Histone Demethylases as Counterbalance to H3K27me3 Silencing in Plants

Dynamics of polycomb group marks in Arabidopsis

Polycomb Group (PcG) histone-modifying system is key in maintaining gene repression, providing a mitotically heritable cellular memory. Nevertheless, to allow plants to transition through distinct transcriptional programs during development or to respond to external cues, PcG-mediated repression requires reversibility. Several data suggest that the dynamics of PcG marks may vary considerably in different cell contexts; however, how PcG marks are established, maintained, or removed in each case is far from clear. In this review, we survey the knowns and unknowns of the molecular mechanisms underlying the maintenance or turnover of PcG marks in different cell stages.

The roles of epigenetic regulators in plant regeneration: Exploring patterns amidst complex conditions

Plants possess remarkable capability to regenerate upon tissue damage or optimal environmental stimuli. This ability not only serves as a crucial strategy for immobile plants to survive through harsh environments, but also made numerous modern plant improvements techniques possible. At the cellular level, this biological process involves dynamic changes in gene expression that redirect cell fate transitions. It is increasingly recognized that chromatin epigenetic modifications, both activating and repressive, intricately interact to regulate this process. Moreover, the outcomes of epigenetic regulation on regeneration are influenced by factors such as the differences in regenerative plant species and donor tissue types, as well as the concentration and timing of hormone treatments. In this review, we focus on several well-characterized epigenetic modifications and their regulatory roles in the expression of widely studied morphogenic regulators, aiming to enhance our understanding of the mechanisms by which epigenetic modifications govern plant regeneration.

Epigenetic regulation and epigenetic memory resetting during plant rejuvenation

Reversal of plant developmental status from the mature to the juvenile phase, thus leading to the restoration of the developmental potential, is referred to as plant rejuvenation. It involves multilayer regulation, including resetting gene expression patterns, chromatin remodeling, and histone modifications, eventually resulting in the restoration of juvenile characteristics. Although plants can be successfully rejuvenated using some forestry practices to restore juvenile morphology, physiology, and reproductive capabilities, studies on the epigenetic mechanisms underlying this process are in the nascent stage. This review provides an overview of the plant rejuvenation process and discusses the key epigenetic mechanisms involved in DNA methylation, histone modification, and chromatin remodeling in the process of rejuvenation, as well as the roles of small RNAs in this process. Additionally, we present new inquiries regarding the epigenetic regulation of plant rejuvenation, aiming to advance our understanding of rejuvenation in sexually and asexually propagated plants. Overall, we highlight the importance of epigenetic mechanisms in the regulation of plant rejuvenation, providing valuable insights into the complexity of this process.

Arabidopsis LSH10 transcription factor and OTLD1 histone deubiquitinase interact and transcriptionally regulate the same target genes

Histone ubiquitylation/deubiquitylation plays a major role in the epigenetic regulation of gene expression. In plants, OTLD1, a member of the ovarian tumor (OTU) deubiquitinase family, deubiquitylates histone 2B and represses the expression of genes involved in growth, cell expansion, and hormone signaling. OTLD1 lacks the intrinsic ability to bind DNA. How OTLD1, as well as most other known plant histone deubiquitinases, recognizes its target genes remains unknown. Here, we show that Arabidopsis transcription factor LSH10, a member of the ALOG protein family, interacts with OTLD1 in living plant cells. Loss-of-function LSH10 mutations relieve the OTLD1-promoted transcriptional repression of the target genes, resulting in their elevated expression, whereas recovery of the LSH10 function results in down-regulated transcription of the same genes. We show that LSH10 associates with the target gene chromatin as well as with DNA sequences in the promoter regions of the target genes. Furthermore, without LSH10, the degree of H2B monoubiquitylation in the target promoter chromatin increases. Hence, our data suggest that OTLD1-LSH10 acts as a co-repressor complex potentially representing a general mechanism for the specific function of plant histone deubiquitinases at their target chromatin.

Methylation hallmarks on the histone tail as a linker of osmotic stress and gene transcription

Plants dynamically manipulate their gene expression in acclimation to the challenging environment. Hereinto, the histone methylation tunes the gene transcription via modulation of the chromatin accessibility to transcription machinery. Osmotic stress, which is caused by water deprivation or high concentration of ions, can trigger remarkable changes in histone methylation landscape and genome-wide reprogramming of transcription. However, the dynamic regulation of genes, especially how stress-inducible genes are timely epi-regulated by histone methylation remains largely unclear. In this review, recent findings on the interaction between histone (de)methylation and osmotic stress were summarized, with emphasis on the effects on histone methylation profiles imposed by stress and how histone methylation works to optimize the performance of plants under stress.

