Molecular mechanisms in the developmental regulation of the maize Suppressor-mutator transposable element

From parasites to partners: exploring the intricacies of host-transposon dynamics and coevolution

Transposable elements, often referred to as "jumping genes," have long been recognized as genomic parasites due to their ability to integrate and disrupt normal gene function and induce extensive genomic alterations, thereby compromising the host's fitness. To counteract this, the host has evolved a plethora of mechanisms to suppress the activity of the transposons. Recent research has unveiled the host-transposon relationships to be nuanced and complex phenomena, resulting in the coevolution of both entities. Transposition increases the mutational rate in the host genome, often triggering physiological pathways such as immune and stress responses. Current gene transfer technologies utilizing transposable elements have potential drawbacks, including off-target integration, induction of mutations, and modifications of cellular machinery, which makes an in-depth understanding of the host-transposon relationship imperative. This review highlights the dynamic interplay between the host and transposable elements, encompassing various factors and components of the cellular machinery. We provide a comprehensive discussion of the strategies employed by transposable elements for their propagation, as well as the mechanisms utilized by the host to mitigate their parasitic effects. Additionally, we present an overview of recent research identifying host proteins that act as facilitators or inhibitors of transposition. We further discuss the evolutionary outcomes resulting from the genetic interactions between the host and the transposable elements. Finally, we pose open questions in this field and suggest potential avenues for future research.

Transgenerational epigenetic control of constitutive heterochromatin, transposons, and centromeres

Epigenetic mechanisms are important not only for development but also for genome stability and chromosome dynamics. The latter types of epigenetic controls can often be transgenerational. Here, we review recent progress in two examples of transgenerational epigenetic control: i) the control of constitutive heterochromatin and transposable elements and ii) epigenetic mechanisms that regulate centromere specification and functions. We also discuss the biological significance of enigmatic associations among centromeres, transposons, and constitutive heterochromatin.

Exploitation of epigenetic variation of crop wild relatives for crop improvement and agrobiodiversity preservation

Crop wild relatives (CWRs) are recognized as the best potential source of traits for crop improvement. However, successful crop improvement using CWR relies on identifying variation in genes controlling desired traits in plant germplasms and subsequently incorporating them into cultivars. Epigenetic diversity may provide an additional layer of variation within CWR and can contribute novel epialleles for key traits for crop improvement. There is emerging evidence that epigenetic variants of functional and/or agronomic importance exist in CWR gene pools. This provides a rationale for the conservation of epigenotypes of interest, thus contributing to agrobiodiversity preservation through conservation and (epi)genetic monitoring. Concepts and techniques of classical and modern breeding should consider integrating recent progress in epigenetics, initially by identifying their association with phenotypic variations and then by assessing their heritability and stability in subsequent generations. New tools available for epigenomic analysis offer the opportunity to capture epigenetic variation and integrate it into advanced (epi)breeding programmes. Advances in -omics have provided new insights into the sources and inheritance of epigenetic variation and enabled the efficient introduction of epi-traits from CWR into crops using epigenetic molecular markers, such as epiQTLs.

Transposable Elements as a Source of Novel Repetitive DNA in the Eukaryote Genome

The impact of transposable elements (TEs) on the evolution of the eukaryote genome has been observed in a number of biological processes, such as the recruitment of the host's gene expression network or the rearrangement of genome structure. However, TEs may also provide a substrate for the emergence of novel repetitive elements, which contribute to the generation of new genomic components during the course of the evolutionary process. In this review, we examine published descriptions of TEs that give rise to tandem sequences in an attempt to comprehend the relationship between TEs and the emergence of de novo satellite DNA families in eukaryotic organisms. We evaluated the intragenomic behavior of the TEs, the role of their molecular structure, and the chromosomal distribution of the paralogous copies that generate arrays of repeats as a substrate for the emergence of new repetitive elements in the genome. We highlight the involvement and importance of TEs in the eukaryote genome and its remodeling processes.

Epigenetics and its role in effecting agronomical traits

Climate-resilient crops with improved adaptation to the changing climate are urgently needed to feed the growing population. Hence, developing high-yielding crop varieties with better agronomic traits is one of the most critical issues in agricultural research. These are vital to enhancing yield as well as resistance to harsh conditions, both of which help farmers over time. The majority of agronomic traits are quantitative and are subject to intricate genetic control, thereby obstructing crop improvement. Plant epibreeding is the utilisation of epigenetic variation for crop development, and has a wide range of applications in the field of crop improvement. Epigenetics refers to changes in gene expression that are heritable and induced by methylation of DNA, post-translational modifications of histones or RNA interference rather than an alteration in the underlying sequence of DNA. The epigenetic modifications influence gene expression by changing the state of chromatin, which underpins plant growth and dictates phenotypic responsiveness for extrinsic and intrinsic inputs. Epigenetic modifications, in addition to DNA sequence variation, improve breeding by giving useful markers. Also, it takes epigenome diversity into account to predict plant performance and increase crop production. In this review, emphasis has been given for summarising the role of epigenetic changes in epibreeding for crop improvement.

The epiallelic potential of transposable elements and its evolutionary significance in plants

DNA provides the fundamental framework for heritability, yet heritable trait variation need not be completely ‘hard-wired’ into the DNA sequence. In plants, the epigenetic machinery that controls transposable element (TE) activity, and which includes DNA methylation, underpins most known cases of inherited trait variants that are independent of DNA sequence changes. Here, we review our current knowledge of the extent, mechanisms and potential adaptive contribution of epiallelic variation at TE-containing alleles in this group of species. For the purpose of this review, we focus mainly on DNA methylation, as it provides an easily quantifiable readout of such variation. The picture that emerges is complex. On the one hand, pronounced differences in DNA methylation at TE sequences can either occur spontaneously or be induced experimentally en masse across the genome through genetic means. Many of these epivariants are stably inherited over multiple sexual generations, thus leading to transgenerational epigenetic inheritance. Functional consequences can be significant, yet they are typically of limited magnitude and although the same epivariants can be found in nature, the factors involved in their generation in this setting remain to be determined. On the other hand, moderate DNA methylation variation at TE-containing alleles can be reproducibly induced by the environment, again usually with mild effects, and most of this variation tends to be lost across generations. Based on these considerations, we argue that TE-containing alleles, rather than their inherited epiallelic variants, are the main targets of natural selection. Thus, we propose that the adaptive contribution of TE-associated epivariation, whether stable or not, lies predominantly in its capacity to modulate TE mobilization in response to the environment, hence providing hard-wired opportunities for the flexible exploration of the phenotypic space.

This article is part of the theme issue ‘How does epigenetics influence the course of evolution?’

