REF6 recognizes a specific DNA sequence to demethylate H3K27me3 and regulate organ boundary formation in Arabidopsis

Post-Embryonic Phase Transitions Mediated by Polycomb Repressive Complexes in Plants

Correct timing of developmental phase transitions is critical for the survival and fitness of plants. Developmental phase transitions in plants are partially promoted by controlling relevant genes into active or repressive status. Polycomb Repressive Complex1 (PRC1) and PRC2, originally identified in Drosophila, are essential in initiating and/or maintaining genes in repressive status to mediate developmental phase transitions. Our review summarizes mechanisms in which the embryo-to-seedling transition, the juvenile-to-adult transition, and vegetative-to-reproductive transition in plants are mediated by PRC1 and PRC2, and suggests that PRC1 could act either before or after PRC2, or that they could function independently of each other. Details of the exact components of PRC1 and PRC2 in each developmental phase transitions and how they are recruited or removed will need to be addressed in the future.

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.

RNA-directed DNA methylation prevents rapid and heritable reversal of transposon silencing under heat stress in Zea mays

In large complex plant genomes, RNA-directed DNA methylation (RdDM) ensures that epigenetic silencing is maintained at the boundary between genes and flanking transposable elements. In maize, RdDM is dependent on Mediator of Paramutation1 (Mop1), a gene encoding a putative RNA dependent RNA polymerase. Here we show that although RdDM is essential for the maintenance of DNA methylation of a silenced MuDR transposon in maize, a loss of that methylation does not result in a restoration of activity. Instead, heritable maintenance of silencing is maintained by histone modifications. At one terminal inverted repeat (TIR) of this element, heritable silencing is mediated via histone H3 lysine 9 dimethylation (H3K9me2), and histone H3 lysine 27 dimethylation (H3K27me2), even in the absence of DNA methylation. At the second TIR, heritable silencing is mediated by histone H3 lysine 27 trimethylation (H3K27me3), a mark normally associated with somatically inherited gene silencing. We find that a brief exposure of high temperature in a mop1 mutant rapidly reverses both of these modifications in conjunction with a loss of transcriptional silencing. These reversals are heritable, even in mop1 wild-type progeny in which methylation is restored at both TIRs. These observations suggest that DNA methylation is neither necessary to maintain silencing, nor is it sufficient to initiate silencing once has been reversed. However, given that heritable reactivation only occurs in a mop1 mutant background, these observations suggest that DNA methylation is required to buffer the effects of environmental stress on transposable elements.

Most plant genomes are mostly transposable elements (TEs), most of which are held in check by modifications of both DNA and histones. The bulk of silenced TEs are associated with methylated DNA and histone H3 lysine 9 dimethylation (H3K9me2). In contrast, epigenetically silenced genes are often associated with histone lysine 27 trimethylation (H3K27me3). Although stress can affect each of these modifications, plants are generally competent to rapidly reset them following that stress. Here we demonstrate that although DNA methylation is not required to maintain silencing of the MuDR element, it is essential for preventing heat-induced, stable and heritable changes in both H3K9me2 and H3K27me3 at this element, and for concomitant changes in transcriptional activity. These finding suggest that RdDM acts to buffer the effects of heat on silenced transposable elements, and that a loss of DNA methylation under conditions of stress can have profound and long-lasting effects on epigenetic silencing in maize.

H3K27me3 demethylases alter HSP22 and HSP17.6C expression in response to recurring heat in Arabidopsis

Acclimation to high temperature increases plants' tolerance of subsequent lethal high temperatures. Although epigenetic regulation of plant gene expression is well studied, how plants maintain a memory of environmental changes over time remains unclear. Here, we show that JUMONJI (JMJ) proteins, demethylases involved in histone H3 lysine 27 trimethylation (H3K27me3), are necessary for Arabidopsis thaliana heat acclimation. Acclimation induces sustained H3K27me3 demethylation at HEAT SHOCK PROTEIN22 (HSP22) and HSP17.6C loci by JMJs, poising the HSP genes for subsequent activation. Upon sensing heat after a 3-day interval, JMJs directly reactivate these HSP genes. Finally, jmj mutants fail to maintain heat memory under fluctuating field temperature conditions. Our findings of an epigenetic memory mechanism involving histone demethylases may have implications for environmental adaptation of field plants.

JMJ Histone Demethylases Balance H3K27me3 and H3K4me3 Levels at the HSP21 Locus during Heat Acclimation in Arabidopsis

Exposure to moderately high temperature enables plants to acquire thermotolerance to high temperatures that might otherwise be lethal. In Arabidopsis thaliana, histone H3 lysine 27 trimethylation (H3K27me3) at the heat shock protein 17.6C (HSP17.6C) and HSP22 loci is removed by Jumonji C domain-containing protein (JMJ) histone demethylases, thus allowing the plant to ‘remember’ the heat experience. Other heat memory genes, such as HSP21, are downregulated in acclimatized jmj quadruple mutants compared to the wild type, but how those genes are regulated remains uncharacterized. Here, we show that histone H3 lysine 4 trimethylation (H3K4me3) at HSP21 was maintained at high levels for at least three days in response to heat. This heat-dependent H3K4me3 accumulation was compromised in the acclimatized jmj quadruple mutant as compared to the acclimatized wild type. JMJ30 directly bound to the HSP21 locus in response to heat and coordinated H3K27me3 and H3K4me3 levels under standard and fluctuating conditions. Our results suggest that JMJs mediate the balance between H3K27me3 and H3K4me3 at the HSP21 locus through proper maintenance of H3K27me3 removal during heat acclimation.

Current understanding of plant Polycomb group proteins and the repressive histone H3 Lysine 27 trimethylation

Polycomb group (PcG) proteins are highly conserved chromatin-modifying complexes that implement gene silencing in higher eukaryotes. Thousands of genes and multiple developmental processes are regulated by PcG proteins. As the first chromatin modifier been identified in model plant Arabidopsis thaliana, the methyltransferase CURLY LEAF (CLF) and its catalyzed histone H3 Lysine 27 trimethylation (H3K27me3) have already become well-established paradigm in plant epigenetic study. Like in animals, PcG proteins mediate plant development and repress homeotic gene expression by antagonizing with trithorax group proteins. Recent researches have advanced our understanding on plant PcG proteins, including the plant-specific components of these well-conserved protein complexes, the close association with transcription factors and noncoding RNA for the spatial and temporal specificity, the dynamic regulation of the repressive mark H3K27me3 and the PcG-mediated chromatin conformation alterations in gene expression. In this review, we will summarize the molecular mechanisms of PcG-implemented gene repression and the relationship between H3K27me3 and another repressive mark histone H2A Lysine 121 mono-ubiquitination (H2A121ub) will also be discussed.

Histone methylation in epigenetic regulation and temperature responses

Methylation of histones on different lysine residues is dynamically added by distinct writer enzymes, interpreted by reader proteins, and removed by eraser enzymes. This epigenetic mark has widespread, dynamic roles in plant development and environmental responses. For example, histone methylation plays a key role in mediating plant responses to temperature, including alterations of flowering time. In this review, we summarize recent advances in understanding the mechanism by which histone methylation regulates these processes, and discuss the role of histone methylation in temperature responses, based on data from Arabidopsis thaliana.

An atlas of wheat epigenetic regulatory elements reveals subgenome divergence in the regulation of development and stress responses

Wheat (Triticum aestivum) has a large allohexaploid genome. Subgenome-divergent regulation contributed to genome plasticity and the domestication of polyploid wheat. However, the specificity encoded in the wheat genome determining subgenome-divergent spatio-temporal regulation has been largely unexplored. The considerable size and complexity of the genome are major obstacles to dissecting the regulatory specificity. Here, we compared the epigenomes and transcriptomes from a large set of samples under diverse developmental and environmental conditions. Thousands of distal epigenetic regulatory elements (distal-epiREs) were specifically linked to their target promoters with coordinated epigenomic changes. We revealed that subgenome-divergent activity of homologous regulatory elements is affected by specific epigenetic signatures. Subgenome-divergent epiRE regulation of tissue specificity is associated with dynamic modulation of H3K27me3 mediated by Polycomb complex and demethylases. Furthermore, quantitative epigenomic approaches detected key stress responsive cis- and trans-acting factors validated by DNA Affinity Purification and sequencing, and demonstrated the coordinated interplay between epiRE sequence contexts, epigenetic factors, and transcription factors in regulating subgenome divergent transcriptional responses to external changes. Together, this study provides a wealth of resources for elucidating the epiRE regulomics and subgenome-divergent regulation in hexaploid wheat, and gives new clues for interpreting genetic and epigenetic interplay in regulating the benefits of polyploid wheat.

