It is generally considered that Polycomb Repressive Complex (PRC)2 deposits the histone mark H3K27me3 on silent protein-coding genes, while transposable elements are repressed by DNA and/or H3K9 ...methylation. Yet, there is increasing evidence that PRC2 also targets and even silences transposable elements in representatives of several distantly related eukaryotic lineages. In plants and animals, H3K27me3 is present on transposable elements in mutants and specific cell types devoid of DNA methylation. In this Opinion, we summarize the experimental evidence for this phenomenon across the eukaryotic kingdom, and discuss its functional and evolutionary significance. We hypothesize that an ancestral role of Polycomb group (PcG) proteins was to silence transposable elements.
It has been generally accepted that H3K9me2-3/DNA methylation and H3K27me3 silent heterochromatin marks are found in mutually exclusive genomic regions and display distinct properties.This view is challenged by recent observations of transposable elements marked by H3K27me3.We hypothesize an ancestral association of H3K27me3 with silencing of transposable elements. As multicellular eukaryotes evolved, H3K27me3 became associated with gene silencing, while H3K9/DNA methylation became dedicated to silencing transposable elements.
The mechanisms underlying induction and suppression of RNA silencing in the ongoing plant-virus arms race are poorly understood. We show here that virus-derived small RNAs produced by Arabidopsis ...Dicer-like 4 (DCL4) program an effector complex conferring antiviral immunity. Inhibition of DCL4 by a viral-encoded suppressor revealed the subordinate antiviral activity of DCL2. Accordingly, inactivating both DCL2 and DCL4 was necessary and sufficient to restore systemic infection of a suppressor-deficient virus. The effects of DCL2 were overcome by increasing viral dosage in inoculated leaves, but this could not surmount additional, non-cell autonomous effects of DCL4 specifically preventing viral unloading from the vasculature. These findings define a molecular framework for studying antiviral silencing and defense in plants.
Mobilization of transposable elements (TEs) in plants has been recognized as a driving force of evolution and adaptation, in particular by providing genes with regulatory modules that impact their ...transcription. In this study, we employed an ATCOPIA93 long‐terminal repeat (LTR) promoter‐GUS fusion to show that this retrotransposon behaves like an immune‐responsive gene during pathogen defense in Arabidopsis. We also showed that the endogenous ATCOPIA93 copy “EVD”, which is activated in the presence of bacterial stress, is negatively regulated by both DNA methylation and polycomb‐mediated silencing, a mode of repression typically found at protein‐coding and microRNA genes. Interestingly, an ATCOPIA93‐derived soloLTR is located upstream of the disease resistance gene RPP4 and is devoid of DNA methylation and H3K27m3 marks. Through loss‐of‐function experiments, we demonstrate that this soloLTR is required for the proper expression of RPP4 during plant defense, thus linking the responsiveness of ATCOPIA93 to biotic stress and the co‐option of its LTR for plant immunity.
Synopsis
The immune‐responsive LTR retrotransposon EVD is dually repressed by DNA methylation and polycomb‐group proteins. This restricts expression and transposition potential for EVD while an unmethylated form of EVD has been exploited to activate resistance gene RPP4 during plant pathogen defense.
An ATCOPIA93 LTR::GUS fusion that lacks promoter DNA methylation behaves like an immune‐responsive gene.
The corresponding endogenous transposable element EVD is dually controlled by DNA methylation and polycomb group proteins during the plant immune response.
A variant ATCOPIA93 soloLTR has a naturally unmethylated promoter and is required to properly induce expression of the RPP4 disease resistance gene during plant defense.
The controlled responsiveness of a retrotransposon to biotic stress is linked to the co‐option of its LTR for plant immunity.
While expression of LTR‐retrotransposon EVD is repressed by DNA methylation and polycomb‐group protein, an unmethylated form of EVD activates resistance gene RPP4 during plant pathogen defense.