MSAP analysis of epigenetic changes reveals the mechanism of bicolor petal formation in Paeonia suffruticosa 'Shima Nishiki'

Paeonia suffruticosa 'Shima Nishiki' is a very valuable bicolor cultivar because of its distinctive and colorful flowers. However, our understanding of the mechanisms underlying bicolor petal formation is limited. In this study, we used the methylation-sensitive amplified polymorphism (MSAP) method to assess the levels and pattern of cytosine methylation in different-colored petals during floral development. Our data showed differences in the methylation levels of red and pink petals. The methylation rate of the red petals was consistently higher than that of the pink petals, with maximum values of 58.45% (red petals) and 44.36% (pink petals) during the S2 developmental stage. However, obvious differences were not observed in the patterns of cytosine methylation in different-colored petals; methylation and demethylation occurred simultaneously and the proportions were similar. In addition, we isolated and sequenced the differentially methylated fragments and found that one fragment was homologous to the bHLH1 gene of P. suffruticosa 'Luoyang Hong'; its expression pattern suggested that the bHLH1 gene may be involved in the regulation of the formation of bicolor flowers in P. suffruticosa 'Shima Nishiki'. These results will provide a valuable resource for further investigation of the genetic mechanisms underlying bicolor petal formation in P. suffruticosa 'Shima Nishiki'.

A CsTu-TS1 regulatory module promotes fruit tubercule formation in cucumber

The fruit epidermal features such as the size of tubercules are important fruit quality traits for cucumber production. But the mechanisms underlying tubercule formation remain elusive. Here, tubercule size locus CsTS1 was identified by map-based cloning and was found to encode an oleosin protein. Allelic variation was identified in the promoter region of CsTS1, resulting in low expression of CsTS1 in all 22 different small-warty or nonwarty cucumber lines. High CsTS1 expression levels were closely correlated with increased fruit tubercule size among 44 different cucumber lines. Transgenic complementation and RNAi-mediated gene silencing of CsTS1 in transgenic cucumber plants demonstrated that CsTS1 positively regulates the development of tubercules. CsTS1 is highly expressed in the peel at fruit tubercule forming and enlargement stage. Auxin content and expression of three auxin signalling pathway genes were altered in the 35S:CsTS1 and CsTS1-RNAi fruit tubercules, a result that was supported by comparing the cell size of the control and transgenic fruit tubercules. CsTu, a C₂ H₂ zinc finger domain transcription factor that regulates tubercule initiation, binds directly to the CsTS1 promoter and promotes its expression. Taken together, our results reveal a novel mechanism in which the CsTu-TS1 complex promotes fruit tubercule formation in cucumber.

The influence of Al3+ on DNA methylation and sequence changes in the triticale (× Triticosecale Wittmack) genome

Abiotic stressors such as drought, salinity, and exposure to heavy metals can induce epigenetic changes in plants. In this study, liquid chromatography (RP-HPLC), methylation amplified fragment length polymorphisms (metAFLP), and methylation-sensitive amplification polymorphisms (MSAP) analysis was used to investigate the effects of aluminum (Al) stress on DNA methylation levels in the crop species triticale. RP-HPLC, but not metAFLP or MSAP, revealed significant differences in methylation between Al-tolerant (T) and non-tolerant (NT) triticale lines. The direction of methylation change was dependent on phenotype and organ. Al treatment increased the level of global DNA methylation in roots of T lines by approximately 0.6%, whereas demethylation of approximately 1.0% was observed in NT lines. DNA methylation in leaves was not affected by Al stress. The metAFLP and MSAP approaches identified DNA alterations induced by Al³⁺ treatment. The metAFLP technique revealed sequence changes in roots of all analyzed triticale lines and few mutations in leaves. MSAP showed that demethylation of CCGG sites reached approximately 3.97% and 3.75% for T and NT lines, respectively, and was more abundant than de novo methylation, which was observed only in two tolerant lines affected by Al stress. Three of the MSAP fragments showed similarity to genes involved in abiotic stress.

Modelling the influence of parental effects on gene-network evolution

Understanding the importance of nongenetic heredity in the evolutionary process is a major topic in modern evolutionary biology. We modified a classical gene-network model by allowing parental transmission of gene expression and studied its evolutionary properties through individual-based simulations. We identified ontogenetic time (i.e. the time gene networks have to stabilize before being submitted to natural selection) as a crucial factor in determining the evolutionary impact of this phenotypic inheritance. Indeed, fast-developing organisms display enhanced adaptation and greater robustness to mutations when evolving in presence of nongenetic inheritance (NGI). In contrast, in our model, long development reduces the influence of the inherited state of the gene network. NGI thus had a negligible effect on the evolution of gene networks when the speed at which transcription levels reach equilibrium is not constrained. Nevertheless, simulations show that intergenerational transmission of the gene-network state negatively affects the evolution of robustness to environmental disturbances for either fast- or slow-developing organisms. Therefore, these results suggest that the evolutionary consequences of NGI might not be sought only in the way species respond to selection, but also on the evolution of emergent properties (such as environmental and genetic canalization) in complex genetic architectures.

Methylome evolution in plants

Despite major progress in dissecting the molecular pathways that control DNA methylation patterns in plants, little is known about the mechanisms that shape plant methylomes over evolutionary time. Drawing on recent intra- and interspecific epigenomic studies, we show that methylome evolution over long timescales is largely a byproduct of genomic changes. By contrast, methylome evolution over short timescales appears to be driven mainly by spontaneous epimutational events. We argue that novel methods based on analyses of the methylation site frequency spectrum (mSFS) of natural populations can provide deeper insights into the evolutionary forces that act at each timescale.

ZmMBD101 is a DNA-binding protein that maintains Mutator elements chromatin in a repressive state in maize

In maize (Zea mays), as well as in other crops, transposable elements (TEs) constitute a great proportion of the genome. Chromatin modifications play a vital role in establishing transposon silencing and perpetuating the acquired repressive state. Nucleosomes associated with TEs are enriched for dimethylation of histone H3 at lysine 9 and 27 (H3K9me2 and H3K27me2, respectively), signals of repressive chromatin. Here, we describe a chromatin protein, ZmMBD101, involved in the regulation of Mutator (Mu) genes in maize. ZmMBD101 is localized to the nucleus and contains a methyl-CpG-binding domain (MBD) and a zinc finger CW (CW) domain. Transgenic lines with reduced levels of ZmMBD101 transcript present enhanced induction of Mu genes when plants are irradiated with UV-B. Chromatin immunoprecipitation analysis with H3K9me2 and H3K27me2 antibodies indicated that ZmMBD101 is required to maintain the levels of these histone repressive marks at Mu terminal inverted repeats (TIRs) under UV-B conditions. Although Mutator inactivity is associated with DNA methylation, cytosine methylation at Mu TIRs is not affected in ZmMBD101 deficient plants. Several plant proteins are predicted to share the simple CW-MBD domain architecture present in ZmMBD101. We hypothesize that plant CW-MBD proteins may also function to protect plant genomes from deleterious transposition.

