DNA methylation in higher plants: past, present and future.

A relatively high degree of nuclear DNA (nDNA) methylation is a specific feature of plant genomes. Targets for cytosine DNA methylation in plant genomes are CG, CHG and CHH (H is A, T, C) sequences. More than 30% total m(5)C in plant DNA is located in non-CG sites. DNA methylation in plants is species-, tissue-, organelle- and age-specific; it is involved in the control of all genetic functions including transcription, replication, DNA repair, gene transposition and cell differentiation. DNA methylation is engaged in gene silencing and parental imprinting, it controls expression of transgenes and foreign DNA in cell. Plants have much more complicated and sophisticated system of the multicomponent genome methylations compared to animals; DNA methylation in plant mitochondria is performed in other fashion as compared to that in nuclei. The nDNA methylation is carried out by cytosine DNA methyltransferases of, at least, three families. In contrast to animals the plants with the major maintenance methyltransferase MET1 (similar to animal Dnmt1) inactivated do survive. One and the same plant gene may be methylated at both adenine and cytosine residues; specific plant adenine DNA methyltransferase was described. Thus, two different systems of the genome modification based on methylation of cytosines and adenines seem to coexist in higher plants. This article is part of a Special Issue entitled: Epigenetic control of cellular and developmental processes in plants.

The gene for domains rearranged methyltransferase (DRM2) in Arabidopsis thaliana plants is methylated at both cytosine and adenine residues.

The methylation patterns of cytosine and adenine residues in the Arabidopsis thaliana gene for domains rearranged methyltransferase (DRM2) were studied in wild-type and several transgene plant lines containing antisense fragments of the cytosine DNA-methyltransferase gene METI under the control of copper-inducible promoters. It was shown that the promoter region of the DRM2 gene is mostly unmethylated at the internal cytosine residue in CCGG sites whereas the 3'-end proximal part of the gene coding region is highly methylated. The DRM2 gene was found to be also methylated at adenine residues in some GATC sequences. Cytosine methylation in CCGG sites and adenine methylation in GATC sites in the DRM2 gene are variable between wild-type and different transgenic plants. The induction of antisense METI constructs with copper ions in transgene plants in most cases leads to further alterations in the DRM2 gene methylation patterns.