Natural depletion of histone H1 in sex cells causes DNA demethylation, heterochromatin decondensation and transposon activation

Linker histone H1 drives heterochromatin condensation via phase separation in Arabidopsis

In the eukaryotic nucleus, heterochromatin forms highly condensed, visible foci known as heterochromatin foci (HF). These HF are enriched with linker histone H1, a key player in heterochromatin condensation and silencing. However, it is unknown how H1 aggregates HF and condenses heterochromatin. In this study, we established that H1 facilitates heterochromatin condensation by enhancing inter- and intrachromosomal interactions between and within heterochromatic regions of the Arabidopsis (Arabidopsis thaliana) genome. We demonstrated that H1 drives HF formation via phase separation, which requires its C-terminal intrinsically disordered region (C-IDR). A truncated H1 lacking the C-IDR fails to form foci or recover HF in the h1 mutant background, whereas C-IDR with a short stretch of the globular domain (18 out of 71 amino acids) is sufficient to rescue both defects. In addition, C-IDR is essential for H1's roles in regulating nucleosome repeat length and DNA methylation in Arabidopsis, indicating that phase separation capability is required for chromatin functions of H1. Our data suggest that bacterial H1-like proteins, which have been shown to condense DNA, are intrinsically disordered and capable of mediating phase separation. Therefore, we propose that phase separation mediated by H1 or H1-like proteins may represent an ancient mechanism for condensing chromatin and DNA.

Potent pollen gene regulation by DNA glycosylases in maize

Although DNA methylation primarily represses transposable elements (TEs) in plants, it also represses select endosperm and pollen genes. These genes, or their cis-regulatory elements, are methylated in plant body tissues but are demethylated by DNA glycosylases (DNGs) in endosperm and pollen, enabling their transcription. Activity of either one of two DNGs, MDR1 or DNG102, is essential for pollen viability in maize. Using single-pollen mRNA sequencing on pollen segregating mutations in both genes, we identified 58 candidate DNG target genes, whose expression is strongly decreased in double mutant pollen (124-fold decrease on average). These genes account for 11.1% of the wild-type pollen polyadenylated transcriptome, but they are silent or barely detectable in the plant body. They are unusual in their tendency to lack introns but even more so in their having TE-like methylation in their coding DNA sequence. Moreover, they are strongly enriched for predicted functions in cell wall modification. While some may support development of the pollen grain cell wall, expansins and pectinases in this set of genes suggest a function in cell wall loosening to support the rapid tip growth characteristic of pollen tubes as they carry the sperm cells through maternal apoplast and extracellular matrix of the pistil. These results suggest a critical role for DNA methylation and demethylation in regulating maize genes with potential for extremely high expression in pollen but constitutive silencing elsewhere.

Sperm-specific histone H1 in highly condensed sperm nucleus of Sargassum horneri

Spermatogenesis is one of the most dramatic changes in cell differentiation. Remarkable chromatin condensation of the nucleus is observed in animal, plant, and algal sperm. Sperm nuclear basic proteins (SNBPs), such as protamine and sperm-specific histone, are involved in chromatin condensation of the sperm nucleus. Among brown algae, sperm of the oogamous Fucales algae have a condensed nucleus. However, the existence of sperm-specific SNBPs in Fucales algae was unclear. Here, we identified linker histone (histone H1) proteins in the sperm and analyzed changes in their gene expression pattern during spermatogenesis in Sargassum horneri. A search of transcriptomic data for histone H1 genes in showed six histone H1 genes, which we named ShH1.1a, ShH1b, ShH1.2, ShH1.3, ShH1.4, and ShH1.5. Analysis of SNBPs using SDS-PAGE and LC-MS/MS showed that sperm nuclei contain histone ShH1.2, ShH1.3, and ShH1.4 in addition to core histones. Both ShH1.2 and ShH1.3 genes were expressed in the vegetative thallus and the male and female receptacles (the organs producing antheridium or oogonium). Meanwhile, the ShH1.4 gene was expressed in the male receptacle but not in the vegetative thallus and female receptacles. From these results, ShH1.4 may be a sperm-specific histone H1 of S. horneri.

Plant histone variants at the nexus of chromatin readouts, stress and development

Histones are crucial proteins that are involved in packaging the DNA as condensed chromatin inside the eukaryotic cell nucleus. Rather than being static packaging units, these molecules undergo drastic variations spatially and temporally to facilitate accessibility of DNA to replication, transcription as well as wide range of gene regulatory machineries. In addition, incorporation of paralogous variants of canonical histones in the chromatin is ascribed to specific functions. Given the peculiar requirement of plants to rapidly modulate gene expression levels on account of their sessile nature, histones and their variants serve as additional layers of gene regulation. This review summarizes the mechanisms and implications of distribution, modifications and differential incorporation of histones and their variants across plant genomes, and outlines emerging themes.

Loops, crosstalk, and compartmentalization: it takes many layers to regulate DNA methylation

DNA methylation is a conserved epigenetic modification associated with transposon silencing and gene regulation. The stability of this modification relies on intimate connections between DNA and histone modifications that generate self-reinforcing loops wherein the presence of one mark promotes the other. However, it is becoming increasingly clear that the efficiency of these loops is affected by cross-talk between pathways and by chromatin accessibility, which is heavily influenced by histone variants. Focusing primarily on plants, this review provides an update on the aforementioned self-reinforcing loops, highlights recent advances in understanding how DNA methylation pathways are restricted to prevent encroachment on genes, and discusses the roles of histone variants in compartmentalizing epigenetic pathways within the genome. This multilayered approach facilitates two essential, yet opposing functions, the ability to maintain heritable DNA methylation patterns while retaining the flexibility to modify these patterns during development.

