Mobilization of transposons by a mutation abolishing full DNA methylation in Arabidopsis

The highest-copy repeats are methylated in the small genome of the early divergent vascular plant Selaginella moellendorffii

Background

The lycophyte Selaginella moellendorffii is a vascular plant that diverged from the fern/seed plant lineage at least 400 million years ago. Although genomic information for S. moellendorffii is starting to be produced, little is known about basic aspects of its molecular biology. In order to provide the first glimpse to the epigenetic landscape of this early divergent vascular plant, we used the methylation filtration technique. Methylation filtration genomic libraries select unmethylated DNA clones due to the presence of the methylation-dependent restriction endonuclease McrBC in the bacterial host.

Results

We conducted a characterization of the DNA methylation patterns of the S. moellendorffii genome by sequencing a set of S. moellendorffii shotgun genomic clones, along with a set of methylation filtered clones. Chloroplast DNA, which is typically unmethylated, was enriched in the filtered library relative to the shotgun library, showing that there is DNA methylation in the extremely small S. moellendorffii genome. The filtered library also showed enrichment in expressed and gene-like sequences, while the highest-copy repeats were largely under-represented in this library. These results show that genes and repeats are differentially methylated in the S. moellendorffii genome, as occurs in other plants studied.

Conclusion

Our results shed light on the genome methylation pattern in a member of a relatively unexplored plant lineage. The DNA methylation data reported here will help understanding the involvement of this epigenetic mark in fundamental biological processes, as well as the evolutionary aspects of epigenetics in land plants.

Virus-like particle formation and translational start site choice of the plant retrotransposon Tto1

Ty1/copia group retrotransposon Tto1 from tobacco was put under control of an inducible promoter for expression in Arabidopsis thaliana. The system was used to analyze intermediates of the transposition process. The Tto1 RNA 5' region has a complex structure and contains several AUG codons. We therefore sought to experimentally define the translation initiation site. Constructs starting at various positions within the structural gag region were expressed in planta and functionally characterized. We found that gag proteins starting at the first ATG of the gag-pol ORF (ATG1), but also those starting at the third ATG of the gag-pol ORF (ATG3), can form virus-like particles (VLPs). However, gag protein expressed by the inducible Tto1 element had a size similar to gag starting at ATG1, and mutation of ATG1 in the inducible element abolished reverse transcription. This suggested that translation initiation at ATG1 is essential for the Tto1 life cycle. To support this conjecture, gag protein starting at ATG1, or gag protein shortened amino-terminally by nine amino acids (starting at the second ATG of the gag region, ATG2), was co-expressed with Tto1 carrying mutations at ATG1 and ATG2. Trans-complementation of the defective Tto element by gag starting at ATG1, but not by gag starting at ATG2, defines ATG1 as the functional translation initiation site.

Evolution and control of imprinted FWA genes in the genus Arabidopsis

A central question in genomic imprinting is how a specific sequence is recognized as the target for epigenetic marking. In both mammals and plants, imprinted genes are often associated with tandem repeats and transposon-related sequences, but the role of these elements in epigenetic gene silencing remains elusive. FWA is an imprinted gene in Arabidopsis thaliana expressed specifically in the female gametophyte and endosperm. Tissue-specific and imprinted expression of FWA depends on DNA methylation in the FWA promoter, which is comprised of two direct repeats containing a sequence related to a SINE retroelement. Methylation of this element causes epigenetic silencing, but it is not known whether the methylation is targeted to the SINE-related sequence itself or the direct repeat structure is also necessary. Here we show that the repeat structure in the FWA promoter is highly diverse in species within the genus Arabidopsis. Four independent tandem repeat formation events were found in three closely related species. Another related species, A. halleri, did not have a tandem repeat in the FWA promoter. Unexpectedly, even in this species, FWA expression was imprinted and the FWA promoter was methylated. In addition, our expression analysis of FWA gene in vegetative tissues revealed high frequency of intra-specific variation in the expression level. In conclusion, we show that the tandem repeat structure is dispensable for the epigenetic silencing of the FWA gene. Rather, SINE-related sequence is sufficient for imprinting, vegetative silencing, and targeting of DNA methylation. Frequent independent tandem repeat formation events in the FWA promoter led us to propose that they may be a consequence, rather than cause, of the epigenetic control. The possible significance of epigenetic variation in reproductive strategies during evolution is also discussed.

Large-scale survey of cytosine methylation of retrotransposons and the impact of readout transcription from long terminal repeats on expression of adjacent rice genes

Transposable elements (TEs) represent approximately 45% of the human genome and 50-90% of some grass genomes. While most elements contain inactivating mutations, others are reversibly inactivated (silenced) by epigenetic mechanisms, including cytosine methylation. Previous studies have shown that retrotransposons can influence the expression of adjacent host genes. In this study, the methylation patterns of TEs and their flanking sequences in different tissues were undertaken using a novel technique called transposon methylation display (TMD). TMD was successfully applied on a highly copied (approximately 1000 copies), newly amplified LTR retrotransposon family in rice called Dasheng. We determined that the methylation status of a subset of LTRs varies in leaves vs. roots. In addition, we determined that tissue-specific LTR methylation correlated with tissue-specific expression of the flanking rice gene. Genes showing tissue-specific expression were in opposite orientation relative to the LTR. Antisense transcripts were detected in the tissue where the sense transcripts from that gene were not detected. Comparative analysis of Dasheng LTR methylation in the two subspecies, japonica vs. indica revealed LTR-mediated differences in subspecies gene expression. Subspecies-specific expression was due either to polymorphic Dasheng insertion sites between the two subspecies or to subspecies-specific methylation of LTRs at the same locus accounted for observed differences in the expression of adjacent genes.

Hypomethylation and transcriptional reactivation of retrotransposon-like sequences in ddm1 transgenic plants of Brassica rapa

DNA methylation and histone modification play important roles in regulating gene expression. The DDM1 gene in Arabidopsis thaliana (AtDDM1) is required for the maintenance of DNA methylation level and histone H3 methylation pattern. We isolated DDM1 homologs of Brassica rapa, BrDDM1a and BrDDM1b, which have 84.4% and 84.1% deduced amino acid sequence identities with AtDDM1, respectively. Both the BrDDM1a and BrDDM1b genes were found to be expressed in vegetative and reproductive tissues. B. rapa ddm1-RNAi transgenic plants with reduced levels of BrDDM1a/BrDDM1b expression showed genome-wide and non-tissue-specific demethylation. These results suggest that the BrDDM1a and BrDDM1b genes are orthologs of AtDDM1 and are required for the maintenance of DNA methylation as is AtDDM1. Despite genome-wide demethylation, developmental abnormalities were not found in the ddm1-RNAi transgenic plants. Dominance relationships of SP11/SCR alleles, the determinant of pollen recognition specificity in Brassica self-incompatibility, in S heterozygotes in B. rapa were not influenced by the low level of the BrDDM1 expression. Transcriptional reactivation of retrotransposon-like sequences observed in the ddm1-RNAi transgenic plants indicates that BrDDM1a and BrDDM1b participate in silencing of retrotransposons. Hypomethylation states of the ddm1-RNAi transgenic plants were inherited by plants of the next generation even by plants which had lost the RNAi construct by segregation. Remethylation was observed in a few progenies. Efficiencies of remethylation in the progenies without the RNAi construct were different between 18S rDNA, BoSTF12a/15a, and BrTto1 sequences.