Methylation affects transposition and splicing of a large CACTA transposon from a MYB transcription factor regulating anthocyanin synthase genes in soybean seed coats

We determined the molecular basis of three soybean lines that vary in seed coat color at the R locus which is thought to encode a MYB transcription factor. RM55-r(m) is homozygous for a mutable allele (r(m)) that specifies black and brown striped seeds; RM30-R* is a stable black revertant isoline derived from the mutable line; and RM38-r has brown seed coats due to a recessive r allele shown to translate a truncated MYB protein. Using long range PCR, 454 sequencing of amplicons, and whole genome re-sequencing, we determined that the variegated RM55-r(m) line had a 13 kb CACTA subfamily transposon insertion (designated TgmR*) at a position 110 bp from the beginning of Intron2 of the R locus, Glyma09g36983. Although the MYB encoded by R was expressed at only very low levels in older seed coats of the black revertant RM30-R* line, it upregulated expression of anthocyanidin synthase genes (ANS2, ANS3) to promote the synthesis of anthocyanins. Surprisingly, the RM30-R* revertant also carried the 13 kb TgmR* insertion in Intron2. Using RNA-Seq, we showed that intron splicing was accurate, albeit at lower levels, despite the presence of the 13 kb TgmR* element. As determined by whole genome methylation sequencing, we demonstrate that the TgmR* sequence was relatively more methylated in RM30-R* than in the mutable RM55-r(m) progenitor line. The stabilized and more methylated RM30-R* revertant line apparently lacks effective binding of a transposae to its subterminal repeats, thus allowing intron splicing to proceed resulting in sufficient MYB protein to stimulate anthocyanin production and thus black seed coats. In this regard, the TgmR* element in soybean resembles McClintock's Spm-suppressible and change-of-state alleles of maize. This comparison explains the opposite effects of the TgmR* element on intron splicing of the MYB gene in which it resides depending on the methylation state of the element.

Single-base-resolution methylomes of Populus trichocarpa reveal the association between DNA methylation and drought stress

Background

DNA methylation is an important biological form of epigenetic modification, playing key roles in plant development and environmental responses.

Results

In this study, we examined single-base resolution methylomes of Populus under control and drought stress conditions using high-throughput bisulfite sequencing for the first time. Our data showed methylation levels of methylated cytosines, upstream 2kp, downstream 2kb, and repeatitive sequences significantly increased after drought treatment in Populus. Interestingly, methylation in 100 bp upstream of the transcriptional start site (TSS) repressed gene expression, while methylations in 100-2000bp upstream of TSS and within the gene body were positively associated with gene expression. Integrated with the transcriptomic data, we found that all cis-splicing genes were non-methylated, suggesting that DNA methylation may not associate with cis-splicing. However, our results showed that 80% of trans-splicing genes were methylated. Moreover, we found 1156 transcription factors (TFs) with reduced methylation and expression levels and 690 TFs with increased methylation and expression levels after drought treatment. These TFs may play important roles in Populus drought stress responses through the changes of DNA methylation.

Conclusions

These findings may provide valuable new insight into our understanding of the interaction between gene expression and methylation of drought responses in Populus.

Analysis of DNA methylation of perennial ryegrass under drought using the methylation-sensitive amplification polymorphism (MSAP) technique

Perennial ryegrass (Lolium perenne), an excellent grass for forage and turf, is widespread in temperate regions. Drought is an important factor that limits its growth, distribution, and yield. DNA methylation affects gene expression and plays an important role in adaptation to adverse environments. In this study, the DNA methylation changes in perennial ryegrass under drought stress were assessed using methylation-sensitive amplified polymorphism (MSAP). After 15 days of drought stress treatment, the plant height was less than half of the control, and the leaves were smaller and darker. Genome-wide, a total of 652 CCGG sites were detected by MSAP. The total methylation level was 57.67 and 47.39 % in the control and drought treatment, respectively, indicating a decrease of 10.28 % due to drought exposure. Fifteen differentially displayed DNA fragments in MSAP profiles were cloned for sequencing analysis. The results showed that most of the genes involved in stress responses. The relative expression levels revealed that three demethylated fragments were up-regulated. The expression of a predicted retrotransposon increased significantly, changing from hypermethylation to non-methylation. Although the extent of methylation in two other genes decreased, the sites of methylation remained, and the expression increased only slightly. All of these results suggested that drought stress decreased the total DNA methylation level in perennial ryegrass and demethylation up-regulated related gene expressions and that the extent of methylation was negatively correlated with expression. Overall, the induced epigenetic changes in genome probably are an important regulatory mechanism for acclimating perennial ryegrass to drought and possibly other environmental stresses.

The initiation of epigenetic silencing of active transposable elements is triggered by RDR6 and 21-22 nucleotide small interfering RNAs

Transposable elements (TEs) are mobile fragments of DNA that are repressed in both plant and animal genomes through the epigenetic inheritance of repressed chromatin and expression states. The epigenetic silencing of TEs in plants is mediated by a process of RNA-directed DNA methylation (RdDM). Two pathways of RdDM have been identified: RNA Polymerase IV (Pol IV)-RdDM, which has been shown to be responsible for the de novo initiation, corrective reestablishment, and epigenetic maintenance of TE and/or transgene silencing; and RNA-dependent RNA Polymerase6 (RDR6)-RdDM, which was recently identified as necessary for maintaining repression for a few TEs. We have further characterized RDR6-RdDM using a genome-wide search to identify TEs that generate RDR6-dependent small interfering RNAs. We have determined that TEs only produce RDR6-dependent small interfering RNAs when transcriptionally active, and we have experimentally identified two TE subfamilies as direct targets of RDR6-RdDM. We used these TEs to test the function of RDR6-RdDM in assays for the de novo initiation, corrective reestablishment, and maintenance of TE silencing. We found that RDR6-RdDM plays no role in maintaining TE silencing. Rather, we found that RDR6 and Pol IV are two independent entry points into RdDM and epigenetic silencing that perform distinct functions in the silencing of TEs: Pol IV-RdDM functions to maintain TE silencing and to initiate silencing in an RNA Polymerase II expression-independent manner, while RDR6-RdDM functions to recognize active Polymerase II-derived TE mRNA transcripts to both trigger and correctively reestablish TE methylation and epigenetic silencing.

Molecular genetics and epigenetics of CACTA elements

The CACTA transposons, so named for a highly conserved motif at element ends, comprise one of the most abundant superfamilies of Class 2 (cut-and-paste) plant transposons. CACTA transposons characteristically include subterminal sequences of several hundred nucleotides containing closely spaced direct and inverted repeats of a short, conserved sequence of 14-15 bp. The Supressor-mutator (Spm) transposon, identified and subjected to detailed genetic analysis by Barbara McClintock, remains the paradigmatic element of the CACTA family. The Spm transposon encodes two proteins required for transposition, the transposase (TnpD) and a regulatory protein (TnpA) that binds to the subterminal repeats. Spm expression is subject to both genetic and epigenetic regulation. The Spm-encoded TnpA serves as an activator of the epigenetically inactivated, methylated Spm, stimulating both transient and heritable activation of the transposon. TnpA also serves as a negative regulator of the demethylated active element promoter and is required, in addition to the TnpD, for transposition.

