DNA methylation and chromatin structure affect transcriptional and post-transcriptional transgene silencing in Arabidopsis

An RRM domain protein SOE suppresses transgene silencing in rice

Gene expression is regulated at multiple levels, including RNA processing and DNA methylation/demethylation. How these regulations are controlled remains unclear. Here, through analysis of a suppressor for the OsEIN2 over-expressor, we identified an RNA recognition motif protein SUPPRESSOR OF EIN2 (SOE). SOE is localized in nuclear speckles and interacts with several components of the spliceosome. We find SOE associates with hundreds of targets and directly binds to a DNA glycosylase gene DNG701 pre-mRNA for efficient splicing and stabilization, allowing for subsequent DNG701-mediated DNA demethylation of the transgene promoter for proper gene expression. The V81M substitution in the suppressor mutant protein mSOE impaired its protein stability and binding activity to DNG701 pre-mRNA, leading to transgene silencing. SOE mutation enhances grain size and yield. Haplotype analysis in c. 3000 rice accessions reveals that the haplotype 1 (Hap 1) promoter is associated with high 1000-grain weight, and most of the japonica accessions, but not indica ones, have the Hap 1 elite allele. Our study discovers a novel mechanism for the regulation of gene expression and provides an elite allele for the promotion of yield potentials in rice.

RTEL1 is required for silencing and epigenome stability

Transcriptional silencing is an essential mechanism for controlling the expression of genes, transgenes and heterochromatic repeats through specific epigenetic marks on chromatin that are maintained during DNA replication. In Arabidopsis, silenced transgenes and heterochromatic sequences are typically associated with high levels of DNA methylation, while silenced genes are enriched in H3K27me3. Reactivation of these loci is often correlated with decreased levels of these repressive epigenetic marks. Here, we report that the DNA helicase REGULATOR OF TELOMERE ELONGATION 1 (RTEL1) is required for transcriptional silencing. RTEL1 deficiency causes upregulation of many genes enriched in H3K27me3 accompanied by a moderate decrease in this mark, but no loss of DNA methylation at reactivated heterochromatic loci. Instead, heterochromatin exhibits DNA hypermethylation and increased H3K27me3 in rtel1. We further find that loss of RTEL1 suppresses the release of heterochromatin silencing caused by the absence of the MOM1 silencing factor. RTEL1 is conserved among eukaryotes and plays a key role in resolving DNA secondary structures during DNA replication. Inducing such aberrant DNA structures using DNA cross-linking agents also results in a loss of transcriptional silencing. These findings uncover unappreciated roles for RTEL1 in transcriptional silencing and in stabilizing DNA methylation and H3K27me3 patterns.

Genome-Wide Tissue-Specific Genes Identification for Novel Tissue-Specific Promoters Discovery in Soybean

Promoters play a crucial role in controlling the spatial and temporal expression of genes at transcriptional levels in the process of higher plant growth and development. The spatial, efficient, and correct regulation of exogenous genes expression, as desired, is the key point in plant genetic engineering research. Constitutive promoters widely used in plant genetic transformation are limited because, sometimes, they may cause potential negative effects. This issue can be solved, to a certain extent, by using tissue-specific promoters. Compared with constitutive promoters, a few tissue-specific promoters have been isolated and applied. In this study, based on the transcriptome data, a total of 288 tissue-specific genes were collected, expressed in seven tissues, including the leaves, stems, flowers, pods, seeds, roots, and nodules of soybean (Glycine max). KEGG pathway enrichment analysis was carried out, and 52 metabolites were annotated. A total of 12 tissue-specific genes were selected via the transcription expression level and validated through real-time quantitative PCR, of which 10 genes showed tissue-specific expression. The 3-kb 5' upstream regions of ten genes were obtained as putative promoters. Further analysis showed that all the 10 promoters contained many tissue-specific cis-elements. These results demonstrate that high-throughput transcriptional data can be used as effective tools, providing a guide for high-throughput novel tissue-specific promoter discovery.

Long distance signalling and epigenetic changes in crop grafting

Humans have used grafting for more than 4000 years to improve plant production, through physically joining two different plants, which can continue to grow as a single organism. Today, grafting is becoming increasingly more popular as a technique to increase the production of herbaceous horticultural crops, where rootstocks can introduce traits such as resistance to several pathogens and/or improving the plant vigour. Research in model plants have documented how long-distance signalling mechanisms across the graft junction, together with epigenetic regulation, can produce molecular and phenotypic changes in grafted plants. Yet, most of the studied examples rely on proof-of-concept experiments or on limited specific cases. This review explores the link between research findings in model plants and crop species. We analyse studies investigating the movement of signalling molecules across the graft junction and their implications on epigenetic regulation. The improvement of genomics analyses and the increased availability of genetic resources has allowed to collect more information on potential benefits of grafting in horticultural crop models. Ultimately, further research into this topic will enhance our ability to use the grafting technique to exploit genetic and epigenetic variation in crops, as an alternative to traditional breeding.

DDM1-Mediated TE Silencing in Plants

Epigenetic modifications are indispensable for regulating gene bodies and TE silencing. DECREASE IN DNA METHYLATION 1 (DDM1) is a chromatin remodeller involved in histone modifications and DNA methylation. Apart from maintaining the epigenome, DDM1 also maintains key plant traits such as flowering time and heterosis. The role of DDM1 in epigenetic regulation is best characterised in plants, especially arabidopsis, rice, maize and tomato. The epigenetic changes induced by DDM1 establish the stable inheritance of many plant traits for at least eight generations, yet DDM1 does not methylate protein-coding genes. The DDM1 TE silencing mechanism is distinct and has evolved independently of other silencing pathways. Unlike the RNA-directed DNA Methylation (RdDM) pathway, DDM1 does not depend on siRNAs to enforce the heterochromatic state of TEs. Here, we review DDM1 TE silencing activity in the RdDM and non-RdDM contexts. The DDM1 TE silencing machinery is strongly associated with the histone linker H1 and histone H2A.W. While the linker histone H1 excludes the RdDM factors from methylating the heterochromatin, the histone H2A.W variant prevents TE mobility. The DDM1-H2A.W strategy alone silences nearly all the mobile TEs in the arabidopsis genome. Thus, the DDM1-directed TE silencing essentially preserves heterochromatic features and abolishes mobile threats to genome stability.

An siRNA-guided ARGONAUTE protein directs RNA polymerase V to initiate DNA methylation

In mammals and plants, cytosine DNA methylation is essential for the epigenetic repression of transposable elements and foreign DNA. In plants, DNA methylation is guided by small interfering RNAs (siRNAs) in a self-reinforcing cycle termed RNA-directed DNA methylation (RdDM). RdDM requires the specialized RNA polymerase V (Pol V), and the key unanswered question is how Pol V is first recruited to new target sites without pre-existing DNA methylation. We find that Pol V follows and is dependent on the recruitment of an AGO4-clade ARGONAUTE protein, and any siRNA can guide the ARGONAUTE protein to the new target locus independent of pre-existing DNA methylation. These findings reject long-standing models of RdDM initiation and instead demonstrate that siRNA-guided ARGONAUTE targeting is necessary, sufficient and first to target Pol V recruitment and trigger the cycle of RdDM at a transcribed target locus, thereby establishing epigenetic silencing.

Comprehensive Mechanism of Gene Silencing and Its Role in Plant Growth and Development

Gene silencing is a negative feedback mechanism that regulates gene expression to define cell fate and also regulates metabolism and gene expression throughout the life of an organism. In plants, gene silencing occurs via transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS). TGS obscures transcription via the methylation of 5' untranslated region (5'UTR), whereas PTGS causes the methylation of a coding region to result in transcript degradation. In this review, we summarized the history and molecular mechanisms of gene silencing and underlined its specific role in plant growth and crop production.