Control of genic DNA methylation by a jmjC domain-containing protein in Arabidopsis thaliana

Differential cytosine methylation of repeats and genes is important for coordination of genome stability and proper gene expression. Through genetic screen of mutants showing ectopic cytosine methylation in a genic region, we identified a jmjC-domain gene, IBM1 (increase in bonsai methylation 1), in Arabidopsis thaliana. In addition to the ectopic cytosine methylation, the ibm1 mutations induced a variety of developmental phenotypes, which depend on methylation of histone H3 at lysine 9. Paradoxically, the developmental phenotypes of the ibm1 were enhanced by the mutation in the chromatin-remodeling gene DDM1 (decrease in DNA methylation 1), which is necessary for keeping methylation and silencing of repeated heterochromatin loci. Our results demonstrate the importance of chromatin remodeling and histone modifications in the differential epigenetic control of repeats and genes.

Epigenetic control of plant stress response

Living organisms have the clearly defined strategies of stress response. These strategies are predefined by a genetic make-up of the organism and depend on a complex regulatory network of molecular interactions. Although in most cases, the plant response to stress based on the mechanisms of tolerance, resistance, and avoidance has clearly defined metabolic pathways, the ability to acclimate/adapt after a single generation exposure previously observed in several studies (Boyko A et al. [2007]: Nucleic Acids Res 35:1714-1725; Boyko and Kovalchuk, unpublished data), represents an interesting phenomenon that cannot be explained by Mendelian genetics. The latest findings in the field of epigenetics and the process of a reversible control over gene expression and inheritance lead to believe that organisms, especially plants, may have a flexible short-term strategy of the response to stress. Indeed, the organisms that can modify gene expression reversibly have an advantage in evolutionary terms, since they can avoid unnecessary excessive rearrangements and population diversification. This review covers various epigenetic processes involved in plant stress response. We focus on the mechanisms of DNA methylation and histone modifications responsible for the protection of somatic cells and inheritance of stress memories.

Host genome surveillance for retrotransposons by transposon-derived proteins

Transposable elements and their remnants constitute a substantial fraction of eukaryotic genomes. Host genomes have evolved defence mechanisms, including chromatin modifications and RNA interference, to regulate transposable elements. Here we describe a genome surveillance mechanism for retrotransposons by transposase-derived centromeric protein CENP-B homologues of the fission yeast Schizosaccharomyces pombe. CENP-B homologues of S. pombe localize at and recruit histone deacetylases to silence Tf2 retrotransposons. CENP-Bs also repress solo long terminal repeats (LTRs) and LTR-associated genes. Tf2 elements are clustered into 'Tf' bodies, the organization of which depends on CENP-Bs that display discrete nuclear structures. Furthermore, CENP-Bs prevent an 'extinct' Tf1 retrotransposon from re-entering the host genome by blocking its recombination with extant Tf2, and silence and immobilize a Tf1 integrant that becomes sequestered into Tf bodies. Our results reveal a probable ancient retrotransposon surveillance pathway important for host genome integrity, and highlight potential conflicts between DNA transposons and retrotransposons, major transposable elements believed to have greatly moulded the evolution of genomes.

Transcript profiling of the hypomethylated hog1 mutant of Arabidopsis

Transcript profiling was used to look for genes that differ in expression between the SAH hydrolase deficient and hypomethylated hog1-1 mutant and the parental (HOG1) line. This analysis identified a subset of gene transcripts that were up-regulated in hog1-1 plants. The majority of these transcripts were from genes located in the pericentromeric heterochromatin. About a third of the genes are annotated as transposons or having transposon homology. Subsequent experiments using Northern blots, RT-PCR and real-time RT-PCR confirmed the up-regulation of 19 of the genes and identified a set of molecular probes for genes that are up-regulated in the hog1-1 background. Six (of six genes tested) of the hog1-1 up-regulated genes are also up-regulated in the hypomethylated ddm1 mutant, three in the hypomethylated met1 mutant and three in the dcl3 mutant. The results suggest that the hypomethylation in the mutant lines may have a causal role in the up-regulation of these transcripts.

An osmium-DNA interstrand complex: application to facile DNA methylation analysis

Nucleic acids often acquire new functions by forming a variety of complexes with metal ions. Osmium, in an oxidized state, also reacts with C5-methylated pyrimidines. However, control of the sequence specificity of osmium complexation with DNA is still immature, and the value of the resulting complexes is unknown. We have designed a bipyridine-attached adenine derivative for sequence-specific osmium complexation. Sequence-specific osmium complexation was achieved by hybridization of a short DNA molecule containing this functional nucleotide to a target DNA sequence and resulted in the formation of a cross-linked structure. The interstrand cross-link clearly distinguished methylated cytosines from unmethylated cytosines and was used to quantify the degree of methylation at a specific cytosine in the genome.

Postintegrative gene silencing within the Sleeping Beauty transposition system

The Sleeping Beauty (SB) transposon represents an important vehicle for in vivo gene delivery because it can efficiently and stably integrate into mammalian genomes. In this report, we examined transposon expression in human cells using a novel nonselective fluorescence-activated cell sorter-based method and discovered that SB integrates approximately 20 times more frequently than previously reported within systems that were dependent on transgene expression and likely subject to postintegrative gene silencing. Over time, phenotypic analysis of clonal integrants demonstrated that SB undergoes additional postintegrative gene silencing, which varied based on the promoter used for transgene expression. Molecular and biochemical studies suggested that transposon silencing was influenced by DNA methylation and histone deacetylation because both 5-aza-2'-deoxycytidine and trichostatin A partially rescued transgene silencing in clonal cell lines. Collectively, these data reveal the existence of a multicomponent postintegrative gene silencing network that efficiently targets invading transposon sequences for transcriptional silencing in mammalian cells.

Heritable epigenetic mutation of a transposon-flanked Arabidopsis gene due to lack of the chromatin-remodeling factor DDM1

Epigenetically silent transposons and repeats constitute a substantial proportion of eukaryotic genomes, but their impact on cellular gene function remains largely unexplored. In Arabidopsis, transposons are silenced by DNA methylation, and this methylation is often abolished by mutations in a chromatin-remodeling gene DDM1 (DECREASE IN DNA METHYLATION 1). The ddm1 mutation induces various types of developmental abnormalities through de-repression of transposons and repeats. Here, we report a novel mechanism for a ddm1-induced syndrome, called bonsai (bns). We identified the gene responsible for the bns phenotypes by genetic linkage analysis and subsequent transcriptional analysis. The bns phenotypes are due to silencing of a putative Anaphase-Promoting Complex (APC) 13 gene. The BNS gene silencing was associated with DNA hypermethylation, which is in contrast to the ddm1-induced hypomethylation in the other genomic regions. This paradoxical BNS hypermethylation was reproducibly induced during self-pollination of the ddm1 mutant, and it was mediated by a long interspersed nuclear element (LINE) retrotransposon flanking the BNS gene. We discuss possible molecular mechanisms and the evolutionary implications of transposon-mediated epigenetic changes in the BNS locus.

