DNA methyltransferase CHROMOMETHYLASE3 prevents ONSEN transposon silencing under heat stress

To infinity and beyond: recent progress, bottlenecks, and potential of clonal seeds by apomixis

Apomixis - clonal seed production in plants - is a rare yet phylogenetically widespread trait that has recurrently evolved in plants to fix hybrid genotypes over generations. Apomixis is absent from major crop species and has been seen as a holy grail of plant breeding due to its potential to propagate hybrid vigor in perpetuity. Here we exhaustively review recent progress, bottlenecks, and potential in the individual components of gametophytic apomixis (avoidance of meiosis, skipping fertilization by parthenogenesis, autonomous endosperm development), and sporophytic apomixis. The Mitosis instead of Meiosis system has now been successfully set up in three species (Arabidopsis, rice, and tomato), yet significant hurdles remain for universal bioengineering of clonal gametes. Parthenogenesis has been engineered in even more species, yet incomplete penetrance still remains an issue; we discuss the choice of parthenogenesis genes (BABY BOOM, PARTHENOGENESIS, WUSCHEL) and also how to drive egg cell-specific expression. The identification of pathways to engineer autonomous endosperm development would allow fully autonomous seed production, yet here significant challenges remain. The recent achievements in the engineering of synthetic apomixis in rice at high penetrance show great potential and the remaining obstacles toward implementation in this crop are addressed. Overall, the recent practical examples of synthetic apomixis suggest the field is flourishing and implementation in agricultural systems could soon take place.

Substrate specificity and protein stability drive the divergence of plant-specific DNA methyltransferases

DNA methylation is an important epigenetic mechanism essential for transposon silencing and genome integrity. Across evolution, the substrates of DNA methylation have diversified between kingdoms. In plants, chromomethylase3 (CMT3) and CMT2 mediate CHG and CHH methylation, respectively. However, how these two methyltransferases diverge on substrate specificities during evolution remains unknown. Here, we reveal that CMT2 originates from a duplication of an evolutionarily ancient CMT3 in flowering plants. Lacking a key arginine residue recognizing CHG in CMT2 impairs its CHG methylation activity in most flowering plants. An engineered V1200R mutation empowers CMT2 to restore CHG and CHH methylations in Arabidopsis cmt2cmt3 mutant, testifying a loss-of-function effect for CMT2 during evolution. CMT2 has evolved a long and unstructured amino terminus critical for protein stability, especially under heat stress, and is plastic to tolerate various natural mutations. Together, this study reveals the mechanism of chromomethylase divergence for context-specific DNA methylation in plants and sheds important lights on DNA methylation evolution and function.

Substrate specificity and protein stability drive the divergence of plant-specific DNA methyltransferases

DNA methylation is an important epigenetic mechanism essential for transposon silencing and genome integrity. Across evolution, the substrates of DNA methylation have diversified between kingdoms to account for genome complexity. In plants, Chromomethylase3 (CMT3) and CMT2 are the major methyltransferases mediating CHG and CHH methylation, respectively. However, how these two enzymes diverge on substrate specificities during evolution remains unknown. Here, we reveal that CMT2 originates from a duplication of the evolutionarily more ancient CMT3 in flowering plants. Lacking a key arginine residue recognizing CHG in CMT2 impairs its CHG methylation activity in most flowering plants. An engineered V1200R mutation empowers CMT2 to restore both CHG and CHH methylation in Arabidopsis cmt2cmt3 mutant, testifying a loss-of-function effect for CMT2 after ∼200 million years of evolution. Interestingly, CMT2 has evolved a long and unstructured N-terminus critical for balancing protein stability, especially under heat stress. Furthermore, CMT2 N-terminus is plastic and can be tolerant to various natural mutations. Together, this study reveals the mechanism of chromomethylase divergence for context-specific DNA methylation in plants and sheds important lights on DNA methylation evolution and function.