GSNOR Contributes to Demethylation and Expression of Transposable Elements and Stress-Responsive Genes

In the past, reactive nitrogen species (RNS) were supposed to be stress-induced by-products of disturbed metabolism that cause oxidative damage to biomolecules. However, emerging evidence demonstrates a substantial role of RNS as endogenous signals in eukaryotes. In plants, S-nitrosoglutathione (GSNO) is the dominant RNS and serves as the ^•NO donor for S-nitrosation of diverse effector proteins. Remarkably, the endogenous GSNO level is tightly controlled by S-nitrosoglutathione reductase (GSNOR) that irreversibly inactivates the glutathione-bound NO to ammonium. Exogenous feeding of diverse RNS, including GSNO, affected chromatin accessibility and transcription of stress-related genes, but the triggering function of RNS on these regulatory processes remained elusive. Here, we show that GSNO reductase-deficient plants (gsnor1-3) accumulate S-adenosylmethionine (SAM), the principal methyl donor for methylation of DNA and histones. This SAM accumulation triggered a substantial increase in the methylation index (MI = [SAM]/[S-adenosylhomocysteine]), indicating the transmethylation activity and histone methylation status in higher eukaryotes. Indeed, a mass spectrometry-based global histone profiling approach demonstrated a significant global increase in H3K9me2, which was independently verified by immunological detection using a selective antibody. Since H3K9me2-modified regions tightly correlate with methylated DNA regions, we also determined the DNA methylation status of gsnor1-3 plants by whole-genome bisulfite sequencing. DNA methylation in the CG, CHG, and CHH contexts in gsnor1-3 was significantly enhanced compared to the wild type. We propose that GSNOR1 activity affects chromatin accessibility by controlling the transmethylation activity (MI) required for maintaining DNA methylation and the level of the repressive chromatin mark H3K9me2.

Insights into the Role of Transcriptional Gene Silencing in Response to Herbicide-Treatments in Arabidopsis thaliana

Herbicide resistance is broadly recognized as the adaptive evolution of weed populations to the intense selection pressure imposed by the herbicide applications. Here, we tested whether transcriptional gene silencing (TGS) and RNA-directed DNA Methylation (RdDM) pathways modulate resistance to commonly applied herbicides. Using Arabidopsis thaliana wild-type plants exposed to sublethal doses of glyphosate, imazethapyr, and 2,4-D, we found a partial loss of TGS and increased susceptibility to herbicides in six out of 11 tested TGS/RdDM mutants. Mutation in REPRESSOR OF SILENCING 1 (ROS1), that plays an important role in DNA demethylation, leading to strongly increased susceptibility to all applied herbicides, and imazethapyr in particular. Transcriptomic analysis of the imazethapyr-treated wild type and ros1 plants revealed a relation of the herbicide upregulated genes to chemical stimulus, secondary metabolism, stress condition, flavonoid biosynthesis, and epigenetic processes. Hypersensitivity to imazethapyr of the flavonoid biosynthesis component TRANSPARENT TESTA 4 (TT4) mutant plants strongly suggests that ROS1-dependent accumulation of flavonoids is an important mechanism for herbicide stress response in A. thaliana. In summary, our study shows that herbicide treatment affects transcriptional gene silencing pathways and that misregulation of these pathways makes Arabidopsis plants more sensitive to herbicide treatment.

DNA polymerase epsilon is required for heterochromatin maintenance in Arabidopsis

BACKGROUND

Chromatin organizes DNA and regulates its transcriptional activity through epigenetic modifications. Heterochromatic regions of the genome are generally transcriptionally silent, while euchromatin is more prone to transcription. During DNA replication, both genetic information and chromatin modifications must be faithfully passed on to daughter strands. There is evidence that DNA polymerases play a role in transcriptional silencing, but the extent of their contribution and how it relates to heterochromatin maintenance is unclear.

RESULTS

We isolate a strong hypomorphic Arabidopsis thaliana mutant of the POL2A catalytic subunit of DNA polymerase epsilon and show that POL2A is required to stabilize heterochromatin silencing genome-wide, likely by preventing replicative stress. We reveal that POL2A inhibits DNA methylation and histone H3 lysine 9 methylation. Hence, the release of heterochromatin silencing in POL2A-deficient mutants paradoxically occurs in a chromatin context of increased levels of these two repressive epigenetic marks. At the nuclear level, the POL2A defect is associated with fragmentation of heterochromatin.

CONCLUSION

These results indicate that POL2A is critical to heterochromatin structure and function, and that unhindered replisome progression is required for the faithful propagation of DNA methylation throughout the cell cycle.

Design of orthogonal regulatory systems for modulating gene expression in plants

Agricultural biotechnology strategies often require the precise regulation of multiple genes to effectively modify complex plant traits. However, most efforts are hindered by a lack of characterized tools that allow for reliable and targeted expression of transgenes. We have successfully engineered a library of synthetic transcriptional regulators that modulate expression strength in planta. By leveraging orthogonal regulatory systems from Saccharomyces spp., we have developed a strategy for the design of synthetic activators, synthetic repressors, and synthetic promoters and have validated their use in Nicotiana benthamiana and Arabidopsis thaliana. This characterization of contributing genetic elements that dictate gene expression represents a foundation for the rational design of refined synthetic regulators. Our findings demonstrate that these tools provide variation in transcriptional output while enabling the concerted expression of multiple genes in a tissue-specific and environmentally responsive manner, providing a basis for generating complex genetic circuits that process endogenous and environmental stimuli.

Comparative analysis of epigenetic inhibitors reveals different degrees of interference with transcriptional gene silencing and induction of DNA damage

Repetitive DNA sequences and some genes are epigenetically repressed by transcriptional gene silencing (TGS). When genetic mutants are not available or problematic to use, TGS can be suppressed by chemical inhibitors. However, informed use of epigenetic inhibitors is partially hampered by the absence of any systematic comparison. In addition, there is emerging evidence that epigenetic inhibitors cause genomic instability, but the nature of this damage and its repair remain unclear. To bridge these gaps, we compared the effects of 5-azacytidine (AC), 2'-deoxy-5-azacytidine (DAC), zebularine and 3-deazaneplanocin A (DZNep) on TGS and DNA damage repair. The most effective inhibitor of TGS was DAC, followed by DZNep, zebularine and AC. We confirmed that all inhibitors induce DNA damage and suggest that this damage is repaired by multiple pathways with a critical role of homologous recombination and of the SMC5/6 complex. A strong positive link between the degree of cytidine analog-induced DNA demethylation and the amount of DNA damage suggests that DNA damage is an integral part of cytidine analog-induced DNA demethylation. This helps us to understand the function of DNA methylation in plants and opens the possibility of using epigenetic inhibitors in biotechnology.