Polycomb Repressive Complex 2 and KRYPTONITE regulate pathogen-induced programmed cell death in Arabidopsis

The Polycomb Repressive Complex 2 (PRC2) is well-known for its role in controlling developmental transitions by suppressing the premature expression of key developmental regulators. Previous work revealed that PRC2 also controls the onset of senescence, a form of developmental programmed cell death (PCD) in plants. Whether the induction of PCD in response to stress is similarly suppressed by the PRC2 remained largely unknown. In this study, we explored whether PCD triggered in response to immunity- and disease-promoting pathogen effectors is associated with changes in the distribution of the PRC2-mediated histone H3 lysine 27 trimethylation (H3K27me3) modification in Arabidopsis thaliana. We furthermore tested the distribution of the heterochromatic histone mark H3K9me2, which is established, to a large extent, by the H3K9 methyltransferase KRYPTONITE, and occupies chromatin regions generally not targeted by PRC2. We report that effector-induced PCD caused major changes in the distribution of both repressive epigenetic modifications and that both modifications have a regulatory role and impact on the onset of PCD during pathogen infection. Our work highlights that the transition to pathogen-induced PCD is epigenetically controlled, revealing striking similarities to developmental PCD.

Genome-wide analysis of SET-domain group histone methyltransferases in apple reveals their role in development and stress responses

Background

Histone lysine methylation plays an important role in plant development and stress responses by activating or repressing gene expression. Histone lysine methylation is catalyzed by a class of SET-domain group proteins (SDGs). Although an increasing number of studies have shown that SDGs play important regulatory roles in development and stress responses, the functions of SDGs in apple remain unclear.

Results

A total of 67 SDG members were identified in the Malus×domestica genome. Syntenic analysis revealed that most of the MdSDG duplicated gene pairs were associated with a recent genome-wide duplication event of the apple genome. These 67 MdSDG members were grouped into six classes based on sequence similarity and the findings of previous studies. The domain organization of each MdSDG class was characterized by specific patterns, which was consistent with the classification results. The tissue-specific expression patterns of MdSDGs among the 72 apple tissues in the different apple developmental stages were characterized to provide insight into their potential functions in development. The expression profiles of MdSDGs were also investigated in fruit development, the breaking of bud dormancy, and responses to abiotic and biotic stress; the results indicated that MdSDGs might play a regulatory role in development and stress responses. The subcellular localization and putative interaction network of MdSDG proteins were also analyzed.

Conclusions

This work presents a fundamental comprehensive analysis of SDG histone methyltransferases in apple and provides a basis for future studies of MdSDGs involved in apple development and stress responses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07596-0.

Systematic analysis of JmjC gene family and stress--response expression of KDM5 subfamily genes in Brassica napus

Background

Jumonji C (JmjC) proteins exert critical roles in plant development and stress response through the removal of lysine methylation from histones. Brassica napus, which originated from spontaneous hybridization by Brassica rapa and Brassica oleracea, is the most important oilseed crop after soybean. In JmjC proteins of Brassica species, the structure and function and its relationship with the parents and model plant Arabidopsis thaliana remain uncharacterized. Systematic identification and analysis for JmjC family in Brassica crops can facilitate the future functional characterization and oilseed crops improvement.

Methods

Basing on the conserved JmjC domain, JmjC homologs from the three Brassica species, B. rapa (AA), B. oleracea (CC) and B. napus, were identified from the Brassica database. Some methods, such as phylogenic analysis, chromosomal mapping, HMMER searching, gene structure display and Logos analysis, were used to characterize relationships of the JmjC homologs. Synonymous and nonsynonymous nucleotide substitutions were used to infer the information of gene duplication among homologs. Then, the expression levels of BnKDM5 subfamily genes were checked under abiotic stress by qRT-PCR.

Results

Sixty-five JmjC genes were identified from B. napus genome, 29 from B. rapa, and 23 from B. oleracea. These genes were grouped into seven clades based on the phylogenetic analysis, and their catalytic activities of demethylation were predicted. The average retention rate of B. napus JmjC genes (B. napus JmjC gene from B. rapa (93.1%) and B. oleracea (82.6%)) exceeded whole genome level. JmjC sequences demonstrated high conservation in domain origination, chromosomal location, intron/exon number and catalytic sites. The gene duplication events were confirmed among the homologs. Many of the BrKDM5 subfamily genes showed higher expression under drought and NaCl treatments, but only a few genes were involved in high temperature stress.

Conclusions

This study provides the first genome-wide characterization of JmjC genes in Brassica species. The BnJmjC exhibits higher conservation during the formation process of allotetraploid than the average retention rates of the whole B. napus genome. Furthermore, expression profiles of many genes indicated that BnKDM5 subfamily genes are involved in stress response to salt, drought and high temperature.

Transcriptional landscapes of floral meristems in barley

Organ development in plants predominantly occurs postembryonically through combinatorial activity of meristems; therefore, meristem and organ fate are intimately connected. Inflorescence morphogenesis in grasses (Poaceae) is complex and relies on a specialized floral meristem, called spikelet meristem, that gives rise to all other floral organs and ultimately the grain. The fate of the spikelet determines reproductive success and contributes toward yield-related traits in cereal crops. Here, we examined the transcriptional landscapes of floral meristems in the temperate crop barley (Hordeum vulgare L.) using RNA-seq of laser capture microdissected tissues from immature, developing floral structures. Our unbiased, high-resolution approach revealed fundamental regulatory networks, previously unknown pathways, and key regulators of barley floral fate and will equally be indispensable for comparative transcriptional studies of grass meristems.

Genome-wide occupancy of Arabidopsis SWI/SNF chromatin remodeler SPLAYED provides insights into its interplay with its close homolog BRAHMA and Polycomb proteins

SPLAYED (SYD) is a SWItch/Sucrose Non-Fermentable (SWI/SNF)-type chromatin remodeler identified in Arabidopsis thaliana (Arabidopsis). It is believed to play both redundant and differential roles with its closest homolog BRAHMA (BRM) in diverse plant growth and development processes. To better understand how SYD functions, we profiled the genome-wide occupancy of SYD and its impact on the global transcriptome and trimethylation of histone H3 on lysine 27 (H3K27me3). To map the global occupancy of SYD, we generated a GFP-tagged transgenic line and used it for chromatin immunoprecipitation experiments followed by next-generation sequencing, by which more than 6000 SYD target genes were identified. Through integrating SYD occupancy and transcriptome profiles, we found that SYD preferentially targets to nucleosome-free regions of expressed genes. Further analysis revealed that SYD occupancy peaks exhibit five distinct patterns, which were also shared by BRM and BAF60, a conserved SWI/SNF complex component, indicating the common target sites of these SWI/SNF chromatin remodelers and the functional relevance of such distinct patterns. To investigate the interplay between SYD and Polycomb-group (PcG) proteins, we performed a genome-wide analysis of H3K27me3 in syd-5. We observed both increases and decreases in H3K27me3 levels at a few hundred genes in syd-5 compared to wild type. Our results imply that SYD can act antagonistically or synergistically with PcG at specific genes. Together, our SYD genome-wide occupancy data and the transcriptome and H3K27me3 profiles provide a much-needed resource for dissecting SYD's crucial roles in the regulation of plant growth and development.