DNA methylation in plants: relationship to small RNAs and histone modifications, and functions in transposon inactivation

DNA methylation is a type of epigenetic marking that strongly influences chromatin structure and gene expression in plants and mammals. Over the past decade, DNA methylation has been intensively investigated in order to elucidate its control mechanisms. These studies have shown that small RNAs are involved in the induction of DNA methylation, that there is a relationship between DNA methylation and histone methylation, and that the base excision repair pathway has an important role in DNA demethylation. Some aspects of DNA methylation have also been shown to be shared with mammals, suggesting that the regulatory pathways are, in part at least, evolutionarily conserved. Considerable progress has been made in elucidating the mechanisms that control DNA methylation; however, many aspects of the mechanisms that read the information encoded by DNA methylation and mediate this into downstream regulation remain uncertain, although some candidate proteins have been identified. DNA methylation has a vital role in the inactivation of transposons, suggesting that DNA methylation is a key factor in the evolution and adaptation of plants.

Activation and epigenetic regulation of DNA transposon nDart1 in rice

A large part of the rice genome is composed of transposons. Since active excision/reintegration of these mobile elements may result in harmful genetic changes, many transposons are maintained in a genetically or epigenetically inactivated state. However, some non-autonomous DNA transposons of the nDart1-3 subgroup, including nDart1-0, actively transpose in specific rice lines, such as pyl-v which carries an active autonomous element, aDart1-27, on chromosome 6. Although nDart1-3 subgroup elements show considerable sequence identity, they display different excision frequencies. The most active element, nDart1-0, had a low cytosine methylation status. The aDart1-27 sequence showed conservation between pyl-stb (pyl-v derivative line) and Nipponbare, which both lack autonomous activity for transposition of nDart1-3 subgroup elements. In pyl-v plants, the promoter region of the aDart1-27 transposase gene was more hypomethylated than in other rice lines. Treatment with the methylation inhibitor 5-azacytidine (5-azaC) induced transposition of nDart1-3 subgroup elements in both pyl-stb and Nipponbare plants; the new insertion sites were frequently located in genic regions. 5-AzaC treatment principally induced expression of Dart1-34 transposase rather than the other 38 aDart1-related elements in both pyl-stb and Nipponbare treatment groups. Our observations show that transposition of nDart1-3 subgroup elements in the nDart1/aDart1 tagging system is correlated with the level of DNA methylation. Our system does not cause somaclonal variation due to an absence of transformed plants, offers the possibility of large-scale screening in the field and can identify dominant mutants. We therefore propose that this tagging system provides a valuable addition to the tools available for rice functional genomics.

TCUP: A Novel hAT Transposon Active in Maize Tissue Culture

Transposable elements (TEs) are capable of inducing heritable de novo genetic variation. The sequences capable of reactivation, and environmental factors that induce mobilization, remain poorly defined even in well-studied genomes such as maize. We treated maize tissue culture with the demethylating agent 5-aza-2-deoxcytidine and examined long-term tissue culture lines to discover silenced TEs that have the potential to induce heritable genetic variation. Through these screens we have identified a novel low copy number hAT transposon, Tissue Culture Up-Regulated (TCUP), which is transcribed at high levels in long-term maize black Mexican sweet (BMS) tissue culture and is transcribed in response to treatment with 5-aza-2-deoxycytidine. Analysis of the TIGR Maize Gene Index revealed that this element is the most frequently represented EST from the BMS cell culture library and is not represented in other tissue libraries, which is the basis for its name. A full-length sequence was assembled in inbred B73 that contains the putative functional motifs required for autonomous movement of a hAT transposon. Transposon display detected novel TCUP insertions in two long-term tissue-cultured cell lines of the genotype Hi-II A × B and BMS. This research implicates TCUP as a transposon that is capable of reactivation and which may also be particularly sensitive to the stress of the tissue culture environment. Our findings are consistent with the hypothesis that epigenetic alterations potentiate genomic responses to stress during clonal propagation of plants.

A "mille-feuille" of silencing: epigenetic control of transposable elements

Despite their abundance in the genome, transposable elements (TEs) and their derivatives are major targets of epigenetic silencing mechanisms, which restrain TE mobility at different stages of the life cycle. DNA methylation, post-translational modification of histone tails and small RNA-based pathways contribute to maintain TE silencing; however, some of these epigenetic marks are tightly interwoven and this complicates the delineation of the exact contribution of each in TE silencing. Recent studies have confirmed that host genomes have evolved versatility in the use of these mechanisms to individualize silencing of particular TEs. These studies also revealed that silencing of TEs is much more dynamic than had been previously thought and can be reversed on the genomic scale in particular cell types or under special environmental conditions. This article is part of a Special Issue entitled "Epigenetic control of cellular and developmental processes in plants".

Epigenetics in plants-vernalisation and hybrid vigour

In this review we have analysed two major biological systems involving epigenetic control of gene activity. In the first system we demonstrate the interplay between genetic and epigenetic controls over the transcriptional activity of FLC, a major repressor of flowering in Arabidopsis. FLC is down-regulated by low temperature treatment (vernalisation) releasing the repressor effect on flowering. We discuss the mechanisms of the reduced transcription and the memory of the vernalisation treatment through vegetative development. We also discuss the resetting of the repressed activity level of the FLC gene, following vernalisation, to the default high activity level and show it occurs during both male and female gametogenesis but with different timing in each. In the second part of the review discussed the complex multigenic system which is responsible for the patterns of gene activity which bring about hybrid vigour in crosses between genetically similar but epigenetically distinct parents. The epigenetic systems that we have identified as contributing to the heterotic phenotype are the 24nt siRNAs and their effects on RNA dependent DNA methylation (RdDM) at the target loci leading to changed expression levels. We conclude that it is likely that epigenetic controls are involved in expression systems in many aspects of plant development and plant function.

Epigenetic control of gene regulation in plants

In eukaryotes, including plants, the genome is compacted into chromatin, which forms a physical barrier for gene transcription. Therefore, mechanisms that alter chromatin structure play an essential role in gene regulation. When changes in the chromatin states are inherited trough mitotic or meiotic cell division, the mechanisms responsible for these changes are defined as epigenetic. In this paper, we review data arising from genome-wide analysis of the epigenetic landscapes in different plant species to establish the correlation between specific epigenetic marks and transcription. In the subsequent sections, mechanisms of epigenetic control of gene regulation mediated by DNA-binding transcription factors and by transposons located in proximity to genes are illustrated. Finally, plant peculiarities for epigenetic control of gene regulation and future perspectives in this research area are discussed. This article is part of a Special Issue entitled: Epigenetic Control of cellular and developmental processes in plants.