Epigenetic natural variation in Arabidopsis thaliana

Cytosine methylation of repetitive sequences is widespread in plant genomes, occurring in both symmetric (CpG and CpNpG) as well as asymmetric sequence contexts. We used the methylation-dependent restriction enzyme McrBC to profile methylated DNA using tiling microarrays of Arabidopsis Chromosome 4 in two distinct ecotypes, Columbia and Landsberg erecta. We also used comparative genome hybridization to profile copy number polymorphisms. Repeated sequences and transposable elements (TEs), especially long terminal repeat retrotransposons, are densely methylated, but one third of genes also have low but detectable methylation in their transcribed regions. While TEs are almost always methylated, genic methylation is highly polymorphic, with half of all methylated genes being methylated in only one of the two ecotypes. A survey of loci in 96 Arabidopsis accessions revealed a similar degree of methylation polymorphism. Within-gene methylation is heritable, but is lost at a high frequency in segregating F(2) families. Promoter methylation is rare, and gene expression is not generally affected by differences in DNA methylation. Small interfering RNA are preferentially associated with methylated TEs, but not with methylated genes, indicating that most genic methylation is not guided by small interfering RNA. This may account for the instability of gene methylation, if occasional failure of maintenance methylation cannot be restored by other means.

Large-scale dissociation and sequential reassembly of pericentric heterochromatin in dedifferentiated Arabidopsis cells

Chromocenters in Arabidopsis thaliana are discrete nuclear domains of mainly pericentric heterochromatin. They are characterized by the presence of repetitive sequences, methylated DNA and dimethylated histone H3K9. Here we show that dedifferentiation of specialized mesophyll cells into undifferentiated protoplasts is accompanied by the disruption of chromocenter structures. The dramatic reduction of heterochromatin involves the decondensation of all major repeat regions, also including the centromeric 180 bp tandem repeats. Only the 45S rDNA repeat remained in a partly compact state in most cells. Remarkably, the epigenetic indicators for heterochromatin, DNA methylation and H3K9 dimethylation, did not change upon decondensation. Furthermore, the decondensation of pericentric heterochromatin did not result in transcriptional reactivation of silent genomic elements. The decondensation process was reversible upon prolonged culturing. Strikingly, recondensation of heterochromatin into chromocenters is a stepwise process. Compaction of the tandemly arranged 45S rDNA regions occurs first, followed by the centromeric 180 bp and the 5S rDNA repeats and finally the dispersed repeats, including transposons. The sequence of reassembly seems to be correlated to the size of the repeat domains. Our results indicate that different types of pericentromeric repeats form different types of heterochromatin, which subsequently merge to form a chromocenter.

Chromatin assembly factor 1 ensures the stable maintenance of silent chromatin states in Arabidopsis

Newly synthesized DNA is rapidly assembled into mature nucleosomes by the deposition of pre-existing and nascent histones, and some parts of this process are facilitated by chromatin assembly factor 1 (CAF-1). Loss-of-function mutants of CAF-1 in Arabidopsis, fasciata (fas), show a variety of morphological abnormalities and unique defects in gene expression in the meristems. In order to clarify the implications of CAF-1 in the maintenance of chromatin states in higher eukaryotes, we investigated transcriptional gene silencing (TGS) of various genes in fas mutants. Here, we show that TGS of endogenous CACTA transposons was released in a stochastic manner in fas. Other endogenous silent genes, a transposon AtMu1 and a hypothetical gene T5L23.26 at a heterochromatin knob, were also transcriptionally activated, and the activation of the three different silent loci at different chromosomal sites occurred non-concomitantly with each other. Furthermore, TGS of the silent beta-glucuronidase (GUS) transgene was also de-repressed randomly in fas. We conclude that CAF-1 ensures the stable inheritance of epigenetic states through growth and development in Arabidopsis.

Distinct expression of endogenous Petunia vein clearing virus and the DNA transposon dTph1 in two Petunia hybrida lines is correlated with differences in histone modification and siRNA production

Endogenous viruses exist in all kingdoms. They usually have active mechanisms of integration, as in bacteriophage lambda and animal retroviruses, and sophisticated mechanisms to maintain a proviral state over decades and generations. Plant para retroviruses, however, neither have an integrase, nor genes for maintaining the proviral state. How are those elements controlled, and under what conditions can they be activated? Here we study the proviral state of endogenous petunia vein clearing virus (ePVCV). Our results support the hypothesis that the proviral state is associated with a host silencing mechanism manifested by DNA methylation, chromatin modification and production of small interfering (si) RNAs. PVCV may be induced by applying abiotic stress, leading to the development of viral symptoms and increased transcript and siRNA levels. Similar levels of ePVCV DNA methylation were observed in two different lines of Petunia hybrida, RdC (rose du ciel) and W138, the latter known for its active version of transposon dTph1. In contrast, significant differences in histone modification were detected. The predominant association of ePVCV sequences with histone H3 methylated at lysine 9 (H3mK9) in RdC and with about equal amounts of H3mK9 and H3mK4 in W138 indicates a less repressive proviral state in the latter cultivar.

Differential epigenetic regulation within an Arabidopsis retroposon family

We previously reported a novel family of Arabidopsis thaliana nonautonomous retroposons, Sadhu, showing epigenetic variation in natural populations. Here, we show that transcripts corresponding to Sadhu elements accumulate in a subset of mutants carrying disruptions in genes encoding chromatin modification enzymes, but are not significantly expressed in mutants defective in RNA silencing pathways, indicating that RNA-directed processes are not necessary to maintain transcriptional suppression of this class of retroelements. We focused our analysis on three representative elements showing differential responses to ddm1, met1, and hda6 mutations. These mutations had differing effects on cytosine methylation depending on the element and the sequence context. Curiously, the Sadhu6-1 element with the strongest CpHpG methylation is expressed in a met1 CpG methyltransferase mutant, but is not expressed in ddm1 or cmt3 mutants. Regardless of the mutant background, H3meK9 was found at silenced loci, while H3meK4 was restricted to expressed alleles. We discuss the different modes of regulation within this family and the potential impact of this regulation on the stability of silencing in natural populations.

Transgenerational changes in the genome stability and methylation in pathogen-infected plants: (virus-induced plant genome instability)

Previously, we reported the generation of a virus-induced systemic signal that increased the somatic and meiotic recombination rates in tobacco mosaic virus (TMV)-infected tobacco plants. Here, we analyzed the progeny of plants that received the signal and found that these plants also have a higher frequency of rearrangements in the loci carrying the homology to LRR region of the gene of resistance to TMV (N-gene). Analysis of the stability of repetitive elements from Nicotiana tabacum loci and 5.8S ribosomal RNA loci did not show any changes. Further analysis of the changes in the progeny of infected plants revealed that they had substantially hypermethylated genomes. At the same time, loci-specific methylation analysis showed: (1) profound hypomethylation in several LRR-containing loci; (2) substantial hypermethylation of actin loci and (3) no change in methylation in the loci of repetitive elements from N. tabacum or 5.8S ribosomal RNA. Global genome hypermethylation of the progeny is believed to be part of a general protection mechanism against stress, whereas locus-specific hypomethylation is associated with a higher frequency of rearrangements. Increased recombination events combined with the specific methylation pattern induced by pathogen attack could be a sign of an adaptive response by plants.