Genome-wide analysis of JMJ-C histone demethylase family involved in salt-tolerance in Gossypium hirsutum L

The jumonji C (JMJ-C) domain-containing protein is a histone demethylase and is involved in plant stress. However, the function of the JMJ-C gene family in cotton is still not confirmed. Herein, 25, 26, 52, and 53 members belonging to the JMJ-C gene family were identified in Gossypium raimondii, Gossypium arboreum, Gossypium hirsutum, and Gossypium barbadense, respectively. Based on phylogenetic relationships and conserved domains, the JMJ-C genes were categorized into five subfamilies, KDM3, KDM4, KDM5, JMJC, and JMJD6. The chromosomal location, gene structure, motif compositions, and cis-elements have been displayed. The collinear investigation showed that whole-genome duplication event is the mainly power to drive JMJ-C gene family expansion. Transcriptome and qRT-PCR analysis revealed that eight GhJMJs were induced by salt and PEG treatment. Further assays confirmed that GhJMJ34/40 greatly improved salt and osmotic tolerance in Saccharomyces cerevisiae. These results help clarify JMJ-C protein functions in preparation for further study.

MtFULc controls inflorescence development by directly repressing MtTFL1 in Medicago truncatula

Flowering plants display a vast diversity of inflorescence architecture, which plays an important role in determining seed yield and fruit production. Unlike the model eudicot Arabidopsis thaliana that has simple inflorescences, most legume plants have compound types of inflorescences. Recent studies in the model legume species Pisum sativum and Medicago truncatula showed that the MADS-box transcription factors VEGETATIVE1/PsFRUITFULc/MtFRUITFULc (VEG1/PsFULc and MtFULc) are essential for the development of compound inflorescences by specifying the secondary inflorescence meristem identity. In this study, we report the isolation and characterization of two new mtfulc alleles by screening the M. truncatula Tnt1 insertion mutant collection. We found that MtFULc specifies M. truncatula secondary inflorescence meristem identity in a dose-dependent manner. Biochemical analysis and chromatin immunoprecipitation (ChIP) assays revealed that MtFULc acts as a transcriptional repressor to directly repress the expression of MtTFL1 through its promoter and 3' intergenic region. Comprehensive genetic analysis suggest MtFULc coordinates with the primary inflorescence meristem maintainer MtTFL1 and floral meristem regulator MtPIM to control M. truncatula inflorescence development. Our findings help to elucidate the mechanism of MtFULc-mediated regulation of secondary inflorescence meristem identity and provide insights into understanding the genetic regulatory network underlying compound inflorescence development in legumes.

Characterization and Stress Response of the JmjC Domain-Containing Histone Demethylase Gene Family in the Allotetraploid Cotton Species Gossypium hirsutum

Histone modification is an important epigenetic modification that controls gene transcriptional regulation in eukaryotes. Histone methylation is accomplished by histone methyltransferase and can occur on two amino acid residues, arginine and lysine. JumonjiC (JmjC) domain-containing histone demethylase regulates gene transcription and chromatin structure by changing the methylation state of the lysine residue site and plays an important role in plant growth and development. In this study, we carried out genome-wide identification and comprehensive analysis of JmjC genes in the allotetraploid cotton species Gossypium hirsutum. In total, 50 JmjC genes were identified and in G. hirsutum, and 25 JmjC genes were identified in its two diploid progenitors, G. arboreum and G. raimondii, respectively. Phylogenetic analysis divided these JmjC genes into five subfamilies. A collinearity analysis of the two subgenomes of G. hirsutum and the genomes of G. arboreum and G. raimondii uncovered a one-to-one relationship between homologous genes of the JmjC gene family. Most homologs in the JmjC gene family between A and D subgenomes of G. hirsutum have similar exon-intron structures, which indicated that JmjC family genes were conserved after the polyploidization. All G. hirsutumJmjC genes were found to have a typical JmjC domain, and some genes also possess other special domains important for their function. Analysis of promoter regions revealed that cis-acting elements, such as those related to hormone and abiotic stress response, were enriched in G. hirsutum JmjC genes. According to a reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis, most G. hirsutumJmjC genes had high abundance expression at developmental stages of fibers, suggesting that they might participate in cotton fiber development. In addition, some G. hirsutumJmjC genes were found to have different degrees of response to cold or osmotic stress, thus indicating their potential role in these types of abiotic stress response. Our results provide useful information for understanding the evolutionary history and biological function of JmjC genes in cotton.

Auxin apical dominance governed by the OsAsp1-OsTIF1 complex determines distinctive rice caryopses development on different branches

In rice (Oryza sativa), caryopses located on proximal secondary branches (CSBs) have smaller grain size and poorer grain filling than those located on apical primary branches (CPBs), greatly limiting grain yield. However, the molecular mechanism responsible for developmental differences between CPBs and CSBs remains elusive. In this transcriptome-wide expression study, we identified the gene Aspartic Protease 1 (OsAsp1), which reaches an earlier and higher transcriptional peak in CPBs than in CSBs after pollination. Disruption of OsAsp1 expression in the heterozygous T-DNA line asp1-1^+/–eliminated developmental differences between CPBs and CSBs. OsAsp1 negatively regulated the transcriptional inhibitor of auxin biosynthesis, OsTAA1 transcriptional inhibition factor 1 (OsTIF1), to preserve indole-3-acetic acid (IAA) apical dominance in CPBs and CSBs. IAA also facilitated OsTIF1 translocation from the endoplasmic reticulum (ER) to the nucleus by releasing the interaction of OsTIF1 with OsAsp1 to regulate caryopses IAA levels via a feedback loop. IAA promoted transcription of OsAsp1 through MADS29 to maintain an OsAsp1 differential between CPBs and CSBs during pollination. Together, these findings provide a mechanistic explanation for the distributed auxin differential between CPBs and CSBs to regulate distinct caryopses development in different rice branches and potential targets for engineering yield improvement in crops.

Rice is a major food crop and an important model plant. Compared with caryopses on apical primary branches (CPBs) of rice, those located on proximal secondary branches (CSBs) display smaller grains and poor grain filling, which greatly limit rice yield potential fulfilment, especially among ‘super’ rice cultivars. In this study, we demonstrated that high indole-3-acetic (IAA) levels upregulated Aspartic Protease 1 (OsAsp1) transcription via MADS29 post-pollination to produce higher OsAsp1 levels in CPBs than in CSBs. OsAsp1 then interacted with OsTAA1 transcriptional inhibition factor 1 (OsTIF1) in the endoplasmic reticulum (ER) to dispel OsTIF1 transcriptional inhibition of OsTAA1, causing IAA content to peak in CPBs at 5 days after fertilisation (DAF). IAA facilitated OsTIF1 translocation from the ER to the nucleus by reducing its interaction with OsAsp1 as feedback regulation of IAA levels in caryopses. Thus, differential auxin levels between CPBs and CSBs are determined by the OsAsp1-OsTIF1 complex, and are essential for the distinct development of CPBs and CSBs, providing potential targets for engineering yield improvement in crops.

A new role for histone demethylases in the maintenance of plant genome integrity

Histone modifications deposited by the Polycomb repressive complex 2 (PRC2) play a critical role in the control of growth, development, and adaptation to environmental fluctuations of most multicellular eukaryotes. The catalytic activity of PRC2 is counteracted by Jumonji-type (JMJ) histone demethylases, which shapes the genomic distribution of H3K27me3. Here, we show that two JMJ histone demethylases in Arabidopsis, EARLY FLOWERING 6 (ELF6) and RELATIVE OF EARLY FLOWERING 6 (REF6), play distinct roles in H3K27me3 and H3K27me1 homeostasis. We show that failure to reset these chromatin marks during sexual reproduction results in the transgenerational inheritance of histone marks, which cause a loss of DNA methylation at heterochromatic loci and transposon activation. Thus, Jumonji-type histone demethylases play a dual role in plants by helping to maintain transcriptional states through development and safeguard genome integrity during sexual reproduction.

