DNA Methylation in Plants

A high degree of nuclear DNA (nDNA) methylation is a specific feature of plant genomes, they do contain 5-methylcytosine (m5C) and N6-methyladenine (m6A). More than 30% m5C is located in CNG sequences. Specific changes in DNA methylation accompany the entire life of a plant starting from seed germination up to the death programmed or induced by various agents and factors of biological or abiotic nature. Modulation of DNA methylation is one of the possible modes of the hormonal action in plant. 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 transgenes and foreign DNA. Plants have much more complicated and sophisticated system of the multi-component and sometimes even conjugated genome (nuclear DNA) methylations compared with animals; besides, unlike animals, they have the plastids with their own unique DNA modification system that may control plastid differentiation and functioning; DNA methylation in plant mitochondria is performed in other fashion compared with it in nuclei. The nuclear DNA methylation system is controlled by three major families of cytosine DNA-methyltransferase genes, at least. In contrast to animals the inactivation of major maintenance methyltransferase MET1 (similar to animal Dnmt1) has no significant consequences for plant survival. Other plant cytosine DNA-methyltransferases have no analogs in animals. Some of them (DRM) are responsible for de novo DNA methylation including asymmetric sequences. Plant gene may be methylated at both adenine and cytosine residues; specific adenine DNA-methyltransferase was described. Adenine DNA methylation may influence cytosine modification and vice versa. Anyway, two different systems of the genome modification based on methylation of adenines and cytosines coexist in higher plants. The specific endonucleases discriminating between methylated and unmethylated DNA are present in plants. Thus, plants may have restriction-modification system. There are peculiar complicated controls for growth and development by DNA methylations in plants; they are well co-ordinated with other epigenetic signals modulating chromatin organisation.