Obesity-Induced Regulator of Calcineurin 1 Overexpression Leads to β-Cell Failure Through Mitophagy Pathway Inhibition

Aims: Type 2 diabetes (T2D) is associated with pancreatic β-cell dysfunction, manifested by reduced glucose-stimulated insulin secretion (GSIS). The regulator of calcineurin 1 (RCAN1) in islets is an endogenous inhibitor of calcium-activated protein phosphatase. Previous studies have indicated that global RCAN1 overexpression under high nutrient stress is involved in insulin resistance in T2D. However, the specific role and mechanism of this gene's overexpression in pancreatic β-cells have not been thoroughly elucidated to date. Results: In this study, we showed that mice overexpressing islet-specific RCAN1 exhibited a prediabetic phenotype with markedly reduced GSIS under nutrient stress. Overexpression of RCAN1 increased the autophagy-associated DNA methylation level of Beclin-1 suppressing the induction of autophagy, affected the protein kinase B, and downregulated the activation of mammalian target of rapamycin, leading to Miro1-mediated mitophagy deficiency. Furthermore, the exacerbated impairment of autophagy induction and mitophagy flux failures induced β-cell apoptosis, resulting in GSIS impairment, lipid imbalance, and NOD-like receptor 3 proinflammation under high nutrient stress in mice. Innovation: Our present data identify a detrimental effect of RCAN1 overexpression on Miro1-mediated mitophagy deficiency and β-cell dysfunction in high-fat diet-fed RCAN1 overexpressing mice. Conclusion: Our results revealed that strategies targeting RCAN1 in vivo may provide a therapeutic target to enhance β-cell mitophagy quality and may determine the crucial factor in T2D development.

Induction of Transgene Suppression in Plants via External Application of Synthetic dsRNA

Recent investigations show that exogenously applied small interfering RNAs (siRNA) and long double-stranded RNA (dsRNA) precursors can be taken up and translocated in plants to induce RNA interference (RNAi) in the plant or in its fungal pathogen. The question of whether genes in the plant genome can undergo suppression as a result of exogenous RNA application on plant surface is almost unexplored. This study analyzed whether it is possible to influence transcript levels of transgenes, as more prone sequences to silencing, in Arabidopsis genome by direct exogenous application of target long dsRNAs. The data revealed that in vitro synthesized dsRNAs designed to target the gene coding regions of enhanced green fluorescent protein (EGFP) or neomycin phosphotransferase II (NPTII) suppressed their transcript levels in Arabidopsis. The fact that, simple exogenous application of polynucleotides can affect mRNA levels of plant transgenes, opens new opportunities for the development of new scientific techniques and crop improvement strategies.

A role for MED14 and UVH6 in heterochromatin transcription upon destabilization of silencing

The TFIIH component UVH6 and the mediator subunit MED14 are differentially required for the release of heterochromatin silencing, and MED14 regulates non-CG DNA methylation in Arabidopsis.

Constitutive heterochromatin is associated with repressive epigenetic modifications of histones and DNA which silence transcription. Yet, particular mutations or environmental changes can destabilize heterochromatin-associated silencing without noticeable changes in repressive epigenetic marks. Factors allowing transcription in this nonpermissive chromatin context remain poorly known. Here, we show that the transcription factor IIH component UVH6 and the mediator subunit MED14 are both required for heat stress–induced transcriptional changes and release of heterochromatin transcriptional silencing in Arabidopsis thaliana. We find that MED14, but not UVH6, is required for transcription when heterochromatin silencing is destabilized in the absence of stress through mutating the MOM1 silencing factor. In this case, our results raise the possibility that transcription dependency over MED14 might require intact patterns of repressive epigenetic marks. We also uncover that MED14 regulates DNA methylation in non-CG contexts at a subset of RNA-directed DNA methylation target loci. These findings provide insight into the control of heterochromatin transcription upon silencing destabilization and identify MED14 as a regulator of DNA methylation.

Effect of modeled microgravity on UV-C-induced interplant communication of Arabidopsis thaliana

Controlled ecological life support systems (CELSS) will be an important feature of long-duration space missions of which higher plants are one of the indispensable components. Because of its pivotal role in enabling plants to cope with environmental stress, interplant communication might have important implications for the ecological stability of such CELSS. However, the manifestations of interplant communication in microgravity conditions have yet to be fully elucidated. To address this, a well-established Arabidopsis thaliana co-culture experimental system, in which UV-C-induced airborne interplant communication is evaluated by the alleviation of transcriptional gene silencing (TGS) in bystander plants, was placed in microgravity modeled by a two-dimensional rotating clinostat. Compared with plants under normal gravity, TGS alleviation in bystander plants was inhibited in microgravity. Moreover, TGS alleviation was also prevented when plants of the pgm-1 line, which are impaired in gravity sensing, were used in either the UV-C-irradiated or bystander group. In addition to the specific TGS-loci, interplant communication-shaped genome-wide DNA methylation in bystander plants was altered under microgravity conditions. These results indicate that interplant communications might be modified in microgravity. Time course analysis showed that microgravity interfered with both the production of communicative signals in UV-C-irradiated plants and the induction of epigenetic responses in bystander plants. This was further confirmed by the experimental finding that microgravity also prevented the response of bystander plants to exogenous methyl jasmonate (JA) and methyl salicylate (SA), two well-known airborne signaling molecules, and down-regulated JA and SA biosynthesis in UV-C-irradiated plants.

Arabidopsis proteins with a transposon-related domain act in gene silencing

Transposable elements (TEs) are prevalent in most eukaryotes, and host genomes have devised silencing strategies to rein in TE activity. One of these, transcriptional silencing, is generally associated with DNA methylation and short interfering RNAs. Here we show that the Arabidopsis genes MAIL1 and MAIN define an alternative silencing pathway independent of DNA methylation and short interfering RNAs. Mutants for MAIL1 or MAIN exhibit release of silencing and appear to show impaired condensation of pericentromeric heterochromatin. Phylogenetic analysis suggests not only that MAIL1 and MAIN encode a retrotransposon-related plant mobile domain, but also that host plant mobile domains were captured by DNA transposons during plant evolution. Our results reveal a role for Arabidopsis proteins with a transposon-related domain in gene silencing.

UV-C-Induced alleviation of transcriptional gene silencing through plant-plant communication: Key roles of jasmonic acid and salicylic acid pathways

Plant stress responses at the epigenetic level are expected to allow more permanent changes of gene expression and potentially long-term adaptation. While it has been reported that plants subjected to adverse environments initiate various stress responses in their neighboring plants, little is known regarding epigenetic responses to external stresses mediated by plant-plant communication. In this study, we show that DNA repetitive elements of Arabidopsis thaliana, whose expression is inhibited epigenetically by transcriptional gene silencing (TGS) mechanism, are activated by UV-C irradiation through airborne plant-plant and plant-plant-plant communications, accompanied by DNA demethylation at CHH sites. Moreover, the TGS is alleviated by direct treatments with exogenous methyl jasmonate (MeJA) and methyl salicylate (MeSA). Further, the plant-plant and plant-plant-plant communications are blocked by mutations in the biosynthesis or signaling of jasmonic acid (JA) or salicylic acid (SA), indicating that JA and SA pathways are involved in the interplant communication for epigenetic responses. For the plant-plant-plant communication, stress cues are relayed to the last set of receiver plants by promoting the production of JA and SA signals in relaying plants, which exhibit upregulated expression of genes for JA and SA biosynthesis and enhanced emanation of MeJA and MeSA.

Hyperosmotic stress memory in Arabidopsis is mediated by distinct epigenetically labile sites in the genome and is restricted in the male germline by DNA glycosylase activity

Inducible epigenetic changes in eukaryotes are believed to enable rapid adaptation to environmental fluctuations. We have found distinct regions of the Arabidopsis genome that are susceptible to DNA (de)methylation in response to hyperosmotic stress. The stress-induced epigenetic changes are associated with conditionally heritable adaptive phenotypic stress responses. However, these stress responses are primarily transmitted to the next generation through the female lineage due to widespread DNA glycosylase activity in the male germline, and extensively reset in the absence of stress. Using the CNI1/ATL31 locus as an example, we demonstrate that epigenetically targeted sequences function as distantly-acting control elements of antisense long non-coding RNAs, which in turn regulate targeted gene expression in response to stress. Collectively, our findings reveal that plants use a highly dynamic maternal 'short-term stress memory' with which to respond to adverse external conditions. This transient memory relies on the DNA methylation machinery and associated transcriptional changes to extend the phenotypic plasticity accessible to the immediate offspring.