MiRNAs differentially expressed in vegetative and reproductive organs of Marchantia polymorpha - insights into their expression pattern, gene structures and function

MicroRNAs regulate gene expression affecting a variety of plant developmental processes. The evolutionary position of Marchantia polymorpha makes it a significant model to understand miRNA-mediated gene regulatory pathways in plants. Previous studies focused on conserved miRNA-target mRNA modules showed their critical role in Marchantia development. Here, we demonstrate that the differential expression of conserved miRNAs among land plants and their targets in selected organs of Marchantia additionally underlines their role in regulating fundamental developmental processes. The main aim of this study was to characterize selected liverwort-specific miRNAs, as there is a limited knowledge on their biogenesis, accumulation, targets, and function in Marchantia. We demonstrate their differential accumulation in vegetative and generative organs. We reveal that all liverwort-specific miRNAs examined are encoded by independent transcriptional units. MpmiR11737a, MpmiR11887 and MpmiR11796, annotated as being encoded within protein-encoding genes, have their own independent transcription start sites. The analysis of selected liverwort-specific miRNAs and their pri-miRNAs often reveal correlation in their levels, suggesting transcriptional regulation. However, MpmiR11796 shows a reverse correlation to its pri-miRNA level, suggesting post-transcriptional regulation. Moreover, we identify novel targets for selected liverwort-specific miRNAs and demonstrate an inverse correlation between their expression and miRNA accumulation. In the case of one miRNA precursor, we provide evidence that it encodes two functional miRNAs with two independent targets. Overall, our research sheds light on liverwort-specific miRNA gene structure, provides new data on their biogenesis and expression regulation. Furthermore, identifying their targets, we hypothesize the potential role of these miRNAs in early land plant development and functioning.

Emerging roles of phase separation in plant transcription and chromatin organization

Transcription is a core step in gene expression. Regulation of transcription is achieved at the level of transcription machinery, local chromatin environment as well as higher-order chromatin organization. Our understanding of transcriptional regulation was advanced by recent introduction of transcription and chromatin-associated condensates, which typically arise via phase separation of proteins and nucleic acids. While studies from mammalian cells are unveiling the mechanisms of phase separation in transcription regulation, those in plants further broaden and deepen our understanding of this process. In this review, we discuss recent progress in plants how phase separation operates in RNA-mediated chromatin silencing, transcription activity, and chromatin compartmentalization.

Histone H1 protects telomeric repeats from H3K27me3 invasion in Arabidopsis

While the pivotal role of linker histone H1 in shaping nucleosome organization is well established, its functional interplays with chromatin factors along the epigenome are just starting to emerge. Here we show that, in Arabidopsis, as in mammals, H1 occupies Polycomb Repressive Complex 2 (PRC2) target genes where it favors chromatin condensation and H3K27me3 deposition. We further show that, contrasting with its conserved function in PRC2 activation at genes, H1 selectively prevents H3K27me3 accumulation at telomeres and large pericentromeric interstitial telomeric repeat (ITR) domains by restricting DNA accessibility to Telomere Repeat Binding (TRB) proteins, a group of H1-related Myb factors mediating PRC2 cis recruitment. This study provides a mechanistic framework by which H1 avoids the formation of gigantic H3K27me3-rich domains at telomeric sequences and contributes to safeguard nucleus architecture.

Plant environmental memory: implications, mechanisms and opportunities for plant scientists and beyond

Plants are extremely plastic organisms. They continuously receive and integrate environmental information and adjust their growth and development to favour fitness and survival. When this integration of information affects subsequent life stages or the development of subsequent generations, it can be considered an environmental memory. Thus, plant memory is a relevant mechanism by which plants respond adaptively to different environments. If the cost of maintaining the response is offset by its benefits, it may influence evolutionary trajectories. As such, plant memory has a sophisticated underlying molecular mechanism with multiple components and layers. Nonetheless, when mathematical modelling is combined with knowledge of ecological, physiological, and developmental effects as well as molecular mechanisms as a tool for understanding plant memory, the combined potential becomes unfathomable for the management of plant communities in natural and agricultural ecosystems. In this review, we summarize recent advances in the understanding of plant memory, discuss the ecological requirements for its evolution, outline the multilayered molecular network and mechanisms required for accurate and fail-proof plant responses to variable environments, point out the direct involvement of the plant metabolism and discuss the tremendous potential of various types of models to further our understanding of the plant's environmental memory. Throughout, we emphasize the use of plant memory as a tool to unlock the secrets of the natural world.

Beyond transcription: compelling open questions in plant RNA biology

The study of RNAs has become one of the most influential research fields in contemporary biology and biomedicine. In the last few years, new sequencing technologies have produced an explosion of new and exciting discoveries in the field but have also given rise to many open questions. Defining these questions, together with old, long-standing gaps in our knowledge, is the spirit of this article. The breadth of topics within RNA biology research is vast, and every aspect of the biology of these molecules contains countless exciting open questions. Here, we asked 12 groups to discuss their most compelling question among some plant RNA biology topics. The following vignettes cover RNA alternative splicing; RNA dynamics; RNA translation; RNA structures; R-loops; epitranscriptomics; long non-coding RNAs; small RNA production and their functions in crops; small RNAs during gametogenesis and in cross-kingdom RNA interference; and RNA-directed DNA methylation. In each section, we will present the current state-of-the-art in plant RNA biology research before asking the questions that will surely motivate future discoveries in the field. We hope this article will spark a debate about the future perspective on RNA biology and provoke novel reflections in the reader.

50 years of sperm cell isolations: from structural to omic studies

The fusion of male and female gametes is a fundamental process in the perpetuation and diversification of species. During the last 50 years, significant efforts have been made to isolate and characterize sperm cells from flowering plants, and to identify how these cells interact with female gametes to achieve double fertilization. The first techniques and analytical approaches not only provided structural and biochemical characterizations of plant sperm cells but also paved the way for in vitro fertilization studies. Further technological advances then led to unique insights into sperm biology at transcriptomic, proteomic and epigenetic level. Starting with a historical overview of sperm cell isolation techniques, we provide examples of how these contributed to create our current knowledge of sperm cell biology, and point out remaining challenges.

