Mutations in the Arabidopsis H3K4me2/3 demethylase JMJ14 suppress posttranscriptional gene silencing by decreasing transgene transcription

ALTERED MERISTEM PROGRAM1 impairs RNA silencing by repressing the biogenesis of a subset of inverted repeat-derived siRNAs

RNA silencing negatively regulates gene expression at the transcriptional and posttranscriptional levels through DNA methylation, histone modification, mRNA cleavage, and translational inhibition. Small interfering RNAs (siRNAs) of 21 to 24 nucleotides are processed from double-stranded RNAs by Dicer-like (DCL) enzymes and play essential roles in RNA silencing in plants. Here, we demonstrated that ALTERED MERISTEM PROGRAM1 (AMP1) and its putative paralog LIKE AMP1 (LAMP1) impair RNA silencing by repressing the biogenesis of a subset of inverted repeat (IR)-derived siRNAs in Arabidopsis (Arabidopsis thaliana). AMP1 and LAMP1 inhibit Pol II-dependent IR gene transcription by suppressing ARGONAUTE 1 (AGO1) protein levels. Genetic analysis indicates that AMP1 acts upstream of RNA polymerase IV subunit 1 (NRPD1), RNA-dependent RNA polymerase 2 (RDR2), and DCL4, which are required for IR-induced RNA silencing. We also show that AMP1 and LAMP1 inhibit siRNA-mediated silencing in a different mechanism from that of AGO4 and DCL3. Together, these results reveal two previously unknown players in siRNA biogenesis from IRs-AGO1, which promotes IR transcription, and AMP1, which inhibits IR transcription indirectly through the repression of AGO1 expression.

The plant siRNA landscape

Whereas micro (mi)RNAs are considered the clean, noble side of the small RNA world, small interfering (si)RNAs are often seen as a noisy set of molecules whose barbarian acronyms reflect a large diversity of often elusive origins and functions. Twenty-five years after their discovery in plants, however, new classes of siRNAs are still being identified, sometimes in discrete tissues or at particular developmental stages, making the plant siRNA world substantially more complex and subtle than originally anticipated. Focusing primarily on the model Arabidopsis, we review here the plant siRNA landscape, including transposable elements (TE)-derived siRNAs, a vast array of non-TE-derived endogenous siRNAs, as well as exogenous siRNAs produced in response to invading nucleic acids such as viruses or transgenes. We primarily emphasize the extraordinary sophistication and diversity of their biogenesis and, secondarily, the variety of their known or presumed functions, including via non-cell autonomous activities, in the sporophyte, gametophyte, and shortly after fertilization.

Involvement of 5mC DNA demethylation via 5-aza-2'-deoxycytidine in regulating gene expression during early somatic embryo development in white spruce (Picea glauca)

DNA methylation plays a crucial role in the development of somatic embryos (SEs) through the regulation of gene expression. To examine the impact of DNA methylation on gene expression during early SE development in Picea glauca, the demethylation reagent 5-aza-dC (5-aza-2'-deoxycytidine) was employed to modify DNA methylation regions and levels during the pre-maturation stage of somatic embryogenesis. The application of 2.0 µM 5-aza-dC did not induce toxicity to SEs in early development. Following treatment, the global DNA methylation level decreased significantly on the 7th day of pre-maturation and the 1st week of maturation. Methylated DNA immunoprecipitation (MeDIP) sequencing revealed that differentially methylated regions, as analyzed through Gene Ontology (GO), were related to plant development and reproduction and that they were hypomethylated on the 3rd day but hypermethylated on the 7th day in 5-aza-dC-treated embryogenic tissues. These findings indicate that 5-aza-dC treatment positively impacts early SE development, which was inhibited following 7 d of treatment. The expression of MSH7, JMJ14, and CalS10 was associated with DNA methylation, epigenetic regulation, and somatic embryogenesis. Further analysis of methylated regions revealed that the expression profiles of MSH7, JMJ14, and CalS10 were correlated with altered DNA methylation, suggesting DNA methylation at 5 mC may play a role in controlling the expression of these genes and regulating the early development of SEs in P. glauca. This study offers new insights into the regulation of somatic embryogenesis in conifers.

Synergistic action of the Arabidopsis spliceosome components PRP39a and SmD1b in promoting posttranscriptional transgene silencing

Besides regulating splicing, the conserved spliceosome component SmD1 (Small nuclear ribonucleoprotein D1)b promotes posttranscriptional silencing of sense transgenes (S-PTGS [post-transcriptional genesilencing]). Here, we show that the conserved spliceosome component PRP39 (Pre-mRNA-processing factor 39)a also plays a role in S-PTGS in Arabidopsis thaliana. However, PRP39a and SmD1b actions appear distinct in both splicing and S-PTGS. Indeed, RNAseq-based analysis of expression level and alternative splicing in prp39a and smd1b mutants identified different sets of deregulated transcripts and noncoding RNAs. Moreover, double mutant analyses involving prp39a or smd1b and RNA quality control (RQC) mutants revealed distinct genetic interactions for SmD1b and PRP39a with nuclear RQC machineries, suggesting nonredundant roles in the RQC/PTGS interplay. Supporting this hypothesis, a prp39a smd1b double mutant exhibited enhanced suppression of S-PTGS compared to the single mutants. Because the prp39a and smd1b mutants (i) showed no major changes in the expression of PTGS or RQC components or in small RNA production and (ii) do not alter PTGS triggered by inverted-repeat transgenes directly producing dsRNA (IR-PTGS), PRP39a, and SmD1b appear to synergistically promote a step specific to S-PTGS. We propose that, independently from their specific roles in splicing, PRP39a and SmD1b limit 3'-to-5' and/or 5'-to-3' degradation of transgene-derived aberrant RNAs in the nucleus, thus favoring the export of aberrant RNAs to the cytoplasm where their conversion into double-stranded RNA (dsRNA) initiates S-PTGS.

Epigenetic management of self and non-self: lessons from 40 years of transgenic plants

Plant varieties exhibiting unstable or variegated phenotypes, or showing virus recovery have long remained a mystery. It is only with the development of transgenic plants 40 years ago that the epigenetic features underlying these phenomena were elucidated. Indeed, the study of transgenic plants that did not express the introduced sequences revealed that transgene loci sometimes undergo transcriptional gene silencing (TGS) or post-transcriptional gene silencing (PTGS) by activating epigenetic defenses that naturally control transposable elements, duplicated genes or viruses. Even when they do not trigger TGS or PTGS spontaneously, stably expressed transgenes driven by viral promoters set apart from endogenous genes in their epigenetic regulation. As a result, transgenes driven by viral promoters are capable of undergoing systemic PTGS throughout the plant, whereas endogenous genes can only undergo local PTGS in cells where RNA quality control is impaired. Together, these results indicate that the host genome distinguishes self from non-self at the epigenetic level, allowing PTGS to eliminate non-self, and preventing PTGS to become systemic and kill the plant when it is locally activated against deregulated self.

