DNA methylation of retrotransposons,DNA transposons and genes in sugar beet (Beta vulgaris L.)

The methylation of cytosines shapes the epigenetic landscape of plant genomes, coordinates transgenerational epigenetic inheritance, represses the activity of transposable elements (TEs), affects gene expression and, hence, can influence the phenotype. Sugar beet (Beta vulgaris ssp. vulgaris), an important crop that accounts for 30% of worldwide sugar needs, has a relatively small genome size (758 Mbp) consisting of approximately 485 Mbp repetitive DNA (64%), in particular satellite DNA, retrotransposons and DNA transposons. Genome‐wide cytosine methylation in the sugar beet genome was studied in leaves and leaf‐derived callus with a focus on repetitive sequences, including retrotransposons and DNA transposons, the major groups of repetitive DNA sequences, and compared with gene methylation. Genes showed a specific methylation pattern for CG, CHG (H = A, C, and T) and CHH sites, whereas the TE pattern differed, depending on the TE class (class 1, retrotransposons and class 2, DNA transposons). Along genes and TEs, CG and CHG methylation was higher than that of adjacent genomic regions. In contrast to the relatively low CHH methylation in retrotransposons and genes, the level of CHH methylation in DNA transposons was strongly increased, pointing to a functional role of asymmetric methylation in DNA transposon silencing. Comparison of genome‐wide DNA methylation between sugar beet leaves and callus revealed a differential methylation upon tissue culture. Potential epialleles were hypomethylated (lower methylation) at CG and CHG sites in retrotransposons and genes and hypermethylated (higher methylation) at CHH sites in DNA transposons of callus when compared with leaves.     

Analysis of target sequences of DDM1s in Brassica rapa by MSAP

DNA methylation is an important epigenetic modification regulating gene expression and transposon silencing. Although epigenetic regulation is involved in some agricultural traits, there has been relatively little research on epigenetic modifications of genes in Brassica rapa, which includes many important vegetables. In B. rapa, orthologs of DDM1, a chromatin remodeling factor required for maintenance of DNA methylation, have been characterized and DNA hypomethylated knock-down plants by RNAi (ddm1-RNAi plants) have been generated. In this study, we investigated differences of DNA methylation status at the genome-wide level between a wild-type (WT) plant and a ddm1-RNAi plant by methylation-sensitive amplification polymorphism (MSAP) analysis. MSAP analysis detected changes of DNA methylation of many repetitive sequences in the ddm1-RNAi plant. Search for body methylated regions in the WT plant revealed no difference in gene body methylation levels between the WT plant and the ddm1-RNAi plant. These results indicate that repetitive sequences are preferentially methylated by DDM1 genes in B. rapa.     

Characterization of a novel satellite DNA sequence from Flying Dragon (Poncirus trifoliata)

Repetitive sequences constitute a significant component of most eukaryotic genomes, and the isolation and characterization of repetitive DNA sequences provide an insight into the organization of the genome of interest. Here, we report the isolation and the molecular analysis and methylation status of a novel tandemly organized repetitive DNA sequence from the genome of Poncirus trifoliata. Digestion of P. trifoliata DNA with Afa I produced a prominent fragment of approximately 400 bp. Southern blotting analysis of genomic DNA digested with the same enzyme revealed a ladder composed of DNA fragments that are multimers of the 400-bp Afa I band, indicating that the repetitive DNA is arrayed in tandem. This suggests that Afa I isolated a novel satellite that we have called Poncirus trifoliata satellite DNA 400 (PN400). This satellite composes 25% of the genome and it is also present in lemon, sour orange and kumquat. Analysis of the methylation status demonstrated that the cytosines in CCGG sequences in this satellite were methylated.     

Rapid changes in amplification and methylation pattern of genomic DNA in cultured carrot root explants (Daucus carota L.)

Summary Rapid genomic DNA variation due to methylation and copy number alteration was observed in carrot root explants 6 h after inoculation and during a 36-h period of exponential callus growth. De novo methylation and amplification of restricted BspNI fragments of low molecular weight occurred before cell cycle activation and should, therefore, be independent of progression through the S-phase of the cell cycle. Growth regulators seemed to influence the amplification pattern indirectly by regulating cell division activity. In exponentially growing callus tissue the copy number of most of the repetitive fragments was dramatically reduced. It is presumed that this reduction in the copy number of repetitive fragments is characteristic of rejuvenilization. 3-Indole-acetic-acid (IAA) and inositol in the medium increased the degree of unspecific genomic DNA methylation in growing rhizogenic carrot callus tissue in the absence of kinetin, which inhibits root induction at that stage. A possible relation to the induction of rhizogenesis is considered. The observed reduction in number of repetitive restriction fragments and the increase in DNA methylation are gross changes covering the total genome. The results are discussed in relation to the controversy concerning the general biological significance of the methylation and amplification of DNA sequences.     