Structure and mechanism of histone methylation dynamics in Arabidopsis

Histone methylation plays a central role in regulating chromatin state and gene expression in Arabidopsis and is involved in a variety of physiological and developmental processes. Dynamic regulation of histone methylation relies on both histone methyltransferase "writer" and histone demethylases "eraser" proteins. In this review, we focus on the four major histone methylation modifications in Arabidopsis H3, H3K4, H3K9, H3K27, and H3K36, and summarize current knowledge of the dynamic regulation of these modifications, with an emphasis on the biochemical and structural perspectives of histone methyltransferases and demethylases.

Conserved and distinct roles of H3K27me3 demethylases regulating flowering time in Brassica rapa

Epigenetic regulation is necessary for optimal organism development and preservation of gene expression profiles in the cell. In plants, the trimethylation of histone H3 lysine 27 (H3K27me3) is a silencing epigenetic mark relevant for developmental transitions like flowering. The floral transition is a key agronomic trait; however, the epigenetic mechanisms of flowering time regulation in crops remain poorly understood. Here we study the Jumonji H3K27me3 demethylases BraA.REF6 and BraA.ELF6 in Brassica rapa. Phenotypic characterization of novel mutant lines and genome-wide H3K27me3 chromatin immunoprecipitation and transcriptomic analyses indicated that BraA.REF6 plays a greater role than BraA.ELF6 in fine-tuning H3K27me3 levels. In addition, we found that braA.elf6 mutants were early flowering due to high H3K27me3 levels at B. rapa homologs of the floral repressor FLC. Unlike mutations in Arabidopsis thaliana, braA.ref6 mutants were late flowering without altering the expression of B. rapa FLC genes. Remarkably, we found that BraA.REF6 regulated a number of gibberellic acid (GA) biosynthetic genes, including a homolog of GA1, and that GA-treatment complemented the late flowering mutant phenotype. This study increases our understanding of the epigenetic regulation of flowering time in B. rapa, highlighting conserved and distinct regulatory mechanisms between model and crop species.

Toxic mechanisms of the trichothecenes T-2 toxin and deoxynivalenol on protein synthesis

The toxic mechanisms of trichothecenes, including T-2 toxin and deoxynivalenol (DON), are closely related with their effects on protein synthesis. Increasing lines of evidence show that T-2 toxin can reduce the levels of tight junction proteins, and nuclear factor erythroid 2-related factor 2 (Nrf2) by disrupting cellular barriers and the cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) and Nrf2/heme oxygenase (HO)-1 pathways. Moreover, it can inhibit aggrecan synthesis, thus causing Kashin-Beck disease. Regarding type B trichothecene, DON inhibits activation marker and β-catenin synthesis by acting on immune cells and the wingless/integrated (Wnt) pathway; it also inhibits cell proliferation and immune surveillance. In addition, DON has been shown to destroy tight junctions, glucose transport, and tumor endothelial marker 8, thus disturbing intestinal function and changing cell migration. This review summarizes the inhibitory effects of the trichothecenes T-2 toxin and DON on different protein synthesis, while discussing their underlying mechanisms. Focus is given to the effects of these toxins on tight junctions, aggrecan, activation markers, and hormones including testosterone under the influence of steroidogenic enzymes. This review can extend the current understanding of the effects of trichothecenes on protein synthesis and help to further understand their toxic mechanisms.

Dynamic epigenetic modifications in plant sugar signal transduction

In eukaryotes, dynamic chromatin states are closely related to changes in gene expression. Epigenetic modifications help plants adapt to their ever-changing environment by modulating gene expression via covalent modification at specific sites on DNA or histones. Sugars provide energy, but also function as signaling molecules to control plant growth and development. Various epigenetic modifications participate in sensing and transmitting sugar signals. Here we summarize recent progress in uncovering the epigenetic mechanisms involved in sugar signal transduction, including histone acetylation and deacetylation, histone methylation and demethylation, and DNA methylation. We also highlight changes in chromatin marks when crosstalk occurs between sugar signaling and the light, temperature, and phytohormone signaling pathways, and describe potential questions and approaches for future research.