Emerging roles of plant transcriptional gene silencing under heat

Plants continuously endure unpredictable environmental fluctuations that upset their physiology, with stressful conditions negatively impacting yield and survival. As a contemporary threat of rapid progression, global warming has become one of the most menacing ecological challenges. Thus, understanding how plants integrate and respond to elevated temperatures is crucial for ensuring future crop productivity and furthering our knowledge of historical environmental acclimation and adaptation. While the canonical heat-shock response and thermomorphogenesis have been extensively studied, evidence increasingly highlights the critical role of regulatory epigenetic mechanisms. Among these, the involvement under heat of heterochromatic suppression mediated by transcriptional gene silencing (TGS) remains the least understood. TGS refers to a multilayered metabolic machinery largely responsible for the epigenetic silencing of invasive parasitic nucleic acids and the maintenance of parental imprints. Its molecular effectors include DNA methylation, histone variants and their post-translational modifications, and chromatin packing and remodeling. This work focuses on both established and emerging insights into the contribution of TGS to the physiology of plants under stressful high temperatures. We summarized potential roles of constitutive and facultative heterochromatin as well as the most impactful regulatory genes, highlighting events where the loss of epigenetic suppression has not yet been associated with corresponding changes in epigenetic marks.

A chromosome-level genome reveals genome evolution and molecular basis of anthraquinone biosynthesis in Rheum palmatum

BACKGROUND

Rhubarb is one of common traditional Chinese medicine with a diverse array of therapeutic efficacies. Despite its widespread use, molecular research into rhubarb remains limited, constraining our comprehension of the geoherbalism.

RESULTS

We assembled the genome of Rheum palmatum L., one of the source plants of rhubarb, to elucidate its genome evolution and unpack the biosynthetic pathways of its bioactive compounds using a combination of PacBio HiFi, Oxford Nanopore, Illumina, and Hi-C scaffolding approaches. Around 2.8 Gb genome was obtained after assembly with more than 99.9% sequences anchored to 11 pseudochromosomes (scaffold N50 = 259.19 Mb). Transposable elements (TE) with a continuous expansion of long terminal repeat retrotransposons (LTRs) is predominant in genome size, contributing to the genome expansion of R. palmatum. Totally 30,480 genes were predicted to be protein-coding genes with 473 significantly expanded gene families enriched in diverse pathways associated with high-altitude adaptation for this species. Two successive rounds of whole genome duplication event (WGD) shared by Fagopyrum tataricum and R. palmatum were confirmed. We also identified 54 genes involved in anthraquinone biosynthesis and other 97 genes entangled in flavonoid biosynthesis. Notably, RpALS emerged as a compelling candidate gene for the octaketide biosynthesis after the key residual screening.

CONCLUSION

Overall, our findings offer not only an enhanced understanding of this remarkable medicinal plant but also pave the way for future innovations in its genetic breeding, molecular design, and functional genomic studies.

METTL4-mediated N6-methyladenine DNA modification regulates thermotolerance in Arabidopsis thaliana

DNA N6-methyladenine (6 mA) is an evolutionarily conserved DNA modification in procaryotes and eukaryotes. The DNA 6 mA methylation is tightly controlled by 6 mA regulatory proteins. DNA N6-adenine methyltransferase 1 (DAMT-1) has been identified as a DNA 6 mA methyltransferase in animals. In plants, DNA 6 mA methylation has been found, however, the DNA 6 mA methyltransferases and their function in plants are largely unknown. In our study, we find METTL4 is a DNA 6 mA methyltransferase in Arabidopsis thaliana. Both in vitro and in vivo evidences support the DNA 6 mA methyltransferase activity of METTL4. mettl4 mutant is hypersensitive to heat stress, suggesting DNA 6 mA methylation plays important role in heat stress adaption. RNA-seq and 6 mA IP-qPCR analysis show that METTL4 participates in heat stress tolerance by regulating expression of heat responsive genes. Our study find METTL4 is a plant DNA 6 mA methyltransferase and illustrates its function in regulating heat stress response.

m6A RNA demethylase AtALKBH9B promotes mobilization of a heat-activated long terminal repeat retrotransposon in Arabidopsis

