Organisms are exposed to different kinds of environmental stresses during their life cycle. To cope up with environment assaults, plants and animals undergo some homeostasis alterations during somatic growth and heritable (transgenerational) gene expression modifications. The heritable changes can occur without any changes in base sequences and is commonly known as epigenetics. Molecular mechanisms of epigenetic modification includes DNA methylation, histone modification (methylation, acetylation, uniquitination, phosphorylation, ribosylation, and biotinylation), small RNA mediated regulation and chromatin remodeling (Wagner, 2003; Vanyushin, 2006). All of these mechanisms may be regulated by different environmental stresses. Studies have revealed altered gene expression in plants, in response to stress conditions that can be fixed epigenetically and inherited to next generation, forming epigenetic stress memories.
2. Molecular Basis
2.1 DNA methylation
The modification at fifth carbon position of a cytosine ring (addition of methyl group to the 5` position of cytosine, thus converting cytosine to 5-methylcytosine (5mC)) is called as DNA methylation. In plants, DNA methylation has usually found in three contexts, methylation at CG, CHG (H replacing A, C or T) and at CHH sites (Chen et al., 2010; Zemach et al., 2010). This DNA methylation accumulated during vegetative phase under influence of environment will be transmitted to next generation through germline