Dimethylation of histone H3 lysine 9 (H3K9m2) and trimethylation of histone H3 lysine 27 (H3K27m3) are two hallmarks of transcriptional repression in many organisms. In Arabidopsis thaliana, H3K27m3 ...is targeted by Polycomb Group (PcG) proteins and is associated with silent protein-coding genes, while H3K9m2 is correlated with DNA methylation and is associated with transposons and repetitive sequences. Recently, ectopic genic DNA methylation in the CHG context (where H is any base except G) has been observed in globally DNA hypomethylated mutants such as met1, but neither the nature of the hypermethylated loci nor the biological significance of this epigenetic phenomenon have been investigated. Here, we generated high-resolution, genome-wide maps of both H3K9m2 and H3K27m3 in wild-type and met1 plants, which we integrated with transcriptional data, to explore the relationships between these two marks. We found that ectopic H3K9m2 observed in met1 can be due to defects in IBM1-mediated H3K9m2 demethylation at some sites, but most importantly targets H3K27m3-marked genes, suggesting an interplay between these two silencing marks. Furthermore, H3K9m2/DNA-hypermethylation at these PcG targets in met1 is coupled with a decrease in H3K27m3 marks, whereas CG/H3K9m2 hypomethylated transposons become ectopically H3K27m3 hypermethylated. Our results bear interesting similarities with cancer cells, which show global losses of DNA methylation but ectopic hypermethylation of genes previously marked by H3K27m3.
In plants and mammals, DNA methylation and histone H3 lysine 27 trimethylation (H3K27me3), which is deposited by the polycomb repressive complex 2, are considered as two specialized systems for the ...epigenetic silencing of transposable element (TE) and genes, respectively. Nevertheless, many TE sequences acquire H3K27me3 when DNA methylation is lost. Here, we show in
that the gain of H3K27me3 observed at hundreds of TEs in the
mutant defective in the maintenance of DNA methylation, essentially depends on CURLY LEAF (CLF), one of two partially redundant H3K27 methyltransferases active in vegetative tissues. Surprisingly, the complete loss of H3K27me3 in
double mutant plants was not associated with further reactivation of TE expression nor with a burst of transposition. Instead,
plants exhibited less activated TEs, and a chromatin recompaction as well as hypermethylation of linker DNA compared with
Thus, a mutation in polycomb repressive complex 2 does not aggravate the molecular phenotypes linked to
but instead partially suppresses them, challenging our assumptions of the relationship between two conserved epigenetic silencing pathways.
DNA methylation is an epigenetic mark that is associated with transcriptional repression of transposable elements and protein-coding genes. Conversely, transcriptionally active regulatory regions are ...strongly correlated with histone 3 lysine 4 di- and trimethylation (H3K4m2/m3). We previously showed that Arabidopsis thaliana plants with mutations in the H3K4m2/m3 demethylase JUMONJI 14 (JMJ14) exhibit a mild reduction in RNA-directed DNA methylation (RdDM) that is associated with an increase in H3K4m2/m3 levels. To determine whether this incomplete RdDM reduction was the result of redundancy with other demethylases, we examined the genetic interaction of JMJ14 with another class of H3K4 demethylases: lysine-specific demethylase 1-like 1 and lysine-specific demethylase 1-like 2 (LDL1 and LDL2). Genome-wide DNA methylation analyses reveal that both families cooperate to maintain RdDM patterns. ChIP-seq experiments show that regions that exhibit an observable DNA methylation decrease are co-incidental with increases in H3K4m2/m3. Interestingly, the impact on DNA methylation was stronger at DNA-methylated regions adjacent to H3K4m2/m3-marked protein-coding genes, suggesting that the activity of H3K4 demethylases may be particularly crucial to prevent spreading of active epigenetic marks. Finally, RNA sequencing analyses indicate that at RdDM targets, the increase of H3K4m2/m3 is not generally associated with transcriptional de-repression. This suggests that the histone mark itself--not transcription--impacts the extent of RdDM.