SlJMJ7 orchestrates tomato fruit ripening via crosstalk between H3K4me3 and DML2-mediated DNA demethylation

The ripening of fleshy fruits is a unique developmental process that Arabidopsis and rice lack. This process is driven by hormones and transcription factors. However, the critical and early regulators of fruit ripening are still poorly understood. Here, we revealed that SlJMJ7, an H3K4 demethylase, is a critical negative regulator of fruit ripening in tomato. Combined genome-wide transcription, binding sites, histone H3K4me3 and DNA methylation analyses demonstrated that SlJMJ7 regulates a key group of ripening-related genes, including ethylene biosynthesis (ACS2, ACS4 and ACO6), transcriptional regulation (RIN and NOR) and DNA demethylation (DML2) genes, by H3K4me3 demethylation. Moreover, loss of SlJMJ7 function leads to increased H3K4me3 levels, which directly activates ripening-related genes, and to global DML2-mediated DNA hypomethylation in fruit, which indirectly prompts expression of ripening-related genes. Together, these effects lead to accelerated fruit ripening in sljmj7 mutant. Our findings demonstrate that SlJMJ7 acts as a master negative regulator of fruit ripening not only through direct removal of H3K4me3 from multiple key ripening-related factors, but also through crosstalk between histone and DNA demethylation. These findings reveal a novel crosstalk between histone methylation and DNA methylation to regulate gene expression in plant developmental processes.

A microProtein repressor complex in the shoot meristem controls the transition to flowering

MicroProteins are potent post-translational regulators. In Arabidopsis (Arabidopsis thaliana), the miP1a/b microProteins delay floral transition by forming a complex with CONSTANS (CO) and the co-repressor protein TOPLESS. To better understand the function of the miP1a microProtein in floral repression, we performed a genetic suppressor screen to identify suppressors of miP1a (sum) function. One mutant, sum1, exhibited strong suppression of the miP1a-induced late-flowering phenotype. Mapping of sum1 identified another allele of the gene encoding the histone H3K4 demethylase JUMONJI14 (JMJ14), which is required for miP1a function. Plants carrying mutations in JMJ14 exhibit an early flowering phenotype that is largely dependent on CO activity, supporting an additional role for CO in the repressive complex. We further investigated whether miP1a function involves chromatin modification, performed whole-genome methylome sequencing studies with plants ectopically expressing miP1a, and identified differentially methylated regions (DMRs). Among these DMRs is the promoter of FLOWERING LOCUS T (FT), the prime target of miP1a that is ectopically methylated in a JMJ14-dependent manner. Moreover, when aberrantly expressed at the shoot apex, CO induces early flowering, but only when JMJ14 is mutated. Detailed analysis of the genetic interaction among CO, JMJ14, miP1a/b, and TPL revealed a potential role for CO as a repressor of flowering in the shoot apical meristem (SAM). Altogether, our results suggest that a repressor complex operates in the SAM, likely to maintain it in an undifferentiated state until leaf-derived florigen signals induce SAM conversion into a floral meristem.

The diversity of post-transcriptional gene silencing mediated by small silencing RNAs in plants

In plants, post-transcriptional gene silencing (PTGS) tightly regulates development, maintains genome stability and protects plant against foreign genes. PTGS can be triggered by virus infection, transgene, and endogenous transcript, thus commonly serves as an RNA-based immune mechanism. Accordingly, based on the initiating factors, PTGS can be divided into viral-PTGS, transgene-PTGS, and endo-gene-PTGS. Unlike the intensely expressed invading transgenes and viral genes that frequently undergo PTGS, most endogenous genes do not trigger PTGS, except for a few that can produce endogenous small RNAs (sRNAs), including microRNA (miRNA) and small interfering RNA (siRNA). Different lengths of miRNA and siRNA, mainly 21-, 22- or 24-nucleotides (nt) exert diverse functions, ranging from target mRNA degradation, translational inhibition, or DNA methylation and chromatin modifications. The abundant 21-nt miRNA or siRNA, processed by RNase-III enzyme DICER-LIKE 1 (DCL1) and DCL4, respectively, have been well studied in the PTGS pathways. By contrast, the scarceness of endogenous 22-nt sRNAs that are primarily processed by DCL2 limits their research, although a few encouraging studies have been reported recently. Therefore, we review here our current understanding of diverse PTGS pathways triggered by a variety of sRNAs and summarize the distinct features of the 22-nt sRNA mediated PTGS.

Contrasting epigenetic control of transgenes and endogenous genes promotes post-transcriptional transgene silencing in Arabidopsis

Transgenes that are stably expressed in plant genomes over many generations could be assumed to behave epigenetically the same as endogenous genes. Here, we report that whereas the histone H3K9me2 demethylase IBM1, but not the histone H3K4me3 demethylase JMJ14, counteracts DNA methylation of Arabidopsis endogenous genes, JMJ14, but not IBM1, counteracts DNA methylation of expressed transgenes. Additionally, JMJ14-mediated specific attenuation of transgene DNA methylation enhances the production of aberrant RNAs that readily induce systemic post-transcriptional transgene silencing (PTGS). Thus, the JMJ14 chromatin modifying complex maintains expressed transgenes in a probationary state of susceptibility to PTGS, suggesting that the host plant genome does not immediately accept expressed transgenes as being epigenetically the same as endogenous genes.

Accumulating evidences point to a discrepancy in the epigenetic behaviour of transgenes and endogenous genes. Here, via characterization of mutants impaired in histone demethylases JMJ14 and IBM1, the authors show that transgenes and endogenous genes are regulated by different epigenetic mechanisms in Arabidopsis.