IQ67 DOMAIN protein 21 is critical for indentation formation in pavement cell morphogenesis

In plants, cortical microtubules anchor to the plasma membrane in arrays and play important roles in cell shape. However, the molecular mechanism of microtubule binding proteins, which connect the plasma membrane and cortical microtubules in cell morphology remains largely unknown. Here, we report that a plasma membrane and microtubule dual-localized IQ67 domain protein, IQD21, is critical for cotyledon pavement cell (PC) morphogenesis in Arabidopsis. iqd21 mutation caused increased indentation width, decreased lobe length, and similar lobe number of PCs, whereas IQD21 overexpression had a different effect on cotyledon PC shape. Weak overexpression led to increased lobe number, decreased indentation width, and similar lobe length, while moderate or great overexpression resulted in decreased lobe number, indentation width, and lobe length of PCs. Live-cell observations revealed that IQD21 accumulation at indentation regions correlates with lobe initiation and outgrowth during PC development. Cell biological and genetic approaches revealed that IQD21 promotes transfacial microtubules anchoring to the plasma membrane via its polybasic sites and bundling at the indentation regions in both periclinal and anticlinal walls. IQD21 controls cortical microtubule organization mainly through promoting Katanin 1-mediated microtubule severing during PC interdigitation. These findings provide the genetic evidence that transfacial microtubule arrays play a determinant role in lobe formation, and the insight into the molecular mechanism of IQD21 in transfacial microtubule organization at indentations and puzzle-shaped PC development.

Extensive de novo activity stabilizes epigenetic inheritance of CG methylation in Arabidopsis transposons

Cytosine methylation within CG dinucleotides (mCG) can be epigenetically inherited over many generations. Such inheritance is thought to be mediated by a semiconservative mechanism that produces binary present/absent methylation patterns. However, we show here that, in Arabidopsis thaliana h1ddm1 mutants, intermediate heterochromatic mCG is stably inherited across many generations and is quantitatively associated with transposon expression. We develop a mathematical model that estimates the rates of semiconservative maintenance failure and de novo methylation at each transposon, demonstrating that mCG can be stably inherited at any level via a dynamic balance of these activities. We find that DRM2-the core methyltransferase of the RNA-directed DNA methylation pathway-catalyzes most of the heterochromatic de novo mCG, with de novo rates orders of magnitude higher than previously thought, whereas chromomethylases make smaller contributions. Our results demonstrate that stable epigenetic inheritance of mCG in plant heterochromatin is enabled by extensive de novo methylation.

DNA methylation dynamics during germline development

DNA methylation plays essential homeostatic functions in eukaryotic genomes. In animals, DNA methylation is also developmentally regulated and, in turn, regulates development. In the past two decades, huge research effort has endorsed the understanding that DNA methylation plays a similar role in plant development, especially during sexual reproduction. The power of whole-genome sequencing and cell isolation techniques, as well as bioinformatics tools, have enabled recent studies to reveal dynamic changes in DNA methylation during germline development. Furthermore, the combination of these technological advances with genetics, developmental biology and cell biology tools has revealed functional methylation reprogramming events that control gene and transposon activities in flowering plant germlines. In this review, we discuss the major advances in our knowledge of DNA methylation dynamics during male and female germline development in flowering plants.

Histone H2B.8 compacts flowering plant sperm through chromatin phase separation

Sperm chromatin is typically transformed by protamines into a compact and transcriptionally inactive state^1,2. Sperm cells of flowering plants lack protamines, yet they have small, transcriptionally active nuclei with chromatin condensed through an unknown mechanism^3,4. Here we show that a histone variant, H2B.8, mediates sperm chromatin and nuclear condensation in Arabidopsis thaliana. Loss of H2B.8 causes enlarged sperm nuclei with dispersed chromatin, whereas ectopic expression in somatic cells produces smaller nuclei with aggregated chromatin. This result demonstrates that H2B.8 is sufficient for chromatin condensation. H2B.8 aggregates transcriptionally inactive AT-rich chromatin into phase-separated condensates, which facilitates nuclear compaction without reducing transcription. Reciprocal crosses show that mutation of h2b.8 reduces male transmission, which suggests that H2B.8-mediated sperm compaction is important for fertility. Altogether, our results reveal a new mechanism of nuclear compaction through global aggregation of unexpressed chromatin. We propose that H2B.8 is an evolutionary innovation of flowering plants that achieves nuclear condensation compatible with active transcription.

Epigenetic dynamics during sexual reproduction: At the nexus of developmental control and genomic integrity

Epigenetic marks influence gene regulation and genomic stability via the repression of transposable elements. During sexual reproduction, tight regulation of the epigenome must take place to maintain the repression of transposable elements while still allowing changes in cell-specific transcriptional programs. In plants, epigenetic marks are reorganized during reproduction and a reinforcing mechanism takes place to ensure transposable elements silencing. In this review, we describe the latest advances in characterizing the cell-specific epigenetic changes occurring from sporogenesis to seed development, with a focus on DNA methylation. We highlight the epigenetic co-regulation between transposable elements and developmental genes at different stages of plant reproduction.

The maize gene maternal derepression of r1 encodes a DNA glycosylase that demethylates DNA and reduces siRNA expression in the endosperm

Demethylation of transposons can activate the expression of nearby genes and cause imprinted gene expression in the endosperm; this demethylation is hypothesized to lead to expression of transposon small interfering RNAs (siRNAs) that reinforce silencing in the next generation through transfer either into egg or embryo. Here we describe maize (Zea mays) maternal derepression of r1 (mdr1), which encodes a DNA glycosylase with homology to Arabidopsis thaliana DEMETER and which is partially responsible for demethylation of thousands of regions in endosperm. Instead of promoting siRNA expression in endosperm, MDR1 activity inhibits it. Methylation of most repetitive DNA elements in endosperm is not significantly affected by MDR1, with an exception of Helitrons. While maternally-expressed imprinted genes preferentially overlap with MDR1 demethylated regions, the majority of genes that overlap demethylated regions are not imprinted. Double mutant megagametophytes lacking both MDR1 and its close homolog DNG102 result in early seed failure, and double mutant microgametophytes fail pre-fertilization. These data establish DNA demethylation by glycosylases as essential in maize endosperm and pollen and suggest that neither transposon repression nor genomic imprinting is its main function in endosperm.