The CHH motif in sugar beet satellite DNA: a modulator for cytosine methylation

Methylation of DNA is important for the epigenetic silencing of repetitive DNA in plant genomes. Knowledge about the cytosine methylation status of satellite DNAs, a major class of repetitive DNA, is scarce. One reason for this is that arrays of tandemly arranged sequences are usually collapsed in next‐generation sequencing assemblies. We applied strategies to overcome this limitation and quantified the level of cytosine methylation and its pattern in three satellite families of sugar beet (Beta vulgaris) which differ in their abundance, chromosomal localization and monomer size. We visualized methylation levels along pachytene chromosomes with respect to small satellite loci at maximum resolution using chromosome‐wide fluorescent in situ hybridization complemented with immunostaining and super‐resolution microscopy. Only reduced methylation of many satellite arrays was obtained. To investigate methylation at the nucleotide level we performed bisulfite sequencing of 1569 satellite sequences. We found that the level of methylation of cytosine strongly depends on the sequence context: cytosines in the CHH motif show lower methylation (44–52%), while CG and CHG motifs are more strongly methylated. This affects the overall methylation of satellite sequences because CHH occurs frequently while CG and CHG are rare or even absent in the satellite arrays investigated. Evidently, CHH is the major target for modulation of the cytosine methylation level of adjacent monomers within individual arrays and contributes to their epigenetic function. This strongly indicates that asymmetric cytosine methylation plays a role in the epigenetic modification of satellite repeats in plant genomes.     

Hypermethylation of tobacco heterochromatic loci in response to osmotic stress

 Plants have to cope with a number of envi-ronmental stresses which may potentially induce genetic and epigenetic changes and thus contribute to genome variability. In the present study we inspected the DNA methylation status of two heterochromatic loci (defined with repetitive DNA sequences HRS60 and GRS) in a tobacco cell culture exposed to osmotic stress. Investigations were performed on a TBY-2 cell suspension culture, and the stress was elicited with NaCl or D-mannitol. Using the restriction enzymes MspI/HpaII and MboI/Sau3AI in combination with Southern hydridization we observed a reversible hypermethylation of the external cytosine at the CpCpG trinucleotides in cells grown under mild osmotic stress equal to a NaCl concentration of 10 g/l. There were no changes in the methylation of the internal cytosine as the CpG dinucleotides within the CCGG motifs (HpaII sites) appeared to be fully methylated in tobacco DNA repetitive sequences under normal physiological conditions. The data suggest epigenetic changes in the plant genome based on de novo methylation of DNA in response to environmental stress. Received: 26 November 1996/Accepted: 20 December 1996     

Characterization of an <Emphasis Type="Italic">Eco</Emphasis>RI family of satellite DNA from two species

We have cloned and sequenced a 321bp band of repetitive DNA from Eptesicus fuscus and E. serotinus observed after gel electrophoresis of EcoRI digested genomic DNA in both species. Southern blot analysis of genomic DNA (from both species) digested with the same enzyme showed the existence of a ladder pattern indicating that the repetitive DNA is arrayed in tandem. The repetitive sequences have a monomer unit of 321bp which is composed of two subunits of 160bp, suggested by the existence of a 160bp band in the ladder of E. fuscus and by the presence of some direct repeats found in the analysis of the consensus sequence. Analysis of the methylation status demonstrated that cytosines in CCGG sequences in this satellite DNA are methylated in E. fuscus but not in the E. serotinus. Alignment of the sequenced clones showed that several nucleotide positions are diagnostic species-specific and consequently the phylogenetic analysis grouped the monomer units from both species in two clearly separated groups.     