Metabolic control of histone demethylase activity involved in plant response to high temperature

Jumonji C (JmjC) domain proteins are histone lysine demethylases that require ferrous iron and alpha-ketoglutarate (or α-KG) as cofactors in the oxidative demethylation reaction. In plants, α-KG is produced by isocitrate dehydrogenases (ICDHs) in different metabolic pathways. It remains unclear whether fluctuation of α-KG levels affects JmjC demethylase activity and epigenetic regulation of plant gene expression. In this work, we studied the impact of loss of function of the cytosolic ICDH (cICDH) gene on the function of histone demethylases in Arabidopsis thaliana. Loss of cICDH resulted in increases of overall histone H3 lysine 4 trimethylation (H3K4me3) and enhanced mutation defects of the H3K4me3 demethylase gene JMJ14. Genetic analysis suggested that the cICDH mutation may affect the activity of other demethylases, including JMJ15 and JMJ18 that function redundantly with JMJ14 in the plant thermosensory response. Furthermore, we show that mutation of JMJ14 affected both the gene activation and repression programs of the plant thermosensory response and that JMJ14 and JMJ15 repressed a set of genes that are likely to play negative roles in the process. The results provide evidence that histone H3K4 demethylases are involved in the plant response to elevated ambient temperature.

Secondary siRNAs in Plants: Biosynthesis, Various Functions, and Applications in Virology

The major components of RNA silencing include both transitive and systemic small RNAs, which are technically called secondary sRNAs. Double-stranded RNAs trigger systemic silencing pathways to negatively regulate gene expression. The secondary siRNAs generated as a result of transitive silencing also play a substantial role in gene silencing especially in antiviral defense. In this review, we first describe the discovery and pathways of transitivity with emphasis on RNA-dependent RNA polymerases followed by description on the short range and systemic spread of silencing. We also provide an in-depth view on the various size classes of secondary siRNAs and their different roles in RNA silencing including their categorization based on their biogenesis. The other regulatory roles of secondary siRNAs in transgene silencing, virus-induced gene silencing, transitivity, and trans-species transfer have also been detailed. The possible implications and applications of systemic silencing and the different gene silencing tools developed are also described. The details on mobility and roles of secondary siRNAs derived from viral genome in plant defense against the respective viruses are presented. This entails the description of other compatible plant-virus interactions and the corresponding small RNAs that determine recovery from disease symptoms, exclusion of viruses from shoot meristems, and natural resistance. The last section presents an overview on the usefulness of RNA silencing for management of viral infections in crop plants.

A G(enomic)P(ositioning)S(ystem) for Plant RNAPII Transcription

Post-translational modifications (PTMs) of histone residues shape the landscape of gene expression by modulating the dynamic process of RNA polymerase II (RNAPII) transcription. The contribution of particular histone modifications to the definition of distinct RNAPII transcription stages remains poorly characterized in plants. Chromatin immunoprecipitation combined with next-generation sequencing (ChIP-seq) resolves the genomic distribution of histone modifications. Here, we review histone PTM ChIP-seq data in Arabidopsis thaliana and find support for a Genomic Positioning System (GPS) that guides RNAPII transcription. We review the roles of histone PTM 'readers', 'writers', and 'erasers', with a focus on the regulation of gene expression and biological functions in plants. The distinct functions of RNAPII transcription during the plant transcription cycle may rely, in part, on the characteristic histone PTM profiles that distinguish transcription stages.

JMJ14 encoded H3K4 demethylase modulates immune responses by regulating defence gene expression and pipecolic acid levels

Epigenetic modifications have emerged as an important mechanism underlying plant defence against pathogens. We examined the role of JMJ14, a Jumonji (JMJ) domain-containing H3K4 demethylase, in local and systemic plant immune responses in Arabidopsis. The function of JMJ14 in local or systemic defence response was investigated by pathogen growth assays and by analysing expression and H3K4me3 enrichments of key defence genes using qPCR and ChIP-qPCR. Salicylic acid (SA) and pipecolic acid (Pip) levels were quantified and function of JMJ14 in SA- and Pip-mediated defences was analysed in Col-0 and jmj14 plants. jmj14 mutants were compromised in both local and systemic defences. JMJ14 positively regulates pathogen-induced H3K4me3 enrichment and expression of defence genes involved in SA- and Pip-mediated defence pathways. Consequently, loss of JMJ14 results in attenuated defence gene expression and reduced Pip accumulation during establishment of systemic acquired resistance (SAR). Exogenous Pip partially restored SAR in jmj14 plants, suggesting that JMJ14 regulated Pip biosynthesis and other downstream factors regulate SAR in jmj14 plants. JMJ14 positively modulates defence gene expressions and Pip levels in Arabidopsis.

Targeted misexpression of NAC052, acting in H3K4 demethylation, alters leaf morphological and anatomical traits in Arabidopsis thaliana

In an effort to identify genetic regulators for the cell ontogeny around the veins in Arabidopsis thaliana leaves, an activation-tagged mutant line with altered leaf morphology and altered bundle sheath anatomy was characterized. This mutant had a small rosette area with wrinkled leaves and chlorotic leaf edges, as well as enhanced chloroplast numbers in the (pre-)bundle sheath tissue. It had a bundle-specific promoter from the gene GLYCINE DECARBOXYLASE SUBUNIT-T from the C4 species Flaveria trinervia (GLDTFt promoter) inserted in the coding region of the transcriptional repressor NAC052, functioning in H3K4 demethylation, in front of an alternative start codon in-frame with the natural start codon. Reconstruction of the mutation event of our activation-tagged line by creating a line expressing an N-terminally truncated sequence of NAC052 under control of the GLDTFt promoter confirmed the involvement of NAC052 in leaf development. Our study not only reveals leaf anatomic and transcriptomic effects of an N-terminally truncated NAC052 under control of the GLDTFt promoter, but also identifies NAC052 as a novel genetic regulator of leaf development.

Conditional effects of the epigenetic regulator JUMONJI 14 in Arabidopsis root growth

Methylation of lysine 4 in histone 3 (H3K4) is a post-translational modification that promotes gene expression. H3K4 methylation can be reversed by specific demethylases with an enzymatic Jumonji C domain. In Arabidopsis thaliana, H3K4-specific JUMONJI (JMJ) proteins distinguish themselves by the association with an F/Y-rich (FYR) domain. Here, we report that jmj14 mutations partially suppress reduced root meristem size and growth vigor of brevis radix (brx) mutants. Similar to its close homologs, JMJ15, JMJ16 and JMJ18, the JMJ14 promoter confers expression in mature root vasculature. Yet, unlike jmj14, neither jmj16 nor jmj18 mutation markedly suppresses brx phenotypes. Domain-swapping experiments suggest that the specificity of JMJ14 function resides in the FYR domain. Despite JMJ14 promoter activity in the mature vasculature, jmj14 mutation affects root meristem size. However, JMJ14 protein is observed throughout the meristem, suggesting that the JMJ14 transcript region contributes substantially to the spatial aspect of JMJ14 expression. In summary, our data reveal a role for JMJ14 in root growth in sensitized genetic backgrounds that depends on its FYR domain and regulatory input from the JMJ14 cistron.