A new role for histone demethylases in the maintenance of plant genome integrity

Histone modifications deposited by the Polycomb repressive complex 2 (PRC2) play a critical role in the control of growth, development, and adaptation to environmental fluctuations of most multicellular eukaryotes. The catalytic activity of PRC2 is counteracted by Jumonji-type (JMJ) histone demethylases, which shapes the genomic distribution of H3K27me3. Here, we show that two JMJ histone demethylases in Arabidopsis, EARLY FLOWERING 6 (ELF6) and RELATIVE OF EARLY FLOWERING 6 (REF6), play distinct roles in H3K27me3 and H3K27me1 homeostasis. We show that failure to reset these chromatin marks during sexual reproduction results in the transgenerational inheritance of histone marks, which cause a loss of DNA methylation at heterochromatic loci and transposon activation. Thus, Jumonji-type histone demethylases play a dual role in plants by helping to maintain transcriptional states through development and safeguard genome integrity during sexual reproduction.

Histone Demethylases as Counterbalance to H3K27me3 Silencing in Plants

The universal importance of epigenetic regulation has become explicit over the last decade. There is now a detailed understanding of the molecular signatures and chromatin-modifying enzymes determining epigenetic regulation. For example, the trimethylation of lysine 27 at histone H3 by Polycomb complexes is a hallmark of silenced gene expression conserved across animal and plant kingdoms. The repressive activity of Polycomb complexes is balanced by the histone demethylase activity of Jumonji C-domain proteins. There has been a lot of research on Polycomb functions and H3K27 methylation; however, until recently, little was known about the role of histone H3K27 demethylases. Here, we review the role of Jumonji C-domain proteins from the plant development perspective. We will recall the history of histone lysine demethylation and explore the recent advances on the H3K27 demethylases in plant biology. Conserved and novel genomic functions of these epigenetic regulators will be discussed.

Histone demethylase SlJMJ6 promotes fruit ripening by removing H3K27 methylation of ripening-related genes in tomato

Fruit ripening is governed by a complex regulatory network. Reversible histone methylation and demethylation regulate chromatin structure and gene expression. However, little is known about the involvement of histone demethylases in regulating fruit ripening. Here, we found that the tomato (Solanum lycopersicum) SlJMJ6 encodes a histone lysine demethylase that specifically demethylates H3K27 methylation. Overexpression of SlJMJ6 accelerates tomato fruit ripening, which is associated with the upregulated expression of a large number of ripening-related genes. Integrated analysis of RNA-seq and chromatin immunoprecipitation followed by sequencing identified 32 genes directly targeted by SlJMJ6 and transcriptionally upregulated with decreased H3K27m3 in SlJMJ6-overexpressed fruit. Numerous SlJMJ6-regulated genes are involved in transcription regulation, ethylene biosynthesis, cell wall degradation and hormone signaling. Eleven ripening-related genes including RIPENING INHIBITOR (RIN), 1-aminocyclopropane 1-carboxylate synthase-4 (ACS4), 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1), pectate lyase (PL) and beta-galactosidase 4 (TBG4), and a DNA demethylase DML2, were confirmed to be regulated directly by SlJMJ6 through removing H3K27me3. Our results demonstrate that SlJMJ6 is a ripening-prompting H3K27me3 demethylase that activates the expression of the ripening-related genes by modulating H3K27me3, thereby facilitating tomato fruit ripening. Our work also reveals a novel link between histone demethylation and DNA demethylation in regulating fruit ripening. To our knowledge, this is the first report of the involvement of a histone lysine demethylase in the regulation of fruit ripening.

Removal of H2Aub1 by ubiquitin-specific proteases 12 and 13 is required for stable Polycomb-mediated gene repression in Arabidopsis

BACKGROUND

Stable gene repression is essential for normal growth and development. Polycomb repressive complexes 1 and 2 (PRC1&2) are involved in this process by establishing monoubiquitination of histone 2A (H2Aub1) and subsequent trimethylation of lysine 27 of histone 3 (H3K27me3). Previous work proposed that H2Aub1 removal by the ubiquitin-specific proteases 12 and 13 (UBP12 and UBP13) is part of the repressive PRC1&2 system, but its functional role remains elusive.

RESULTS

We show that UBP12 and UBP13 work together with PRC1, PRC2, and EMF1 to repress genes involved in stimulus response. We find that PRC1-mediated H2Aub1 is associated with gene responsiveness, and its repressive function requires PRC2 recruitment. We further show that the requirement of PRC1 for PRC2 recruitment depends on the initial expression status of genes. Lastly, we demonstrate that removal of H2Aub1 by UBP12/13 prevents loss of H3K27me3, consistent with our finding that the H3K27me3 demethylase REF6 is positively associated with H2Aub1.

CONCLUSIONS

Our data allow us to propose a model in which deposition of H2Aub1 permits genes to switch between repression and activation by H3K27me3 deposition and removal. Removal of H2Aub1 by UBP12/13 is required to achieve stable PRC2-mediated repression.

Transcriptional regulation of strigolactone signalling in Arabidopsis

Plant hormones known as strigolactones control plant development and interactions between host plants and symbiotic fungi or parasitic weeds^1-4. In Arabidopsis thaliana and rice, the proteins DWARF14 (D14), MORE AXILLARY GROWTH 2 (MAX2), SUPPRESSOR OF MAX2-LIKE 6, 7 and 8 (SMXL6, SMXL7 and SMXL8) and their orthologues form a complex upon strigolactone perception and play a central part in strigolactone signalling^5-10. However, whether and how strigolactones activate downstream transcription remains largely unknown. Here we use a synthetic strigolactone to identify 401 strigolactone-responsive genes in Arabidopsis, and show that these plant hormones regulate shoot branching, leaf shape and anthocyanin accumulation mainly through transcriptional activation of the BRANCHED 1, TCP DOMAIN PROTEIN 1 and PRODUCTION OF ANTHOCYANIN PIGMENT 1 genes. We find that SMXL6 targets 729 genes in the Arabidopsis genome and represses the transcription of SMXL6, SMXL7 and SMXL8 by binding directly to their promoters, showing that SMXL6 serves as an autoregulated transcription factor to maintain the homeostasis of strigolactone signalling. These findings reveal an unanticipated mechanism through which a transcriptional repressor of hormone signalling can directly recognize DNA and regulate transcription in higher plants.

Gene expression trajectories during male and female reproductive development in balsam poplar (Populus balsamifera L.)

Plant reproductive development from the first appearance of reproductively committed axes through to floral maturation requires massive and rapid remarshalling of gene expression. In dioecious species such as poplar this is further complicated by divergent male and female developmental programs. We used seven time points in male and female balsam poplar (Populus balsamifera L.) buds and catkins representing the full annual flowering cycle, to elucidate the effects of time and sex on gene expression during reproductive development. Time (developmental stage) is dominant in patterning gene expression with the effect of sex nested within this. Here, we find (1) evidence for five successive waves of alterations to the chromatin landscape which may be important in setting the overall reproductive trajectory, regardless of sex. (2) Each individual developmental stage is further characterized by marked sex-differential gene expression. (3) Consistent sexually differentiated gene expression regardless of developmental stage reveal candidates for high-level regulators of sex and include the female-specific poplar ARR17 homologue. There is also consistent male-biased expression of the MADS-box genes PISTILLATA and APETALA3. Our work provides insights into expression trajectories shaping reproductive development, its potential underlying mechanisms, and sex-specific translation of the genome information into reproductive structures in balsam poplar.

Crystal structures of REF6 and its complex with DNA reveal diverse recognition mechanisms

Relative of Early Flowing 6 (REF6) is a DNA-sequence-specific H3K27me3/2 demethylase that contains four zinc finger (ZnF) domains and targets several thousand genes in Arabidopsis thaliana. The ZnF domains are essential for binding target genes, but the structural basis remains unclear. Here, we determined crystal structures of the ZnF domains and REF6-DNA complex, revealing a unique REF6-family-specific half-cross-braced ZnF (RCZ) domain and two C2H2-type ZnFs. DNA-binding induces a profound conformational change in the hinge region of REF6. Each REF6 recognizes six bases and DNA methylation reduces the binding affinity. Both the acidic region and basic region are important for the self-association of REF6. The REF6 DNA-binding affinity is determined by the sequence-dependent conformations of DNA and also the cooperativity in different target motifs. The conformational plasticity enables REF6 to function as a global transcriptional regulator that directly binds to many diverse genes, revealing the structural basis for the epigenetic modification recognition.