A transposon-based activation tagging system for gene function discovery in Arabidopsis

Activation tagging is a powerful strategy to find new gene functions, especially from genes that are redundant or show lethal knock-out phenotypes. It has been applied using T-DNA or transposons. En/Spm-I/dSpm engineered transposons are efficient activation tags in Arabidopsis. An immobilized transposase source and an enhancer-bearing non-autonomous element are used in combination with positive and negative selectable markers to generate a population of single- or low-copy, stable insertions. This method describes the steps required for selection of parental lines, generation of a population of stable insertions, and gene identification.

Paramutation and development

Paramutation describes a heritable change of gene expression that is brought about through interactions between homologous chromosomes. Genetic analyses in plants and, more recently, in mouse indicate that genomic sequences related to transcriptional control and molecules related to small RNA biology are necessary for specific examples of paramutation. Some of the molecules identified in maize are also required for normal plant development. These observations indicate a functional relationship between the nuclear mechanisms responsible for paramutation and modes of developmental gene control.

An endogenous, systemic RNAi pathway in plants

Recent work on metazoans has uncovered the existence of an endogenous RNA-silencing pathway that functionally recapitulates the effects of experimental RNA interference (RNAi) used for gene knockdown in organisms such as Caenorhabditis elegans and Drosophila. The endogenous short interfering (si)RNA involved in this pathway are processed by Dicer-like nucleases from genomic loci re-arranged to form extended inverted repeats (IRs) that produce perfect or near-perfect dsRNA molecules. Although such IR loci are commonly detected in plant genomes, their genetics, evolution and potential contribution to plant biology through endogenous silencing have remained largely unexplored. Through an exhaustive analysis performed using Arabidopsis, we provide here evidence that at least two such endogenous IRs are genetically virtually indistinguishable from the transgene constructs commonly used for RNAi in plants. We show how these loci can be useful probes of the cellular mechanism and fluidity of RNA-silencing pathways in plants, and provide evidence that they may arise and disappear on an ecotype scale, show highly cell-specific expression patterns and respond to various stresses. IR loci thus have the potential to act as molecular sensors of the local environments found within distinct ecological plant niches. We further show that the various siRNA size classes produced by at least one of these IR loci are functionally loaded into cognate effector proteins and mediate both post-transcriptional gene silencing and RNA-directed DNA methylation (RdDM) of endogenous as well as exogenous targets. Finally, and as previously reported during plant experimental RNAi, we provide evidence that endogenous IR-derived siRNAs of all size classes are not cell-autonomous and can be transported through graft junctions over long distances, in target tissues where they are functional, at least in mediating RdDM. Collectively, these results define the existence of a bona fide, endogenous and systemic RNAi pathway in plants that may have implications in adaptation, epiallelism and trans-generational memory.

Enhancer/Suppressor mutator (En/Spm)-like transposable elements of cassava (Manihot esculenta) are transcriptionally inactive

Transposable elements contribute to the size, structure, variation, and diversity of the genome and have major effects on gene function. Sequencing projects have revealed the diversity of transposable elements in many organisms and have shown that they constitute a high percentage of the genome. PCR-based techniques using degenerate primers designed from conserved enzyme domains of transposable elements can provide quick and extensive surveys, making study of diversity and abundance and their applications possible in species where full genome sequence data are not yet available. We studied cassava (Manihot esculenta) En/Spm-like transposons (Meens) with regard to genomic distribution, sequence diversity and methylation status. Cassava transposase fragments characteristic of En/Spm-like transposons were isolated, cloned and characterized. Sequence analysis showed that cassava En/Spm-like elements are highly conserved, with overall identity in the range of 68-98%. Southern hybridization supports the presence of multiple copies of En/Spm-like transposons integrated in the genome of all cassava cultivars that we tested. Hybridization patterns of HpaII- and MspI-digested cassava genomic DNA revealed highly methylated sequences. There were no clear differences in hybridization pattern between the cultivars. We did not detect RNA transcripts of Meens by Northern procedures. We examined the possibility of recent transposition activities of the cassava En/Spm-like elements.

Excision of an active CACTA-like transposable element from DFR2 causes variegated flowers in soybean [Glycine max (L.) Merr.]

Active endogenous transposable elements, useful tools for gene isolation, have not been reported from any legume species. An active transposable element was suggested to reside in the W4 locus that governs flower color in soybean. Through biochemical and molecular analyses of several revertants of the w4-m allele, we have shown that the W4 locus encodes dihydroflavonol-4-reductase 2 (DFR2). w4-m has arisen through insertion of Tgm9, a 20,548-bp CACTA-like transposable element, into the second intron of DFR2. Tgm9 showed high nucleic acid sequence identity to Tgmt*. Its 5' and 3' terminal inverted repeats start with conserved CACTA sequence. The 3' subterminal region is highly repetitive. Tgm9 carries TNP1- and TNP2-like transposase genes that are expressed in the mutable line, T322 (w4-m). The element excises at a high frequency from both somatic and germinal tissues. Following excision, reinsertions of Tgm9 into the DFR2 promoter generated novel stable alleles, w4-dp (dilute purple flowers) and w4-p (pale flowers). We hypothesize that the element is fractured during transposition, and truncated versions of the element in new insertion sites cause stable mutations. The highly active endogenous transposon, Tgm9, should facilitate genomics studies specifically that relate to legume biology.

Transgenerational epigenetic effects

Transgenerational epigenetic effects include all processes that have evolved to achieve the nongenetic determination of phenotype. There has been a long-standing interest in this area from evolutionary biologists, who refer to it as non-Mendelian inheritance. Transgenerational epigenetic effects include both the physiological and behavioral (intellectual) transfer of information across generations. Although in most cases the underlying molecular mechanisms are not understood, modifications of the chromosomes that pass to the next generation through gametes are sometimes involved, which is called transgenerational epigenetic inheritance. There is a trend for those outside the field of molecular biology to assume that most cases of transgenerational epigenetic effects are the result of transgenerational epigenetic inheritance, in part because of a misunderstanding of the terms. Unfortunately, this is likely to be far from the truth.

The art and design of genetic screens: maize

Maize (Zea mays) is an excellent model for basic research. Genetic screens have informed our understanding of developmental processes, meiosis, epigenetics and biochemical pathways--not only in maize but also in other cereal crops. We discuss the forward and reverse genetic screens that are possible in this organism, and emphasize the available tools. Screens exploit the well-studied behaviour of transposon systems, and the distinctive chromosomes allow an integration of cytogenetics into mutagenesis screens and analyses. The imminent completion of the maize genome sequence provides the essential resource to move seamlessly from gene to phenotype and back.