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.

Maintenance DNA methyltransferase (Met1) and silencing of CpG-methylated foreign DNA in Volvox carteri

DNA methylation plays an important role in the gene-silencing network of higher eukaryotes. We have analyzed the 21.5-kb maintenance methyltransferase (M-MTase) gene, met1, of the multicellular green alga Volvox carteri. The met1 transcript was detected only during the period when DNA replication and cell division are taking place. It encodes a 238 kDa protein containing eight C-terminal activity domains typical of M-MTases, plus upstream DNA-binding domains including the ProDom domain PD003757, which experimental analyses in animal systems have indicated is required for targeting the enzyme to DNA-replication foci. Several insertions of unknown function make Volvox Met1 the largest known member of the Met1/Dnmt1 family. Here we also show that several endogenous transposon families are CpG-methylated in Volvox, which we think causes them to be inactive. This view is supported by the observation that an in vitro CpG-methylated gene introduced into Volvox was maintained in the methylated and silent state over >100 generations. Thus, we believe that Met1 recognizes and perpetuates the in vitro methylation signal, and that the silencing machinery is then able to transduce such a methylation-only signal into a stable heterochromatic (and silent) state.

Molecular basis of late-flowering phenotype caused by dominant epi-alleles of the FWA locus in Arabidopsis

The late-flowering phenotype of dominant fwa mutants is caused by hypomethylation in the FWA locus leading to ectopic expression of a homeodomain leucine zipper (HD-ZIP) protein. However, little is known about whether FWA has any role in regulation of flowering and how ectopically expressed FWA delays flowering. Through analysis of FWA expression in wild-type seedlings, it was shown that FWA is not expressed during the vegetative phase. This suggests that FWA has no role in flowering. The previous reports that fwa suppressed the precocious-flowering phenotype of plants overexpressing FLOWERING LOCUS T (FT) suggest that the flowering pathway(s) either at and/or downstream of FT is blocked by FWA. Comparison of gene expression profiles in three genetic backgrounds ectopically expressing FWA and their respective wild types failed to detect common changes, ruling out the possibility that FWA acts through transcriptional misregulation. Yeast two-hybrid analysis and in vitro pull-down assay showed that FWA protein can specifically interact with FT protein. The importance of protein interaction with FT in delaying flowering was supported by studies involving N-terminal and C-terminal truncations of FWA. The C-terminal truncation with abolished interaction did not delay flowering when overexpressed, while the N-terminal truncation, which retains interaction, did. Specific interaction of FWA with FT enabled us to use FWA protein as a specific inhibitor of FT protein function. Through tissue-specific ectopic expression of FWA, further support for the shoot apex being the site of action of FT protein was provided.

Genetic analysis and epigenetic silencing of At4CL1 and At4CL2 expression in transgenic Arabidopsis

4-coumarate::CoA ligase (4CL) gene family members are involved in channeling carbon flow into branch pathways of phenylpropanoid metabolism. Transgenic Arabidopsis plants containing the At4CL1 or At4CL2 promoter fused to the beta-glucuronidase (GUS) reporter gene show developmentally regulated GUS expression in the xylem tissues of the root and shoot. To identify regulatory genes involved in the developmental regulation of At4CL and other phenylpropanoid-specific genes, we generated ethyl methyl sulfate mutagenized populations of At4CL1::GUS and At4CL2::GUS transgenic lines and screened approximately 16,000 progeny for reduced or altered GUS expression. Several lines with reproducible patterns of reduced GUS expression were identified. However, the GUS-expression phenotype segregated in a non-Mendelian manner in all of the identified lines. Also, GUS expression was restored by 5-azacytidine (aza) treatment, suggesting inhibitory DNA methylation of the transgene. Southern analysis confirmed DNA methylation of the proximal promoter sequences of the transgene only in the mutant lines. In addition, retransformation of At4CL::GUS lines with further At4CL promoter constructs enhanced the GUS-silencing phenotype. Taken together, these results suggest that the isolated mutants are epimutants. Apparently, two different modes of silencing were engaged in the At4CL1::GUS and At4CL2::GUS silenced lines. While silencing in the seedlings of the At4CL1::GUS lines was root specific in seedlings, it affected all organs in the At4CL2::GUS lines. Also, At4CL1::GUS transgene silencing was confined to the transgene but At4CL2::GUS silencing extended to the endogenous At4CL2 gene. Organ-specific silencing of the At4CL1::GUS transgene cannot be explained by current models in the literature.

SDG714, a histone H3K9 methyltransferase, is involved in Tos17 DNA methylation and transposition in rice

Although the role of H3K9 methylation in rice (Oryza sativa) is unclear, in Arabidopsis thaliana the loss of histone H3K9 methylation by mutation of Kryptonite [also known as SU(VAR)3-9 homolog] reduces genome-wide DNA methylation and increases the transcription of transposable elements. Here, we report that rice SDG714 (for SET Domain Group Protein714) encodes a histone H3K9-specific methyltransferase. The C terminus of SDG714 confers enzymatic activity and substrate specificity, whereas the N terminus localizes it in the nucleus. Loss-of-function mutants of SDG714 (SDG714IR transformants) generated by RNA interference display a mostly glabrous phenotype as a result of the lack of macro trichomes in glumes, leaves, and culms compared with control plants. These mutants also show decreased levels of CpG and CNG cytosine methylation as well as H3K9 methylation at the Tos17 locus, a copia-like retrotransposon widely used for the generation of rice mutants. Most interestingly, loss of function of SDG714 can enhance transcription and cause the transposition of Tos17. Together, these results suggest that histone H3K9 methylation mediated by SDG714 is involved in DNA methylation, the transposition of transposable elements, and genome stability in rice.

Control of FWA gene silencing in Arabidopsis thaliana by SINE-related direct repeats

A unique feature of late-flowering fwa epigenetic mutations is that the phenotype is caused by ectopic expression of the homeobox gene FWA. During normal development the FWA gene is expressed specifically in the endosperm in an imprinted manner. Ectopic FWA expression and disruption of imprinting can be induced in mutants of a CG methyltransferase MET1 (methyltransferase 1) or a chromatin-remodeling gene DDM1 (decrease in DNA methylation 1), suggesting that the proper FWA expression depends on cytosine methylation. However, critical methylated residues controlling FWA silencing are not pinpointed. Nor is it understood how the FWA gene is initially methylated and silenced in wild-type plants. Here we mapped sequences critical for FWA silencing by application of RdDM (RNA-directed DNA methylation) to a ddm1-induced stable fwa epiallele. Transcription of double-stranded RNA corresponding to the tandem direct repeats around the FWA transcription start site induced de novo DNA methylation, transcriptional suppression and phenotypic reversion. The induced changes were heritable even without the transgene, which correlates with inheritance of CG methylation in the direct repeats. The newly silenced FWA allele was transcribed in an endosperm-specific and imprinted manner, as is the case for the wild-type FWA gene. The results indicate that methylation of the direct repeats, which presumably originated from a short interspersed nuclear element (SINE), is sufficient to induce proper epigenetic control of the FWA gene.