Genome modifications in protoplast-derived tobacco plants: Contents of repetitive DNA sequences

Plasticity of the tobacco genome was studied by testing the DNAs of protoplast-derived regenerants with three different repetitive DNA sequences by the method of quantitative DNA/DNA hybridizations. A large population of 91 regenerants belonging to 35 different protoclones was analysed and a high degree of heterogeneity in the contents of the different DNA repeats was detected. The contents of middle repetitive sequences of two types were more stable or changed in the same direction, while the highly repetitive sequence varied independently and displayed a significant reduction in comparison with the two other sequences. Comparing the variation within the subpopulations of plants of the same clonal origin and the variation among the protoclones led to a conclusion that the pre-existing DNA variability in the starting plant material and/or thein vitro stress during the very early stages of protoclone regeneration played a decisive role in the formation of modified genomes in regenerants.     

Analysis of DNA methylation during the germination of wheat seeds

DNA methylation is known to play a crucial role in regulating plant development and organ or tissue differentiation. Here, we focused on the DNA methylation dynamics during the germination of wheat seeds using the adapted AFLP technique so called methylation-sensitive amplified polymorphism (MSAP). The MSAP profiles of genomic DNA in embryo and endosperm tissues of germinating seeds, as well as dry seeds were characterized and notable changes of cytosine methylation were detected. Comparisons of MSAP profiles in different tissues tested showed that the methylation level in dry seeds is the highest. The alteration analysis of cytosine methylation displayed that the number of demethylation events were three times higher than that of de novo methylation, which indicated that the demethylation was predominant in germinating wheat seeds, though the methylation events occurred as well. Sixteen differentially displayed DNA fragments in MSAP profiles were cloned and the sequencing analysis confirmed that nine of them contained CCGG sites. The further BLAST search showed that four of the cloned sequences were located in coding regions. Interestingly, three of the sixteen candidates were homologous to retrotransposons, which indicated that switches between DNA methylation and demethylation occurred in retrotransposon elements along with the germination of wheat seeds.     

Meiotic transmission of a hypomethylated repetitive DNA family in tobacco

We have recently shown that hypomethylation of cytosine residues in the HRS60 family of repetitive DNA sequences can be induced with 5-azacytidine (5-azaC) in tobacco tissue cultures. We have also proven that such a DNA methylation status is maintained during the recovery of protoplasts, plant regeneration, and vegetative development. In the present paper we follow meiotic transmission of hypomethylated HRS60 DNA. Plants obtained from seeds treated with 5-azaC were either self pollinated or crossed with a non-treated control in a reciprocal way. Analysis of the methylation status of the HRS60 DNA revealed that these sequences were hypomethylated in the progenies up to the extent found in the parental 5-azaC-treated plant. Since no parent-of-origin effect was observed, we presume that both male and female gametes transmit an artificial methylation imprint to a similar extent. This result is supported by methylcytosine evaluation in the total genomic DNA samples. A temporal analysis of 5-azaC effects on germinating seeds and a phenotypic evaluation of 5-azaC-treated tobacco plants are also presented.On leave from the School of Biology and Biochemistry, University of Bath, England     

Different methylation pattern of melon satellite DNA sequences in hypocotyl and callus tissues

Satellite DNA sequences from Cucumis melo have been examined with respect to modification at CCGG sequences in hypocotyls and in callus tissues. For this purpose, restriction fragments given by HpaII and MspI were compared (both enzymes recognize CCGG sequences but have different sensitivity to methylation at this site). Whereas the methylation level of satellite DNA sequences is on average higher in hypocotyls than in callus tissues, the comparison of partially methylated repeat units of satellite DNA reveals that in callus tissues, all methylated restriction sites are doubly methylated.     

Hypomethylated sequences: Characterization of the duplicate soybean genome

Soybean is believed to be a diploidized tetraploid generated from an allotetraploid ancestor. In this study, we used hypomethylated genomic DNA as a source of probes to investigate the genomic structure and methylation patterns of duplicated sequences. Forty-five genomic clones from Phaseolus vulgaris and 664 genomic clones from Glycine max were used to examine the duplicated regions in the soybean genome. Southern analysis of genomic DNA using probes from both sources revealed that greater than 15% of the hypomethylated genomic regions were only present once in the soybean genome. The remaining ca. 85% of the hypomethylated regions comprise duplicated or middle repetitive DNA sequences. If only the ratio of single to duplicate probe patterns is considered, it appears that 25% of the single-copy sequences have been lost. By using a subset of probes that only detected duplicated sequences, we examined the methylation status of the homeologous genomes with the restriction enzymes MspI and HpaII. We found that in all cases both copies of these regions were hypomethylated, although there were examples of low-level methylation. It appears that duplicate sequences are being eliminated in the diploidization process. Our data reveal no evidence that duplicated sequences are being silenced by inactivation correlated with methylation patterns.     

