Basics of epigenetics
Gene expression can be controlled by changes in structure of chromatin without changing the DNA sequence and this phenomenon called epigenetic control. DNA methylation and histone and histone variation are well known epigenetic modification[Ryo F. et. al.2012]. Methylation of fifth carbon of cytosine residue is one of the most extensively studied epigenetic modification in plants[Tatsuo Kanno et. al 2012].
DNA methylation in the promoter region leads to down regulation of gene. In the methylated DNA regions transcription factor can not bind and RNA polymerase activity is also inhibited. so there is no transcription of gene in to RNA and hence there is no expression.
Abiotic stresses such as chilling, planting density, rubbing, cutting, and successive rounds of subculture generally decrease the
levels of DNA methylation. The opposite effect is seen for salt stress, and the effects of heavy-metal stress are species specific. Biotic stresses such as pathogenic infection can lead to two contrasting effects on the levels of methylation in plants: hypermethylation on the genome wide level and hypomethylation of resistance-related genes.[Hai Feng and Ting Zhang 2008]
Histone and chromatin protein are critical responders to stress alters gene expression that alter gene expression. Nucleosomal comprises four major histone proteins Histone H3, H4,. H2A and H2B. Further package in to increasing order chromatin. This packaging not only serves to physically fit the DNA into the limited volume of the nucleus, but also acts as a principal regulator of transcription by acting in concert with transcription factor(Karan and Jerry 2012). Many chromatin modifying proteins put specific chemical marks on Histone and these marks can in turn be recognized by specific chromatin binding protein that alter other aspects of chromatin structure or behavior.
Recent studies have reported that histone modifications such as H3K4me3, H3K9ac, H3K23ac, H3K27ac, H3K27met3 and H4ac can be correlated with gene expression in response to abiotic stress such as water deficit, high salinity and temperature shifts. some histone modification change rapidly in response to environmental change rapidly in response to environmental changes, while other changes gradually along with changes in gene expression to control physio-homeostasis and development under environmental stress[K. Sako et. al. 2015].
Link for reference
http://www.qiagen.com/in/products/genes%20and%20pathways/complete%20biology%20list/epigenetic%20regulation for educational pourpose
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