Abstract

Field crops and plants are adversely affected by biotic stress like pests and pathogens and abiotic stress including heat, drought, salt, cold. These conditions adversely reduce plant growth, development, and yield. Animals move from one place to another to cope with biotic and abiotic stress but plants are fixed to one position. Plants cope with these stresses by the mechanism of epigenetic modification. Various epigenetic modifications help plants to respond to various stress. DNA methylation is one of the most important and conserved epigenetic modifications that regulate, stabilizes, and maintains genomic integrity. Loss of DNA methylation or aberrant patterns of DNA methylation causes abnormalities in the gene regulation of plants. DNA methylation in plants is regulated by the combined action of de novo methylation, maintenance of methylation, and demethylation. In plants, DNA methylation is established via RNA directed DNA methylation pathway. In plants de novo methylation in all sequences is mediated by an enzyme - Domain Rearranged Methyltransferase 2. The enzymes that regulate DNA methylation in plants are different but have some homology to that of mammalian DNA methylation enzymes. Maintenance of methylation during the semiconservative DNA methylation is done by MET1 whereas asymmetrical methylation is maintained by constant de novo methylation by DRM2 and RdMD pathway. DNA demethylation is done actively by DNA glycosylase in combination with the base excision pathway. DNA methylation helps to develop adaptation mechanisms towards various biotic and abiotic stresses. Still, the exact mechanism of DNA methylation is not known fully. In de novo methylation by RdDM pathway, small RNAs guide the target specificity for methylation but this small RNA is also involved in demethylation at several ROS1 dependent genomic regions. How this small RNA guides both methylation and demethylation is still the subject of study. The DNA methylation in the promoter region inactivates genes where some other genes are activated, the mechanism underlying activation and deactivation is the key point for further study of DNA methylation in plants.