Transposons are mobile and ubiquitous DNA molecules that can cause vast genomic alterations. In plants, it is well documented that transposon mobilization is strongly repressed by DNA methylation; however, its regulation at the posttranscriptional level remains relatively uninvestigated. Here, we suggest that transposon RNA is marked by m6A RNA methylation and can be localized in stress granules (SGs). Intriguingly, SG-localized AtALKBH9B selectively demethylates a heat-activated retroelement, Onsen, and thereby releases it from spatial confinement, allowing for its mobilization. In addition, we show evidence that m6A RNA methylation contributes to transpositional suppression by inhibiting virus-like particle assembly and extrachromosomal DNA production. In summary, this study unveils a previously unknown role for m6A in the suppression of transposon mobility and provides insight into how transposons counteract the m6A-mediated repression mechanism by hitchhiking the RNA demethylase of the host.

Integrative transcriptome and whole-genome bisulfite sequencing analyses of a temperature-sensitive albino tea plant cultivar

Green tea made from albino buds and leaves has a strong umami taste and aroma. The cultivar 'Zhonghuang 2' (ZH2, Camellia sinensis) is a natural mutant with young shoots that are yellow in spring and green or yellow-green in summer. However, the mechanism of leaf color change remains unclear. Here, we found that young shoots of ZH2 were yellow at low temperature (LT) and green at high temperature (HT), indicating that ZH2 is a temperature-sensitive cultivar. Transmission electron microscopy analysis showed that the grana in the chloroplasts of young shoots grown at LT were poorly stacked, which caused a lack of photoreactions and chlorophyll. RNA-seq results showed 1279 genes differentially expressed in the young shoots grown at LT compared with those at HT, including genes related to cytochrome synthesis, chloroplast development, photosynthesis, and DNA methylation. A whole-genome bisulfite sequencing assay revealed that the dynamics of DNA methylation levels in the CG, CHG, and CHH contexts decreased under LT, and the change was most obvious in the CHH context. Furthermore, 72 genes showed significant changes in both expression and DNA methylation levels, and most of them were related to cytochrome synthesis, chloroplast development, photosynthesis, transcription factors, and signaling pathways. These results demonstrate that DNA methylation is involved in the LT-regulated albino processes of ZH2. Changes in DNA methylation levels were associated with changes in gene expression levels, affecting the structure and function of chloroplasts, which may have a phenotypic impact on shoot and leaf color.

Databases and prospects of dynamic gene regulation in eukaryotes: A mini review

In eukaryotes, dynamic regulation enables DNA polymerases to catalyze a variety of RNA products in spatial and temporal patterns. Dynamic gene expression is regulated by transcription factors (TFs) and epigenetics (DNA methylation and histone modification). The applications of biochemical technology and high-throughput sequencing enhance the understanding of mechanisms of these regulations and affected genomic regions. To provide a searchable platform for retrieving such metadata, numerous databases have been developed based on the integration of genome-wide maps (e.g., ChIP-seq, whole-genome bisulfite sequencing, RNA-seq, ATAC-seq, DNase-seq, and MNase-seq data) and functionally genomic annotation. In this mini review, we summarize the main functions of TF-related databases and outline the prevalent approaches used in inferring epigenetic regulations, their associated genes, and functions. We review the literature on crosstalk between TF and epigenetic regulation and the properties of non-coding RNA regulation, which are challenging topics that promise to pave the way for advances in database development.

Regulatory mechanism of a heat-activated retrotransposon by DDR complex in Arabidopsis thaliana

The RNA-directed DNA methylation (RdDM) pathway plays an essential role in the transposon silencing mechanism; the DDR complex, consisting of DRD1, DMS3, and RDM1, is an essential component of the RdDM pathway. ONSEN, identified in Arabidopsis, is a retrotransposon activated by heat stress at 37°C; however, studies on the regulation of ONSEN are limited. In this study, we analyzed the regulation of ONSEN activity by the DDR complex in Arabidopsis. We elucidated that loss of any component of the DDR complex increased ONSEN transcript levels. Transgenerational transposition of ONSEN was observed in the DDR-complex mutants treated with heat stress for 48 h. Furthermore, the DDR complex components DRD1, DMS3, and RDM1 played independent roles in suppressing ONSEN transcription and transposition. Moreover, we found that the duration of heat stress affects ONSEN activity. Therefore, the results of this study provide new insights into the retrotransposon regulatory mechanisms of the DDR complex in the RdDM pathway.