Topoisomerase VI participates in an insulator-like function that prevents H3K9me2 spreading

The organization of the genome into transcriptionally active and inactive chromatin domains requires well-delineated chromatin boundaries and insulator functions in order to maintain the identity of adjacent genomic loci with antagonistic chromatin marks and functionality. In plants that lack known chromatin insulators, the mechanisms that prevent heterochromatin spreading into euchromatin remain to be identified. Here, we show that DNA Topoisomerase VI participates in a chromatin boundary function that safeguards the expression of genes in euchromatin islands within silenced heterochromatin regions. While some transposable elements are reactivated in mutants of the Topoisomerase VI complex, genes insulated in euchromatin islands within heterochromatic regions of the Arabidopsis thaliana genome are specifically down-regulated. H3K9me2 levels consistently increase at euchromatin island loci and decrease at some transposable element loci. We further show that Topoisomerase VI physically interacts with S-adenosylmethionine synthase methionine adenosyl transferase 3 (MAT3), which is required for H3K9me2. A Topoisomerase VI defect affects MAT3 occupancy on heterochromatic elements and its exclusion from euchromatic islands, thereby providing a possible mechanistic explanation to the essential role of Topoisomerase VI in the delimitation of chromatin domains.

Histone Variants in the Specialization of Plant Chromatin

The basic unit of chromatin, the nucleosome, is an octamer of four core histone proteins (H2A, H2B, H3, and H4) and serves as a fundamental regulatory unit in all DNA-templated processes. The majority of nucleosome assembly occurs during DNA replication when these core histones are produced en masse to accommodate the nascent genome. In addition, there are a number of nonallelic sequence variants of H2A and H3 in particular, known as histone variants, that can be incorporated into nucleosomes in a targeted and replication-independent manner. By virtue of their sequence divergence from the replication-coupled histones, these histone variants can impart unique properties onto the nucleosomes they occupy and thereby influence transcription and epigenetic states, DNA repair, chromosome segregation, and other nuclear processes in ways that profoundly affect plant biology. In this review, we discuss the evolutionary origins of these variants in plants, their known roles in chromatin, and their impacts on plant development and stress responses. We focus on the individual and combined roles of histone variants in transcriptional regulation within euchromatic and heterochromatic genome regions. Finally, we highlight gaps in our understanding of plant variants at the molecular, cellular, and organismal levels, and we propose new directions for study in the field of plant histone variants.

Multi-omics data integration reveals link between epigenetic modifications and gene expression in sugar beet (Beta vulgaris subsp. vulgaris) in response to cold

Background

DNA methylation is thought to influence the expression of genes, especially in response to changing environmental conditions and developmental changes. Sugar beet (Beta vulgaris ssp. vulgaris), and other biennial or perennial plants are inevitably exposed to fluctuating temperatures throughout their lifecycle and might even require such stimulus to acquire floral competence. Therefore, plants such as beets, need to fine-tune their epigenetic makeup to ensure phenotypic plasticity towards changing environmental conditions while at the same time steering essential developmental processes. Different crop species may show opposing reactions towards the same abiotic stress, or, vice versa, identical species may respond differently depending on the specific kind of stress.

Results

In this study, we investigated common effects of cold treatment on genome-wide DNA methylation and gene expression of two Beta vulgaris accessions via multi-omics data analysis. Cold exposure resulted in a pronounced reduction of DNA methylation levels, which particularly affected methylation in CHH context (and to a lesser extent CHG) and was accompanied by transcriptional downregulation of the chromomethyltransferase CMT2 and strong upregulation of several genes mediating active DNA demethylation.

Conclusion

Integration of methylomic and transcriptomic data revealed that, rather than methylation having directly influenced expression, epigenetic modifications correlated with changes in expression of known players involved in DNA (de)methylation. In particular, cold triggered upregulation of genes putatively contributing to DNA demethylation via the ROS1 pathway. Our observations suggest that these transcriptional responses precede the cold-induced global DNA-hypomethylation in non-CpG, preparing beets for additional transcriptional alterations necessary for adapting to upcoming environmental changes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08312-2.

Epigenetic Regulation of Megaspore Mother Cell Formation

In flowering plants, the female gametophyte (FG) initiates from the formation of the megaspore mother cell (MMC). Among a pool of the somatic cells in the ovule primordium, only one hypodermal cell undergoes a transition of cell fate to become the MMC. Subsequently, the MMC undergoes a series of meiosis and mitosis to form the mature FG harboring seven cells with eight nuclei. Although SPL/NZZ, the core transcription factor for MMC formation, was identified several decades ago, which and why only one somatic cell is chosen as the MMC have long remained mysterious. A growing body of evidence reveal that MMC formation is associated with epigenetic regulation at multiple layers, including dynamic distribution of histone variants and histone modifications, small RNAs, and DNA methylation. In this review, we summarize the progress of epigenetic regulation in the MMC formation, emphasizing the roles of chromosome condensation, histone variants, histone methylation, small RNAs, and DNA methylation.

The epigenetic origin of life history transitions in plants and algae

Plants and algae have a complex life history that transitions between distinct life forms called the sporophyte and the gametophyte. This phenomenon-called the alternation of generations-has fascinated botanists and phycologists for over 170 years. Despite the mesmerizing array of life histories described in plants and algae, we are only now beginning to learn about the molecular mechanisms controlling them and how they evolved. Epigenetic silencing plays an essential role in regulating gene expression during multicellular development in eukaryotes, raising questions about its impact on the life history strategy of plants and algae. Here, we trace the origin and function of epigenetic mechanisms across the plant kingdom, from unicellular green algae through to angiosperms, and attempt to reconstruct the evolutionary steps that influenced life history transitions during plant evolution. Central to this evolutionary scenario is the adaption of epigenetic silencing from a mechanism of genome defense to the repression and control of alternating generations. We extend our discussion beyond the green lineage and highlight the peculiar case of the brown algae. Unlike their unicellular diatom relatives, brown algae lack epigenetic silencing pathways common to animals and plants yet display complex life histories, hinting at the emergence of novel life history controls during stramenopile evolution.