Locus- and Site-Specific DNA Methylation of 19 kDa Zein Genes in Maize

An interesting question in maize development is why only a single zein gene is highly expressed in each of the 19-kDa zein gene clusters (A and B types), z1A2-1 and z1B4, in the immature endosperm. For instance, epigenetic marks could provide a structural difference. Therefore, we investigated the DNA methylation of the arrays of gene copies in both promoter and gene body regions of leaf (non-expressing tissue as a control), normal endosperm, and cultured endosperm. Although we could show that expressed genes have much lower methylation levels in promoter regions than silent ones in both leaf and normal endosperm, there was surprisingly also a difference in the pattern of the z1A and z1B gene clusters. The expression of z1B gene is suppressed by increased DNA methylation and activated with reduced DNA methylation, whereas z1A gene expression is not. DNA methylation in gene coding regions is higher in leaf than in endosperm, whereas no significant difference is observed in gene bodies between expressed and non-expressed gene copies. A median CHG methylation (25–30%) appears to be optimal for gene expression. Moreover, tissue-cultured endosperm can reset the DNA methylation pattern and tissue-specific gene expression. These results reveal that DNA methylation changes of the 19-kDa zein genes is subject to plant development and tissue culture treatment, but varies in different chromosomal locations, indicating that DNA methylation changes do not apply to gene expression in a uniform fashion. Because tissue culture is used to produce transgenic plants, these studies provide new insights into variation of gene expression of integrated sequences.

Essential RNA-Based Technologies and Their Applications in Plant Functional Genomics

Genome sequencing has not only extended our understanding of the blueprints of many plant species but has also revealed the secrets of coding and non-coding genes. We present here a brief introduction to and personal account of key RNA-based technologies, as well as their development and applications for functional genomics of plant coding and non-coding genes, with a focus on short tandem target mimics (STTMs), artificial microRNAs (amiRNAs), and CRISPR/Cas9. In addition, their use in multiplex technologies for the functional dissection of gene networks is discussed.

Evolutionary transgenomics: prospects and challenges

Many advances in our understanding of the genetic basis of species differences have arisen from transformation experiments, which allow us to study the effect of genes from one species (the donor) when placed in the genetic background of another species (the recipient). Such interspecies transformation experiments are usually focused on candidate genes - genes that, based on work in model systems, are suspected to be responsible for certain phenotypic differences between the donor and recipient species. We suggest that the high efficiency of transformation in a few plant species, most notably Arabidopsis thaliana, combined with the small size of typical plant genes and their cis-regulatory regions allow implementation of a screening strategy that does not depend upon a priori candidate gene identification. This approach, transgenomics, entails moving many large genomic inserts of a donor species into the wild type background of a recipient species and then screening for dominant phenotypic effects. As a proof of concept, we recently conducted a transgenomic screen that analyzed more than 1100 random, large genomic inserts of the Alabama gladecress Leavenworthia alabamica for dominant phenotypic effects in the A. thaliana background. This screen identified one insert that shortens fruit and decreases A. thaliana fertility. In this paper we discuss the principles of transgenomic screens and suggest methods to help minimize the frequencies of false positive and false negative results. We argue that, because transgenomics avoids committing in advance to candidate genes it has the potential to help us identify truly novel genes or cryptic functions of known genes. Given the valuable knowledge that is likely to be gained, we believe the time is ripe for the plant evolutionary community to invest in transgenomic screens, at least in the mustard family Brassicaceae where many species are amenable to efficient transformation.

Inverted-Repeat RNAs Targeting FT Intronic Regions Promote FT Expression in Arabidopsis

Transcriptional gene silencing (TGS) is often associated with promoter methylation in both animals and plants. However, the function of DNA methylation in the intragenic region remains unclear. Here, we confirmed that promoter methylation of FLOWERING LOCUS T (FT) led to gene silencing; in contrast, we found that intragenic methylation triggered by RNA-directed DNA methylation (RdDM) promoted FT expression. DNA methylation of the FT gene body blocked FLC repressor binding to the CArG boxes. However, when the boxes were not directly targeted by inverted-repeat RNAs (IRs), FLC binding blocked spreading of DNA methylation to theses sequences. Notwithstanding the FLC binding, FT was still activated under this condition. The DNA methylation was accompanied by elevated H3K9 methylation levels on the FT gene body. More importantly, the FT diurnal and organ-specific expression pattern was preserved in the activated plants. Our data demonstrate that the same type of epigenetic modification can lead to an opposite genetic outcome depending on the location of the modification on the gene locus. Moreover, we highlight a novel strategy to activate gene expression without changing its spatio-temporal regulatory patterns.

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.

Small RNA deep sequencing identifies novel and salt-stress-regulated microRNAs from roots of Medicago sativa and Medicago truncatula

Small 21- to 24-nucleotide (nt) ribonucleic acids (RNAs), notably the microRNA (miRNA), are emerging as a posttranscriptional regulation mechanism. Salt stress is one of the primary abiotic stresses that cause the crop losses worldwide. In saline lands, root growth and function of plant are determined by the action of environmental salt stress through specific genes that adapt root development to the restrictive condition. To elucidate the role of miRNAs in salt stress regulation in Medicago, we used a high-throughput sequencing approach to analyze four small RNA libraries from roots of Zhongmu-1 (Medicago sativa) and Jemalong A17 (Medicago truncatula), which were treated with 300 mM NaCl for 0 and 8 h. Each library generated about 20 million short sequences and contained predominantly small RNAs of 24-nt length, followed by 21-nt and 22-nt small RNAs. Using sequence analysis, we identified 385 conserved miRNAs from 96 families, along with 68 novel candidate miRNAs. Of all the 68 predicted novel miRNAs, 15 miRNAs were identified to have miRNA*. Statistical analysis on abundance of sequencing read revealed specific miRNA showing contrasting expression patterns between M. sativa and M. truncatula roots, as well as between roots treated for 0 and 8 h. The expression of 10 conserved and novel miRNAs was also quantified by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). The miRNA precursor and target genes were predicted by bioinformatics analysis. We concluded that the salt stress related conserved and novel miRNAs may have a large variety of target mRNAs, some of which might play key roles in salt stress regulation of Medicago.

The histone chaperone complex HIR maintains nucleosome occupancy and counterbalances impaired histone deposition in CAF-1 complex mutants

Chromatin organization is essential for coordinated gene expression, genome stability, and inheritance of epigenetic information. The main components involved in chromatin assembly are specific complexes such as Chromatin Assembly Factor 1 (CAF-1) and Histone Regulator (HIR), which deposit histones in a DNA synthesis-dependent or -independent manner, respectively. Here, we characterize the role of the plant orthologs Histone Regulator A (HIRA), Ubinuclein (UBN) and Calcineurin Binding protein 1 (CABIN1), which constitute the HIR complex. Arabidopsis loss-of-function mutants for the various subunits of the complex are viable, but hira mutants show reduced fertility. We show that loss of HIRA reduces extractable histone H3 protein levels and decreases nucleosome occupancy at both actively transcribed genes and heterochromatic regions. Concomitantly, HIRA contributes to maintenance of silencing of pericentromeric repeats and certain transposons. A genetic analysis based on crosses between mutants deficient in subunits of the CAF-1 and HIR complexes showed that simultaneous loss of both the CAF-1 and HIR histone H3 chaperone complexes severely affects plant survival, growth and reproductive development. Our results suggest that HIRA partially rescues impaired histone deposition in fas mutants to preserve nucleosome occupancy, implying plasticity in histone variant interaction and deposition.