Perspectives of CRISPR/Cas-mediated cis-engineering in horticulture: unlocking the neglected potential for crop improvement

Directed breeding of horticultural crops is essential for increasing yield, nutritional content, and consumer-valued characteristics such as shape and color of the produce. However, limited genetic diversity restricts the amount of crop improvement that can be achieved through conventional breeding approaches. Natural genetic changes in cis-regulatory regions of genes play important roles in shaping phenotypic diversity by altering their expression. Utilization of CRISPR/Cas editing in crop species can accelerate crop improvement through the introduction of genetic variation in a targeted manner. The advent of CRISPR/Cas-mediated cis-regulatory region engineering (cis-engineering) provides a more refined method for modulating gene expression and creating phenotypic diversity to benefit crop improvement. Here, we focus on the current applications of CRISPR/Cas-mediated cis-engineering in horticultural crops. We describe strategies and limitations for its use in crop improvement, including de novo cis-regulatory element (CRE) discovery, precise genome editing, and transgene-free genome editing. In addition, we discuss the challenges and prospects regarding current technologies and achievements. CRISPR/Cas-mediated cis-engineering is a critical tool for generating horticultural crops that are better able to adapt to climate change and providing food for an increasing world population.

Histone tales: lysine methylation, a protagonist in Arabidopsis development

Histone methylation plays a fundamental role in the epigenetic regulation of gene expression driven by developmental and environmental cues in plants, including Arabidopsis. Histone methyltransferases and demethylases act as 'writers' and 'erasers' of methylation at lysine and/or arginine residues of core histones, respectively. A third group of proteins, the 'readers', recognize and interpret the methylation marks. Emerging evidence confirms the crucial roles of histone methylation in multiple biological processes throughout the plant life cycle. In this review, we summarize the regulatory mechanisms of lysine methylation, especially at histone H3 tails, and focus on the recent advances regarding the roles of lysine methylation in Arabidopsis development, from seed performance to reproductive development, and in callus formation.

Unwinding BRAHMA Functions in Plants

The ATP-dependent Switch/Sucrose non-fermenting (SWI/SNF) chromatin remodeling complex (CRC) regulates the transcription of many genes by destabilizing interactions between DNA and histones. In plants, BRAHMA (BRM), one of the two catalytic ATPase subunits of the complex, is the closest homolog of the yeast and animal SWI2/SNF2 ATPases. We summarize here the advances describing the roles of BRM in plant development as well as its recently reported chromatin-independent role in pri-miRNA processing in vitro and in vivo. We also enlighten the roles of plant-specific partners that physically interact with BRM. Three main types of partners can be distinguished: (i) DNA-binding proteins such as transcription factors which mostly cooperate with BRM in developmental processes, (ii) enzymes such as kinases or proteasome-related proteins that use BRM as substrate and are often involved in response to abiotic stress, and (iii) an RNA-binding protein which is involved with BRM in chromatin-independent pri-miRNA processing. This overview contributes to the understanding of the central position occupied by BRM within regulatory networks controlling fundamental biological processes in plants.

CURLY LEAF Regulates MicroRNA Activity by Controlling ARGONAUTE 1 Degradation in Plants

CURLY LEAF (CLF) encodes the methyltransferase subunit of the Polycomb Repressor Complex 2 (PRC2), which regulates the expression of target genes through H3K27 trimethylation. We isolated a new CLF mutant allele (clf-78) using a genetic screen designed to identify microRNA (miRNA) deficient mutants. CLF mutant plants showed impaired miRNA activity caused by increased ubiquitination and enhanced degradation of ARGONAUTE 1 (AGO1) in specific tissues. Such CLF-mediated AGO1 regulation was evident when plants were exposed to UV radiation, which caused increased susceptibility of clf mutants to some UV-induced responses. Furthermore, we showed that CLF directly regulates FBW2, which in turn triggers AGO1 degradation in the clf mutants. Interestingly, AGO1 bound to a target appeared particularly prone to degradation in the mutant plants, a process that was exacerbated when the complex bound a non-cleavable target. Thus, prolonged AGO1-target interaction seems to favor AGO1 degradation, suggesting that non-cleavable miRNA targets may overcome translation inhibition by modulating AGO1 stability in plants.

Reprogramming of Cell Fate During Root Regeneration by Transcriptional and Epigenetic Networks

Many plant species are able to regenerate adventitious roots either directly from aerial organs such as leaves or stems, in particularly after detachment (cutting), or indirectly, from over-proliferating tissue termed callus. In agriculture, this capacity of de novo root formation from cuttings can be used to clonally propagate several important crop plants including cassava, potato, sugar cane, banana and various fruit or timber trees. Direct and indirect de novo root regeneration (DNRR) originates from pluripotent cells of the pericycle tissue, from other root-competent cells or from non-root-competent cells that first dedifferentiate. Independently of their origin, the cells convert into root founder cells, which go through proliferation and differentiation subsequently forming functional root meristems, root primordia and the complete root. Recent studies in the model plants Arabidopsis thaliana and rice have identified several key regulators building in response to the phytohormone auxin transcriptional networks that are involved in both callus formation and DNRR. In both cases, epigenetic regulation seems essential for the dynamic reprogramming of cell fate, which is correlated with local and global changes of the chromatin states that might ensure the correct spatiotemporal expression pattern of the key regulators. Future approaches might investigate in greater detail whether and how the transcriptional key regulators and the writers, erasers, and readers of epigenetic modifications interact to control DNRR.

Molecular characterization and expression analysis reveal the roles of Cys2/His2 zinc-finger transcription factors during flower development of Brassica rapa subsp. chinensis

Conserved motif, gene structure, expression and interaction analysis of C2H2-ZFPs in Brassica rapa, and identified types of genes may play essential roles in flower development, and BrZFP38 was proved to function in flower development by affecting pollen formation. Flower development plays a central role in determining the reproduction of higher plants, and Cys₂/His₂ zinc-finger proteins (C2H2-ZFPs) widely participate in the transcriptional regulation of flower development. C2H2-ZFPs with various structures are the most widespread DNA-binding transcription factors in plants. In this study, conserved protein motif and gene structures were analyzed to investigate systematically the molecular features of Brassica rapa C2H2-ZFP genes. Expression of B. rapa C2H2-ZFPs in multiple tissues showed that more than half of the family members with different types ZFs were expressed in flowers. The specific expression profiles of these C2H2-ZFPs in different B. rapa floral bud stages were further evaluated to identify their potential roles in flower development. Interaction networks were constructed in B. rapa based on the orthology of flower-related C2H2-ZFP genes in Arabidopsis. The putative cis-regulatory elements in the promoter regions of these C2H2-ZFP genes were thoroughly analyzed to elucidate their transcriptional regulation. Results showed that the orthologs of known-function flower-related C2H2-ZFP genes were conserved and differentiated in B. rapa. A C2H2-ZFP was proved to function in B. rapa flower development. Our study provides a systematic investigation of the molecular characteristics and expression profiles of C2H2-ZFPs in B. rapa and promotes further work in function and transcriptional regulation of flower development.

Histone modifications and their regulatory roles in plant development and environmental memory

Plants grow in dynamic environments where they receive diverse environmental signals. Swift and precise control of gene expression is essential for plants to align their development and metabolism with fluctuating surroundings. Modifications on histones serve as "histone code" to specify chromatin and gene activities. Different modifications execute distinct functions on the chromatin, promoting either active transcription or gene silencing. Histone writers, erasers, and readers mediate the regulation of histone modifications by catalyzing, removing, and recognizing modifications, respectively. Growing evidence indicates the important function of histone modifications in plant development and environmental responses. Histone modifications also serve as environmental memory for plants to adapt to environmental changes. Here we review recent progress on the regulation of histone modifications in plants, the impact of histone modifications on environment-controlled developmental transitions including germination and flowering, and the role of histone modifications in environmental memory.