Post-transcriptional gene silencing triggers dispensable DNA methylation in gene body in Arabidopsis

Spontaneous post-transcriptional silencing of sense transgenes (S-PTGS) is established in each generation and is accompanied by DNA methylation, but the pathway of PTGS-dependent DNA methylation is unknown and so is its role. Here we show that CHH and CHG methylation coincides spatially and temporally with RDR6-dependent products derived from the central and 3' regions of the coding sequence, and requires the components of the RNA-directed DNA methylation (RdDM) pathway NRPE1, DRD1 and DRM2, but not CLSY1, NRPD1, RDR2 or DCL3, suggesting that RDR6-dependent products, namely long dsRNAs and/or siRNAs, trigger PTGS-dependent DNA methylation. Nevertheless, none of these RdDM components are required to establish S-PTGS or produce a systemic silencing signal. Moreover, preventing de novo DNA methylation in non-silenced transgenic tissues grafted onto homologous silenced tissues does not inhibit the triggering of PTGS. Overall, these data indicate that gene body DNA methylation is a consequence, not a cause, of PTGS, and rule out the hypothesis that a PTGS-associated DNA methylation signal is transmitted independent of a PTGS signal.

RST1 and RIPR connect the cytosolic RNA exosome to the Ski complex in Arabidopsis

The RNA exosome is a key 3’−5’ exoribonuclease with an evolutionarily conserved structure and function. Its cytosolic functions require the co-factors SKI7 and the Ski complex. Here we demonstrate by co-purification experiments that the ARM-repeat protein RESURRECTION1 (RST1) and RST1 INTERACTING PROTEIN (RIPR) connect the cytosolic Arabidopsis RNA exosome to the Ski complex. rst1 and ripr mutants accumulate RNA quality control siRNAs (rqc-siRNAs) produced by the post-transcriptional gene silencing (PTGS) machinery when mRNA degradation is compromised. The small RNA populations observed in rst1 and ripr mutants are also detected in mutants lacking the RRP45B/CER7 core exosome subunit. Thus, molecular and genetic evidence supports a physical and functional link between RST1, RIPR and the RNA exosome. Our data reveal the existence of additional cytosolic exosome co-factors besides the known Ski subunits. RST1 is not restricted to plants, as homologues with a similar domain architecture but unknown function exist in animals, including humans.

Cytosolic RNA degradation by the RNA exosome requires the Ski complex. Here the authors show that the proteins RST1 and RIPR assist the RNA exosome and the Ski complex in RNA degradation, thereby preventing the production of secondary siRNAs from endogenous mRNAs.

Diurnal regulation of SDG2 and JMJ14 by circadian clock oscillators orchestrates histone modification rhythms in Arabidopsis

Background

Circadian rhythms modulate growth and development in all organisms through interlocking transcriptional-translational feedback loops. The transcriptional loop involves chromatin modifications of central circadian oscillators in mammals and plants. However, the molecular basis for rhythmic epigenetic modifications and circadian regulation is poorly understood.

Results

Here we report a feedback relationship between diurnal regulation of circadian clock genes and histone modifications in Arabidopsis. On one hand, the circadian oscillators CCA1 and LHY regulate diurnal expression of genes coding for the eraser (JMJ14) directly and writer (SDG2) indirectly for H3K4me3 modification, leading to rhythmic H3K4me3 changes in target genes. On the other hand, expression of circadian oscillator genes including CCA1 and LHY is associated with H3K4me3 levels and decreased in the sdg2 mutant but increased in the jmj14 mutant. At the genome-wide level, diurnal rhythms of H3K4me3 and another histone mark H3K9ac are associated with diurnal regulation of 20–30% of the expressed genes. While the majority (86%) of H3K4me3 and H3K9ac target genes overlap, only 13% of morning-phased and 22% of evening-phased genes had both H3K4me3 and H3K9ac peaks, suggesting specific roles of different histone modifications in diurnal gene expression.

Conclusions

Circadian clock genes promote diurnal regulation of SDG2 and JMJ14 expression, which in turn regulate rhythmic histone modification dynamics for the clock and its output genes. This reciprocal regulatory module between chromatin modifiers and circadian clock oscillators orchestrates diurnal gene expression that governs plant growth and development.

Electronic supplementary material

The online version of this article (10.1186/s13059-019-1777-1) contains supplementary material, which is available to authorized users.

Tracing the origin and evolution history of methylation-related genes in plants

BACKGROUND

DNA methylation is a crucial epigenetic modification, which is involved in many biological processes, including gene expression regulation, embryonic development, cell differentiation and genomic imprinting etc. And it also involves many key regulatory genes in eukaryotes. By tracing the evolutionary history of methylation-related genes, we can understand the origin and expansion time of these genes, which helps to understand the evolutionary history of plants, and we can also understand the changes of DNA methylation patterns in different species. However, most studies on the evolution of methylation-related genes failed to be carried out for the whole DNA methylation pathway.

RESULTS

In this study, we conducted a comprehensive identification of 33 methylation-related genes in 77 species, and investigated gene origin and evolution throughout the plant kingdom. We found that the origin of genes responsible for methylation maintenance and demethylation evolved early, while most de novo methylation-related genes appeared late. The methylation-related genes were expanded by whole genome duplication and tandem replication, but were also accompanied by a large number of gene absence events in different species. The gene length and intron length varied a lot in different species, but exon structure and functional domains were relatively conserved. The phylogenetic relationships of methylation-related genes were traced to reveal the evolution history of DNA methylation in different species. The expression patterns of methylation-related genes have changed during the evolution of species, and the expression patterns of these genes in different species can be clustered into four categories.

CONCLUSIONS

The study describes a global characterization of DNA methylation-related genes in the plant kingdom. The similarities and differences in origin time, gene structure and phylogenetic relationship of these genes lead us to understand the evolutionary conservation and dynamics of DNA methylation in plants.

The transcription factor OsSUF4 interacts with SDG725 in promoting H3K36me3 establishment

The different genome-wide distributions of tri-methylation at H3K36 (H3K36me3) in various species suggest diverse mechanisms for H3K36me3 establishment during evolution. Here, we show that the transcription factor OsSUF4 recognizes a specific 7-bp DNA element, broadly distributes throughout the rice genome, and recruits the H3K36 methyltransferase SDG725 to target a set of genes including the key florigen genes RFT1 and Hd3a to promote flowering in rice. Biochemical and structural analyses indicate that several positive residues within the zinc finger domain are vital for OsSUF4 function in planta. Our results reveal a regulatory mechanism contributing to H3K36me3 distribution in plants.