The function of RNAi in plant development

The morphological phenotype of mutations in genes required for posttranscriptional gene silencing (PTGS) or RNA interference (RNAi) in Arabidopsis demonstrates that this process is critical for normal development. One way in which RNAi contributes to gene regulation is through its involvement in the biogenesis of trans-acting small interfering RNAs (siRNAs). These endogenous siRNAs are derived from noncoding transcripts that are cleaved by a microRNA (miRNA) and mediate the silencing of protein-coding transcripts. Some protein-coding genes are also subject to miRNA-initiated transitive silencing. Several developmentally important transcription factors regulated by these silencing mechanisms have been identified.

Allele-specific chromatin immunoprecipitation studies show genetic influence on chromatin state in human genome

Several recent studies have shown a genetic influence on gene expression variation, including variation between the two chromosomes within an individual and variation between individuals at the population level. We hypothesized that genetic inheritance may also affect variation in chromatin states. To test this hypothesis, we analyzed chromatin states in 12 lymphoblastoid cells derived from two Centre d'Etude du Polymorphisme Humain families using an allele-specific chromatin immunoprecipitation (ChIP-on-chip) assay with Affymetrix 10K SNP chip. We performed the allele-specific ChIP-on-chip assays for the 12 lymphoblastoid cells using antibodies targeting at RNA polymerase II and five post-translation modified forms of the histone H3 protein. The use of multiple cell lines from the Centre d'Etude du Polymorphisme Humain families allowed us to evaluate variation of chromatin states across pedigrees. These studies demonstrated that chromatin state clustered by family. Our results support the idea that genetic inheritance can determine the epigenetic state of the chromatin as shown previously in model organisms. To our knowledge, this is the first demonstration in humans that genetics may be an important factor that influences global chromatin state mediated by histone modification, the hallmark of the epigenetic phenomena.

Human health and disease are determined by an interaction between genetic background and environmental exposures. Both normal development and disease are mediated by epigenetic regulation of gene expression. The epigenetic regulation causes heritable changes in gene expression, which is not associated with DNA sequence changes. Instead, it is mediated by chemical modification of DNA such as DNA methylation or by protein modifications such as histone acetylation and methylation. Although much has been known about epigenetic inheritance during development, little is known about the influence of the genetic background on epigenetic processes such as histone modifications. In this report the authors studied five histone modifications on a genome-wide level in cells from different families. Global epigenetic states, as measured by these histone modifications, showed a similar pattern for cells derived from the same family. This study demonstrates that genetic inheritance may be an important factor influencing global chromatin states mediated by histone modifications in humans. These observations illustrate the importance of integrating genetic and epigenetic information into studies of human health and complex diseases.

DNA methylation dynamics in plant genomes

Cytosine bases are extensively methylated in the DNA of plant genomes. DNA methylation has been implicated in the silencing of transposable elements and genes, and loss of methylation can have severe consequences for the organism. The recent methylation profiling of the entire Arabidopsis genome has provided insight into the extent of DNA methylation and its functions in silencing and gene transcription. Patterns of DNA methylation are faithfully maintained across generations, but some changes in DNA methylation are observed in terminally differentiated tissues. Demethylation by a DNA glycosylase is required for the expression of imprinted genes in the endosperm and de novo methylation might play a role in the selective silencing of certain self-incompatibility alleles in the tapetum. Because DNA methylation patterns are faithfully inherited, changes in DNA methylation that arise somatically during the plant life cycle have the possibility of being propagated. Therefore, epimutations might be an important source of variation during plant evolution.

Epigenetic modifications of distinct sequences of the p1 regulatory gene specify tissue-specific expression patterns in maize

Tandemly repeated endogenous genes are common in plants, but their transcriptional regulation is not well characterized. In maize, the P1-wr allele of pericarp color1 is composed of multiple copies arranged in a head-to-tail fashion. P1-wr confers a white kernel pericarp and red cob glume pigment phenotype that is stably inherited over generations. To understand the molecular mechanisms that regulate tissue-specific expression of P1-wr, we have characterized P1-wr*, a spontaneous loss-of-function epimutation that shows a white kernel pericarp and white cob glume phenotype. As compared to its progenitor P1-wr, the P1-wr* is hypermethylated in exon 1 and intron 2 regions. In the presence of the epigenetic modifier Ufo1 (Unstable factor for orange1), P1-wr* plants exhibit a range of cob glume pigmentation whereas pericarps remain colorless. In these plants, the level of cob pigmentation directly correlates with the degree of DNA demethylation in the intron 2 region of p1. Further, genomic bisulfite sequencing indicates that a 168-bp region of intron 2 is significantly hypomethylated in both CG and CNG context in P1-wr* Ufo1 plants. Interestingly, P1-wr* Ufo1 plants did not show any methylation change in a distal enhancer region that has previously been implicated in Ufo1-induced gain of pericarp pigmentation of the P1-wr allele. These results suggest that distinct regulatory sequences in the P1-wr promoter and intron 2 regions can undergo independent epigenetic modifications to generate tissue-specific expression patterns.

Inherited epigenetic variation--revisiting soft inheritance

Phenotypic variation is traditionally parsed into components that are directed by genetic and environmental variation. The line between these two components is blurred by inherited epigenetic variation, which is potentially sensitive to environmental inputs. Chromatin and DNA methylation-based mechanisms mediate a semi-independent epigenetic inheritance system at the interface between genetic control and the environment. Should the existence of inherited epigenetic variation alter our thinking about evolutionary change?

Rejuvenation by shoot apex culture recapitulates the developmental increase of methylation at the maize gene Pl-Blotched

Cytosine methylation provides an attractive epigenetic modification for the global maintenance of phases in plant development; however, there are few known examples of specific genes whose methylation status changes in a developmentally regulated manner. Pl-Blotched, an allele of purple plant1 (pl1), which encodes a myb-like transcription factor that regulates anthocyanin production in maize, is one such gene: certain cytosines at the 3' end of this allele are hypomethylated in seedlings, become hypermethylated in organs formed in the adult phase, and are hypomethylated again in the next generation. We tested whether this developmental pattern of low juvenile cytosine methylation followed by higher methylation in adult tissues could also be observed in plants "rejuvenated" via shoot apex culture. We found that cytosine methylation patterns at Pl-Blotched were indeed recapitulated in culture-rejuvenated plants, showing hypomethylation in leaves with juvenile patterns of differentiation (even though they were made by an old meristem) followed by hypermethylation in later-formed leaves. Our results show that methylation status at that locus is determined by the developmental phase of the shoot, rather than by the age of the meristem forming it. These results support the hypothesis that DNA methylation is employed by the plant to maintain an epigenetic state.