DNA transposons and the evolution of eukaryotic genomes

Transposable elements are mobile genetic units that exhibit broad diversity in their structure and transposition mechanisms. Transposable elements occupy a large fraction of many eukaryotic genomes and their movement and accumulation represent a major force shaping the genes and genomes of almost all organisms. This review focuses on DNA-mediated or class 2 transposons and emphasizes how this class of elements is distinguished from other types of mobile elements in terms of their structure, amplification dynamics, and genomic effect. We provide an up-to-date outlook on the diversity and taxonomic distribution of all major types of DNA transposons in eukaryotes, including Helitrons and Mavericks. We discuss some of the evolutionary forces that influence their maintenance and diversification in various genomic environments. Finally, we highlight how the distinctive biological features of DNA transposons have contributed to shape genome architecture and led to the emergence of genetic innovations in different eukaryotic lineages.

Sleeping beauty transposase has an affinity for heterochromatin conformation

The Sleeping Beauty (SB) transposase reconstructed from salmonid fish has high transposition activity in mammals and has been a useful tool for insertional mutagenesis and gene delivery. However, the transposition efficiency has varied significantly among studies. Our previous study demonstrated that the introduction of methylation into the SB transposon enhanced transposition, suggesting that transposition efficiency is influenced by the epigenetic status of the transposon region. Here, we examined the influence of the chromatin status on SB transposition in mouse embryonic stem cells. Heterochromatin conformation was introduced into the SB transposon by using a tetracycline-controlled transrepressor (tTR) protein, consisting of a tetracycline repressor (TetR) fused to the Kruppel-associated box (KRAB) domain of human KOX1 through tetracycline operator (tetO) sequences. The excision frequency of the SB transposon, which is the first step of the transposition event, was enhanced by approximately 100-fold. SB transposase was found to be colocalized with intense DAPI (4',6'-diamidino-2-phenylindole) staining and with the HP1 family by biochemical fractionation analyses. Furthermore, chromatin immunoprecipitation analysis revealed that SB transposase was recruited to tTR-induced heterochromatic regions. These data suggest that the high affinity of SB transposase for heterochromatin conformation leads to enhancement of SB transposition efficiency.

Regulation of seed size by hypomethylation of maternal and paternal genomes

DNA methylation is an epigenetic modification of cytosine that is important for silencing gene transcription and transposons, gene imprinting, development, and seed viability. DNA METHYLTRANSFERASE1 (MET1) is the primary maintenance DNA methyltransferase in Arabidopsis (Arabidopsis thaliana). Reciprocal crosses between antisense MET1 transgenic and wild-type plants show that DNA hypomethylation has a parent-of-origin effect on seed size. However, due to the dominant nature of the antisense MET1 transgene, the parent with a hypomethylated genome, its gametophyte, and both the maternal and paternal genomes of the F(1) seed become hypomethylated. Thus, the distinct role played by hypomethylation at each generation is not known. To address this issue, we examined F(1) seed from reciprocal crosses using a loss-of-function recessive null allele, met1-6. Crosses between wild-type and homozygous met1-6 parents show that hypomethylated maternal and paternal genomes result in significantly larger and smaller F(1) seeds, respectively. Our analysis of crosses between wild-type and heterozygous MET1/met1-6 parents revealed that hypomethylation in the female or male gametophytic generation was sufficient to influence F(1) seed size. A recessive mutation in another gene that dramatically reduces DNA methylation, DECREASE IN DNA METHYLATION1, also causes parent-of-origin effects on F(1) seed size. By contrast, recessive mutations in genes that regulate a smaller subset of DNA methylation (CHROMOMETHYLASE3 and DOMAINS REARRANGED METHYLTRANSFERASES1 and 2) had little effect on seed size. Collectively, these results show that maternal and paternal genomes play distinct roles in the regulation of seed size in Arabidopsis.

Restriction landmark genome scanning method using isoschizomers (MspI/HpaII) for DNA methylation analysis

Restriction landmark genome scanning (RLGS) is a 2-DE of genomic DNA, which visualizes thousands of loci. In a conventional RLGS method for methylation analysis, we have used a methylation sensitive restriction enzyme, NotI as a landmark. However, it was unable to discriminate methylation polymorphism from sequence polymorphism. Here, we report an improved RLGS method to detect methylated sites directly. We employed isoschizomers, MspI and HpaII, that recognize the same sequence (CCGG) but have different methylation sensitivity. We carried out the RLGS analysis of Arabidopsis thaliana ecotype Columbia, and obtained a pair of spot patterns with MspI and HpaII. We detected 22 spots in both patterns. In comparison of them, 18% of the spots were polymorphic, which indicated the methylation of C(5m)CGG sites. Further analyses revealed an additional methylated site of NotI. Moreover, 52 and 54 restriction enzyme sites were also analyzed in two other ecotypes, Wassilewskija and Landsberg erecta, respectively. Consequently, 15% of the 52 common sites showed methylation polymorphism among the three ecotypes. The restriction sites analyzed in this study were located in or near genes, and contribute new data about the correlation between methylation status and gene expression. Therefore, this result strongly indicates that the improved RLGS method is readily applicable to practical analyses of methylation dynamics, and provides clues to the relationship between methylation and gene expression.

A histone methylation-dependent DNA methylation pathway is uniquely impaired by deficiency in Arabidopsis S-adenosylhomocysteine hydrolase

S-adenosylhomocysteine hydrolase (SAH) is a key enzyme in the maintenance of methylation homeostasis in eukaryotes because it is needed to metabolize the by-product of transmethylation reactions, S-adenosylhomocysteine (AdoHcy), which causes by-product inhibition of methyltransferases (MTase's). Complete loss of SAH function is lethal. Partial loss of SAH function causes pleiotropic effects including developmental abnormalities and reduced cytosine methylation. Here we describe a novel partial-function missense allele of the Arabidopsis SAH1 gene that causes loss of cytosine methylation specifically in non-CG contexts controlled by the CMT3 DNA MTase and transcriptional reactivation of a silenced reporter gene, without conferring developmental abnormalities. The CMT3 pathway depends on histone H3 lysine 9 methylation (H3 mK9) to guide DNA methylation. Our results suggest that this pathway is uniquely sensitive to SAH impairment because of its requirement for two transmethylation reactions that can both be inhibited by AdoHcy. Our results further suggest that gene silencing pathways involving an interplay between histone and DNA methylation in other eukaryotes can be selectively impaired by controlled SAH downregulation.