The distribution of H3K36me3 varies between species. Here Liu et al. show that the OsSUF4 transcription factor binds its target motif via a zinc finger domain to promote H3K36 methyltransferase targeting close to the transcription start site of genes including the flowering regulators RFT1 and Hd3a.

The seekers: how epigenetic modifying enzymes find their hidden genomic targets in Arabidopsis

Epigenetic regulation plays fundamental roles in modulating chromatin-based processes and shaping the epigenome in multicellular eukaryotes, including plants. How epigenetic factors recognize their target loci hiding in the vast genomic DNA sequence remains a long-standing mystery. During the past several years, a growing body of work has revealed the complex, dynamic, and diverse chromatin-targeting mechanisms of these epigenetic factors. In this review, we focus on recent advances in understanding the recruitment of epigenetic factors to specific genomic regions, based on data from Arabidopsis thaliana.

Structure of the Arabidopsis JMJ14-H3K4me3 Complex Provides Insight into the Substrate Specificity of KDM5 Subfamily Histone Demethylases

In chromatin, histone methylation affects the epigenetic regulation of multiple processes in animals and plants and is modulated by the activities of histone methyltransferases and histone demethylases. The jumonji domain-containing histone demethylases have diverse functions and can be classified into several subfamilies. In humans, the jumonji domain-containing Lysine (K)-Specific Demethylase 5/Jumonji and ARID Domain Protein (KDM5/JARID) subfamily demethylases are specific for histone 3 lysine 4 trimethylation (H3K4me3) and are important drug targets for cancer treatment. In Arabidopsis thaliana, the KDM5/JARID subfamily H3K4me3 demethylase JUMONJI14 (JMJ14) plays important roles in flowering, gene silencing, and DNA methylation. Here, we report the crystal structures of the JMJ14 catalytic domain in both substrate-free and bound forms. The structures reveal that the jumonji and C5HC2 domains contribute to the specific recognition of the H3R2 and H3Q5 to facilitate H3K4me3 substrate specificity. The critical acidic residues are conserved in plants and animals with the corresponding mutations impairing the enzyme activity of both JMJ14 and human KDM5B, indicating a common substrate recognition mechanism for KDM5 subfamily demethylases shared by plants and animals and further informing efforts to design targeted inhibitors of human KDM5.

sgs1: a neomorphic nac52 allele impairing post-transcriptional gene silencing through SGS3 downregulation

Post-transcriptional gene silencing (PTGS) is a defense mechanism that targets invading nucleic acids from endogenous (transposons) or exogenous (pathogens, transgenes) sources. Genetic screens based on the reactivation of silenced transgenes have long been used to identify cellular components and regulators of PTGS. Here we show that the first isolated PTGS-deficient mutant, sgs1, is impaired in the transcription factor NAC52. This mutant exhibits striking similarities to a mutant impaired in the H3K4me3 demethylase JMJ14 isolated from the same genetic screen. These similarities include increased transgene promoter DNA methylation, reduced H3K4me3 and H3K36me3 levels, reduced PolII occupancy and reduced transgene mRNA accumulation. It is likely that increased DNA methylation is the cause of reduced transcription because the effect of jmj14 and sgs1 on transgene transcription is suppressed by drm2, a mutation that compromises de novo DNA methylation, suggesting that the JMJ14-NAC52 module promotes transgene transcription by preventing DNA methylation. Remarkably, sgs1 has a stronger effect than jmj14 and nac52 null alleles on PTGS systems requiring siRNA amplification, and this is due to reduced SGS3 mRNA levels in sgs1. Given that the sgs1 mutation changes a conserved amino acid of the NAC proteins involved in homodimerization, we propose that sgs1 corresponds to a neomorphic nac52 allele encoding a mutant protein that lacks wild-type NAC52 activity but promotes SGS3 downregulation. Together, these results indicate that impairment of PTGS in sgs1 is due to its dual effect on transgene transcription and SGS3 transcription, thus compromising siRNA amplification.

[Reciprocity between active transcription and histone methylation]

In the nucleus of eukaryotic cells, the chromatin states dictated by the different combinations of histone post-translational modifications, such as the methylation of lysine residues, are an integral part of the multitude of epigenomes involved in the fine tuning of all genome functions, and in particular transcription. Over the last decade, an increasing number of factors have been identified as regulators involved in the establishment, reading or erasure of histone methylations. Their characterization in model organisms such as Arabidopsis has thus unraveled their fundamental roles in the control and regulation of essential developmental processes such as the floral transition, cell differentiation, gametogenesis, and/or the response/adaptation of plants to environmental stresses. In this review, we will focus on the methylation of histones functioning as a mark of activate transcription and we will try to highlight, based on recent findings, the more or less direct links between this mark and gene expression. Thus, we will discuss the different mechanisms allowing the dynamics and the integration of the chromatin states resulting from the different histone methylations in connection with the transcriptional machinery of the RNA polymerase II.

RNA Quality Control as a Key to Suppressing RNA Silencing of Endogenous Genes in Plants

RNA quality control of endogenous RNAs is an integral part of eukaryotic gene expression and often relies on exonucleolytic degradation to eliminate dysfunctional transcripts. In parallel, exogenous and selected endogenous RNAs are degraded through RNA silencing, which is a genome defense mechanism used by many eukaryotes. In plants, RNA silencing is triggered by the production of double-stranded RNAs (dsRNAs) by RNA-DEPENDENT RNA POLYMERASEs (RDRs) and proceeds through small interfering (si) RNA-directed, ARGONAUTE (AGO)-mediated cleavage of homologous transcripts. Many studies revealed that plants avert inappropriate posttranscriptional gene silencing of endogenous coding genes by using RNA surveillance mechanisms as a safeguard to protect their transcriptome profiles. The tug of war between RNA surveillance and RNA silencing ensures the appropriate partitioning of endogenous RNA substrates among these degradation pathways. Here we review recent advances on RNA quality control and its role in the suppression of RNA silencing at endogenous genes and discuss the mechanisms underlying the crosstalk among these pathways.