The temperature-dependent change in methylation of the Antirrhinum transposon Tam3 is controlled by the activity of its transposase

The Antirrhinum majus transposon Tam3 undergoes low temperature-dependent transposition (LTDT). Growth at 15 degrees C permits transposition, whereas growth at 25 degrees C strongly suppresses it. The degree of Tam3 DNA methylation is altered somatically and positively correlated with growth temperature, an exceptional epigenetic system in plants. Using a Tam3-inactive line, we show that methylation change depends on Tam3 activity. Random binding site selection analysis and electrophoretic mobility shift assays revealed that the Tam3 transposase (TPase) binds to the major repeat in the subterminal regions of Tam3, the site showing the biggest temperature-dependent change in methylation state. Methylcytosines in the motif impair the binding ability of the TPase. Proteins in a nuclear extract from plants grown at 15 degrees C but not 25 degrees C bind to this motif in Tam3. The decrease in Tam3 DNA methylation at low temperature also requires cell division. Thus, TPase binding to Tam3 occurs only during growth at low temperature and immediately after DNA replication, resulting in a Tam3-specific decrease in methylation of transposon DNA. Consequently, the Tam3 methylation level in LTDT is regulated by Tam3 activity, which is dependent on the ability of its TPase to bind DNA and affected by growth temperature. Thus, the methylation/demethylation of Tam3 is the consequence, not the cause, of LTDT.

A two-edged role for the transposable element Kiddo in the rice ubiquitin2 promoter

Miniature inverted repeat transposable elements (MITEs) are thought to be a driving force for genome evolution. Although numerous MITEs are found associated with genes, little is known about their function in gene regulation. Whereas the rice ubiquitin2 (rubq2) promoter in rice (Oryza sativa) line IR24 contains two nested MITEs (Kiddo and MDM1), that in line T309 has lost Kiddo, providing an opportunity to understand the role of MITEs in promoter function. No difference in endogenous rubq2 transcript levels between T309 and IR24 was evident using RT-PCR. However, promoter analysis using both transient and stably transformed calli revealed that Kiddo contributed some 20% of the total expression. Bisulfite genomic sequencing of the rubq2 promoters revealed specific DNA methylation at both symmetric and asymmetric cytosine residues on the MITE sequences, possibly induced by low levels of homologous transcripts. When methylation of the MITEs was blocked by 5-azacytidine treatment, a threefold increase in the endogenous rubq2 transcript level was detected in IR24 compared with that in T309. Together with the observed MITE methylation pattern, the detection of low levels of transcripts, but not small RNAs, corresponding to Kiddo and MDM1 suggested that RNA-dependent DNA methylation is induced by MITE transcripts. We conclude that, although Kiddo enhances transcription from the rubq2 promoter, this effect is mitigated by sequence-specific epigenetic modification.

Mobilization of the active MITE transposons mPing and Pong in rice by introgression from wild rice (Zizania latifolia Griseb.)

Hybridization between different species plays an important role in plant genome evolution, as well as is a widely used approach for crop improvement. McClintock has predicted that plant wide hybridization constitutes a "genomic shock" whereby cryptic transposable elements may be activated. However, direct experimental evidence showing a causal relationship between plant wide hybridization and transposon mobilization has not yet been reported. The miniature-Ping (mPing) is a recently isolated active miniature inverted-repeat transposable element transposon from rice, which is mobilized by tissue culture and gamma-ray irradiation. We show herein that mPing, together with its putative transposase-encoding partner, Pong, is mobilized in three homologous recombinant inbred lines (RILs), derived from hybridization between rice (cultivar Matsumae) and wild rice (Zizania latifolia Griseb.), harboring introgressed genomic DNA from wild rice. In contrast, both elements remain immobile in two lines sharing the same parentage to the RILs but possessing no introgressed DNA. Thus, we have presented direct evidence that is consistent with McClintock's insight by demonstrating a causal link between wide hybridization and transposon mobilization in rice. In addition, we report an atypical behavior of mPing/Pong mobilization in these lines, i.e., the exclusive absence of footprints after excision.

Epigenetic control of CACTA transposon mobility in Arabidopsis thaliana

Epigenetic mutation, heritable developmental variation not based on a change in nucleotide sequence, is widely reported in plants. However, the developmental and evolutionary significance of such mutations remains enigmatic. On the basis of our studies of the endogenous Arabidopsis transposon CACTA, we propose that the inheritance of epigenetic gene silencing over generations can function as a transgenerational genome defense mechanism against deleterious movement of transposons. We previously reported that silent CACTA1 is mobilized by the DNA hypomethylation mutation ddm1 (decrease in DNA methylation). In this study, we report that CACTA activated by the ddm1 mutation remains mobile in the presence of the wild-type DDM1 gene, suggesting that de novo silencing is not efficient for the defense of the genome against CACTA movement. The defense depends on maintenance of transposon silencing over generations. In addition, we show that the activated CACTA1 element transposes throughout the genome in DDM1 plants, as reported previously for ddm1 backgrounds. Furthermore, the CACTA1 element integrated into both the ddm1-derived and the DDM1-derived chromosomal regions in the DDM1 wild-type plants, demonstrating that this class of transposons does not exhibit targeted integration into heterochromatin, despite its accumulation in the pericentromeric regions in natural populations. The possible contribution of natural selection as a mechanism for the accumulation of transposons and evolution of heterochromatin is discussed.

SGS3 and SGS2/SDE1/RDR6 are required for juvenile development and the production of trans-acting siRNAs in Arabidopsis

Higher plants undergo a transition from a juvenile to an adult phase of vegetative development prior to flowering. Screens for mutants that undergo this transition precociously produced alleles of two genes required for posttranscriptional gene silencing (PTGS)--SUPPRESSOR OF GENE SILENCING3 (SGS3) and SUPPRESSOR OF GENE SILENCING2(SGS2)/SILENCING DEFECTIVE1 (SDE1)/RNA-DEPENDENT POLYMERASE6 (RDR6). Loss-of-function mutations in these genes have a phenotype similar to that of mutations in the Argonaute gene ZIPPY (ZIP). Epistasis analysis suggests that ZIP, SGS3, SGS2/SDE1/RDR6, and the putative miRNA export receptor, HASTY (HST), operate in the same pathway(s). Microarray analysis revealed a small number of genes whose mRNA is increased in ZIP, SGS3, and SGS2/SDE1/RDR6 mutants, as well as genes that are up-regulated in SGS3 and SGS2/SDE1/RDR6 mutants, but not in ZIP mutants. One of these latter genes (At5g18040) is silenced posttranscriptionally in trans by the sRNA255 family of endogenous, noncoding, small interfering RNAs (siRNAs). The increase in At5g18040 mRNA in SGS3 and SGS2/SDE1/RDR6 mutants is attributable to the absence of sRNA255-like siRNAs in these mutants. These results demonstrate a role for endogenous siRNAs in the regulation of gene expression, and suggest that PTGS plays a central role in the temporal control of shoot development in plants.