Diversification of chromatin organization in eukaryotes

Our knowledge of the chromatin landscape and its regulation was originally discovered using yeast and a limited number of animals models. A wealth of studies in model plants now strongly demonstrates the conservation of certain features while illuminating the diversification of others. Here we summarize recent advances that describe specific features of chromatin organization of transcriptional units and specific regulation of heterochromatin in flowering plants. We highlight the importance of transcriptional regulation in plant chromatin organization and the need to investigate a more diverse range of species to understand the chromatin landscape in eukaryotes.

Reprogramming of Histone H3 Lysine Methylation During Plant Sexual Reproduction

Plants undergo extensive reprogramming of chromatin status during sexual reproduction, a process vital to cell specification and pluri- or totipotency establishment. As a crucial way to regulate chromatin organization and transcriptional activity, histone modification can be reprogrammed during sporogenesis, gametogenesis, and embryogenesis in flowering plants. In this review, we first introduce enzymes required for writing, recognizing, and removing methylation marks on lysine residues in histone H3 tails, and describe their differential expression patterns in reproductive tissues, then we summarize their functions in the reprogramming of H3 lysine methylation and the corresponding chromatin re-organization during sexual reproduction in Arabidopsis, and finally we discuss the molecular significance of histone reprogramming in maintaining the pluri- or totipotency of gametes and the zygote, and in establishing novel cell fates throughout the plant life cycle. Despite rapid achievements in understanding the molecular mechanism and function of the reprogramming of chromatin status in plant development, the research in this area still remains a challenge. Technological breakthroughs in cell-specific epigenomic profiling in the future will ultimately provide a solution for this challenge.

Expression profiling of H3K27me3 demethylase genes during plant development and in response to environmental stress in Arabidopsis

Histone modification influences gene expression. Among histone modifications, H3K27me3 is associated with downregulation of nearby genes via chromatin compaction. In Arabidopsis thaliana, a subset of JUMONJI C DOMAIN-CONTAINING PROTEIN (JMJ) proteins play a critical role in removal of H3K27me3 during plant development or in response to environmental cues. However, the regulation of H3K27me3 demethylase gene expression is not yet fully characterized. In this study, we computationally characterized the expression patterns of JMJ H3K27me3 demethylase genes using public transcriptome datasets created across plant development and after various environmental cues. Consistent with the available transcriptome datasets, GUS staining validated that JMJ30 was highly expressed in the L1 layer of the shoot apical meristem. Furthermore, expression data for panel of five H3K27me3 demethylase genes revealed JMJ30 to be the most highly affected by abiotic and biotic stress. In addition, JMJ30 expression was variable between Arabidopsis thaliana accessions. Finally, the expression of a JMJ30 orthologue from the related species Arabidopsis halleri, AhgJMJ30, fluctuated under field conditions. Taken together, our results suggest that transcriptional changes of H3K27me3 demethylase genes may play key roles in development and environmental responses.

Unraveling the Genetic Basis of Fertility Restoration for Cytoplasmic Male Sterile Line WNJ01A Originated From Brassica juncea in Brassica napus

Crosses that lead to heterosis have been widely used in the rapeseed (Brassica napus L.) industry. Cytoplasmic male sterility (CMS)/restorer-of-fertility (Rf) systems represent one of the most useful tools for rapeseed production. Several CMS types and their restorer lines have been identified in rapeseed, but there are few studies on the mechanisms underlying fertility restoration. Here, we performed morphological observation, map-based cloning, and transcriptomic analysis of the F₂ population developed by crossing the CMS line WNJ01A with its restorer line Hui01. Paraffin-embedded sections showed that the sporogenous cell stage was the critical pollen degeneration period, with major sporogenous cells displaying loose and irregular arrangement in sterile anthers. Most mitochondrial electron transport chain (mtETC) complex genes were upregulated in fertile compared to sterile buds. Using bulked segregant analysis (BSA)-seq to analyze mixed DNA pools from sterile and fertile F₂ buds, respectively, we identified a 6.25 Mb candidate interval where Rfw is located. Using map-based cloning experiments combined with bacterial artificial chromosome (BAC) clone sequencing, the candidate interval was reduced to 99.75 kb and two pentatricopeptide repeat (PPR) genes were found among 28 predicted genes in this interval. Transcriptome sequencing showed that there were 1679 DEGs (1023 upregulated and 656 downregulated) in fertile compared to sterile F₂ buds. The upregulated differentially expressed genes (DEGs) were enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG) lysine degradation pathway and phenylalanine metabolism, and the downregulated DEGs were enriched in cutin, suberine, and wax biosynthesis. Furthermore, 44 DEGs were involved in pollen and anther development, such as tapetum, microspores, and pollen wall development. All of them were upregulated except a few such as POE1 genes (which encode Pollen Ole e I allergen and extensin family proteins). There were 261 specifically expressed DEGs (9 and 252 in sterile and fertile buds, respectively). Regarding the fertile bud-specific upregulated DEGs, the ubiquitin-proteasome pathway was enriched. The top four hub genes in the protein-protein interaction network (BnaA09g56400D, BnaA10g18210D, BnaA10g18220D, and BnaC09g41740D) encode RAD23d proteins, which deliver ubiquitinated substrates to the 26S proteasome. These findings provide evidence on the pathways regulated by Rfw and improve our understanding of fertility restoration.