A Key Silencing Histone Mark on Chromatin Is Lost When Colorectal Adenocarcinoma Cells Are Depleted of Methionine by Methionine γ-Lyase

Methionine is an essential amino acid used, beyond protein synthesis, for polyamine formation and DNA/RNA/protein methylation. Cancer cells require particularly high methionine supply for their homeostasis. A successful approach for decreasing methionine concentration is based on the systemic delivery of methionine γ-lyase (MGL), with in vitro and in vivo studies demonstrating its efficacy in cancer therapy. However, the mechanisms explaining how cancer cells suffer from the absence of methionine more significantly than non-malignant cells are still unclear. We analyzed the outcome of the human colorectal adenocarcinoma cancer cell line HT29 to the exposure of MGL for up to 72 h by monitoring cell viability, proteome expression, histone post-translational modifications, and presence of spurious transcription. The rationale of this study was to verify whether reduced methionine supply would affect chromatin decondensation by changing the levels of histone methylation and therefore increasing genomic instability. MGL treatment showed a time-dependent cytotoxic effect on HT29 cancer cells, with an IC₅₀ of 30 µg/ml, while Hs27 normal cells were less affected, with an IC₅₀ of >460 µg/ml. Although the levels of total histone methylation were not altered, a loss of the silencing histone mark H3K9me2 was observed, as well as a decrease in H4K20me3. Since H3K9me2/3 decorate repetitive DNA elements, we proved by qRT-PCR that MGL treatment leads to an increased expression of major satellite units. Our data indicate that selected histone methylation marks may play major roles in the mechanism of methionine starvation in cancer cells, proving that MGL treatment directly impacts chromatin homeostasis.

From birth to function: Male gametophyte development in flowering plants

Male germline development in flowering plants involves two distinct and successive phases, microsporogenesis and microgametogenesis, which involve one meiosis followed by two rounds of mitosis. Many aspects of distinctions after mitosis between the vegetative cell and the male germ cells are seen, from morphology to structure, and the differential functions of the two cell types in the male gametophyte are differentially needed and required for double fertilization. The two sperm cells, carriers of the hereditary substances, depend on the vegetative cell/pollen tube to be delivered to the female gametophyte for double fertilization. Thus, the intercellular communication and coordinated activity within the male gametophyte probably represent the most subtle regulation in flowering plants to guarantee the success of reproduction. This review will focus on what we have known about the differentiation process and the functional diversification of the vegetative cell and the male germ cell, the most crucial cell types for plant fertility and crop production.

The rice VCS1 is identified as a molecular tool to mark and visualize the vegetative cell of pollen

Cell-type-specific markers are valuable tools to reveal developmental processes underlying cell differentiation during plant reproduction. Here we report the pollen vegetative cell marker gene VCS1 (Vegetative Cell Specific 1) of rice (Oryza sativa japonica). VCS1 was expressed specifically in late pollen and was predicted to encode a small FAF domain-containing protein of 205 amino acid residues (aa). The expression of reporter fusion proteins showed that VCS1 was exclusively targeted to the vegetative nucleus of pollen. Upon pollen germination, VCS1 lost vegetative nucleus localization, and appeared diffused in the vegetative cytoplasm of pollen grain but not in the pollen tube. T-DNA insertional mutation which disrupted the carboxyl-terminal 21 aa of VCS1 did not affect plant vegetative growth and pollen development, while destruction of VCS1 by CRISPR/Cas9 only moderately affect pollen viability. VCS1 is evolutionally conserved in monocots but appeared absent in dicotyledons. This study reveals a molecular tool for visualizing the vegetative cell of rice and possible other monocots, which has potential values in the genetic engineering of male-sterile lines.

Nurse cell--derived small RNAs define paternal epigenetic inheritance in Arabidopsis

The plant male germline undergoes DNA methylation reprogramming, which methylates genes de novo and thereby alters gene expression and regulates meiosis. Here, we reveal the molecular mechanism underlying this reprogramming. We demonstrate that genic methylation in the male germline, from meiocytes to sperm, is established by 24-nucleotide small interfering RNAs (siRNAs) transcribed from transposons with imperfect sequence homology. These siRNAs are synthesized by meiocyte nurse cells (tapetum) through activity of CLSY3, a chromatin remodeler absent in other anther cells. Tapetal siRNAs govern germline methylation throughout the genome, including the inherited methylation patterns in sperm. Tapetum-derived siRNAs also silence germline transposons, safeguarding genome integrity. Our results reveal that tapetal siRNAs are sufficient to reconstitute germline methylation patterns and drive functional methylation reprogramming throughout the male germline.

The histone variant H2A.W and linker histone H1 co-regulate heterochromatin accessibility and DNA methylation

In flowering plants, heterochromatin is demarcated by the histone variant H2A.W, elevated levels of the linker histone H1, and specific epigenetic modifications, such as high levels of DNA methylation at both CG and non-CG sites. How H2A.W regulates heterochromatin organization and interacts with other heterochromatic features is unclear. Here, we create a h2a.w null mutant via CRISPR-Cas9, h2a.w-2, to analyze the in vivo function of H2A.W. We find that H2A.W antagonizes deposition of H1 at heterochromatin and that non-CG methylation and accessibility are moderately decreased in h2a.w-2 heterochromatin. Compared to H1 loss alone, combined loss of H1 and H2A.W greatly increases accessibility and facilitates non-CG DNA methylation in heterochromatin, suggesting co-regulation of heterochromatic features by H2A.W and H1. Our results suggest that H2A.W helps maintain optimal heterochromatin accessibility and DNA methylation by promoting chromatin compaction together with H1, while also inhibiting excessive H1 incorporation.

Male gametophyte development in flowering plants: A story of quarantine and sacrifice

Over 160 years ago, scientists made the first microscopic observations of angiosperm pollen. Unlike in animals, male meiosis in angiosperms produces a haploid microspore that undergoes one asymmetric division to form a vegetative cell and a generative cell. These two cells have distinct fates: the vegetative cell exits the cell cycle and elongates to form a tip-growing pollen tube; the generative cell divides once more in the pollen grain or within the growing pollen tube to form a pair of sperm cells. The concept that male germ cells are less active than the vegetative cell came from biochemical analyses and pollen structure anatomy early in the last century and is supported by the pollen transcriptome data of the last decade. However, the mechanism of how and when the transcriptional repression in male germ cells occurs is still not fully understood. In this review, we provide a brief account of the cytological and metabolic differentiation between the vegetative cell and male germ cells, with emphasis on the role of temporary callose walls, dynamic nuclear pore density, transcription repression, and histone variants. We further discuss the intercellular movement of small interfering RNA (siRNA) derived from transposable elements (TEs) and reexamine the function of TE expression in male germ cells.

