The role of ATXR6 expression in modulating genome stability and transposable element repression in Arabidopsis

H3.1K27M-induced misregulation of the TONSOKU-H3.1 pathway causes genomic instability

The oncomutation lysine 27-to-methionine in histone H3 (H3K27M) is frequently identified in tumors of patients with diffuse midline glioma-H3K27 altered (DMG-H3K27a). H3K27M inhibits the deposition of the histone mark H3K27me3, which affects the maintenance of transcriptional programs and cell identity. Cells expressing H3K27M are also characterized by defects in genome integrity, but the mechanisms linking expression of the oncohistone to DNA damage remain mostly unknown. In this study, we demonstrate that expression of H3.1K27M in the model plant Arabidopsis thaliana interferes with post-replicative chromatin maturation mediated by the H3.1K27 methyltransferases ATXR5 and ATXR6. As a result, H3.1 variants on nascent chromatin remain unmethylated at K27 (H3.1K27me0), leading to ectopic activity of TONSOKU (TSK/TONSL), which induces DNA damage and genomic alterations. Elimination of TSK activity suppresses the genome stability defects associated with H3.1K27M expression, while inactivation of specific DNA repair pathways prevents survival of H3.1K27M-expressing plants. Overall, our results suggest that H3.1K27M disrupts the chromatin-based mechanisms regulating TSK activity, which causes genomic instability and may contribute to the etiology of DMG-H3K27a.

The role of DNA content in shaping chromatin architecture and gene expression

Whole-genome duplication is an evolutionary force that drives speciation in all living kingdoms and is notably prevalent in plants. The evolutionary history of plants involved at least two genomic duplications that significantly expanded the plant morphology and physiology spectrum. Many important crops are polyploids, showing valuable features relative to morphological and stress response traits. After genome duplication, diploidization processes facilitate genomic adjustments to restore disomic inheritance. However, little is known about the chromatin changes triggered by nuclear DNA content alterations. Here, we report that synthetically induced genome duplication leads to chromatinization and significant changes in gene expression, resulting in a transcriptional landscape resembling a natural tetraploid. Interestingly, synthetic diploidization elicits only minor alterations in transcriptional activity and chromatin accessibility compared to the more pronounced effects of tetraploidization. We identified epigenetic factors, including specific histone variants, that showed increased expression following genome duplication and decreased expression after genome reduction. These changes may play a key role in the epigenetic mechanisms underlying the phenotypic complexity after tetraploidization in plants. Our findings shed light on the mechanisms that modulate chromatin accessibility remodeling and gene transcription regulation underlying plant genome adaptation in response to changes in genome size.

Genome-wide profiling and functional study of short N-terminal H2B variants in Arabidopsis

INTRODUCTION

Nucleosomes harboring specific histone variants show distinct chromatin localization patterns and regulatory functions, thereby playing crucial roles in epigenetic regulation. Compared to the well-understood variants of H2A and H3, the study about H2B variants is emerging. Deciphering the roles and regulatory mechanisms of H2B variants in plants will provide more knowledges about epigenetic regulations in plant biology.

OBJECTIVES

Using the model plant Arabidopsis thaliana as the research subject, we systematically analyzed histone H2B variants, four short N-terminal histone H2B variants (snH2Bs) were identified. The genomic distribution characteristics of these snH2Bs, their impact on plant growth, and the potential regulatory mechanisms were studied.

METHODS

By integrating whole-genome chromatin immunoprecipitation sequencing (ChIP-seq) and fluorescence microscopy localization analysis, the distribution of snH2Bs across the genome was identified. Single, double, and triple knock-out mutants were constructed using CRISPR-Cas9 to further explore the functions of snH2Bs in the growth process of Arabidopsis thaliana, the possible mechanisms were also discussed.

RESULTS

These snH2B variants are preferentially expressed in reproductive tissues and are detected in the nuclei of pollen grains. Further genome-wide profiling indicates that the snH2Bs distribute at active chromatin regions and are positively correlated with gene expression. By creating knock-out single, double, and triple mutants for these snH2Bs, we demonstrate that H2B.5 influences vegetative to reproductive transition. We also show that H2B.5 is required for proper accumulation of H3 lysine 9 acetylation and H2B mono-ubiquitination.