Development of a VIGS vector based on the β-satellite DNA associated with bhendi yellow vein mosaic virus

Bhendi yellow vein mosaic virus (BYMV) is a monopartite begomovirus with an associated β-satellite. βC1 ORF encoded by the β-satellite is the symptom determinant and a strong suppressor of post transcriptional gene silencing. To create a virus induced gene silencing vector based upon the β-satellite associated with BYVMV the βC1 ORF was replaced with multiple cloning sites. GFP transgene and plant endogenous genes Su, PDS, PCNA and AGO1 were cloned into β-satellite based VIGS vector. GFP expression was silenced in the GFP expressing transgenic 16c Nicotiana benthamiana plants infiltrated with VIGS vector carrying GFP gene inside. N. benthamiana plants infiltrated with the VIGS vector harboring the endogenous genes Su, PDS, PCNA and AGO1 produced the phenotypic symptoms yellowing of the veins, photobleaching of the veins, stunting of the plant and upward leaf curling, respectively. Real time PCR analyses revealed a reduction in the levels of the corresponding transgene or endogenous target mRNA. The β-satellite based VIGS vector was able to silence the target genes effectively. Hence, BYVMV β-satellite based VIGS vector can be used in functional genomics studies.

Panspecies small-molecule disruptors of heterochromatin-mediated transcriptional gene silencing

Heterochromatin underpins gene repression, genome integrity, and chromosome segregation. In the fission yeast Schizosaccharomyces pombe, conserved protein complexes effect heterochromatin formation via RNA interference-mediated recruitment of a histone H3 lysine 9 methyltransferase to cognate chromatin regions. To identify small molecules that inhibit heterochromatin formation, we performed an in vivo screen for loss of silencing of a dominant selectable kanMX reporter gene embedded within fission yeast centromeric heterochromatin. Two structurally unrelated compounds, HMS-I1 and HMS-I2, alleviated kanMX silencing and decreased repressive H3K9 methylation levels at the transgene. The decrease in methylation caused by HMS-I1 and HMS-I2 was observed at all loci regulated by histone methylation, including centromeric repeats, telomeric regions, and the mating-type locus, consistent with inhibition of the histone deacetylases (HDACs) Clr3 and/or Sir2. Chemical-genetic epistasis and expression profiles revealed that both compounds affect the activity of the Clr3-containing Snf2/HDAC repressor complex (SHREC). In vitro HDAC assays revealed that HMS-I1 and HMS-I2 inhibit Clr3 HDAC activity. HMS-I1 also alleviated transgene reporter silencing by heterochromatin in Arabidopsis and a mouse cell line, suggesting a conserved mechanism of action. HMS-I1 and HMS-I2 bear no resemblance to known inhibitors of chromatin-based activities and thus represent novel chemical probes for heterochromatin formation and function.

Heat-induced release of epigenetic silencing reveals the concealed role of an imprinted plant gene

Epigenetic mechanisms suppress the transcription of transposons and DNA repeats; however, this suppression can be transiently released under prolonged heat stress. Here we show that the Arabidopsis thaliana imprinted gene SDC, which is silent during vegetative growth due to DNA methylation, is activated by heat and contributes to recovery from stress. SDC activation seems to involve epigenetic mechanisms but not canonical heat-shock perception and signaling. The heat-mediated transcriptional induction of SDC occurs particularly in young developing leaves and is proportional to the level of stress. However, this occurs only above a certain window of absolute temperatures and, thus, resembles a thermal-sensing mechanism. In addition, the re-silencing kinetics during recovery can be entrained by repeated heat stress cycles, suggesting that epigenetic regulation in plants may conserve memory of stress experience. We further demonstrate that SDC contributes to the recovery of plant biomass after stress. We propose that transcriptional gene silencing, known to be involved in gene imprinting, is also co-opted in the specific tuning of SDC expression upon heat stress and subsequent recovery. It is therefore possible that dynamic properties of the epigenetic landscape associated with silenced or imprinted genes may contribute to regulation of their expression in response to environmental challenges.

In plants, expression of certain imprinted genes is restricted to embryo nourishing tissue, the endosperm. Since these genes are silenced by epigenetic mechanisms during vegetative growth, it has been assumed that they have no role in this phase of the plant life cycle. Here, we report on heat-mediated release of epigenetic silencing and ectopic activation of the Arabidopsis thaliana endosperm-imprinted gene SDC. The stress induced activation of SDC involves epigenetic regulation but not the canonical heat-shock perception and signaling, and it seems to be required for efficient growth recovery after the stress. Our results exemplify a potential concealed role of an imprinted gene in plant responses to environmental challenges.

Identification of genes preventing transgenerational transmission of stress-induced epigenetic states

Examples of transgenerational transmission of environmentally induced epigenetic traits remain rare and disputed. Abiotic stress can release the transcription of epigenetically suppressed transposons and, noticeably, this activation is only transient. Therefore, it is likely that mechanisms countering the mitotic and meiotic inheritance of stress-triggered chromatin changes must exist but are undefined. To reveal these mechanisms, we screened for Arabidopsis mutants impaired in the resetting of stress-induced loss of epigenetic silencing and found that two chromatin regulators, Decrease in DNA methylation1 (DDM1) and Morpheus' Molecule1 (MOM1), act redundantly to restore prestress state and thus erase "epigenetic stress memory". In ddm1 mutants, stress hyperactivates heterochromatic transcription and transcription persists longer than in the wild type. However, this newly acquired state is not transmitted to the progeny. Strikingly, although stress-induced transcription in mom1 mutants is as rapidly silenced as in wild type, in ddm1 mom1 double mutants, transcriptional signatures of stress are able to persist and are found in the progeny of plants stressed as small seedlings. Our results reveal an important, previously unidentified function of DDM1 and MOM1 in rapid resetting of stress induced epigenetic states, and therefore also in preventing their mitotic propagation and transgenerational inheritance.

Expression analysis of Arabidopsis XH/XS-domain proteins indicates overlapping and distinct functions for members of this gene family

RNA-directed DNA methylation (RdDM) is essential for de novo DNA methylation in higher plants, and recent reports established novel elements of this silencing pathway in the model organism Arabidopsis thaliana. Involved in de novo DNA methylation 2 (IDN2) and the closely related factor of DNA methylation (FDM) are members of a plant-specific family of dsRNA-binding proteins characterized by conserved XH/XS domains and implicated in the regulation of RdDM at chromatin targets. Genetic analyses have suggested redundant as well as non-overlapping activities for different members of the gene family. However, detailed insights into the function of XH/XS-domain proteins are still elusive. By the generation and analysis of higher-order mutant combinations affected in IDN2 and further members of the gene family, we have provided additional evidence for their redundant activity. Distinct roles for members of the XH/XS-domain gene family were indicated by differences in their expression and subcellular localization. Fluorescent protein-tagged FDM genes were expressed either in nuclei or in the cytoplasm, suggestive of activities of XH/XS-domain proteins in association with chromatin as well as outside the nuclear compartment. In addition, we observed altered location of a functional FDM1-VENUS reporter from the nucleus into the cytoplasm under conditions when availability of further FDM proteins was limited. This is suggestive of a mechanism by which redistribution of XH/XS-domain proteins could compensate for the loss of closely related proteins.