Epigenetic regulation and intercellular communication during male gametophyte development

The male gametophyte of angiosperms has long been recognized as an ideal system for the study of the molecular mechanisms regulating cell fate determination. Recent findings on histone variants in two cell lineages, vegetative-cell-derived small interfering RNA and transposable element expression provide new power for relevant investigations.

Heterochromatic silencing is reinforced by ARID1-mediated small RNA movement in Arabidopsis pollen

In the plant male germline, transposable elements (TEs) are reactivated in the companion vegetative nucleus, resulting in siRNA production and the intercellular movement of these siRNAs to reinforce TE silencing in sperm. However, the mechanism by which siRNA movement is regulated remains unexplored. Here we show that ARID1, a transcription factor which is constitutively expressed in the vegetative nucleus but dynamically accumulates in the generative cell (the progenitor of sperm) to promote the second pollen mitosis, mediates siRNA movement to reinforce heterochromatic silencing in the male germline. We looked for regulators involved in the accumulation of ARID1 in the generative cell, and found that AGO9, a germline-specific AGO in Arabidopsis, is required for the accumulation of ARID1 in the generative cell. Mutations in either ARID1 or AGO9 lead to the interruption of not only the second pollen mitosis but also the movement of siRNA from the vegetative nucleus to the male germline, resulting in the release of heterochromatic silencing in the male germline. Moreover, conditional knockdown of ARID1 in the generative cell causes reduced heterochromatic silencing in both bicellular and mature pollen. This study provides insights into how a spatiotemporal transcription factor coordinates heterochromatic silencing and male germline maturation.

Role of H1 and DNA methylation in selective regulation of transposable elements during heat stress

Heat-stressed Arabidopsis plants release heterochromatin-associated transposable element (TE) silencing, yet it is not accompanied by major reductions of epigenetic repressive modifications. In this study, we explored the functional role of histone H1 in repressing heterochromatic TEs in response to heat stress. We generated and analyzed RNA and bisulfite-sequencing data of wild-type and h1 mutant seedlings before and after heat stress. Loss of H1 caused activation of pericentromeric Gypsy elements upon heat treatment, despite these elements remaining highly methylated. By contrast, nonpericentromeric Copia elements became activated concomitantly with loss of DNA methylation. The same Copia elements became activated in heat-treated chromomethylase 2 (cmt2) mutants, indicating that H1 represses Copia elements through maintaining DNA methylation under heat. We discovered that H1 is required for TE repression in response to heat stress, but its functional role differs depending on TE location. Strikingly, H1-deficient plants treated with the DNA methyltransferase inhibitor zebularine were highly tolerant to heat stress, suggesting that both H1 and DNA methylation redundantly suppress the plant response to heat stress.

Epigenetic reprogramming rewires transcription during the alternation of generations in Arabidopsis

Alternation between morphologically distinct haploid and diploid life forms is a defining feature of most plant and algal life cycles, yet the underlying molecular mechanisms that govern these transitions remain unclear. Here, we explore the dynamic relationship between chromatin accessibility and epigenetic modifications during life form transitions in Arabidopsis. The diploid-to-haploid life form transition is governed by the loss of H3K9me2 and DNA demethylation of transposon-associated cis-regulatory elements. This event is associated with dramatic changes in chromatin accessibility and transcriptional reprogramming. In contrast, the global loss of H3K27me3 in the haploid form shapes a chromatin accessibility landscape that is poised to re-initiate the transition back to diploid life after fertilisation. Hence, distinct epigenetic reprogramming events rewire transcription through major reorganisation of the regulatory epigenome to guide the alternation of generations in flowering plants.

Male fertility in Arabidopsis requires active DNA demethylation of genes that control pollen tube function

Active DNA demethylation is required for sexual reproduction in plants but the molecular determinants underlying this epigenetic control are not known. Here, we show in Arabidopsis thaliana that the DNA glycosylases DEMETER (DME) and REPRESSOR OF SILENCING 1 (ROS1) act semi-redundantly in the vegetative cell of pollen to demethylate DNA and ensure proper pollen tube progression. Moreover, we identify six pollen-specific genes with increased DNA methylation as well as reduced expression in dme and dme;ros1. We further show that for four of these genes, reinstalling their expression individually in mutant pollen is sufficient to improve male fertility. Our findings demonstrate an essential role of active DNA demethylation in regulating genes involved in pollen function.

Polycomb mutant partially suppresses DNA hypomethylation-associated phenotypes in Arabidopsis

A mutation in Arabidopsis polycomb repressive complex 2 partially suppresses the transposon activity observed in a DNA methylation mutant, challenging expectations.

In plants and mammals, DNA methylation and histone H3 lysine 27 trimethylation (H3K27me3), which is deposited by the polycomb repressive complex 2, are considered as two specialized systems for the epigenetic silencing of transposable element (TE) and genes, respectively. Nevertheless, many TE sequences acquire H3K27me3 when DNA methylation is lost. Here, we show in Arabidopsis thaliana that the gain of H3K27me3 observed at hundreds of TEs in the ddm1 mutant defective in the maintenance of DNA methylation, essentially depends on CURLY LEAF (CLF), one of two partially redundant H3K27 methyltransferases active in vegetative tissues. Surprisingly, the complete loss of H3K27me3 in ddm1 clf double mutant plants was not associated with further reactivation of TE expression nor with a burst of transposition. Instead, ddm1 clf plants exhibited less activated TEs, and a chromatin recompaction as well as hypermethylation of linker DNA compared with ddm1. Thus, a mutation in polycomb repressive complex 2 does not aggravate the molecular phenotypes linked to ddm1 but instead partially suppresses them, challenging our assumptions of the relationship between two conserved epigenetic silencing pathways.

Chromatin regulates expression of small RNAs to help maintain transposon methylome homeostasis in Arabidopsis

BACKGROUND

Eukaryotic genomes are partitioned into euchromatic and heterochromatic domains to regulate gene expression and other fundamental cellular processes. However, chromatin is dynamic during growth and development and must be properly re-established after its decondensation. Small interfering RNAs (siRNAs) promote heterochromatin formation, but little is known about how chromatin regulates siRNA expression.