CONCLUSION

Overall, our study not only provide insights into the functions and chromatin characteristics of plant snH2Bs, but also supplies examples that illustrate the interplay between histone variants and histone modification. These findings contribute to the understanding of the fundamental principles of epigenetic regulation in eukaryotes and also highlight potential targets for crop improvement.

H3.1K27M-induced misregulation of the TSK/TONSL-H3.1 pathway causes genomic instability

The oncomutation lysine 27-to-methionine in histone H3 (H3K27M) is frequently identified in tumors of patients with diffuse midline glioma-H3K27 altered (DMG-H3K27a). H3K27M inhibits the deposition of the histone mark H3K27me3, which affects the maintenance of transcriptional programs and cell identity. Cells expressing H3K27M are also characterized by defects in genome integrity, but the mechanisms linking expression of the oncohistone to DNA damage remain mostly unknown. In this study, we demonstrate that expression of H3.1K27M in the model plant Arabidopsis thaliana interferes with post-replicative chromatin maturation mediated by the H3.1K27 methyltransferases ATXR5 and ATXR6. As a result, H3.1 variants on nascent chromatin remain unmethylated at K27 (H3.1K27me0), leading to ectopic activity of TONSOKU (TSK), which induces DNA damage and genomic alterations. Elimination of TSK activity suppresses the genome stability defects associated with H3.1K27M expression, while inactivation of specific DNA repair pathways prevents survival of H3.1K27M-expressing plants. Overall, our results suggest that H3.1K27M disrupts the chromatin-based mechanisms regulating TSK/TONSL activity, which causes genomic instability and may contribute to the etiology of DMG-H3K27a.

Heterochromatin in plant meiosis

Heterochromatin is an organizational property of eukaryotic chromosomes, characterized by extensive DNA and histone modifications, that is associated with the silencing of transposable elements and repetitive sequences. Maintaining heterochromatin is crucial for ensuring genomic integrity and stability during the cell cycle. During meiosis, heterochromatin is important for homologous chromosome synapsis, recombination, and segregation, but our understanding of meiotic heterochromatin formation and condensation is limited. In this review, we focus on the dynamics and features of heterochromatin and how it condenses during meiosis in plants. We also discuss how meiotic heterochromatin influences the interaction and recombination of homologous chromosomes during prophase I.

Diurnal control of H3K27me1 deposition shapes expression of a subset of cell cycle and DNA damage response genes

Rhythmic oscillation of biological processes helps organisms adapt their physiological responses to the most appropriate time of the day. Chromatin remodeling has been described as one of the molecular mechanisms controlling these oscillations. The importance of these changes in transcriptional activation as well as in the maintenance of heterochromatic regions has been widely demonstrated. However, little is still known on how diurnal changes can impact the global status of chromatin modifications and, hence, control gene expression. In plants, the repressive mark H3K27me1, deposited by ARABIDOPSIS TRITHORAX-RELATED PROTEIN 5 and 6 (ATXR5 and 6) methyltransferases, is largely associated with transposable elements but also covers lowly expressed genes. Here we show that this histone modification is preferentially deposited during the night. In euchromatic regions, it is found along the bodies of DNA damage response genes (DDR), where it is needed for their proper expression. The absence of H3K27me1 translates into an enhanced expression of DDR genes that follows a rhythmic oscillation pattern. This evidences a link between chromatin modifications and their synchronization with the diurnal cycle in order to accurately modulate the activation of biological processes to the most appropriate time of the day.

Establishment and Maintenance of Heat-Stress Memory in Plants

Among the rich repertoire of strategies that allow plants to adapt to high-temperature stress is heat-stress memory. The mechanisms underlying the establishment and maintenance of heat-stress memory are poorly understood, although the chromatin opening state appears to be an important structural basis for maintaining heat-stress memory. The chromatin opening state is influenced by epigenetic modifications, making DNA and histone modifications important entry points for understanding heat-shock memory. Current research suggests that traditional heat-stress signaling pathway components might be involved in chromatin opening, thereby promoting the establishment of heat-stress memory in plants. In this review, we discuss the relationship between chromatin structure-based maintenance and the establishment of heat-stress memory. We also discuss the association between traditional heat-stress signals and epigenetic modifications. Finally, we discuss potential research ideas for exploring plant adaptation to high-temperature stress in the future.