Eukaryotic DNA cytosine methylation can be used to transcriptionally silence repetitive sequences, including transposons and retroviruses. This silencing is stable between cell generations as ...cytosine methylation is maintained epigenetically through DNA replication. The Arabidopsis thaliana Dnmt3 cytosine methyltransferase ortholog DOMAINS rearranged methyltransferase2 (DRM2) is required for establishment of small interfering RNA (siRNA) directed DNA methylation. In mammals PIWI proteins and piRNA act in a convergently evolved RNA-directed DNA methylation system that is required to repress transposon expression in the germ line. De novo methylation may also be independent of RNA interference and small RNAs, as in Neurospora crassa. Here we identify a clade of catalytically mutated DRM2 paralogs in flowering plant genomes, which in A.thaliana we term domains rearranged methyltransferase3 (DRM3). Despite being catalytically mutated, DRM3 is required for normal maintenance of non-CG DNA methylation, establishment of RNA-directed DNA methylation triggered by repeat sequences and accumulation of repeat-associated small RNAs. Although the mammalian catalytically inactive Dnmt3L paralogs act in an analogous manner, phylogenetic analysis indicates that the DRM and Dnmt3 protein families diverged independently in plants and animals. We also show by site-directed mutagenesis that both the DRM2 N-terminal UBA domains and C-terminal methyltransferase domain are required for normal RNA-directed DNA methylation, supporting an essential targeting function for the UBA domains. These results suggest that plant and mammalian RNA-directed DNA methylation systems consist of a combination of ancestral and convergent features.
Histone demethylases—both lysine‐specific demethylase 1 (LSD1) and Jumonji‐C (JmjC) domain‐containing proteins—are broadly implicated in the regulation of chromatin‐dependent processes. In ...Arabidopsis thaliana, histone marks directly affect DNA methylation, and mutations in LSD1 homologues show reduced DNA methylation at some loci. We screened transfer DNA mutations in genes encoding JmjC domains for defects in DNA methylation. Mutations in jmj14 result in reduced DNA methylation in non‐CG contexts at targets of DRM2 (domains rearranged methyltransferase 2)‐mediated RNA‐directed DNA methylation (RdDM), which is associated with an increase in H3K4m3. Unlike other components of RdDM, JMJ14 is not required for de novo methylation of a transgene, suggesting that JMJ14 is specifically involved in the maintenance phase of DRM2‐mediated RdDM.
This work identifies an H3K4m3 demethylase from the Jumonji family (JMJ14) that is required for DRM2‐mediated DNA methylation in Arabidopsis thaliana, showing that H3K4m3 marks have a negative impact on the siRNA‐driven DRM2 pathway.
DNA methylation is an epigenetic mark that is associated with transcriptional repression of transposable elements and protein-coding genes. Conversely, transcriptionally active regulatory regions are ...strongly correlated with histone 3 lysine 4 di- and trimethylation (H3K4m2/m3). We previously showed that Arabidopsis thaliana plants with mutations in the H3K4m2/m3 demethylase JUMONJI 14 (JMJ14) exhibit a mild reduction in RNA-directed DNA methylation (RdDM) that is associated with an increase in H3K4m2/m3 levels. To determine whether this incomplete RdDM reduction was the result of redundancy with other demethylases, we examined the genetic interaction of JMJ14 with another class of H3K4 demethylases: LYSINE-SPECIFIC DEMETHYLASE 1-LIKE 1 and LYSINE-SPECIFIC DEMETHYLASE 1-LIKE 2 (LDL1 and LDL2). Genome-wide DNA methylation analyses reveal that both families cooperate to maintain RdDM patterns. ChIP-seq experiments show that regions that exhibit an observable DNA methylation decrease are co-incidental with increases in H3K4m2/m3. Interestingly, the impact on DNA methylation was stronger at DNA-methylated regions adjacent to H3K4m2/m3-marked protein-coding genes, suggesting that the activity of H3K4 demethylases may be particularly crucial to prevent spreading of active epigenetic marks. Finally, RNA sequencing analyses indicate that at RdDM targets, the increase of H3K4m2/m3 is not generally associated with transcriptional de-repression. This suggests that the histone mark itself--not transcription--impacts the extent of RdDM.
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