*NATURAL GENETIC VARIATION* þ *EPIGENETIC VARIATION* ¼ *PHENOTYPIC* & *FUNCTIONAL DIVERSITY* ð*origin of 'phenotypic plasticity'*Þ

However, not all epigenetic marks are stable; some of them are transient, i.e. in case of DNA repair and/or cell cycle phases. To delineate the epigenetic mechanisms of regulating gene expression, it is important to understand how the epigenetic changes work. Briefly, the genomic DNA in eukaryotes is packaged into organised structure called the chromatin. The chromatin is composed of basic units called nucleosome (as shown below):

*Understanding the Impact of Global Climate Change on Abiotic Stress in Plants… DOI: http://dx.doi.org/10.5772/intechopen.109618*

The targets for epigenetic regulation are as follows:


These mechanisms, also sometimes known as the 'epigenetic code' directly regulate the activity of a gene by influencing the arrangement of nuleosome and consequently the compactness of chromatin. If the chromatin is tightly packed ('heterochromatin'), it is less accessible for expression, and if the chromatin is loosely packed ('euchromatin'), it is readily accessible by RNA Pol II for carrying out gene transcription. It has been indicated that under abiotic stress, plants can display three different kinds of epigenetic memories (**Figure 4**) [40, 45].

There is growing evidence that plants from diverse genera, including forest trees, respond to abiotic stresses of drought, salinity and UV through epigenetic modulation of gene expression [40, 46]. Additionally, these epigenetic imprints are also linked with adaptation, acclimation as well as acclimatisation under stressful conditions.

**Figure 4.**

*Different types of epigenetic memories (Extracted from Müller-Xing* et al*. [40]).*