Meristem-specific expression of epigenetic regulators safeguards transposon silencing in Arabidopsis

In plants, transposable elements (TEs) are kept inactive by transcriptional gene silencing (TGS). TGS is established and perpetuated by RNA-directed DNA methylation (RdDM) and maintenance methylation pathways, respectively. Here, we describe a novel RdDM function specific for shoot apical meristems that reinforces silencing of TEs during early vegetative growth. In meristems, RdDM counteracts drug-induced interference with TGS maintenance and consequently prevents TE activation. Simultaneous disturbance of both TGS pathways leads to transcriptionally active states of repetitive sequences that are inherited by somatic tissues and partially by the progeny. This apical meristem-specific mechanism is mediated by increased levels of TGS factors and provides a checkpoint for correct epigenetic inheritance during the transition from vegetative to reproductive phase and to the next generation.

Construction of Differential-Methylation Subtractive Library

Stress-induced ROS changes DNA methylation patterns. A protocol combining methylation-sensitive restriction endonuclease (MS-RE) digestion with suppression subtractive hybridization (SSH) to construct the differential-methylation subtractive library was developed for finding genes regulated by methylation mechanism under cold stress. The total efficiency of target fragment detection was 74.64%. DNA methylation analysis demonstrated the methylation status of target fragments changed after low temperature or DNA methyltransferase inhibitor treatment. Transcription level analysis indicated that demethylation of DNA promotes gene expression level. The results proved that our protocol was reliable and efficient to obtain gene fragments in differential-methylation status.

The role of small RNAs in vaccination

The concept of vaccination came to light following Edward Jenner's classical observation on milkmaids who were protected against smallpox. However, plants lack the cellular based immunity system and thus it was not appreciated earlier that plants can also be protected from their pathogens. But phenomena like cross-protection, pathogen derived resistance (PDR), viral recovery, etc. in plants suggested that plants have also evolved immunity against their pathogens. The further advances in the field revealed that an endogenous defense system could have multiple prongs. With the advent of RNAi, it was clear that the antiviral immune responses are related to the induction of specific small RNAs. The detection of virus specific small RNAs (vsiRNA) in immunized plants confirmed their roles in the immunity against pathogens. Although many issues related to antiviral mechanisms are yet to be addressed, the existing tools of RNAi can be efficiently used to control the invading viruses in transgenic plants. It is also possible that the microRNA(s) induced in infected plants impart immunity against viral pathogens. So the small RNA molecules play a vital role in defense system and these can be engineered to enhance the immunity against specific viral pathogens.

Geminivirus Rep protein interferes with the plant DNA methylation machinery and suppresses transcriptional gene silencing

Cytosine methylation is an epigenetic mark that promotes gene silencing and plays an important role in genome defence against transposons and invading DNA viruses. Previous data showed that the largest family of single-stranded DNA viruses, Geminiviridae, prevents methylation-mediated transcriptional gene silencing (TGS) by interfering with the proper functioning of the plant methylation cycle. Here, we describe a novel counter-defence strategy used by geminiviruses, which reduces the expression of the plant maintenance DNA methyltransferases, METHYLTRANSFERASE 1 (MET1) and CHROMOMETHYLASE 3 (CMT3), in both locally and systemically infected tissues. We demonstrated that the virus-mediated repression of these two maintenance DNA methyltransferases is widespread among geminivirus species. Additionally, we identified Rep (Replication associated protein) as the geminiviral protein responsible for the repression of MET1 and CMT3, and another viral protein, C4, as an ancillary player in MET1 down-regulation. The presence of Rep suppressed TGS of an Arabidopsis thaliana transgene and of host loci whose expression was strongly controlled by CG methylation. Bisulfite sequencing analyses showed that the expression of Rep caused a substantial reduction in the levels of DNA methylation at CG sites. Our findings suggest that Rep, the only viral protein essential for replication, displays TGS suppressor activity through a mechanism distinct from that thus far described for geminiviruses.

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.

Arabidopsis HIT4 encodes a novel chromocentre-localized protein involved in the heat reactivation of transcriptionally silent loci and is essential for heat tolerance in plants

The Arabidopsis mutant heat-intolerant 4-1 (hit4-1) was isolated from an ethyl methanesulphonate-mutagenized M2 population on the basis of its inability to withstand prolonged heat stress (4 days at 37°C). Further characterization indicated that hit4-1 was impaired specifically in terms of basal but not acquired thermotolerance. Map-based cloning revealed that the HIT4 gene encoded a plant-specific protein for which the molecular function has yet to be studied. To investigate the cellular role of HIT4 and hence elucidate better its protective function in heat tolerance in plants, a GFP-HIT4 reporter construct was created for a protoplast transient expression assay. Results showed that fluorescently tagged HIT4 was localized to the chromocentre, a condensed heterochromatin domain that harbours repetitive elements for which transcription is normally suppressed by transcriptional gene silencing (TGS). DAPI-staining analysis and FISH with a probe that targeted centromeric repeats showed that heat-induced chromocentre decondensation was inhibited in nuclei of hit4-1 subjected to direct heat treatment, but not in those that were allowed to acquire thermotolerance. Moreover, heat reactivation of various TGS loci, regardless of whether they were endogenous or transgenic, or existed as a single copy or as repeats, was found to be attenuated in hit4-1. Meanwhile, the levels of transcripts of heat shock protein genes in response to heat stress were similar in both hit4-1 and wild-type plants. Collectively, these results demonstrated that HIT4 defines a new TGS regulator that acts at the level of heterochromatin organization and is essential for basal thermotolerance in plants.

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.

Role of RNA interference (RNAi) in the Moss Physcomitrella patens

RNA interference (RNAi) is a mechanism that regulates genes by either transcriptional (TGS) or posttranscriptional gene silencing (PTGS), required for genome maintenance and proper development of an organism. Small non-coding RNAs are the key players in RNAi and have been intensively studied in eukaryotes. In plants, several classes of small RNAs with specific sizes and dedicated functions have evolved. The major classes of small RNAs include microRNAs (miRNAs) and small interfering RNAs (siRNAs), which differ in their biogenesis. miRNAs are synthesized from a short hairpin structure while siRNAs are derived from long double-stranded RNAs (dsRNA). Both miRNA and siRNAs control the expression of cognate target RNAs by binding to reverse complementary sequences mediating cleavage or translational inhibition of the target RNA. They also act on the DNA and cause epigenetic changes such as DNA methylation and histone modifications. In the last years, the analysis of plant RNAi pathways was extended to the bryophyte Physcomitrella patens, a non-flowering, non-vascular ancient land plant that diverged from the lineage of seed plants approximately 450 million years ago. Based on a number of characteristic features and its phylogenetic key position in land plant evolution P. patens emerged as a plant model species to address basic as well as applied topics in plant biology. Here we summarize the current knowledge on the role of RNAi in P. patens that shows functional overlap with RNAi pathways from seed plants, and also unique features specific to this species.