Differentiation of metaxylem cell line in the root ofAllium cepa L.

Summary The differentiation processes of the metaxylem cell line in the root ofAllium cepa are characterized by amplification phenomena of repetitive DNA sequences mainly localized in heterochromatic regions of metaphase chromosomes. Moreover, these sequences are heavily methylated. This paper presents additional results on variation in endogenous DNA methylation in different developing root segments. The results show that methylation is higher in apical meristematic cells than the differentiating segments; contrastingly, total RNA synthesis seems to be correlated with undermethylation. Addition of labelled methyl groups to DNA by eukaryotic methylase, DNA digestions with different restriction enzymes specific for methylated sites and HPLC analysis confirmed the above results. Moreover, variation in methylation levels during differentiation occur not only at the internal cytosine of the-CCG-sites, but also at external cytosine. Furthermore, methylation affects other sites containing the trinucleotides-CXG-. In conclusion, root differentiation inAllium cepa seems to be correlated with gene activation modulated by the methylation/demethylation of particular DNA sequences.     

DNA Methylation in Genomes of Several Annual Herbaceous and Woody Perennial Plants of Varying Ploidy as Detected by MSAP

Polyploidization is known to accompany altered DNA methylation in higher plants, which plays an important role in gene expression regulation and maintaining genome stability. While the characteristics of DNA methylation in different polyploid plants are still to be elucidated; here, status of genomic DNA methylation in a series of diploid, triploid, and tetraploid annual herbaceous plants (watermelon and Salvia) and woody perennials (pear, Poplar, and loquat) were explored by methylation-specific amplified polymorphism analysis. The results indicated that levels of DNA methylation in triploid watermelon and Salvia were lower than their diploid parents. In triploid Poplar and pear, higher levels of DNA methylation were detected, and no significant difference was observed between triploid and tetraploid in all tested materials. Further data analysis suggested that about half of the total detected sites underwent changes of DNA methylation patterns in triploid watermelons and Salvia, as well as an obvious trend towards demethylation. However, the changes of DNA methylation patterns in three triploid woody perennials were only 17.54–33.40%. This implied that the characteristics of DNA methylation are significantly different during the polyploidization of different plant species. Furthermore, the results suggested that the level of DNA methylation was nonlinearly related to the ploidy level, and triploid plants displayed more interesting DNA methylation status. The characteristics and possible functions of DNA methylation in different ploidy series are further discussed.     

A repetitive DNA fragment carrying a hot spot for de novo DNA methylation enhances expression variegation in tobacco and petunia

A 1.6 kb r ep etitive DNA s equence (RPS) from Petunia hybrida was identified that destabilizes expression of a GUS marker transgene. Following polyethylene glycol (PEG)-mediated tobacco and petunia protoplast transformations, GUS expression patterns analysed on callus and plant levels were clearly more variable when constructs contained the RPS sequence. The effect on transgens expression required chromosomal integration since the two different RPS constructs employed did not exhibit reduced levels of GUS activities in transient assays. DNA methylation analysis implies a hypermethylated default state of endogenous RPS copies present in the petunia genome. Analysis of the transgens DNA in different transgenic tobacco plants showed almost complete hypermethylation of a particular Hhal site of the RPS sequence. It is proposed that, due to the presence of specific signals within the RPS region or based on interaction of RPS with other endogenous homologous sequences, RPS functions as an initiation region for de novo methylation and induces expression variegation in adjacent sequences.     

Tissue culture-induced DNA methylation polymorphisms in repetitive DNA of tomato calli and regenerated plants