RESULTS

We demonstrate that thousands of transposable elements (TEs) produce exceptionally high levels of siRNAs in Arabidopsis thaliana embryos. TEs generate siRNAs throughout embryogenesis according to two distinct patterns depending on whether they are located in euchromatic or heterochromatic regions of the genome. siRNA precursors are transcribed in embryos, and siRNAs are required to direct the re-establishment of DNA methylation on TEs from which they are derived in the new generation. Decondensed chromatin also permits the production of 24-nt siRNAs from heterochromatic TEs during post-embryogenesis, and siRNA production from bipartite-classified TEs is controlled by their chromatin states.

CONCLUSIONS

Decondensation of heterochromatin in response to developmental, and perhaps environmental, cues promotes the transcription and function of siRNAs in plants. Our results indicate that chromatin-mediated siRNA transcription provides a cell-autonomous homeostatic control mechanism to help reconstitute pre-existing chromatin states during growth and development including those that ensure silencing of TEs in the future germ line.

Mass Spectrometry to Study Chromatin Compaction

Chromatin accessibility is a major regulator of gene expression. Histone writers/erasers have a critical role in chromatin compaction, as they “flag” chromatin regions by catalyzing/removing covalent post-translational modifications on histone proteins. Anomalous chromatin decondensation is a common phenomenon in cells experiencing aging and viral infection. Moreover, about 50% of cancers have mutations in enzymes regulating chromatin state. Numerous genomics methods have evolved to characterize chromatin state, but the analysis of (in)accessible chromatin from the protein perspective is not yet in the spotlight. We present an overview of the most used approaches to generate data on chromatin accessibility and then focus on emerging methods that utilize mass spectrometry to quantify the accessibility of histones and the rest of the chromatin bound proteome. Mass spectrometry is currently the method of choice to quantify entire proteomes in an unbiased large-scale manner; accessibility on chromatin of proteins and protein modifications adds an extra quantitative layer to proteomics dataset that assist more informed data-driven hypotheses in chromatin biology. We speculate that this emerging new set of methods will enhance predictive strength on which proteins and histone modifications are critical in gene regulation, and which proteins occupy different chromatin states in health and disease.

Heat stress-induced transposon activation correlates with 3D chromatin organization rearrangement in Arabidopsis

In higher eukaryotes, heterochromatin is mainly composed of transposable elements (TEs) silenced by epigenetic mechanisms. But, the silencing of certain heterochromatin-associated TEs is disrupted by heat stress. By comparing genome-wide high-resolution chromatin packing patterns under normal or heat conditions obtained through Hi-C analysis, we show here that heat stress causes global rearrangement of the 3D genome in Arabidopsis thaliana. Contacts between pericentromeric regions and distal chromosome arms, as well as proximal intra-chromosomal interactions along the chromosomes, are enhanced. However, interactions within pericentromeres and those between distal intra-chromosomal regions are decreased. Many inter-chromosomal interactions, including those within the KNOT, are also reduced. Furthermore, heat activation of TEs exhibits a high correlation with the reduction of chromosomal interactions involving pericentromeres, the KNOT, the knob, and the upstream and downstream flanking regions of the activated TEs. Together, our results provide insights into the relationship between TE activation and 3D genome reorganization.

Polymerase IV Plays a Crucial Role in Pollen Development in Capsella

In Arabidopsis (Arabidopsis thaliana), DNA-dependent RNA polymerase IV (Pol IV) is required for the formation of transposable element (TE)-derived small RNA transcripts. These transcripts are processed by DICER-LIKE3 into 24-nucleotide small interfering RNAs (siRNAs) that guide RNA-directed DNA methylation. In the pollen grain, Pol IV is also required for the accumulation of 21/22-nucleotide epigenetically activated siRNAs, which likely silence TEs via post-transcriptional mechanisms. Despite this proposed role of Pol IV, its loss of function in Arabidopsis does not cause a discernible pollen defect. Here, we show that the knockout of NRPD1, encoding the largest subunit of Pol IV, in the Brassicaceae species Capsella (Capsella rubella), caused postmeiotic arrest of pollen development at the microspore stage. As in Arabidopsis, all TE-derived siRNAs were depleted in Capsella nrpd1 microspores. In the wild-type background, the same TEs produced 21/22-nucleotide and 24-nucleotide siRNAs; these processes required Pol IV activity. Arrest of Capsella nrpd1 microspores was accompanied by the deregulation of genes targeted by Pol IV-dependent siRNAs. TEs were much closer to genes in Capsella compared with Arabidopsis, perhaps explaining the essential role of Pol IV in pollen development in Capsella. Our discovery that Pol IV is functionally required in Capsella microspores emphasizes the relevance of investigating different plant models.

High expression in maize pollen correlates with genetic contributions to pollen fitness as well as with coordinated transcription from neighboring transposable elements

In flowering plants, gene expression in the haploid male gametophyte (pollen) is essential for sperm delivery and double fertilization. Pollen also undergoes dynamic epigenetic regulation of expression from transposable elements (TEs), but how this process interacts with gene expression is not clearly understood. To explore relationships among these processes, we quantified transcript levels in four male reproductive stages of maize (tassel primordia, microspores, mature pollen, and sperm cells) via RNA-seq. We found that, in contrast with vegetative cell-limited TE expression in Arabidopsis pollen, TE transcripts in maize accumulate as early as the microspore stage and are also present in sperm cells. Intriguingly, coordinate expression was observed between highly expressed protein-coding genes and their neighboring TEs, specifically in mature pollen and sperm cells. To investigate a potential relationship between elevated gene transcript level and pollen function, we measured the fitness cost (male-specific transmission defect) of GFP-tagged coding sequence insertion mutations in over 50 genes identified as highly expressed in the pollen vegetative cell, sperm cell, or seedling (as a sporophytic control). Insertions in seedling genes or sperm cell genes (with one exception) exhibited no difference from the expected 1:1 transmission ratio. In contrast, insertions in over 20% of vegetative cell genes were associated with significant reductions in fitness, showing a positive correlation of transcript level with non-Mendelian segregation when mutant. Insertions in maize gamete expressed2 (Zm gex2), the sole sperm cell gene with measured contributions to fitness, also triggered seed defects when crossed as a male, indicating a conserved role in double fertilization, given the similar phenotype previously demonstrated for the Arabidopsis ortholog GEX2. Overall, our study demonstrates a developmentally programmed and coordinated transcriptional activation of TEs and genes in pollen, and further identifies maize pollen as a model in which transcriptomic data have predictive value for quantitative phenotypes.