Genome-wide high-resolution mapping and functional analysis of DNA methylation in arabidopsis

Cytosine methylation is important for transposon silencing and epigenetic regulation of endogenous genes, although the extent to which this DNA modification functions to regulate the genome is still unknown. Here we report the first comprehensive DNA methylation map of an entire genome, at 35 base pair resolution, using the flowering plant Arabidopsis thaliana as a model. We find that pericentromeric heterochromatin, repetitive sequences, and regions producing small interfering RNAs are heavily methylated. Unexpectedly, over one-third of expressed genes contain methylation within transcribed regions, whereas only approximately 5% of genes show methylation within promoter regions. Interestingly, genes methylated in transcribed regions are highly expressed and constitutively active, whereas promoter-methylated genes show a greater degree of tissue-specific expression. Whole-genome tiling-array transcriptional profiling of DNA methyltransferase null mutants identified hundreds of genes and intergenic noncoding RNAs with altered expression levels, many of which may be epigenetically controlled by DNA methylation.

Epigenetic regulation of the rice retrotransposon Tos17

Transposable elements are major components of plant genomes. Their activity seems to be epigenetically regulated by gene silencing systems. Here we report epigenetic variation in the retrotransposon Tos17 activity in rice varieties. Of the two copies of Tos17 present in chromosome 7 (Tos17 (chr.7)) and chromosome 10 (Tos17 (chr.10)), Tos17 (chr.7) is strongly activated by tissue culture in most varieties including Nipponbare except for Moritawase, despite the identity of the DNA sequences in Moritawase and Nipponbare. Tos17 (chr.7) activity correlated with its methylation status, and Tos17 (chr.7 )in Moritawase was heavily methylated and activated by treatment of 5-azacytidine (5-azaC), a DNA methylation inhibitor. Although the original copies of Tos17 are methylated to some extent in all varieties examined, the transposed copies in calli mostly are not methylated. When plants were regenerated from calli, the degree of methylation of the Tos17 DNA increased gradually with the growth of plants, and a significant progress of DNA methylation occurred in the next generation after a completed reproductive cycle. With increasing DNA methylation, the transcription of transposed and original Tos17 copies driven by its own as well as by a flanking gene promoter were suppressed. We conclude that Tos17 DNA methylation controls the transpositional activity of Tos17, and modulates the activity of neighboring genes. Based on the analysis of the inactive Tos17 (chr.10), we propose that another mechanism, called transcriptional interference, is involved in the control of Tos17 activity.

Mobilized retrotransposon Tos17 of rice by alien DNA introgression transposes into genes and causes structural and methylation alterations of a flanking genomic region

Tos17 is a copia-like endogenous retrotransposon of rice, which can be activated by various stresses such as tissue culture and alien DNA introgression. To confirm element mobilization by introgression and to study possible structural and epigenetic effects of Tos17 insertion on its target sequences, we isolated all flanking regions of Tos17 in an introgressed rice line (Tong35) that contains minute amount of genomic DNA from wild rice (Zizania latifolia). It was found that there has been apparent but limited mobilization of Tos17 in this introgression line, as being reflected by increased but stable copy number of the element in progeny of the line. Three of the five activated copies of the element have transposed into genes. Based on sequence analysis and Southern blot hybridization with several double-enzyme digests, no structural change in Tos17 could be inferred in the introgression line. Cytosine methylation status at all seven CCGG sites within Tos17 was also identical between the introgression line and its rice parent (Matsumae)-all sites being heavily methylated. In contrast, changes in structure and cytosine methylation patterns were detected in one of the three low-copy genomic regions that flank newly transposed Tos17, and all changes are stably inherited through selfed generations.

Centromere locations and associated chromosome rearrangements in Arabidopsis lyrata and A. thaliana

We analyzed linkage and chromosomal positions of genes in A. lyrata ssp. petraea that are located near the centromere (CEN) regions of A. thaliana, using at least two genes from the short and long arms of each chromosome. In our map, genes from all 10 A. thaliana chromosome arms are also tightly linked in A. lyrata. Genes from the regions on the two sides of CEN5 have distant map localizations in A. lyrata (genes on the A. thaliana short-arm genes are on linkage group AL6, and long-arm genes are on AL7), but genes from the other four A. thaliana centromere regions remain closely linked in A. lyrata. The observation of complete linkage between short- and long-arm centromere genes, but not between genes in other genome regions that are separated by similar physical distances, suggests that crossing-over frequencies near the A. lyrata ssp. petraea centromere regions are low, as in A. thaliana. Thus, the centromere positions appear to be conserved between A. thaliana and A. lyrata, even though three centromeres have been lost in A. thaliana, and the core satellite sequences in the two species are very different. We can now definitively identify the three centromeres that were eliminated in the fusions that formed the A. thaliana chromosomes. However, we cannot tell whether genes were lost along with these centromeres, because such genes are absent from the A. thaliana genome, which is the sole source of markers for our mapping.

Regulation of imprinted DNA methylation

DNA methylation is an essential enzymatic modification in mammals. This common epigenetic mark occurs predominantly at the fifth carbon of cytosines within the palindromic dinucleotide 5'-CpG-3'. The majority of methylated CpGs are located within repetitive elements including centromeric repeats, satellite sequences and gene repeats encoding ribosomal RNAs. CpG islands, frequently located at the 5' end of genes, are typically unmethylated. DNA methylation also occurs at imprinted genes which exhibit parent-of-origin-specific patterns of methylation and expression. Imprinted methylation at differentially methylated domains (DMDs) is one of the regulatory mechanisms controlling the allele-specific expression of imprinted genes. Proper control of DNA methylation is needed for normal development and loss of methylation control can contribute to initiation and progression of tumorigenesis (reviewed in Plass and Soloway, 2002). Because patterns of imprinted DNA methylation are highly reproducible, imprinted loci make useful models for studying regulation of DNA methylation and may provide insights into how this regulation goes awry in cancer. Here, we review what is currently known about the mechanisms regulating imprinted DNA methylation. We will focus on cis-acting DNA sequences, trans-acting protein factors and the possible involvement of RNAs in control of imprinted DNA methylation.

Transposon-mediated expansion and diversification of a family of ULP-like genes

Transposons comprise a major component of eukaryotic genomes, yet it remains controversial whether they are merely genetic parasites or instead significant contributors to organismal function and evolution. In plants, thousands of DNA transposons were recently shown to contain duplicated cellular gene fragments, a process termed transduplication. Although transduplication is a potentially rich source of novel coding sequences, virtually all appear to be pseudogenes in rice. Here we report the results of a genome-wide survey of transduplication in Mutator-like elements (MULEs) in Arabidopsis thaliana, which shows that the phenomenon is generally similar to rice transduplication, with one important exception: KAONASHI (KI). A family of more than 97 potentially functional genes and apparent pseudogenes, evidently derived at least 15 MYA from a cellular small ubiquitin-like modifier-specific protease gene, KI is predominantly located in potentially autonomous non-terminal inverted repeat MULEs and has evolved under purifying selection to maintain a conserved peptidase domain. Similar to the associated transposase gene but unlike cellular genes, KI is targeted by small RNAs and silenced in most tissues but has elevated expression in pollen. In an Arabidopsis double mutant deficient in histone and DNA methylation with elevated KI expression compared to wild type, at least one KI-MULE is mobile. The existence of KI demonstrates that transduplicated genes can retain protein-coding capacity and evolve novel functions. However, in this case, our evidence suggests that the function of KI may be selfish rather than cellular.