Activation of a rice endogenous retrotransposon Tos17 in tissue culture is accompanied by cytosine demethylation and causes heritable alteration in methylation pattern of flanking genomic regions

Tos17 is a copia-like, cryptic retrotransposon of rice, but can be activated by tissue culture. To study possible epigenetic mechanism controlling activity of Tos17, we subjected three rice lines (the parental line cv. Matsumae and two introgression lines, RZ2 and RZ35) that harbor different copies of the element to tissue culture. For each line, we investigated transcription and transposition of Tos17 in seed plants, calli and regenerated plants, cytosine-methylation status at CG and CNG positions within Tos17, effect of 5-azacytidine on methylation status and activity of Tos17, and cytosine-methylation states in genomic regions flanking original and some newly transposed copies of Tos17 in calli and regenerated plants. We found that only in introgression line RZ35 was Tos17 transcriptionally activated and temporarily mobilized by tissue culture, which was followed by repression before or upon plant regeneration. The activity and inactivity of Tos17 in calli and regenerated plants of RZ35 are accompanied by hypo- and hyper-CG methylation and hemi- and full CNG methylation, respectively, within the element, whereas immobilization of the element in the other two lines is concomitant with near-constant, full hypermethylation. Treatment with 5-azacytidine induced both CG and CNG partial hypomethylation of Tos17 in two lines (Matsumae and RZ35), which, however, was not accompanied by activation of Tos17 in any line. Heritable alteration in cytosine-methylation patterns occurred in three of seven genomic regions flanking Tos17 in calli and regenerated plants of RZ35, but in none of the five regions flanking dormant Tos17 in the other two lines.

Distinct mechanisms determine transposon inheritance and methylation via small interfering RNA and histone modification

Heritable, but reversible, changes in transposable element activity were first observed in maize by Barbara McClintock in the 1950s. More recently, transposon silencing has been associated with DNA methylation, histone H3 lysine-9 methylation (H3mK9), and RNA interference (RNAi). Using a genetic approach, we have investigated the role of these modifications in the epigenetic regulation and inheritance of six Arabidopsis transposons. Silencing of most of the transposons is relieved in DNA methyltransferase (met1), chromatin remodeling ATPase (ddm1), and histone modification (sil1) mutants. In contrast, only a small subset of the transposons require the H3mK9 methyltransferase KRYPTONITE, the RNAi gene ARGONAUTE1, and the CXG methyltransferase CHROMOMETHYLASE3. In crosses to wild-type plants, epigenetic inheritance of active transposons varied from mutant to mutant, indicating these genes differ in their ability to silence transposons. According to their pattern of transposon regulation, the mutants can be divided into two groups, which suggests that there are distinct, but interacting, complexes or pathways involved in transposon silencing. Furthermore, different transposons tend to be susceptible to different forms of epigenetic regulation.

Rob Martienssen and colleagues report that different transposonsrespond to different types of epigenetic regulation andspeculate that two distinct mechanisms of transposon silencing are likely to interact in a common pathway

Rapid inactivation of the maize transposable element En/Spm in Medicago truncatula

Transposable elements have been widely used as mutagens in many organisms. Among them, the maize transposable element En/Spm has been shown to transpose efficiently in several plant species including the model plant Arabidopsis, where it has been used for large-scale mutagenesis. To determine whether we could use this transposon as a mutagen in the model legume plant Medicago truncatula, we tested the activity of the autonomous element, as well as two defective elements, in this plant, and in Arabidopsis as a positive control. In agreement with previous reports, we observed efficient excision of the autonomous En/Spm element in A. thaliana. This element was also active in M. truncatula, but the transposition activity was low and was apparently restricted to the tissue culture step necessary for the production of transgenic plants. The activity of one of the defective transposable elements, dSpm, was very low in A. thaliana and even lower in M. truncatula. The use of different sources of transposases suggested that this defect in transposition was associated with the dSpm element itself. Transposition of the other defective element, I6078, was also detected in M. truncatula, but, as observed with the autonomous element, transposition events were very rare and occurred during tissue culture. These results suggest that the En/Spm element is rapidly inactivated in the regenerated plants and their progeny, and therefore is not suitable for routine insertion mutagenesis in M. truncatula.

Role of CG and non-CG methylation in immobilization of transposons in Arabidopsis

Methylation of cytosine residues in eukaryotic genomes is often associated with repeated sequences including transposons and their derivatives. Methylation has been implicated in control of two potential deleterious effects of these repeats: (1) uncontrolled transcription, which often disturbs proper expression of nearby host genes, and (2) changes in genome structure by transposition and ectopic recombination. Arabidopsis thaliana provides a genetically tractable system to examine these possibilities, since viable mutants in DNA methyltransferases are available. Arabidopsis MET1 (METHYLTRANSFERASE1, ortholog of mammalian DNA methyltransferase Dnmt1) is necessary for maintaining genomic cytosine methylation at 5'-CG-3' sites. Arabidopsis additionally methylates non-CG sites using CHROMOMETHYLASE3 (CMT3). We examined the mobility of endogenous CACTA transposons in met1, cmt3, and cmt3-met1 mutants. High-frequency transposition of CACTA elements was detected in cmt3-met1 double mutants. Single mutants in either met1 or cmt3 were much less effective in mobilization, despite significant induction of CACTA transcript accumulation. These results lead us to conclude that CG and non-CG methylation systems redundantly function for immobilization of transposons. Non-CG methylation in plants may have evolved as an additional epigenetic tag dedicated to transposon control. This view is consistent with the recent finding that CMT3 preferentially methylates transposon-related sequences.

Epi-alleles in plants: inheritance of epigenetic information over generations

Epigenetic modification of plant gene and transposon activity, which correlates with their methylation, is often heritable over many generations. Such heritable properties allow conventional genetic linkage analysis to identify the sequences affected in epigenetic variants. Machinery controlling the establishment of the epigenetic state and role of the epigenetic controls in plant development are also discussed.

Plant transposable elements: where genetics meets genomics

Transposable elements are the single largest component of the genetic material of most eukaryotes. The recent availability of large quantities of genomic sequence has led to a shift from the genetic characterization of single elements to genome-wide analysis of enormous transposable-element populations. Nowhere is this shift more evident than in plants, in which transposable elements were first discovered and where they are still actively reshaping genomes.

Regulation of activator/dissociation transposition by replication and DNA methylation

In maize the transposable elements Activator/Dissociation (Ac/Ds) transpose shortly after replication from one of the two resulting chromatids ("chromatid selectivity"). A model has been suggested that explains this phenomenon as a consequence of different affinity for Ac transposase binding to holo-, hemi-, and unmethylated transposon ends. Here we demonstrate that in petunia cells a holomethylated Ds is unable to excise from a nonreplicating vector and that replication restores excision. A Ds element hemi-methylated on one DNA strand transposes in the absence of replication, whereas hemi-methylation of the complementary strand causes a >6.3-fold inhibition of Ds excision. Consistently in the active hemi-methylated state, the Ds ends have a high binding affinity for the transposase, whereas binding to inactive ends is strongly reduced. These results provide strong evidence for the above-mentioned model. Moreover, in the absence of DNA methylation, replication enhances Ds transposition in petunia protoplasts >8-fold and promotes formation of a predominant excision footprint. Accordingly, replication also has a methylation-independent regulatory effect on transposition.