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Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

environment. Plants require a continual change at the molecular level and

efficiently modify their metabolic pathways (Gallo-Franco et al., 2020).

Stress is a situation that can deteriorate plant’s fitness to grow and survive

that can reduce the yield almost 50% of crop productivity, resulting in scarcity

of food (Boyer, 1982). Epigenetic mechanisms regulate the stress-induced

genes at the transcriptional and post-transcriptional level (Kim et al., 2015;

Luo & He, 2020; Lämke & Bäurle, 2017). Epigenetic regulation enhances

the existence of plants by enhancing their forbearance for stresses. Stress

signals induce DNA methylation that alter the promoter regions of stress-

responsive genes leading to alteration of their expression pattern (Chin­

nusamy & Zhu, 2009), modification of histone proteins (Gallo-Franco et al.,

2020). The enzyme cytosine methyltransferase methylates cytosine residue

within the CpG dinucleotide (Bewick et al., 2016; Niederhuth & Schmitz,

2017). DNA covers the histone octamer (H2A, H2B, H3, and H4) while the

amino acids of the amino-terminal chain of the globular histone proteins are

exposed outside for the post-translational modification (PTMs) and result in

alteration of gene expression (Kouzarides, 2007). The expression of genes

is suppressed by sumoylation and biotinylation of the amino acids, while

acetylation and phosphorylation activate gene expression. DNA replication,

transcription, and DNA repair are greatly controlled by histone modification

(Nathan et al., 2006; Ransom et al., 2010). DNA methylation is related to

the prolonged transgenerational preservation of epigenetic modification and

methylation influences the mode of transcription (Gallo-Franco et al., 2020).

DNA methylation is the most frequent stress-responsive epigenetic modifi­

cation, which occurs across plant species and strengthens the significance

of this process (Feng et al., 2010). Most of the epigenetic modifications are

inherited and perform a pivotal part in plant adaptability (Robertson & Wolf,

2012). Methylation in genomic DNA is transmitted over generations without

having any modification in the acquired methylation pattern, this leads to

the formation of “epialleles” (Kalisz & Purugganan, 2004). Epialleles may

exist above stress-associated genes but maybe even exist in genomic areas

which have no relation with the stress response (Verhoeven et al., 2010).

Gene expression may be suppressed by transposons, this may be caused by

the methylated state of transposons that are located near the genes and even

regulate the expression of genes through methylation spread mechanism

(Saze & Kakutani, 2007). sRNAs of diverse types, as well as lncRNAs, play

an important role in epigenetic processes. Small RNAs change the chromatin

conformation and regulates gene silencing through Argonaute mediated

complexes along with complementary emerging RNA platform resulting