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

Posttranscriptional gene silencing (PTGS) mediated by sense transgenes (S-PTGS) results in RNA degradation and DNA methylation of the transcribed region. Through a forward genetic screen, a mutant defective in the Histone3 Lysine4 di/trimethyl (H3K4me2/3) demethylase Jumonji-C (JmjC) domain-containing protein14 (JMJ14) was identified. This mutant reactivates various transgenes silenced by S-PTGS and shows reduced Histone3 Lysine9 Lysine14 acetylation (H3K9K14Ac) levels, reduced polymerase II occupancy, reduced transgene transcription, and increased DNA methylation in the promoter region, consistent with the hypothesis that high levels of transcription are required to trigger S-PTGS. The jmj14 mutation also reduces the expression of transgenes that do not trigger S-PTGS. Moreover, expression of transgenes that undergo S-PTGS in a wild-type background is reduced in jmj14 sgs3 double mutants compared with PTGS-deficient sgs3 mutants, indicating that JMJ14 is required for high levels of transcription in a PTGS-independent manner. Whereas endogenous loci regulated by JMJ14 exhibit increased H3K4me2 and H3K4me3 levels in the jmj14 mutant, transgene loci exhibit unchanged H3K4me2 and decreased H3K4me3 levels. Because jmj14 mutations impair PTGS of transgenes expressed under various plant or viral promoters, we hypothesize that JMJ14 demethylation activity is prevented by antagonistic epigenetic marks specifically imposed at transgene loci. Removing JMJ14 likely allows other H3K4 demethylases encoded by the Arabidopsis thaliana genome to act on transgenes and reduce transcription levels, thus preventing the triggering of S-PTGS.

RNA interference-mediated gene knockdown within specific cell types

In plants, RNA interference (RNAi)-induced gene silencing can spread from the initiation site to nearby cells. The silencing signal moves from cell-to-cell through plasmodesmata and, over long distances, through the phloem. In this study, we employed a nuclear-localized GFP fusion protein to visualize the pattern of gene silencing induced by three different transgenes expressing double-stranded RNA (dsRNA) in Arabidopsis root tips. In all cases, we found that dsRNA-induced silencing did not spread from the silencing initiation site to adjacent cells. In the first set of experiments, in a transgenic background expressing nuclear-localized GFP within a contiguous cell layer that included endodermis, cortex/endodermis (joint) initial (CEI) cells and the quiescent center (QC) cells, expression of the marker gene was silenced specifically in the QC cells without affecting gene expression in the adjacent CEI and endodermal cells. The next two sets of experiments examined the knockdown of two endogenous genes. We observed that silencing was completely restricted to the QC and endodermal cells within which the dsRNA transgenes were expressed. Overall, these results accentuate one important aspect of RNAi-induced gene silencing, that it can be cell autonomous, and demonstrated the feasibility of selective gene knockdown within specific cell types.

NERD, a plant-specific GW protein, defines an additional RNAi-dependent chromatin-based pathway in Arabidopsis

In Arabidopsis, transcriptional gene silencing (TGS) can be triggered by 24 nt small-interfering RNAs (siRNAs) through the RNA-directed DNA methylation (RdDM) pathway. By functional analysis of NERD, a GW repeat- and PHD finger-containing protein, we demonstrate that Arabidopsis harbors a second siRNA-dependent DNA methylation pathway targeting a subset of nonconserved genomic loci. The activity of the NERD-dependent pathway differs from RdDM by the fact that it relies both on silencing-related factors previously implicated only in posttranscriptional gene silencing (PTGS), including RNA-DEPENDENT RNA POLYMERASE1/6 and ARGONAUTE2, and most likely on 21 nt siRNAs. A central role for NERD in integrating RNA silencing and chromatin signals in transcriptional silencing is supported by data showing that it binds both to histone H3 and AGO2 proteins and contributes to siRNA accumulation at a NERD-targeted locus. Our results unravel the existence of a conserved chromatin-based RNA silencing pathway encompassing both PTGS and TGS components in plants.

A novel HSI2 mutation in Arabidopsis affects the PHD-like domain and leads to derepression of seed-specific gene expression

Two related B3 domain transcriptional repressors, HSI2 (HIGH-LEVEL EXPRESSION OF SUGAR-INDUCIBLE GENE2)/VAL1 (VP1/ABI3-LIKE1) and HSL1 (HSI2-LIKE1)/VAL2, function redundantly to repress key transcriptional regulators of seed maturation genes in Arabidopsis thaliana seedlings. Using a forward genetic screen designed to isolate trans-acting mutants that affected expression of a transgene containing the glutathione S-transferase F8 promoter::luciferase (GSTF8::LUC) reporter, we identified a novel HSI2 mutant allele, hsi2-4, that exhibits constitutively elevated luciferase expression while expression of the endogenous GSTF8 transcript remains unchanged. The hsi2-4 lesion was found to be a missense mutation that results in the substitution of a conserved cysteine within the plant homeodomain-like (PHD) motif of HSI2. Microarray analysis of hsi2-4 and hsi2-4 hsl1 mutants indicated that the HSI2 PHD-like domain functions non-redundantly to repress a subset of seed maturation genes, including those that encode AGL15 (AGAMOUS-LIKE15), FUSCA3 (FUS3), cruciferins, cupin family proteins, late-embryogenesis abundant protein, oleosins, 2S albumins and other seed-specific proteins in Arabidopsis seedlings. Many genes that are responsive to this mutation in the HSI2 PHD-like domain are enriched in histone H3 trimethylation on lysine 27 residues (H3K27me3), a repressive epigenetic mark. Chromatin immunoprecipitation analysis showed that sequences of the GSTF8::LUC transgene are enriched in H3K27me3 in a HSI2 PHD domain-dependent manner. These results indicate that the transcriptional repression activity of the HSI2 PHD domain could be mediated, at least in part, by its participation in the deposition of H3K27me3 on the chromatin of specific target genes.

Structural basis of transcriptional gene silencing mediated by Arabidopsis MOM1

Shifts between epigenetic states of transcriptional activity are typically correlated with changes in epigenetic marks. However, exceptions to this rule suggest the existence of additional, as yet uncharacterized, layers of epigenetic regulation. MOM1, a protein of 2,001 amino acids that acts as a transcriptional silencer, represents such an exception. Here we define the 82 amino acid domain called CMM2 (Conserved MOM1 Motif 2) as a minimal MOM1 fragment capable of transcriptional regulation. As determined by X-ray crystallography, this motif folds into an unusual hendecad-based coiled-coil. Structure-based mutagenesis followed by transgenic complementation tests in plants demonstrate that CMM2 and its dimerization are effective for transcriptional suppression at chromosomal loci co-regulated by MOM1 and the siRNA pathway but not at loci controlled by MOM1 in an siRNA–independent fashion. These results reveal a surprising separation of epigenetic activities that enable the single, large MOM1 protein to coordinate cooperating mechanisms of epigenetic regulation.

Epigenetic shifts in transcriptional activities are usually correlated with changes in chromatin properties and covalent modification of DNA and/or histones. There are, however, exceptional regulators that are able to switch epigenetic states without the apparent involvement of changes in chromatin or DNA modifications. MOM1 protein, derived from CHD3 chromatin remodelers, belongs to this group. Here we defined a very small domain of MOM1 (less than 5% of its total sequence) that is sufficient for epigenetic regulation. We solved the structure of this domain and found that it forms a dimer with each monomer consisting of unusual consecutive 11 amino-acid hendecad repeats folding into an antiparallel coiled-coil. In vivo experiments demonstrated that the formation of this coiled-coil is essential for silencing activity; however, it is effective only at loci co-silenced by MOM1 and small RNAs. At loci not controlled by small RNAs, the entire MOM1 protein is required. Our results demonstrate that a single epigenetic regulator is able to differentially use its domains to control diverse chromosomal targets. The acquisition of the coiled-coil domain of MOM1 reflects a neofunctionalization of CHD3 proteins, which allowed MOM1 to broaden its activity and to provide input into multiple epigenetic pathways.