H3.1K27me1 loss confers Arabidopsis resistance to Geminivirus by sequestering DNA repair proteins onto host genome

The H3 methyltransferases ATXR5 and ATXR6 deposit H3.1K27me1 to heterochromatin to prevent genomic instability and transposon re-activation. Here, we report that atxr5 atxr6 mutants display robust resistance to Geminivirus. The viral resistance is correlated with activation of DNA repair pathways, but not with transposon re-activation or heterochromatin amplification. We identify RAD51 and RPA1A as partners of virus-encoded Rep protein. The two DNA repair proteins show increased binding to heterochromatic regions and defense-related genes in atxr5 atxr6 vs wild-type plants. Consequently, the proteins have reduced binding to viral DNA in the mutant, thus hampering viral amplification. Additionally, RAD51 recruitment to the host genome arise via BRCA1, HOP2, and CYCB1;1, and this recruitment is essential for viral resistance in atxr5 atxr6. Thus, Geminiviruses adapt to healthy plants by hijacking DNA repair pathways, whereas the unstable genome, triggered by reduced H3.1K27me1, could retain DNA repairing proteins to suppress viral amplification in atxr5 atxr6.

Molecular, hormonal, and metabolic mechanisms of fruit set, the ovary-to-fruit transition, in horticultural crops

Fruit set is the process by which the ovary develops into a fruit and is an important factor in determining fruit yield. Fruit set is induced by two hormones, auxin and gibberellin, and the activation of their signaling pathways, partly by suppressing various negative regulators. Many studies have investigated the structural changes and gene networks in the ovary during fruit set, revealing the cytological and molecular mechanisms. In tomato (Solanum lycopersicum), SlIAA9 and SlDELLA/PROCERA act as auxin and gibberellin signaling repressors, respectively, and are important regulators of the activity of transcription factors and downstream gene expression involved in fruit set. Upon pollination, SlIAA9 and SlDELLA are degraded, which subsequently activates downstream cascades and mainly contributes to active cell division and cell elongation, respectively, in ovaries during fruit setting. According to current knowledge, the gibberellin pathway functions as the most downstream signal in fruit set induction, and therefore its role in fruit set has been extensively explored. Furthermore, multi-omics analysis has revealed the detailed dynamics of gene expression and metabolites downstream of gibberellins, highlighting the rapid activation of central carbon metabolism. This review will outline the relevant mechanisms at the molecular and metabolic levels during fruit set, particularly focusing on tomato.

Mitotic inheritance of genetic and epigenetic information via the histone H3.1 variant

The replication-dependent histone H3.1 variant, ubiquitous in multicellular eukaryotes, has been proposed to play key roles during chromatin replication due to its unique expression pattern restricted to the S phase of the cell cycle. Here, we describe recent discoveries in plants regarding molecular mechanisms and cellular pathways involving H3.1 that contribute to the maintenance of genomic and epigenomic information. First, we highlight new advances concerning the contribution of the histone chaperone CAF-1 and the TSK-H3.1 DNA repair pathway in preventing genomic instability during replication. We then summarize the evidence connecting H3.1 to specific roles required for the mitotic inheritance of epigenetic states. Finally, we discuss the recent identification of a specific interaction between H3.1 and DNA polymerase epsilon and its functional implications.

Dynamics of epigenetic control in plants via SET domain containing proteins: Structural and functional insights

Plants control expression of their genes in a way that involves manipulating the chromatin structural dynamics in order to adapt to environmental changes and carry out developmental processes. Histone modifications like histone methylation are significant epigenetic marks which profoundly and globally modify chromatin, potentially affecting the expression of several genes. Methylation of histones is catalyzed by histone lysine methyltransferases (HKMTs), that features an evolutionary conserved domain known as SET [Su(var)3-9, E(Z), Trithorax]. This methylation is directed at particular lysine (K) residues on H3 or H4 histone. Plant SET domain group (SDG) proteins are categorized into different classes that have been conserved through evolution, and each class have specificity that influences how the chromatin structure operates. The domains discovered in plant SET domain proteins have typically been linked to protein-protein interactions, suggesting that majority of the SDGs function in complexes. Additionally, SDG-mediated histone mark deposition also affects alternative splicing events. In present review, we discussed the diversity of SDGs in plants including their structural properties. Additionally, we have provided comprehensive summary of the functions of the SDG-domain containing proteins in plant developmental processes and response to environmental stimuli have also been highlighted.