A JUMONJI Protein with E3 Ligase and Histone H3 Binding Activities Affects Transposon Silencing in Arabidopsis

Transposable elements (TEs) make up a large proportion of eukaryotic genomes. As their mobilization creates genetic variation that threatens genome integrity, TEs are epigenetically silenced through several pathways, and this may spread to neighboring sequences. JUMONJI (JMJ) proteins can function as antisilencing factors and prevent silencing of genes next to TEs Whether TE silencing is counterbalanced by the activity of antisilencing factors is still unclear. Here, we characterize JMJ24 as a regulator of TE silencing. We show that loss of JMJ24 results in increased silencing of the DNA transposon AtMu1c, while overexpression of JMJ24 reduces silencing. JMJ24 has a JumonjiC (JmjC) domain and two RING domains. JMJ24 autoubiquitinates in vitro, demonstrating E3 ligase activity of the RING domain(s). JMJ24-JmjC binds the N-terminal tail of histone H3, and full-length JMJ24 binds histone H3 in vivo. JMJ24 activity is anticorrelated with histone H3 Lys 9 dimethylation (H3K9me2) levels at AtMu1c Double mutant analyses with epigenetic silencing mutants suggest that JMJ24 antagonizes histone H3K9me2 and requires H3K9 methyltransferases for its activity on AtMu1c Genome-wide transcriptome analysis indicates that JMJ24 affects silencing at additional TEs Our results suggest that the JmjC domain of JMJ24 has lost demethylase activity but has been retained as a binding domain for histone H3. This is in line with phylogenetic analyses indicating that JMJ24 (with the mutated JmjC domain) is widely conserved in angiosperms. Taken together, this study assigns a role in TE silencing to a conserved JmjC-domain protein with E3 ligase activity, but no demethylase activity.

RNA silencing movement in plants

Multicellular organisms, like higher plants, need to coordinate their growth and development and to cope with environmental cues. To achieve this, various signal molecules are transported between neighboring cells and distant organs to control the fate of the recipient cells and organs. RNA silencing produces cell non-autonomous signal molecules that can move over short or long distances leading to the sequence specific silencing of a target gene in a well defined area of cells or throughout the entire plant, respectively. The nature of these signal molecules, the route of silencing spread, and the genes involved in their production, movement and reception are discussed in this review. Additionally, a short section on features of silencing spread in animal models is presented at the end of this review.

The Nuclear Ribonucleoprotein SmD1 Interplays with Splicing, RNA Quality Control, and Posttranscriptional Gene Silencing in Arabidopsis

RNA quality control (RQC) eliminates aberrant RNAs based on their atypical structure, whereas posttranscriptional gene silencing (PTGS) eliminates both aberrant and functional RNAs through the sequence-specific action of short interfering RNAs (siRNAs). The Arabidopsis thaliana mutant smd1b was identified in a genetic screen for PTGS deficiency, revealing the involvement of SmD1, a component of the Smith (Sm) complex, in PTGS. The smd1a and smd1b single mutants are viable, but the smd1a smd1b double mutant is embryo-lethal, indicating that SmD1 function is essential. SmD1b resides in nucleoli and nucleoplasmic speckles, colocalizing with the splicing-related factor SR34. Consistent with this, the smd1b mutant exhibits intron retention at certain endogenous mRNAs. SmD1 binds to RNAs transcribed from silenced transgenes but not nonsilenced ones, indicating a direct role in PTGS. Yet, mutations in the RQC factors UPFRAMESHIFT3, EXORIBONUCLEASE2 (XRN2), XRN3, and XRN4 restore PTGS in smd1b, indicating that SmD1 is not essential for but rather facilitates PTGS. Moreover, the smd1b mtr4 double mutant is embryo-lethal, suggesting that SmD1 is essential for mRNA TRANSPORT REGULATOR4-dependent RQC. These results indicate that SmD1 interplays with splicing, RQC, and PTGS. We propose that SmD1 facilitates PTGS by protecting transgene-derived aberrant RNAs from degradation by RQC in the nucleus, allowing sufficient amounts to enter cytoplasmic siRNA bodies to activate PTGS.

BnSGS3 Has Differential Effects on the Accumulation of CMV, ORMV and TuMV in Oilseed Rape

Virus diseases greatly affect oilseed rape (Brassica napus) production. Investigating antiviral genes may lead to the development of disease-resistant varieties of oilseed rape. In this study, we examined the effects of the suppressor of gene silencing 3 in Brassica napus (BnSGS3, a putative antiviral gene) with different genus viruses by constructing BnSGS3-overexpressing (BnSGS3-Ov) and BnSGS3-silenced (BnSGS3-Si) oilseed rape (cv. Zhongshuang No. 6) plants. These three viruses are Oilseed rape mosaic virus (ORMV), Turnip mosaic virus (TuMV) and Cucumber mosaic virus (CMV). The native BnSGS3 expressed in all examined tissues with the highest expression in siliques. All three viruses induced BnSGS3 expression, but ORMV induced a dramatic increase in the BnSGS3-Ov plants, followed by TuMV and CMV. Upon inoculation with three different viruses, transcript abundance of BnSGS3 gene follows: BnSGS3-Ov > non-transgenic plants > BnSGS3-Si. The accumulation quantities of ORMV and TuMV exhibited a similar trend. However, CMV accumulation showed an opposite trend where virus accumulations were negatively correlated with BnSGS3 expression. The results suggest that BnSGS3 selectively inhibits CMV accumulation but promotes ORMV and TuMV accumulation. BnSGS3 should be used in different ways (up- and down-regulation) for breeding virus-resistant oilseed rape varieties.

C-terminal domains of a histone demethylase interact with a pair of transcription factors and mediate specific chromatin association

Jumonji C (JmjC) domain-containing protein 14 (JMJ14) is an H3K4-specific histone demethylase that has important roles in RNA-mediated gene silencing and flowering time regulation in Arabidopsis. However, how JMJ14 is recruited to its target genes remains unclear. Here, we show that the C-terminal FYRN (F/Y-rich N terminus) and FYRC (F/Y-rich C terminus) domains of JMJ14 are required for RNA silencing and flowering time regulation. Chromatin binding of JMJ14 is lost upon deletion of its FYRN and FYRC domains, and H3K4me3 is increased. FYRN and FYRC domains interact with a pair of NAC (NAM, ATAF, CUC) domain-containing transcription factors, NAC050 and NAC052. Genome-wide chromatin immunoprecipitation analysis revealed that JMJ14 and NAC050/052 share a set of common target genes with CTTGNNNNNCAAG consensus sequences. Mutations in either NAC052 or NAC050 impair RNA-mediated gene silencing. Together, our findings demonstrate an important role of FYRN and FYRC domains in targeting JMJ14 through direct interaction with NAC050/052 proteins, which reveals a novel mechanism of histone demethylase recruitment.