The MYB Activator WHITE PETAL1 Associates with MtTT8 and MtWD40-1 to Regulate Carotenoid-Derived Flower Pigmentation in Medicago truncatula

Carotenoids are a group of natural tetraterpenoid pigments with indispensable roles in the plant life cycle and the human diet. Although the carotenoid biosynthetic pathway has been well characterized, the regulatory mechanisms that control carotenoid metabolism, especially in floral organs, remain poorly understood. In this study, we identified an anthocyanin-related R2R3-MYB protein, WHITE PETAL1 (WP1), that plays a critical role in regulating floral carotenoid pigmentation in Medicago truncatula Carotenoid analyses showed that the yellow petals of the wild-type M. truncatula contained high concentrations of carotenoids that largely consisted of esterified lutein and that disruption of WP1 function via Tnt1 insertion led to substantially reduced lutein accumulation. WP1 mainly functions as a transcriptional activator and directly regulates the expression of carotenoid biosynthetic genes including MtLYCe and MtLYCb through its C-terminal acidic activation motif. Further molecular and genetic analyses revealed that WP1 physically interacts with MtTT8 and MtWD40-1 proteins and that this interaction facilitates WP1's function in the transcriptional activation of both carotenoid and anthocyanin biosynthetic genes. Our findings demonstrate the molecular mechanism of WP1-mediated regulation of floral carotenoid pigmentation and suggest that the conserved MYB-basic-helix-loop-helix-WD40 regulatory module functions in carotenoid biosynthesis in M. truncatula, with specificity imposed by the MYB partner.

Two Loci, RiAF3 and RiAF4, Contribute to the Annual-Fruiting Trait in Rubus

Most Rubus species have a biennial cycle of flowering and fruiting with an intervening period of winter dormancy, in common with many perennial fruit crops. Annual-fruiting (AF) varieties of raspberry (Rubus idaeus and Rubus occidentalis L.) and blackberry (Rubus subgenus Rubus) are able to flower and fruit in one growing season, without the intervening dormant period normally required in biennial-fruiting (BF) varieties. We used a red raspberry (R. idaeus) population segregating for AF obtained from a cross between NC493 and 'Chilliwack' to identify genetic factors controlling AF. Genotyping by sequencing (GBS) was used to generate saturated linkage maps in both parents. Trait mapping in this population indicated that AF is controlled by two newly identified loci (RiAF3 and RiAF4) located on Rubus linkage groups (LGs) 3 and 4. The location of these loci was analyzed using single-nucleotide polymorphism (SNP) markers on independent red raspberry and blackberry populations segregating for the AF trait. This confirmed that AF in Rubus is regulated by loci on LG 3 and 4, in addition to a previously reported locus on LG 7. Comparative RNAseq analysis at the time of floral bud differentiation in an AF and a BF variety revealed candidate genes potentially regulating the trait.

Epigenetic silencing of a multifunctional plant stress regulator

The central regulator of the ethylene (ET) signaling pathway, which controls a plethora of developmental programs and responses to environmental cues in plants, is ETHYLENE-INSENSITIVE2 (EIN2). Here we identify a chromatin-dependent regulatory mechanism at EIN2 requiring two genes: ETHYLENE-INSENSITIVE6 (EIN6), which is a H3K27me3 demethylase also known as RELATIVE OF EARLY FLOWERING6 (REF6), and EIN6 ENHANCER (EEN), the Arabidopsis homolog of the yeast INO80 chromatin remodeling complex subunit IES6 (INO EIGHTY SUBUNIT). Strikingly, EIN6 (REF6) and the INO80 complex redundantly control the level and the localization of the repressive histone modification H3K27me3 and the histone variant H2A.Z at the 5' untranslated region (5'UTR) intron of EIN2. Concomitant loss of EIN6 (REF6) and the INO80 complex shifts the chromatin landscape at EIN2 to a repressive state causing a dramatic reduction of EIN2 expression. These results uncover a unique type of chromatin regulation which safeguards the expression of an essential multifunctional plant stress regulator.

Arabidopsis Cys2/His2 zinc-finger protein MAZ1 is essential for intine formation and exine pattern

Cys₂/His₂ zinc-finger protein (C2H2-ZFP) is widely involved in the reproductive development of plants, but its role in pollen development is still elusive. Here, we identified a pollen-related C2H2-ZFP gene named as MALE FERTILITY-ASSOCIATED ZINC FINGER PROTEIN 1 (MAZ1), which was first isolated from Arabidopsis thaliana. MAZ1 showed a preferential expression pattern in early anther development. Its mutation resulted in aberrant primexine deposition at the tetrad stage, followed by a defective multiple-layer pattern of exine with irregular baculum and no tectum. Furthermore, microspore development was arrested, and no intine layer was formed. These developmental defects led to fertility reduction and pollen abortion. This study reveals the essential role of MAZ1 in pollen wall development.

JASPAR 2018: update of the open-access database of transcription factor binding profiles and its web framework

JASPAR (http://jaspar.genereg.net) is an open-access database of curated, non-redundant transcription factor (TF)-binding profiles stored as position frequency matrices (PFMs) and TF flexible models (TFFMs) for TFs across multiple species in six taxonomic groups. In the 2018 release of JASPAR, the CORE collection has been expanded with 322 new PFMs (60 for vertebrates and 262 for plants) and 33 PFMs were updated (24 for vertebrates, 8 for plants and 1 for insects). These new profiles represent a 30% expansion compared to the 2016 release. In addition, we have introduced 316 TFFMs (95 for vertebrates, 218 for plants and 3 for insects). This release incorporates clusters of similar PFMs in each taxon and each TF class per taxon. The JASPAR 2018 CORE vertebrate collection of PFMs was used to predict TF-binding sites in the human genome. The predictions are made available to the scientific community through a UCSC Genome Browser track data hub. Finally, this update comes with a new web framework with an interactive and responsive user-interface, along with new features. All the underlying data can be retrieved programmatically using a RESTful API and through the JASPAR 2018 R/Bioconductor package.

Arabidopsis histone H3K4 demethylase JMJ17 functions in dehydration stress response

Under dehydration in plants, antagonistic activities of histone 3 lysine 4 (H3K4) methyltransferase and histone demethylase maintain a dynamic and homeostatic state of gene expression by orientating transcriptional reprogramming toward growth or stress tolerance. However, the histone demethylase that specifically controls histone methylation homeostasis under dehydration stress remains unknown. Here, we document that a histone demethylase, JMJ17, belonging to the KDM5/JARID1 family, plays crucial roles in response to dehydration stress and abscisic acid (ABA) in Arabidopsis thaliana. jmj17 loss-of-function mutants displayed dehydration stress tolerance and ABA hypersensitivity in terms of stomatal closure. JMJ17 specifically demethylated H3K4me1/2/3 via conserved iron-binding amino acids in vitro and in vivo. Moreover, H3K4 demethylase activity of JMJ17 was required for dehydration stress response. Systematic combination of genome-wide chromatin immunoprecipitation coupled with massively parallel DNA sequencing (ChIP-seq) and RNA-sequencing (RNA-seq) analyses revealed that a loss-of-function mutation in JMJ17 caused an ectopic increase in genome-wide H3K4me3 levels and activated a plethora of dehydration stress-responsive genes. Importantly, JMJ17 bound directly to the chromatin of OPEN STOMATA 1 (OST1) and demethylated H3K4me3 for the regulation of OST1 mRNA abundance, thereby modulating the dehydration stress response. Our results demonstrate a new function of a histone demethylase under dehydration stress in plants.