Arabidopsis RNA Polymerase IV generates 21-22 nucleotide small RNAs that can participate in RNA-directed DNA methylation and may regulate genes

The plant-specific RNA Polymerase IV (Pol IV) transcribes heterochromatic regions, including many transposable elements (TEs), with the well-described role of generating 24 nucleotide (nt) small interfering RNAs (siRNAs). These siRNAs target DNA methylation back to TEs to reinforce the boundary between heterochromatin and euchromatin. In the male gametophytic phase of the plant life cycle, pollen, Pol IV switches to generating primarily 21-22 nt siRNAs, but the biogenesis and function of these siRNAs have been enigmatic. In contrast to being pollen-specific, we identified that Pol IV generates these 21-22 nt siRNAs in sporophytic tissues, likely from the same transcripts that are processed into the more abundant 24 nt siRNAs. The 21-22 nt forms are specifically generated by the combined activities of DICER proteins DCL2/DCL4 and can participate in RNA-directed DNA methylation. These 21-22 nt siRNAs are also loaded into ARGONAUTE1 (AGO1), which is known to function in post-transcriptional gene regulation. Like other plant siRNAs and microRNAs incorporated into AGO1, we find a signature of genic mRNA cleavage at the predicted target site of these siRNAs, suggesting that Pol IV-generated 21-22 nt siRNAs may function to regulate gene transcript abundance. Our data provide support for the existing model that in pollen Pol IV functions in gene regulation. This article is part of a discussion meeting issue 'Crossroads between transposons and gene regulation'.

Epigenetics Regulates Reproductive Development in Plants

Seed, resulting from reproductive development, is the main nutrient source for human beings, and reproduction has been intensively studied through genetic, molecular, and epigenetic approaches. However, how different epigenetic pathways crosstalk and integrate to regulate seed development remains unknown. Here, we review the recent progress of epigenetic changes that affect chromatin structure, such as DNA methylation, polycomb group proteins, histone modifications, and small RNA pathways in regulating plant reproduction. In gametogenesis of flowering plants, epigenetics is dynamic between the companion cell and gametes. Cytosine DNA methylation occurs in CG, CHG, CHH contexts (H = A, C, or T) of genes and transposable elements, and undergoes dynamic changes during reproduction. Cytosine methylation in the CHH context increases significantly during embryogenesis, reaches the highest levels in mature embryos, and decreases as the seed germinates. Polycomb group proteins are important transcriptional regulators during seed development. Histone modifications and small RNA pathways add another layer of complexity in regulating seed development. In summary, multiple epigenetic pathways are pivotal in regulating seed development. It remains to be elucidated how these epigenetic pathways interplay to affect dynamic chromatin structure and control reproduction.

Linker histones are fine-scale chromatin architects modulating developmental decisions in Arabidopsis

BACKGROUND

Chromatin provides a tunable platform for gene expression control. Besides the well-studied core nucleosome, H1 linker histones are abundant chromatin components with intrinsic potential to influence chromatin function. Well studied in animals, little is known about the evolution of H1 function in other eukaryotic lineages for instance plants. Notably, in the model plant Arabidopsis, while H1 is known to influence heterochromatin and DNA methylation, its contribution to transcription, molecular, and cytological chromatin organization remains elusive.

RESULTS

We provide a multi-scale functional study of Arabidopsis linker histones. We show that H1-deficient plants are viable yet show phenotypes in seed dormancy, flowering time, lateral root, and stomata formation-complemented by either or both of the major variants. H1 depletion also impairs pluripotent callus formation. Fine-scale chromatin analyses combined with transcriptome and nucleosome profiling reveal distinct roles of H1 on hetero- and euchromatin: H1 is necessary to form heterochromatic domains yet dispensable for silencing of most transposable elements; H1 depletion affects nucleosome density distribution and mobility in euchromatin, spatial arrangement of nanodomains, histone acetylation, and methylation. These drastic changes affect moderately the transcription but reveal a subset of H1-sensitive genes.

CONCLUSIONS

H1 variants have a profound impact on the molecular and spatial (nuclear) chromatin organization in Arabidopsis with distinct roles in euchromatin and heterochromatin and a dual causality on gene expression. Phenotypical analyses further suggest the novel possibility that H1-mediated chromatin organization may contribute to the epigenetic control of developmental and cellular transitions.

Natural depletion of histone H1 in sex cells causes DNA demethylation, heterochromatin decondensation and transposon activation

Transposable elements (TEs), the movement of which can damage the genome, are epigenetically silenced in eukaryotes. Intriguingly, TEs are activated in the sperm companion cell - vegetative cell (VC) - of the flowering plant Arabidopsis thaliana. However, the extent and mechanism of this activation are unknown. Here we show that about 100 heterochromatic TEs are activated in VCs, mostly by DEMETER-catalyzed DNA demethylation. We further demonstrate that DEMETER access to some of these TEs is permitted by the natural depletion of linker histone H1 in VCs. Ectopically expressed H1 suppresses TEs in VCs by reducing DNA demethylation and via a methylation-independent mechanism. We demonstrate that H1 is required for heterochromatin condensation in plant cells and show that H1 overexpression creates heterochromatic foci in the VC progenitor cell. Taken together, our results demonstrate that the natural depletion of H1 during male gametogenesis facilitates DEMETER-directed DNA demethylation, heterochromatin relaxation, and TE activation.

Plant Chromatin Catches the Sun

Plants use solar radiation as energy source for photosynthesis. They also take advantage of the information provided by the varying properties of sunlight, such as wavelength, orientation, and periodicity, to trigger physiological and developmental adaptations to a changing environment. After more than a century of research efforts in plant photobiology, multiple light signaling pathways converging onto chromatin-based mechanisms have now been identified, which in some instances play critical roles in plant phenotypic plasticity. In addition to locus-specific changes linked to transcription regulation, light signals impact higher-order chromatin organization. Here, we summarize current knowledge on how light can affect the global composition and the spatial distribution of chromatin domains. We introduce emerging questions on the functional links between light signaling and the epigenome, and further discuss how different chromatin regulatory layers may interconnect during plant adaptive responses to light.