Retrotransposons: How the continuous evolutionary front shapes plant genomes for response to heat stress

Long terminal repeat retrotransposons (LTR retrotransposons) are the most abundant group of mobile genetic elements in eukaryotic genomes and are essential in organizing genomic architecture and phenotypic variations. The diverse families of retrotransposons are related to retroviruses. As retrotransposable elements are dispersed and ubiquitous, their "copy-out and paste-in" life cycle of replicative transposition leads to new genome insertions without the excision of the original element. The overall structure of retrotransposons and the domains responsible for the various phases of their replication is highly conserved in all eukaryotes. The two major superfamilies of LTR retrotransposons, Ty1/Copia and Ty3/Gypsy, are distinguished and dispersed across the chromosomes of higher plants. Members of these superfamilies can increase in copy number and are often activated by various biotic and abiotic stresses due to retrotransposition bursts. LTR retrotransposons are important drivers of species diversity and exhibit great variety in structure, size, and mechanisms of transposition, making them important putative actors in genome evolution. Additionally, LTR retrotransposons influence the gene expression patterns of adjacent genes by modulating potential small interfering RNA (siRNA) and RNA-directed DNA methylation (RdDM) pathways. Furthermore, comparative and evolutionary analysis of the most important crop genome sequences and advanced technologies have elucidated the epigenetics and structural and functional modifications driven by LTR retrotransposon during speciation. However, mechanistic insights into LTR retrotransposons remain obscure in plant development due to a lack of advancement in high throughput technologies. In this review, we focus on the key role of LTR retrotransposons response in plants during heat stress, the role of centromeric LTR retrotransposons, and the role of LTR retrotransposon markers in genome expression and evolution.

Insights into the expression of DNA (de)methylation genes responsive to nitric oxide signaling in potato resistance to late blight disease

Our previous study concerning the pathogen-induced biphasic pattern of nitric oxide (NO) burst revealed that the decline phase and a low level of NO, due to S-nitrosoglutathione reductase (GSNOR) activity, might be decisive in the upregulation of stress-sensitive genes via histone H3/H4 methylation in potato leaves inoculated with avr P. infestans. The present study refers to the NO-related impact on genes regulating DNA (de)methylation, being in dialog with histone methylation. The excessive amounts of NO after the pathogen or GSNO treatment forced the transient upregulation of histone SUVH4 methylation and DNA hypermethylation. Then the diminished NO bioavailability reduced the SUVH4-mediated suppressive H3K9me2 mark on the R3a gene promoter and enhanced its transcription. However, we found that the R3a gene is likely to be controlled by the RdDM methylation pathway. The data revealed the time-dependent downregulation of the DCL3, AGO4, and miR482e genes, exerting upregulation of the targeted R3a gene correlated with ROS1 overexpression. Based on these results, we postulate that the biphasic waves of NO burst in response to the pathogen appear crucial in establishing potato resistance to late blight through the RdDM pathway controlling R gene expression.

Structure and Mechanism of Plant DNA Methyltransferases

DNA methylation is an important epigenetic mark conserved in eukaryotes from fungi to animals and plants, where it plays a crucial role in regulating gene expression and transposon silencing. Once the methylation mark is established by de novo DNA methyltransferases, specific regulatory mechanisms are required to maintain the methylation state during chromatin replication, both during meiosis and mitosis. Plant DNA methylation is found in three contexts; CG, CHG, and CHH (H = A, T, C), which are established and maintained by a unique set of DNA methyltransferases and are regulated by plant-specific pathways. DNA methylation in plants is often associated with other epigenetic modifications, such as noncoding RNA and histone modifications. This chapter focuses on the structure, function, and regulatory mechanism of plant DNA methyltransferases and their crosstalk with other epigenetic pathways.