WOX11 recruits a histone H3K27me3 demethylase to promote gene expression during shoot development in rice

WUSCHEL-related homeobox (WOX) genes are key regulators of meristem activity and plant development, the chromatin mechanism of which to reprogram gene expression remains unclear. Histone H3K27me3 is a chromatin mark of developmentally repressed genes. How the repressive mark is removed from specific genes during plant development is largely unknown. Here, we show that WOX11 interacts with the H3K27me3 demethylase JMJ705 to activate gene expression during shoot development in rice. Genetic analysis indicates that WOX11 and JMJ705 cooperatively control shoot growth and commonly regulate the expression of a set of genes involved in meristem identity, chloroplast biogenesis, and energy metabolism in the shoot apex. Loss of WOX11 led to increased H3K27me3 and overexpression of JMJ705 decreased the methylation levels at a subset of common targets. JMJ705 is associated with most of the WOX11-binding sites found in the tested common targets in vivo, regardless of presence or absence of the JMJ705-binding motif. Furthermore, wox11 mutation reduced JMJ705-binding to many targets genome-wide. The results suggest that recruitment of JMJ705 to specific developmental pathway genes is promoted by DNA-binding transcription factors and that WOX11 functions to stimulate shoot growth through epigenetic reprogramming of genes involved in meristem development and energy-generating pathways.

The transcriptome of potato tuber phellogen reveals cellular functions of cork cambium and genes involved in periderm formation and maturation

The periderm is a protective corky tissue that is formed through the cambial activity of phellogen cells, when the outer epidermis is damaged. Timely periderm formation is critical to prevent pathogen invasion and water loss. The outer layers of the potato periderm, the tuber skin, serves as a model to study cork development. Early in tuber development the phellogen becomes active and produces the skin. During tuber maturation it becomes inactive and the skin adheres to the tuber flesh. The characterization of potato phellogen may contribute to the management of costly agricultural problems related to incomplete skin-set and the resulting skinning injuries, and provide us with new knowledge regarding cork development in planta. A transcriptome of potato tuber phellogen isolated by laser capture microdissection indicated similarity to vascular cambium and the cork from trees. Highly expressed genes and transcription factors indicated that phellogen activation involves cytokinesis and gene reprograming for the establishment of a dedifferentiation state; whereas inactivation is characterized by activity of genes that direct organ identity in meristem and cell-wall modifications. The expression of selected genes was analyzed using qPCR in native and wound periderm at distinct developmental stages. This allowed the identification of genes involved in periderm formation and maturation.

DNA methylation repels targeting of Arabidopsis REF6

RELATIVE OF EARLY FLOWERING 6 (REF6/JMJ12), a Jumonji C (JmjC)-domain-containing H3K27me3 histone demethylase, finds its target loci in Arabidopsis genome by directly recognizing the CTCTGYTY motif via its zinc-finger (ZnF) domains. REF6 tends to bind motifs located in active chromatin states that are depleted for heterochromatic modifications. However, the underlying mechanism remains unknown. Here, we show that REF6 preferentially bind to hypo-methylated CTCTGYTY motifs in vivo, and that CHG methylation decreases REF6 DNA binding affinity in vitro. In addition, crystal structures of ZnF-clusters in complex with DNA oligonucleotides reveal that 5-methylcytosine is unfavorable for REF6 binding. In drm1 drm2 cmt2 cmt3 (ddcc) quadruple mutants, in which non-CG methylation is significantly reduced, REF6 can ectopically bind a small number of new target loci, most of which are located in or neighbored with short TEs in euchromatic regions. Collectively, our findings reveal that DNA methylation, likely acting in combination with other epigenetic modifications, may partially explain why REF6 binding is depleted in heterochromatic loci.

REF6 is a H3K27me3 histone demethylase that targets regions of active chromatin in the Arabidopsis genome. Here Qiu et al. show that REF6 preferentially binds unmethylated DNA via its zinc-finger domains suggesting that DNA methylation might deter REF6 binding in heterochromatin.

The H3K27me3 demethylase REF6 promotes leaf senescence through directly activating major senescence regulatory and functional genes in Arabidopsis

The roles of histone demethylation in the regulation of plant flowering, disease resistance, rhythmical response, and seed germination have been elucidated recently; however, how histone demethylation affects leaf senescence remains largely unclear. In this study, we exploited yeast one-hybrid (Y1H) to screen for the upstream regulators of NONYELLOWING1 (NYE1), and identified RELATIVE OF EARLY FLOWERING6 (REF6), a histone H3 lysine 27 tri-methylation (H3K27me3) demethylase, as a putative binding protein of NYE1 promoter. By in vivo and in vitro analyses, we demonstrated that REF6 directly binds to the motif CTCGYTY in NYE1/2 promoters through its zinc finger domain and positively regulates their expression. Loss-of-function of REF6 delayed chlorophyll (Chl) degradation, whereas overexpression of REF6 accelerated Chl degradation. Subsequently, we revealed that REF6 positively regulates the general senescence process by directly up-regulating ETHYLENE INSENSITIVE 2 (EIN2), ORESARA1 (ORE1), NAC-LIKE, ACTIVATED BY AP3/PI (NAP), PYRUVATE ORTHOPHOSPHATE DIKINASE (PPDK), PHYTOALEXIN DEFICIENT 4 (PAD4), LIPOXYGENASE 1 (LOX1), NAC DOMAIN CONTAINING PROTEIN 3 (AtNAC3), and NAC TRANSCRIPTION FACTOR-LIKE 9 (NTL9), the key regulatory and functional genes predominantly involved in the regulation of developmental leaf senescence. Importantly, loss-of-function of REF6 increased H3K27me3 levels at all the target Senescence associated genes (SAGs). We therefore conclusively demonstrate that H3K27me3 methylation represents an epigenetic mechanism prohibiting the premature transcriptional activation of key developmentally up-regulated senescence regulatory as well as functional genes in Arabidopsis.

Leaves of higher plants start yellowing and subsequently die (senescence) at particular developmental stages as a result of both internal and external regulations. Leaf senescence is evolved to facilitate nutrient remobilization to young/important organs to meet their rapid development, and a large number of genes (Senescence associated genes, SAGs) are activated to regulate/facilitate the process. It has been intriguing how these genes are kept transcriptionally inactive to ensure an effective photosynthesis before the initiation of leaf senescence. Here, we reveal an epigenetic mechanism responsible for the prohibition of their premature transcription. We found that an H3K27me3 demethylase, RELATIVE OF EARLY FLOWERING 6 (REF6), directly promotes the expression of its ten target senescence regulatory and functional genes (EIN2, ORE1, NAP, AtNAC3, NTL9, NYE1/2, LOX1, PAD4, and PPDK), which are involved in major phytohormones’ signaling, biosynthesis, and chlorophyll degradation. Crucially, REF6 is substantially involved in promoting the H3K27me3 demethylation of both their promoter and/or coding regions during the aging process of leaves. We therefore provide conclusive evidence that H3K27me3 methylation is an epigenetic mechanism hindering the premature transcriptional activation of key SAGs, which helps to explain the “aging effect” of senescence initiation.

Floral regulators FLC and SOC1 directly regulate expression of the B3-type transcription factor TARGET OF FLC AND SVP 1 at the Arabidopsis shoot apex via antagonistic chromatin modifications

Integration of environmental and endogenous cues at plant shoot meristems determines the timing of flowering and reproductive development. The MADS box transcription factor FLOWERING LOCUS C (FLC) of Arabidopsis thaliana is an important repressor of floral transition, which blocks flowering until plants are exposed to winter cold. However, the target genes of FLC have not been thoroughly described, and our understanding of the mechanisms by which FLC represses transcription of these targets and how this repression is overcome during floral transition is still fragmentary. Here, we identify and characterize TARGET OF FLC AND SVP1 (TFS1), a novel target gene of FLC and its interacting protein SHORT VEGETATIVE PHASE (SVP). TFS1 encodes a B3-type transcription factor, and we show that tfs1 mutants are later flowering than wild-type, particularly under short days. FLC and SVP repress TFS1 transcription leading to deposition of trimethylation of Iysine 27 of histone 3 (H3K27me3) by the Polycomb Repressive Complex 2 at the TFS1 locus. During floral transition, after downregulation of FLC by cold, TFS1 transcription is promoted by SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1), a MADS box protein encoded by another target of FLC/SVP. SOC1 opposes PRC function at TFS1 through recruitment of the histone demethylase RELATIVE OF EARLY FLOWERING 6 (REF6) and the SWI/SNF chromatin remodeler ATPase BRAHMA (BRM). This recruitment of BRM is also strictly required for SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9 (SPL9) binding at TFS1 to coordinate RNAPII recruitment through the Mediator complex. Thus, we show that antagonistic chromatin modifications mediated by different MADS box transcription factor complexes play a crucial role in defining the temporal and spatial patterns of transcription of genes within a network of interactions downstream of FLC/SVP during floral transition.