Cloning of a CACTA transposon-like insertion in intron I of tomato invertase Lin5 gene and identification of transposase-like sequences of Solanaceae species

Very few CACTA transposon-like sequences have been described in Solanaceae species. Sequence information has been restricted to partial transposase (TPase)-like fragments, and no target gene of CACTA-like transposon insertion has been described in tomato to date. In this manuscript, we report on a CACTA transposon-like insertion in intron I of tomato (Lycopersicon esculentum) invertase gene Lin5 and TPase-like sequences of several Solanaceae species. Consensus primers deduced from the TPase region of the tomato CACTA transposon-like element allowed the amplification of similar sequences from various Solanaceae species of different subfamilies including Solaneae (Solanum tuberosum), Cestreae (Nicotiana tabacum) and Datureae (Datura stramonium). This demonstrates the ubiquitous presence of CACTA-like elements in Solanaceae genomes. The obtained partial sequences are highly conserved, and allow further detection and detailed analysis of CACTA-like transposons throughout Solanaceae species. CACTA-like transposon sequences make possible the evaluation of their use for genome analysis, functional studies of genes and the evolutionary relationships between plant species.

Meiotically stable natural epialleles of Sadhu, a novel Arabidopsis retroposon

Epigenetic variation is a potential source of genomic and phenotypic variation among different individuals in a population, and among different varieties within a species. We used a two-tiered approach to identify naturally occurring epigenetic alleles in the flowering plant Arabidopsis: a primary screen for transcript level polymorphisms among three strains (Col, Cvi, Ler), followed by a secondary screen for epigenetic alleles. Here, we describe the identification of stable, meiotically transmissible epigenetic alleles that correspond to one member of a previously uncharacterized non-LTR retroposon family, which we have designated Sadhu. The pericentromeric At2g10410 element is highly expressed in strain Col, but silenced in Ler and 18 other strains surveyed. Transcription of this locus is inversely correlated with cytosine methylation and both the expression and DNA methylation states map in a Mendelian manner to stable cis-acting variation. The silent Ler allele can be converted by the epigenetic modifier mutation ddm1 to a meiotically stable expressing allele with an identical primary nucleotide sequence, demonstrating that the variation responsible for transcript level polymorphism among Arabidopsis strains is epigenetic. We extended our characterization of the Sadhu family members and show that different elements are subject to both genetic and epigenetic variation in natural populations. These findings support the view that an important component of natural variation in retroelements is epigenetic.

DNA Methylation in Plants

DNA in plants is highly methylated, containing 5-methylcytosine (m5C) and N6-methyladenine (m6A); m5C is located mainly in symmetrical CG and CNG sequences but it may occur also in other non-symmetrical contexts. m6A but not m5C was found in plant mitochondrial DNA. DNA methylation in plants is species-, tissue-, organelle- and age-specific. It is controlled by phytohormones and changes on seed germination, flowering and under the influence of various pathogens (viral, bacterial, fungal). DNA methylation controls plant growth and development, with particular involvement in regulation of gene expression and DNA replication. DNA replication is accompanied by the appearance of under-methylated, newly formed DNA strands including Okazaki fragments; asymmetry of strand DNA methylation disappears until the end of the cell cycle. A model for regulation of DNA replication by methylation is suggested. Cytosine DNA methylation in plants is more rich and diverse compared with animals. It is carried out by the families of specific enzymes that belong to at least three classes of DNA methyltransferases. Open reading frames (ORF) for adenine DNA methyltransferases are found in plant and animal genomes, and a first eukaryotic (plant) adenine DNA methyltransferase (wadmtase) is described; the enzyme seems to be involved in regulation of the mitochondria replication. Like in animals, DNA methylation in plants is closely associated with histone modifications and it affects binding of specific proteins to DNA and formation of respective transcription complexes in chromatin. The same gene (DRM2) in Arabidopsis thaliana is methylated both at cytosine and adenine residues; thus, at least two different, and probably interdependent, systems of DNA modification are present in plants. Plants seem to have a restriction-modification (R-M) system. RNA-directed DNA methylation has been observed in plants; it involves de novo methylation of almost all cytosine residues in a region of siRNA-DNA sequence identity; therefore, it is mainly associated with CNG and non-symmetrical methylations (rare in animals) in coding and promoter regions of silenced genes. Cytoplasmic viral RNA can affect methylation of homologous nuclear sequences and it maybe one of the feedback mechanisms between the cytoplasm and the nucleus to control gene expression.

Lower levels of transgene silencing in roots is associated with reduced DNA methylation levels at non-symmetrical sites but not at symmetrical sites

Transgene transcripts were recently shown to accumulate at higher levels in roots, relative to leaves, of silenced-transgenic Nicotiana benthamiana plants and to be inversely related with the accumulation of small interfering RNAs (siRNAs), suggesting that RNA silencing is less active in roots than in leaves (Andika et al., 2005. Mol. Plant-Microbe Interact. 18: 194). Here we show that the lower transgene RNA silencing activity in roots was associated with lower transgene methylation levels at non-symmetrical CpNpN context but not at symmetrical CpG or CpNpG context in three sets of transformant plants with different exogenous genes. In contrast, such a difference between roots and leaves was not observed for the Tnt1 retrotransposon: no Tnt1 transcript was detected in roots or in leaves of N. benthamiana, while equal levels of Tnt1-derived siRNA accumulation and Tnt1 methylation were found. From our data and previously reported information, we suggest that roots have less of an activity that acts at the step of generation of siRNAs.

Transposon-derived repeats in the human genome and 5-methylcytosine-associated mutations in adjacent genes

Transposon-derived repeats (TDR) represent approximately 50% of the human genome. A transposon suppression system has been proposed to explain why transposon-derived repeats (TDR) seldom cause mutations in humans. If this system is based on DNA methylation, a correlation might exist between amount of TDR adjacent to genes and frequency of coding sequence mutations due to m5C deaminations. To test this hypothesis we selected 385 genes based on availability of accurate information on their genome structure and mutation patterns (at least 10 mutations described in the Human Gene Mutation Database (HGMD)). The CENSOR program was used to estimate amount and class of TDR for the gene region and an arbitrarily selected 1 KB from each end. We assumed all C --> T transitions to be possible 5-methylcytosine-associated mutations (MAM) and calculated the number and proportion of MAM in the 385 genes. If there is a strong correlation between methylation of certain CpX dinuclecotides and TDR we might be able to detect it despite limitations of available data for this analysis. We found statistically significant correlations between: i) TDR and number of MAM in genes (r = 0.118, p = 0.02), ii) SINE-TDR and proportion of CpG --> TpG (r = 0.11, p = 0.03); limited to MIR elements only (r = 0.14, p = 0.006), and iii) LINE-TDR and proportion of CpT --> TpT (r = 0.166, p = 0.04). The group of genes with no TDR had a statistically significant lower proportion of MAM (184/479, 0.38 vs. 6466/14524, 0.46; p = 0.009) with differences noted for CpA --> TpA (35/479, 0.073 vs. 1380/11474; p = 0.003). In addition, CpT --> TpT were least common in genes with no TDR (8/479, 0.017), intermediate in genes with TDR in genomic sequence but not mRNA (337/11474, 0.029) and most common in genes with TDR within mature mRNA (121/3050, 0.040; p for trend = 0.003). Our data suggest that TDR adjacent to genes may sometimes influence methylation of cytosines in coding sequences to a degree that it affects mutation patterns. These observations should be followed up with further database analysis and biochemical studies.