Characterization of expansin genes and their transcriptional regulation by histone modifications in strawberry

The possible candidate expansin genes, which may be important for strawberry fruit softening, have been identified in the diploid woodland strawberry Fragaria vesca and the octoploid cultivated strawberry Fragaria × ananassa and their transcriptional regulation by histone modifications has been studied. Softening process greatly affects fruit texture and shelf life. Expansins (EXPs) are a group of structural proteins participating in cell wall loosening, which break the hydrogen bonding between cellulose microfibrils and hemicelluloses. However, our knowledge on how EXP genes are regulated in fruit ripening, especially in non-climacteric fleshy fruits, is limited. Here, we have identified the EXP genes in both the octoploid cultivated strawberry (Fragaria × ananassa) and one of its diploid progenitor species, woodland strawberry (Fragaria vesca). We found that EXP proteins in F. × ananassa were structurally more divergent than the ones in F. vesca. Transcriptome data suggested that FaEXP88, FaEXP114, FveEXP11 and FveEXP33 were the four candidate EXP genes more likely involved in fruit softening, whose transcript levels dramatically increased when firmness decreased during fruit maturation. Phylogenetic analyses showed that those candidate genes were closely clustered, indicating the presence of homoeolog expression dominance in the EXP gene family in strawberry. Moreover, we have performed chromatin immunoprecipitation (ChIP) experiments to investigate the distribution of histone modifications along the promoters and genic regions of the EXP genes in F. vesca. ChIP data revealed that the transcript levels of EXP genes were highly correlated with the enrichment of H3K9/K14 acetylation and H3K27 tri-methylation. Collectively, this study identifies the key EXP genes involved in strawberry fruit softening and reveals a regulatory role of histone modifications in their transcriptional regulation, which would facilitate functional studies of the EXP genes in the ripening of non-climacteric fruits.

Removal of H3K27me3 by JMJ Proteins Controls Plant Development and Environmental Responses in Arabidopsis

Trimethylation of histone H3 lysine 27 (H3K27me3) is a highly conserved repressive histone modification that signifies transcriptional repression in plants and animals. In Arabidopsis thaliana, the demethylation of H3K27 is regulated by a group of JUMONJI DOMAIN-CONTANING PROTEIN (JMJ) genes. Transcription of JMJ genes is spatiotemporally regulated during plant development and in response to the environment. Once JMJ genes are transcribed, recruitment of JMJs to target genes, followed by demethylation of H3K27, is critically important for the precise control of gene expression. JMJs function synergistically and antagonistically with transcription factors and/or other epigenetic regulators on chromatin. This review summarizes the latest advances in our understanding of Arabidopsis H3K27me3 demethylases that provide robust and flexible epigenetic regulation of gene expression to direct appropriate development and environmental responses in plants.

Dynamics of H3K27me3 Modification on Plant Adaptation to Environmental Cues

Given their sessile nature, plants have evolved sophisticated regulatory networks to confer developmental plasticity for adaptation to fluctuating environments. Epigenetic codes, like tri-methylation of histone H3 on Lys27 (H3K27me3), are evidenced to account for this evolutionary benefit. Polycomb repressive complex 2 (PRC2) and PRC1 implement and maintain the H3K27me3-mediated gene repression in most eukaryotic cells. Plants take advantage of this epigenetic machinery to reprogram gene expression in development and environmental adaption. Recent studies have uncovered a number of new players involved in the establishment, erasure, and regulation of H3K27me3 mark in plants, particularly highlighting new roles in plants’ responses to environmental cues. Here, we review current knowledge on PRC2-H3K27me3 dynamics occurring during plant growth and development, including its writers, erasers, and readers, as well as targeting mechanisms, and summarize the emerging roles of H3K27me3 mark in plant adaptation to environmental stresses.