The initiation of flowering in plants is exquisitely sensitive to environmental signals, ensuring that reproduction occurs at the appropriate time of year. The sensitivity of these responses depends upon strong repression of flowering under inappropriate conditions. FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE (SVP) are related transcription factors that act in concert to strongly inhibit flowering in crucifer plants through repressing transcription of their target genes. Many direct FLC/ SVP targets have been identified in genome-wide studies, however few of these genes have been characterized for their roles in regulating flowering time or other aspects of reproductive development. Here, we characterize TARGET OF FLC AND SVP1 (TFS1) as a novel target of FLC and SVP, and demonstrate that TFS1 contributes to proper flowering-time control. Moreover, we provide a detailed mechanistic view of how TFS1 transcription is controlled during reproductive development through the repressive activity of FLC/SVP being overcome by the transcriptional activator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1. Thus we further elucidate the network of genes repressed by FLC/SVP to block flowering and determine mechanisms by which their repressive activity is overcome during the initiation of flowering.

REF6 promotes lateral root formation through de-repression of PIN1/3/7 genes

The H3K27 methyltransferase CLF inhibits lateral root (LR) formation through depositing the repressive H3K27me3 mark to the chromatin of PIN1, a key polar auxin transporter gene. Here, we show that the H3K27me3 demethylase REF6 promotes lateral root primordium initiation and LR emergence. REF6 directly binds to the chromatin of PIN1/3/7. Dysfunction in REF6 results in increased levels of H3K27me3 on PIN1/3/7 and suppressed expression of PIN genes. Genetic analysis of the clf ref6 double mutant revealed an antagonistic action between CLF and REF6, in terms of LR formation. Our findings indicate that H3K27 methylation and demethylation activities are likely coordinated to ensure proper LR organogenesis.

When to stop: an update on molecular mechanisms of floral meristem termination

Flowers have fascinated humans for millennia, not only because of their beauty, but also because they give rise to fruits, from which most agricultural products are derived. In most angiosperms, the number and position of floral organs are morphologically and genetically defined, and their development is tightly controlled by complex regulatory networks to ensure reproductive success. How flower development is temporally initiated and spatially maintained has been widely researched. As the flower develops, the balance between proliferation and differentiation dynamically shifts towards organogenesis and termination of floral stem cell maintenance. In this review, we focus on recent findings that further reveal the intricate molecular mechanisms for precise timing of floral meristem termination.

The Arabidopsis H3K27me3 demethylase JUMONJI 13 is a temperature and photoperiod dependent flowering repressor

In plants, flowering time is controlled by environmental signals such as day-length and temperature, which regulate the floral pathway integrators, including FLOWERING LOCUS T (FT), by genetic and epigenetic mechanisms. Here, we identify an H3K27me3 demethylase, JUMONJI 13 (JMJ13), which regulates flowering time in Arabidopsis. Structural characterization of the JMJ13 catalytic domain in complex with its substrate peptide reveals that H3K27me3 is specifically recognized through hydrogen bonding and hydrophobic interactions. Under short-day conditions, the jmj13 mutant flowers early and has increased FT expression at high temperatures, but not at low temperatures. In contrast, jmj13 flowers early in long-day conditions regardless of temperature. Long-day condition and higher temperature induce the expression of JMJ13 and increase accumulation of JMJ13. Together, our data suggest that the H3K27me3 demethylase JMJ13 acts as a temperature- and photoperiod-dependent flowering repressor.

An H3K27me3 demethylase-HSFA2 regulatory loop orchestrates transgenerational thermomemory in Arabidopsis

Global warming has profound effects on plant growth and fitness. Plants have evolved sophisticated epigenetic machinery to respond quickly to heat, and exhibit transgenerational memory of the heat-induced release of post-transcriptional gene silencing (PTGS). However, how thermomemory is transmitted to progeny and the physiological relevance are elusive. Here we show that heat-induced HEAT SHOCK TRANSCRIPTION FACTOR A2 (HSFA2) directly activates the H3K27me3 demethylase RELATIVE OF EARLY FLOWERING 6 (REF6), which in turn derepresses HSFA2. REF6 and HSFA2 establish a heritable feedback loop, and activate an E3 ubiquitin ligase, SUPPRESSOR OF GENE SILENCING 3 (SGS3)-INTERACTING PROTEIN 1 (SGIP1). SGIP1-mediated SGS3 degradation leads to inhibited biosynthesis of trans-acting siRNA (tasiRNA). The REF6-HSFA2 loop and reduced tasiRNA converge to release HEAT-INDUCED TAS1 TARGET 5 (HTT5), which drives early flowering but attenuates immunity. Thus, heat induces transmitted phenotypes via a coordinated epigenetic network involving histone demethylases, transcription factors, and tasiRNAs, ensuring reproductive success and transgenerational stress adaptation.

Genome-wide occupancy of histone H3K27 methyltransferases CURLY LEAF and SWINGER in Arabidopsis seedlings

The Polycomb Group (PcG) proteins form two protein complexes, PcG Repressive Complex 1 (PRC1) and PRC2, which are key epigenetic regulators in eukaryotes. PRC2 represses gene expression by catalyzing the trimethylation of histone H3 lysine 27 (H3K27me3). In Arabidopsis (Arabidopsis thaliana), CURLY LEAF (CLF) and SWINGER (SWN) are two major H3K27 methyltransferases and core components of PRC2, playing essential roles in plant growth and development. Despite their importance, genome-wide binding profiles of CLF and SWN have not been determined and compared yet. In this study, we generated transgenic lines expressing GFP-tagged CLF/SWN under their respective native promoters and used them for ChIP-seq analyses to profile the genome-wide distributions of CLF and SWN in Arabidopsis seedlings. We also profiled and compared the global H3K27me3 levels in wild-type (WT) and PcG mutants (clf, swn, and clf swn). Our data show that CLF and SWN bind to almost the same set of genes, except that SWN has a few hundred more targets. Two short DNA sequences, the GAGA-like and Telo-box-like motifs, were found enriched in the CLF and SWN binding regions. The H3K27me3 levels in clf, but not in swn, were markedly reduced compared with WT; and the mark was undetectable in the clf swn double mutant. Further, we profiled the transcriptomes in clf, swn, and clf swn, and compared that with WT. Thus this work provides a useful resource for the plant epigenetics community for dissecting the functions of PRC2 in plant growth and development.

Dynamic Changes in Genome-Wide Histone3 Lysine27 Trimethylation and Gene Expression of Soybean Roots in Response to Salt Stress

Soybean is an important economic crop for human diet, animal feeds and biodiesel due to high protein and oil content. Its productivity is significantly hampered by salt stress, which impairs plant growth and development by affecting gene expression, in part, through epigenetic modification of chromatin status. However, little is known about epigenetic regulation of stress response in soybean roots. Here, we used RNA-seq and ChIP-seq technologies to study the dynamics of genome-wide transcription and histone methylation patterns in soybean roots under salt stress. Eight thousand seven hundred ninety eight soybean genes changed their expression under salt stress treatment. Whole-genome ChIP-seq study of an epigenetic repressive mark, histone H3 lysine 27 trimethylation (H3K27me3), revealed the changes in H3K27me3 deposition during the response to salt stress. Unexpectedly, we found that most of the inactivation of genes under salt stress is strongly correlated with the de novo establishment of H3K27me3 in various parts of the promoter or coding regions where there is no H3K27me3 in control plants. In addition, the soybean histone modifiers were identified which may contribute to de novo histone methylation and gene silencing under salt stress. Thus, dynamic chromatin regulation, switch between active and inactive modes, occur at target loci in order to respond to salt stress in soybean. Our analysis demonstrates histone methylation modifications are correlated with the activation or inactivation of salt-inducible genes in soybean roots.