High-resolution structures of complexes of plant S-adenosyl-L-homocysteine hydrolase (Lupinus luteus)

S-Adenosyl-L-homocysteine hydrolase (SAHase) catalyzes the reversible breakdown of S-adenosyl-L-homocysteine (SAH) to adenosine and homocysteine. SAH is formed in methylation reactions that utilize S-adenosyl-L-methionine (SAM) as a methyl donor. By removing the SAH byproduct, SAHase serves as a major regulator of SAM-dependent biological methylation reactions. Here, the first crystal structure of SAHase of plant origin, that from the legume yellow lupin (LlSAHase), is presented. Structures have been determined at high resolution for three complexes of the enzyme: those with a reaction byproduct/substrate (adenosine), with its nonoxidizable analog (cordycepin) and with a product of inhibitor cleavage (adenine). In all three cases the enzyme has a closed conformation. A sodium cation is found near the active site, coordinated by residues from a conserved loop that hinges domain movement upon reactant binding. An insertion segment that is present in all plant SAHases is located near a substrate-pocket access channel and participates in its formation. In contrast to mammalian and bacterial SAHases, the channel is open when adenosine or cordycepin is bound and is closed in the adenine complex. In contrast to SAHases from other organisms, which are active as tetramers, the plant enzyme functions as a homodimer in solution.

RDR2 partially antagonizes the production of RDR6-dependent siRNA in sense transgene-mediated PTGS

Background

RNA-DEPENDENT RNA POLYMERASE6 (RDR6) and SUPPRESSOR of GENE SILENCING 3 (SGS3) are required for DNA methylation and post-transcriptional gene silencing (PTGS) mediated by 21-nt siRNAs produced by sense transgenes (S-PTGS). In contrast, RDR2, but not RDR6, is required for DNA methylation and TGS mediated by 24-nt siRNAs, and for cell-to-cell spreading of IR-PTGS mediated by 21-nt siRNAs produced by inverted repeat transgenes under the control of a phloem-specific promoter.

Principal Findings

In this study, we examined the role of RDR2 and RDR6 in S-PTGS. Unlike RDR6, RDR2 is not required for DNA methylation of transgenes subjected to S-PTGS. RDR6 is essential for the production of siRNAs by transgenes subjected to S-PTGS, but RDR2 also contributes to the production of transgene siRNAs when RDR6 is present because rdr2 mutations reduce transgene siRNA accumulation. However, the siRNAs produced via RDR2 likely are counteractive in wildtype plants because impairement of RDR2 increases S-PTGS efficiency at a transgenic locus that triggers limited silencing, and accelerates S-PTGS at a transgenic locus that triggers efficient silencing.

Conclusions/Significance

These results suggest that RDR2 and RDR6 compete for RNA substrates produced by transgenes subjected to S-PTGS. RDR2 partially antagonizes RDR6 because RDR2 action likely results in the production of counteractive siRNA. As a result, S-PTGS efficiency is increased in rdr2 mutants.

An assessment of transgenomics as a tool for identifying genes involved in the evolutionary differentiation of closely related plant species

• Transgenomics is the process of introducing genomic clones from a donor species into a recipient species and then screening the resultant transgenic lines for phenotypes of interest. This method might allow us to find genes involved in the evolution of phenotypic differences between species as well as genes that have the potential to contribute to reproductive isolation: potential speciation genes. • More than 1100 20-kbp genomic clones from Leavenworthia alabamica were moved into Arabidopsis thaliana by transformation. After screening a single primary transformant for each line, clones associated with mutant phenotypes were tested for repeatability and co-segregation. • We found 84 clones with possible phenotypic effects, of which eight were repeatedly associated with the same phenotype. One clone, 11_11B, co-segregated with a short fruit phenotype. Further study showed that 11_11B affects seed development, with as much as one-third of the seeds aborted in some fruit. • Transgenomics is a viable strategy for discovering genes of evolutionary interest. We identify methods to reduce false positives and false negatives in the future. 11_11B can be viewed as a potential speciation gene, illustrating the value of transgenomics for studying the molecular basis of reproductive isolation.

Genetic rearrangements can modify chromatin features at epialleles

Analogous to genetically distinct alleles, epialleles represent heritable states of different gene expression from sequence-identical genes. Alleles and epialleles both contribute to phenotypic heterogeneity. While alleles originate from mutation and recombination, the source of epialleles is less well understood. We analyze active and inactive epialleles that were found at a transgenic insert with a selectable marker gene in Arabidopsis. Both converse expression states are stably transmitted to progeny. The silent epiallele was previously shown to change its state upon loss-of-function of trans-acting regulators and drug treatments. We analyzed the composition of the epialleles, their chromatin features, their nuclear localization, transcripts, and homologous small RNA. After mutagenesis by T-DNA transformation of plants carrying the silent epiallele, we found new active alleles. These switches were associated with different, larger or smaller, and non-overlapping deletions or rearrangements in the 3' regions of the epiallele. These cis-mutations caused different degrees of gene expression stability depending on the nature of the sequence alteration, the consequences for transcription and transcripts, and the resulting chromatin organization upstream. This illustrates a tight dependence of epigenetic regulation on local structures and indicates that sequence alterations can cause epigenetic changes at some distance in regions not directly affected by the mutation. Similar effects may also be involved in gene expression and chromatin changes in the vicinity of transposon insertions or excisions, recombination events, or DNA repair processes and could contribute to the origin of new epialleles.

Epigenomic and RNA structural correlates of polyadenylation

Polyadenylation (poly(A)) of mRNA plays a critical role in regulating gene expression. Identifying the sequence, structural, and epigenomic determinants of poly(A) site usage is an important long term goal. Several cis elements that mediate poly(A) regulation have been identified. Highly used poly(A) sites are also known to have a greater nucleosome occupancy in the immediate downstream. However, a detailed exploration of additional epigenomic and mRNA structural correlates of poly(A) site usage has not been reported. Importantly, functional interaction between sequence, structure, and the epigenome in determining the poly(A) site usage is not known. We show that highly used poly(A) sites are positively associated with an mRNA structure that is energetically more favorable and one that better exposes a critical polyadenylation cis element. In exploring potential interplay between RNA and chromatin structure, we found that a stronger nucleosome occupancy downstream of poly(A) site strongly correlated with (1) a more favorable mRNA structure, and (2) a greater accumulation of RNA Polymerase II (PolII) at the poly(A) site. Further analysis suggested a causal relationship pointing from PolII accumulation to a stable RNA structure. Additionally, we found that distinct patterns of histone modifications characterize poly(A) sites and these epigenetic patterns alone can distinguish true poly(A) sites with ~76% accuracy and also discriminate between high and low usage poly(A) sites with ~74% accuracy. Our results suggest a causative link between chromatin structure and mRNA structure whereby a compacted chromatin downstream of the poly(A) site slows down the elongating transcript, thus facilitating the folding of nascent mRNA in a favorable structure at poly(A) site during transcription. Additionally we report hitherto unknown epigenomic correlates for poly(A) site usage.

Transgenerational epigenetic inheritance in plants

Interest in transgenerational epigenetic inheritance has intensified with the boosting of knowledge on epigenetic mechanisms regulating gene expression during development and in response to internal and external signals such as biotic and abiotic stresses. Starting with an historical background of scantily documented anecdotes and their consequences, we recapitulate the information gathered during the last 60 years on naturally occurring and induced epialleles and paramutations in plants. We present the major players of epigenetic regulation and their importance in controlling stress responses. The effect of diverse stressors on the epigenetic status and its transgenerational inheritance is summarized from a mechanistic viewpoint. The consequences of transgenerational epigenetic inheritance are presented, focusing on the knowledge about its stability, and in relation to genetically fixed mutations, recombination, and genomic rearrangement. We conclude with an outlook on the importance of transgenerational inheritance for adaptation to changing environments and for practical applications. This article is part of a Special Issue entitled "Epigenetic control of cellular and developmental processes in plants".