Epigenetic marks for mitigating abiotic stresses in plants

Abiotic stressors are one of the major factors affecting agricultural output. Plants have evolved adaptive systems to respond appropriately to various environmental cues. These responses can be accomplished by modulating or fine-tuning genetic and epigenetic regulatory mechanisms. Understanding the response of plants' molecular features to abiotic stress is a priority in the current period of continued environmental changes. Epigenetic modifications are necessary that control gene expression by changing chromatin status and recruiting various transcription regulators. The present study summarized the current knowledge on epigenetic modifications concerning plant responses to various environmental stressors. The functional relevance of epigenetic marks in regulating stress tolerance has been revealed, and epigenetic changes impact the effector genes. This study looks at the epigenetic mechanisms that govern plant abiotic stress responses, especially DNA methylation, histone methylation/acetylation, chromatin remodeling, and various metabolites. Plant breeders will benefit from a thorough understanding of these processes to create alternative crop improvement approaches. Genome editing with clustered regularly interspaced short palindromic repeat/CRISPR-associated proteins (CRISPR/Cas) provides genetic tools to make agricultural genetic engineering more sustainable and publicly acceptable.

Epigenetic regulation of ecotype-specific expression of the heat-activated transposon ONSEN

Transposable elements are present in a wide variety of organisms; however, our understanding of the diversity of mechanisms involved in their activation is incomplete. In this study, we analyzed the transcriptional activation of the ONSEN retrotransposon, which is activated by high-temperature stress in Arabidopsis thaliana. We found that its transcription is significantly higher in the Japanese ecotype Kyoto. Considering that transposons are epigenetically regulated, DNA methylation levels were analyzed, revealing that CHH methylation was reduced in Kyoto compared to the standard ecotype, Col-0. A mutation was also detected in the Kyoto CMT2 gene, encoding a CHH methyltransferase, suggesting that it may be responsible for increased expression of ONSEN. CHH methylation is controlled by histone modifications through a self-reinforcing loop between DNA methyltransferase and histone methyltransferase. Analysis of these modifications revealed that the level of H3K9me2, a repressive histone marker for gene expression, was lower in Kyoto than in Col-0. The level of another repressive histone marker, H3K27me1, was decreased in Kyoto; however, it was not impacted in a Col-0 cmt2 mutant. Therefore, in addition to the CMT2 mutation, other factors may reduce repressive histone modifications in Kyoto.

Specificities and Dynamics of Transposable Elements in Land Plants

Simple Summary

Transposable elements are dynamic components of plant genomes, and display a high diversity of lineages and distribution as the result of evolutionary driving forces and overlapping mechanisms of genetic and epigenetic regulation. They are now regarded as main contributors for genome evolution and function, and important regulators of endogenous gene expression. In this review, we survey recent progress and current challenges in the identification and classification of transposon lineages in complex plant genomes, highlighting the molecular specificities that may explain the expansion and diversification of mobile genetic elements in land plants.

Abstract

Transposable elements (TEs) are important components of most plant genomes. These mobile repetitive sequences are highly diverse in terms of abundance, structure, transposition mechanisms, activity and insertion specificities across plant species. This review will survey the different mechanisms that may explain the variability of TE patterns in land plants, highlighting the tight connection between TE dynamics and host genome specificities, and their co-evolution to face and adapt to a changing environment. We present the current TE classification in land plants, and describe the different levels of genetic and epigenetic controls originating from the plant, the TE itself, or external environmental factors. Such overlapping mechanisms of TE regulation might be responsible for the high diversity and dynamics of plant TEs observed in nature.

The fifth Japanese meeting on biological function and evolution through interactions between hosts and transposable elements

The fifth Japanese meeting on host–transposon interactions, titled “Biological Function and Evolution through Interactions between Hosts and Transposable Elements (TEs),” was held online on August 26–27, 2021. The meeting was supported by National Institute of Genetics and aimed to bring together researchers studying the diverse roles of TEs in genome function and evolution, as well as host defense systems against TE mobility by chromatin and RNA modifications and protein-protein interactions. Here, we present the highlights of the talks.