The propagation of plants through tissue culture can induce a variety of genetic and epigenetic changes. Variation in DNA methylation has been proposed as a mechanism that may explain at least a part of these changes. In the present study, the methylation of tomato callus DNA was compared with that of leaf DNA, from control or regenerated plants, at MspI/HpaII sites around five middle-repetitive sequences. Although the methylation of the internal cytosine in the recognition sequence CCGG varied from zero to nearly full methylation, depending on the probe used, no differences were found between callus and leaf DNA. For the external cytosine, small differences were revealed between leaf and callus DNA with two probes, but no polymorphisms were detected among DNA samples of calli or DNA samples of leaves of regenerated plants. When callus DNA cut with HindIII was studied with one of the probes, H9D9, most of the signal was found in high-molecular-weight DNA, as opposed to control leaf DNA where almost all the signal was in a fragment of 530 bp. Also, an extra fragment of 630 bp was found in the callus DNA that was not present in control leaf DNA. Among leaves of plants regenerated from tissue culture, the 630-bp fragment was found in 10 of 68 regenerated plants. This 630-bp fragment was present among progeny of only 4 of these 10 plants after selfing, i.e. it was partly inherited. In these cases, the fragment was not found in all progeny plants, indicating heterozygosity of the regenerated plants. The data are interpreted as indicating that a HindIII site becomes methylated in callus tissue, and that some of this methylation persists in regenerated plants and is partly transmitted to their progeny.     

Cytological characterization of the tandem repetitive sequences and their methylation status in the Antirrhinum majus genome

Tandem repetitive sequences are DNA motifs common in the genomes of eukaryotic species and are often embedded in heterochromatic regions. In most eukaryotes, ribosomal genes, as well as centromeres and telomeres or subtelomeres, are associated with abundant tandem arrays of repetitive sequences and typically represent the final barriers to completion of whole-genome sequencing. The nature of these repeats makes it difficult to estimate their actual sizes. In this study, combining the two cytological techniques DNA fiber-FISH and pachytene chromosome FISH allowed us to characterize the tandem repeats distributed genome wide in Antirrhinum majus and identify four types of tandem repeats, 45S rDNA, 5S rDNA, CentA1, and CentA2, representing the major tandem repetitive components, which were estimated to have a total length of 18.50 Mb and account for 3.59% of the A. majus genome. FISH examination revealed that all the tandem repeats correspond to heterochromatic knobs along the pachytene chromosomes. Moreover, the methylation status of the tandem repeats was investigated in both somatic cells and pollen mother cells from anther tissues using an antibody against 5-methylcytosine combined with sequential FISH analyses. Our results showed that these repeats were hypomethylated in anther tissues, especially in the pollen mother cells at pachytene stage.     

Variable copy number DNA sequences in rice

Summary We have cloned two types of variable copy number DNA sequences from the rice embryo genome. One of these sequences, which was cloned in pRB301, was amplified about 50-fold during callus formation and diminished in copy number to the embryonic level during regeneration. The other clone, named pRB401, showed the reciprocal pattern. The copy numbers of both sequences were changed even in the early developmental stage and eliminated from nuclear DNA along with growth of the plant. Sequencing analysis of the pRB301 insert revealed some open reading frames and direct repeat structures, but corresponding sequences were not identified in the EMBL and LASL DNA databases. Sequencing of the nuclear genomic fragment cloned in pRB401 revealed the presence of the 3rps12-rps7 region of rice chloroplast DNA. Our observations suggest that during callus formation (dedifferentiation), regeneration and the growth process the copy numbers of some DNA sequences are variable and that nuclear integrated chloroplast DNA acts as a variable copy number sequence in the rice genome. Based on data showing a common sequence in mitochondria and chloroplast DNA of maize (Stern and Lonsdale 1982) and that the rps12 gene of tobacco chloroplast DNA is a divided gene (Torazawa et al. 1986), it is suggested that the sequence on the inverted repeat structure of chloroplast DNA may have the character of a movable genetic element.     

Activation of the maize transposable element Suppressor-mutator (Spm) in tissue culture

Summary Previous experiments have revealed that the maize transposable element Activator (Ac) may become active during tissue culture. The objective of the present study was to determine whether a second transposable element, Suppressor-mutator (Spm), could also be activated in tissue culture and detected in regenerated maize plants. Approximately 500 R₁ progeny of 143 regenerated plants (derived from 49 embryo cell lines) were crossed as males onto an Spm-responsive tester stock. Spm activity was observed in two R₁ progeny of a single regenerated plant. This plant had been regenerated from Type II (friable embryogenic) callus of an A188 × B73 genetic background after 8 months in culture; the absence of Spm activity in four other plants regenerated from this same callus demonstrates that Spm activity was not present before culturing. Approximately 20 Spm-homologous DNA sequences were detected in each of the inbreds used to initiate the tissue cultures; it is presumed that one of these became active to give rise to Spm activity.