Differential methylation of genes and repeats in land plants

The hypomethylated fraction of plant genomes is usually enriched in genes and can be selectively cloned using methylation filtration (MF). Therefore, MF has been used as a gene enrichment technology in sorghum and maize, where gene enrichment was proportional to genome size. Here we apply MF to a broad variety of plant species spanning a wide range of genome sizes. Differential methylation of genic and non-genic sequences was observed in all species tested, from non-vascular to vascular plants, but in some cases, such as wheat and pine, a lower than expected level of enrichment was observed. Remarkably, hexaploid wheat and pine show a dramatically large number of gene-like sequences relative to other plants. In hexaploid wheat, this apparent excess of genes may reflect an abundance of methylated pseudogenes, which may thus be more prevalent in recent polyploids.

Epigenetics and its implications for plant biology 2. The 'epigenetic epiphany': epigenetics, evolution and beyond

SCOPE

In the second part of a two-part review, the ubiquity and universality of epigenetic systems is emphasized, and attention is drawn to the key roles they play, ranging from transducing environmental signals to altering gene expression, genomic architecture and defence.

KEY ISSUES

The importance of transience versus heritability in epigenetic marks is examined, as are the potential for stable epigenetic marks to contribute to plant evolution, and the mechanisms generating novel epigenetic variation, such as stress and interspecific hybridization.

FUTURE PROSPECTS

It is suggested that the ramifications of epigenetics in plant biology are immense, yet unappreciated. In contrast to the ease with which the DNA sequence can be studied, studying the complex patterns inherent in epigenetics poses many problems. Greater knowledge of patterns of epigenetic variation may be informative in taxonomy and systematics, as well as population biology and conservation.

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.

Two different transposable elements inserted in flavonoid 3',5'-hydroxylase gene contribute to pink flower coloration in Gentiana scabra

Pink-flowered gentian plants (Gentiana scabra) have been bred from spontaneous mutations of blue-flowered gentian plants, but the formation mechanism(s) is unknown so far. To investigate the process, two independent pink-flowered gentian plant lines were analyzed by a molecular biological approach. HPLC analysis showed that petals of the blue-flowered cultivar contained a small amount of cyanidin derivatives and major delphinidin derivatives, whereas pink petals had only a small amount of cyanidin derivatives. To find the causal factor(s) of this change, we focused on flavonoid 3',5'-hydroxylase (F3',5'H), which is a key enzyme for delphinidin biosynthesis in the flavonoid biosynthetic pathway. Molecular analyses confirmed that the loss of delphinidin synthesis could be attributed to the insertions of different transposable elements in the F3',5'H gene in each independent pink-flowered gentian plant. Sequence analysis showed that these transposable elements were classified into an hAT superfamily and terminal-repeat retrotransposon in miniature (TRIM), by which normal F3',5'H transcripts were interrupted. Southern blot analysis indicated that they belong to high copy number elements and are also found in a related gentian species (G. triflora). These results suggest that the transposable elements inserted in F3',5'H are the source of the mutations and may also play a substantial role in the genomic evolution of the genus Gentiana.

The Arabidopsis heterochromatin protein1 homolog (TERMINAL FLOWER2) silences genes within the euchromatic region but not genes positioned in heterochromatin

TERMINAL FLOWER2 (TFL2) is the only homolog of heterochromatin protein1 (HP1) in the Arabidopsis genome. Because proteins of the HP1 family in fission yeast and animals act as key components of gene silencing in heterochromatin by binding to histone H3 methylated on lysine 9 (K9), here we examined whether TFL2 has a similar role in Arabidopsis. Unexpectedly, genes positioned in heterochromatin were not activated in tfl2 mutants. Moreover, the TFL2 protein localized preferentially to euchromatic regions and not to heterochromatic chromocenters, where K9-methylated histone H3 is clustered. Instead, TFL2 acts as a repressor of genes related to plant development, i.e. flowering, floral organ identity, meiosis and seed maturation. Up-regulation of the floral homeotic genes PISTILLATA, APETALA3, AGAMOUS and SEPALLATA3 in tfl2 mutants was independent of LEAFY or APETALA3, known activators of the above genes. In addition, transduced APETALA3 promoter fragments as short as 500 bp were sufficient for TFL2-mediated gene repression. Taken together, TFL2 silences specific genes within euchromatin but not genes positioned in heterochromatin of Arabidopsis.

Mobile genetic elements and sexual reproduction

Transposable elements (TE) are prominent components of most eukaryotic genomes. In addition to their possible participation in the origin of sexual reproduction in eukaryotes, they may be also involved in its maintenance as important contributors to the deleterious mutation load. Comparative analyses of transposon content in the genomes of sexually reproducing and anciently asexual species may help to understand the contribution of different TE classes to the deleterious load. The apparent absence of deleterious retrotransposons from the genomes of ancient asexuals is in agreement with the hypothesis that they may play a special role in the maintenance of sexual reproduction and in early extinction for which most species are destined upon the abandonment of sex.

DNA methylation and epigenetic inheritance during plant gametogenesis

In plants, newly acquired epigenetic states of transcriptional gene activity are readily transmitted to the progeny. This is in contrast to mammals, where only rare cases of transgenerational inheritance of new epigenetic traits have been reported (FASEB J 12:949-957, 1998; Nat Genet 23:314-318, 1999; Proc Natl Acad Sci U S A 100:2538-2543, 2003). Epigenetic inheritance in plants seems to rely on cytosine methylation maintained through meiosis and postmeiotic mitoses, giving rise to gametophytes. In particular, maintenance of CpG methylation ((m)CpG) appears to play a central role, guiding the distribution of other epigenetic signals such as histone H3 methylation and non-CpG DNA methylation. The evolutionarily conserved DNA methyltransferase MET1 is responsible for copying (m)CpG patterns through DNA replication in the gametophytic phase. The importance of gametophytic MET1 activity is illustrated by the phenotypes of met1 mutants that are severely compromised in the accuracy of epigenetic inheritance during gametogenesis. This includes elimination of imprinting at paternally silent loci such as FWA or MEDEA (MEA). The importance of DNA methylation in gametophytic imprinting has been reinforced by the discovery of DEMETER (DME), encoding putative DNA glycosylase involved in the removal of (m)C. DME opposes transcriptional silencing associated with imprinting activities of the MEA/FIE polycomb group complex.