Knockdown of a JmjC domain-containing gene JMJ524 confers altered gibberellin responses by transcriptional regulation of GRAS protein lacking the DELLA domain genes in tomato

Plants integrate responses to independent hormonal and environmental signals to survive adversity. In particular, the phytohormone gibberellin (GA) regulates a variety of developmental processes and stress responses. In this study, the Jumonji-C (JmjC) domain-containing gene JMJ524 was characterized in tomato. JMJ524 responded to circadian rhythms and was upregulated by GA treatment. Knockdown of JMJ524 by RNAi caused a GA-insensitive dwarf phenotype with shrunken leaves and shortened internodes. However, in these transgenic plants, higher levels of endogenous GAs were detected. A genome-wide gene expression analysis by RNA-seq indicated that the expression levels of two DELLA-like genes, SlGLD1 ('GRAS protein Lacking the DELLA domain') and SlGLD2, were increased in JMJ524-RNAi transgenic plants. Nevertheless, only the overexpression of SlGLD1 in tomato resulted in a GA-insensitive dwarf phenotype, suggesting that SlGLD1 acts as a repressor of GA signalling. This study proposes that JMJ524 is required for stem elongation by altering GA responses, at least partially by regulating SlGLD1.

Two novel NAC transcription factors regulate gene expression and flowering time by associating with the histone demethylase JMJ14

The histone demethylase JMJ14 catalyzes histone demethylation at lysine 4 of histone 3 and is involved in transcriptional repression and flowering time control in Arabidopsis. Here, we report that JMJ14 is physically associated with two previously uncharacterized NAC transcription factors, NAC050 and NAC052. The NAC050/052-RNAi plants and the CRISPR-CAS9-mediated nac050/052 double mutant plants show an early flowering phenotype, which is similar to the phenotype of jmj14, suggesting a functional association between JMJ14 and NAC050/052. RNA-seq data indicated that hundreds of common target genes are co-regulated by JMJ14 and NAC50/052. Our ChIP analysis demonstrated that JMJ14 and NAC050 directly bind to co-upregulated genes shared in jmj14 and NAC050/052-RNAi, thereby facilitating H3K4 demethylation and transcriptional repression. The NAC050/052 recognition DNA cis-element was identified by an electrophoretic mobility shift assay at the promoters of its target genes. Together, our study identifies two novel NAC transcription repressors and demonstrates that they are involved in transcriptional repression and flowering time control by associating with the histone demethylase JMJ14.

eQTL mapping of transposon silencing reveals a position-dependent stable escape from epigenetic silencing and transposition of AtMu1 in the Arabidopsis lineage

Transposons are massively abundant in all eukaryotic genomes and are suppressed by epigenetic silencing. Transposon activity contributes to the evolution of species; however, it is unclear how much transposition-induced variation exists at a smaller scale and how transposons are targeted for silencing. Here, we exploited differential silencing of the AtMu1c transposon in the Arabidopsis thaliana accessions Columbia (Col) and Landsberg erecta (Ler). The difference persisted in hybrids and recombinant inbred lines and was mapped to a single expression quantitative trait locus within a 20-kb interval. In Ler only, this interval contained a previously unidentified copy of AtMu1c, which was inserted at the 3' end of a protein-coding gene and showed features of expressed genes. By contrast, AtMu1c(Col) was intergenic and associated with heterochromatic features. Furthermore, we identified widespread natural AtMu1c transposition from the analysis of over 200 accessions, which was not evident from alignments to the reference genome. AtMu1c expression was highest for insertions within 3' untranslated regions, suggesting that this location provides protection from silencing. Taken together, our results provide a species-wide view of the activity of one transposable element at unprecedented resolution, showing that AtMu1c transposed in the Arabidopsis lineage and that transposons can escape epigenetic silencing by inserting into specific genomic locations, such as the 3' end of genes.

The RNA helicases AtMTR4 and HEN2 target specific subsets of nuclear transcripts for degradation by the nuclear exosome in Arabidopsis thaliana

The RNA exosome is the major 3'-5' RNA degradation machine of eukaryotic cells and participates in processing, surveillance and turnover of both nuclear and cytoplasmic RNA. In both yeast and human, all nuclear functions of the exosome require the RNA helicase MTR4. We show that the Arabidopsis core exosome can associate with two related RNA helicases, AtMTR4 and HEN2. Reciprocal co-immunoprecipitation shows that each of the RNA helicases co-purifies with the exosome core complex and with distinct sets of specific proteins. While AtMTR4 is a predominantly nucleolar protein, HEN2 is located in the nucleoplasm and appears to be excluded from nucleoli. We have previously shown that the major role of AtMTR4 is the degradation of rRNA precursors and rRNA maturation by-products. Here, we demonstrate that HEN2 is involved in the degradation of a large number of polyadenylated nuclear exosome substrates such as snoRNA and miRNA precursors, incompletely spliced mRNAs, and spurious transcripts produced from pseudogenes and intergenic regions. Only a weak accumulation of these exosome substrate targets is observed in mtr4 mutants, suggesting that MTR4 can contribute, but plays rather a minor role for the degradation of non-ribosomal RNAs and cryptic transcripts in Arabidopsis. Consistently, transgene post-transcriptional gene silencing (PTGS) is marginally affected in mtr4 mutants, but increased in hen2 mutants, suggesting that it is mostly the nucleoplasmic exosome that degrades aberrant transgene RNAs to limit their entry in the PTGS pathway. Interestingly, HEN2 is conserved throughout green algae, mosses and land plants but absent from metazoans and other eukaryotic lineages. Our data indicate that, in contrast to human and yeast, plants have two functionally specialized RNA helicases that assist the exosome in the degradation of specific nucleolar and nucleoplasmic RNA populations, respectively.

Arabidopsis miR156 Regulates Tolerance to Recurring Environmental Stress through SPL Transcription Factors

Plants are sessile organisms that gauge stressful conditions to ensure survival and reproductive success. While plants in nature often encounter chronic or recurring stressful conditions, the strategies to cope with those are poorly understood. Here, we demonstrate the involvement of ARGONAUTE1 and the microRNA pathway in the adaptation to recurring heat stress (HS memory) at the physiological and molecular level. We show that miR156 isoforms are highly induced after HS and are functionally important for HS memory. miR156 promotes sustained expression of HS-responsive genes and is critical only after HS, demonstrating that the effects of modulating miR156 on HS memory do not reflect preexisting developmental alterations. miR156 targets SPL transcription factor genes that are master regulators of developmental transitions. SPL genes are posttranscriptionally downregulated by miR156 after HS, and this is critical for HS memory. Altogether, the miR156-SPL module mediates the response to recurring HS in Arabidopsis thaliana and thus may serve to integrate stress responses with development.