The genomic and epigenetic footprint of local adaptation to variable climates in kiwifruit

A full understanding of adaptive genetic variation at the genomic level will help address questions of how organisms adapt to diverse climates. Actinidia eriantha is a shade-tolerant species, widely distributed in the southern tropical region of China, occurring in spatially heterogeneous environments. In the present study we combined population genomic, epigenomic, and environmental association analyses to infer population genetic structure and positive selection across a climatic gradient, and to assess genomic offset to climatic change for A. eriantha. The population structure is strongly shaped by geography and influenced by restricted gene flow resulting from isolation by distance due to habitat fragmentation. In total, we identified 102 outlier loci and annotated 455 candidate genes associated with the genomic basis of climate adaptation, which were enriched in functional categories related to development processes and stress response; both temperature and precipitation are important factors driving adaptive variation. In addition to single-nucleotide polymorphisms (SNPs), a total of 27 single-methylation variants (SMVs) had significant correlation with at least one of four climatic variables and 16 SMVs were located in or adjacent to genes, several of which were predicted to be involved in plant response to abiotic or biotic stress. Gradient forest analysis indicated that the central/east populations were predicted to be at higher risk of future population maladaptation under climate change. Our results demonstrate that local climate factors impose strong selection pressures and lead to local adaptation. Such information adds to our understanding of adaptive mechanisms to variable climates revealed by both population genome and epigenome analysis.

Distinct roles of Arabidopsis ORC1 proteins in DNA replication and heterochromatic H3K27me1 deposition

Most cellular proteins involved in genome replication are conserved in all eukaryotic lineages including yeast, plants and animals. However, the mechanisms controlling their availability during the cell cycle are less well defined. Here we show that the Arabidopsis genome encodes for two ORC1 proteins highly similar in amino acid sequence and that have partially overlapping expression domains but with distinct functions. The ancestral ORC1b gene, present before the partial duplication of the Arabidopsis genome, has retained the canonical function in DNA replication. ORC1b is expressed in both proliferating and endoreplicating cells, accumulates during G1 and is rapidly degraded upon S-phase entry through the ubiquitin-proteasome pathway. In contrast, the duplicated ORC1a gene has acquired a specialized function in heterochromatin biology. ORC1a is required for efficient deposition of the heterochromatic H3K27me1 mark by the ATXR5/6 histone methyltransferases. The distinct roles of the two ORC1 proteins may be a feature common to other organisms with duplicated ORC1 genes and a major difference with animal cells.

The effect of DNA polymorphisms and natural variation on crossover hotspot activity in Arabidopsis hybrids

In hybrid organisms, genetically divergent homologous chromosomes pair and recombine during meiosis; however, the effect of specific types of polymorphisms on crossover is poorly understood. Here, to analyze this in Arabidopsis, we develop the seed-typing method that enables the massively parallel fine-mapping of crossovers by sequencing. We show that structural variants, observed in one of the generated intervals, do not change crossover frequency unless they are located directly within crossover hotspots. Both natural and Cas9-induced deletions result in lower hotspot activity but are not compensated by increases in immediately adjacent hotspots. To examine the effect of single nucleotide polymorphisms on crossover formation, we analyze hotspot activity in mismatch detection-deficient msh2 mutants. Surprisingly, polymorphic hotspots show reduced activity in msh2. In lines where only the hotspot-containing interval is heterozygous, crossover numbers increase above those in the inbred (homozygous). We conclude that MSH2 shapes crossover distribution by stimulating hotspot activity at polymorphic regions.

The Role of the TSK/TONSL-H3.1 Pathway in Maintaining Genome Stability in Multicellular Eukaryotes

Replication-dependent histone H3.1 and replication-independent histone H3.3 are nearly identical proteins in most multicellular eukaryotes. The N-terminal tails of these H3 variants, where the majority of histone post-translational modifications are made, typically differ by only one amino acid. Despite extensive sequence similarity with H3.3, the H3.1 variant has been hypothesized to play unique roles in cells, as it is specifically expressed and inserted into chromatin during DNA replication. However, identifying a function that is unique to H3.1 during replication has remained elusive. In this review, we discuss recent findings regarding the involvement of the H3.1 variant in regulating the TSK/TONSL-mediated resolution of stalled or broken replication forks. Uncovering this new function for the H3.1 variant has been made possible by the identification of the first proteins containing domains that can selectively bind or modify the H3.1 variant. The functional characterization of H3-variant-specific readers and writers reveals another layer of chromatin-based information regulating transcription, DNA replication, and DNA repair.