicate that the removal of active histone modifications is involved in TE silencing and that different subsets of TEs may be regulated by distinct epigenetic pathways.

Reverse genetics in rice using Tos17

Transposon of Oryza sativa 17 (Tos17), a Ty1-Copia Class I retroelement, is one of the few active retroelements identified in rice, the main cereal crop of human consumption and the model genome for cereals. Tos17 exists in two copies in the standard Nipponbare japonica genome (n = 12 and 379 Mb). Tos17 copies are inactive in the plant grown under normal conditions. However, the copy located on chromosome 7 can be activated upon tissue culture. Plants regenerated from 3- and 5-month-old tissue cultures harbor, respectively, an average of 3.5 and 8 newly transposed copies that are stably inserted at new positions in the genome. Due to its favorable features, Tos17 has been extensively used for insertion mutagenesis of the model genome and 31,403 sequence indexed inserts harbored by regenerants/T-DNA plants are available in the databases. The corresponding seed stocks can be ordered from the laboratories which generated them. Both forward genetics and reverse genetics approaches using these lines have allowed the deciphering of gene function in rice. We report here two protocols for ascertaining the presence of a Tos17 insertion in a gene of interest among R2/T2 seeds received from Tos17 mutant stock centers: The first protocol is PCR-based and allows the identification of azygous, heterozygous and homozygous plants among progenies segregating the insertion. The second protocol is based on DNA blot analysis and can be used to identify homozygous plants carrying the Tos17 copy responsible for gene disruption while cleaning the mutant background from other unwitting mutagen inserts.

Molecular genetics and epigenetics of CACTA elements

The CACTA transposons, so named for a highly conserved motif at element ends, comprise one of the most abundant superfamilies of Class 2 (cut-and-paste) plant transposons. CACTA transposons characteristically include subterminal sequences of several hundred nucleotides containing closely spaced direct and inverted repeats of a short, conserved sequence of 14-15 bp. The Supressor-mutator (Spm) transposon, identified and subjected to detailed genetic analysis by Barbara McClintock, remains the paradigmatic element of the CACTA family. The Spm transposon encodes two proteins required for transposition, the transposase (TnpD) and a regulatory protein (TnpA) that binds to the subterminal repeats. Spm expression is subject to both genetic and epigenetic regulation. The Spm-encoded TnpA serves as an activator of the epigenetically inactivated, methylated Spm, stimulating both transient and heritable activation of the transposon. TnpA also serves as a negative regulator of the demethylated active element promoter and is required, in addition to the TnpD, for transposition.