Suppression of RNA silencing by a plant DNA virus satellite requires a host calmodulin-like protein to repress RDR6 expression

In plants, RNA silencing plays a key role in antiviral defense. To counteract host defense, plant viruses encode viral suppressors of RNA silencing (VSRs) that target different effector molecules in the RNA silencing pathway. Evidence has shown that plants also encode endogenous suppressors of RNA silencing (ESRs) that function in proper regulation of RNA silencing. The possibility that these cellular proteins can be subverted by viruses to thwart host defense is intriguing but has not been fully explored. Here we report that the Nicotiana benthamiana calmodulin-like protein Nbrgs-CaM is required for the functions of the VSR βC1, the sole protein encoded by the DNA satellite associated with the geminivirus Tomato yellow leaf curl China virus (TYLCCNV). Nbrgs-CaM expression is up-regulated by the βC1. Transgenic plants over-expressing Nbrgs-CaM displayed developmental abnormities reminiscent of βC1-associated morphological alterations. Nbrgs-CaM suppressed RNA silencing in an Agrobacterium infiltration assay and, when over-expressed, blocked TYLCCNV-induced gene silencing. Genetic evidence showed that Nbrgs-CaM mediated the βC1 functions in silencing suppression and symptom modulation, and was required for efficient virus infection. Moreover, the tobacco and tomato orthologs of Nbrgs-CaM also possessed ESR activity, and were induced by betasatellite to promote virus infection in these Solanaceae hosts. We further demonstrated that βC1-induced Nbrgs-CaM suppressed the production of secondary siRNAs, likely through repressing RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) expression. RDR6-deficient N. benthamiana plants were defective in antiviral response and were hypersensitive to TYLCCNV infection. More significantly, TYLCCNV could overcome host range restrictions to infect Arabidopsis thaliana when the plants carried a RDR6 mutation. These findings demonstrate a distinct mechanism of VSR for suppressing PTGS through usurpation of a host ESR, and highlight an essential role for RDR6 in RNA silencing defense response against geminivirus infection.

Over-expression of histone H3K4 demethylase gene JMJ15 enhances salt tolerance in Arabidopsis

Histone H3 lysine 4 trimethylation (H3K4me3) has been shown to be involved in stress-responsive gene expression and gene priming in plants. However, the role of H3K4me3 resetting in the processes is not clear. In this work we studied the expression and function of Arabidopsis H3K4 demethylase gene JMJ15. We show that the expression of JMJ15 was relatively low and was limited to a number of tissues during vegetative growth but was higher in young floral organs. Over-expression of the gene in gain-of-function mutants reduced the plant height with accumulation of lignin in stems, while the loss-of-function mutation did not produce any visible phenotype. The gain-of-function mutants showed enhanced salt tolerance, whereas the loss-of-function mutant was more sensitive to salt compared to the wild type. Transcriptomic analysis revealed that over-expression of JMJ15 down-regulated many genes which are preferentially marked by H3K4me3 and H3K4me2. Many of the down-regulated genes encode transcription regulators involved in stress responses. The data suggest that increased JMJ15 levels may regulate the gene expression program that enhances stress tolerance.

Gene silencing in plants: a diversity of pathways

Eukaryotic organisms have evolved a variety of gene silencing pathways in which small RNAs, 20- to 30-nucleotides in length, repress the expression of sequence homologous genes at the transcriptional or post-transcriptional levels. In plants, RNA silencing pathways play important roles in regulating development and response to both biotic and abiotic stresses. The molecular basis of these complex and interconnected pathways has emerged only in recent years with the identification of many of the genes necessary for the biogenesis and action of small RNAs. This review covers the diversity of RNA silencing pathways identified in plants.

Epigenetic switches of tobacco transgenes associate with transient redistribution of histone marks in callus culture

In plants, silencing is usually accompanied by DNA methylation and heterochromatic histone marks. We studied these epigenetic modifications in different epialleles of 35S promoter (P35S)-driven tobacco transgenes. In locus 1, the T-DNA was organized as an inverted repeat, and the residing neomycin phosphotransferase II reporter gene (P35S-nptII) was silenced at the posttranscriptional (PTGS) level. Transcriptionally silenced (TGS) epialleles were generated by trans-acting RNA signals in hybrids or in a callus culture. PTGS to TGS conversion in callus culture was accompanied by loss of the euchromatic H3K4me3 mark in the transcribed region of locus 1, but this change was not transmitted to the regenerated plants from these calli. In contrast, cytosine methylation that spread from the transcribed region into the promoter was maintained in regenerants. Also, the TGS epialleles generated by trans-acting siRNAs did not change their active histone modifications. Thus, both TGS and PTGS epialleles exhibit euchromatic (H3K4me3 and H3K9ac) histone modifications despite heavy DNA methylation in the promoter and transcribed region, respectively. However, in the TGS locus (271), abundant heterochromatic H3K9me2 marks and DNA methylation were present on P35S. Heterochromatic histone modifications are not automatically installed on transcriptionally silenced loci in tobacco, suggesting that repressive histone marks and cytosine methylation may be uncoupled. However, transient loss of euchromatic modifications may guide de novo DNA methylation leading to formation of stable repressed epialleles with recovered eukaryotic marks. Compilation of available data on epigenetic modification of inactivated P35S in different systems is provided.

Cytoplasmic and nuclear quality control and turnover of single-stranded RNA modulate post-transcriptional gene silencing in plants

Eukaryotic RNA quality control (RQC) uses both endonucleolytic and exonucleolytic degradation to eliminate dysfunctional RNAs. In addition, endogenous and exogenous RNAs are degraded through post-transcriptional gene silencing (PTGS), which is triggered by the production of double-stranded (ds)RNAs and proceeds through short-interfering (si)RNA-directed ARGONAUTE-mediated endonucleolytic cleavage. Compromising cytoplasmic or nuclear 5'-3' exoribonuclease function enhances sense-transgene (S)-PTGS in Arabidopsis, suggesting that these pathways compete for similar RNA substrates. Here, we show that impairing nonsense-mediated decay, deadenylation or exosome activity enhanced S-PTGS, which requires host RNA-dependent RNA polymerase 6 (RDR6/SGS2/SDE1) and SUPPRESSOR OF GENE SILENCING 3 (SGS3) for the transformation of single-stranded RNA into dsRNA to trigger PTGS. However, these RQC mutations had no effect on inverted-repeat-PTGS, which directly produces hairpin dsRNA through transcription. Moreover, we show that these RQC factors are nuclear and cytoplasmic and are found in two RNA degradation foci in the cytoplasm: siRNA-bodies and processing-bodies. We propose a model of single-stranded RNA tug-of-war between RQC and S-PTGS that ensures the correct partitioning of RNA substrates among these RNA degradation pathways.