Epigenetics – The Molecular Tool in Understanding Abiotic Stress Response in Plants

369

change chromatin organization, DNA accessibility, and gene activity, and

hence influence a range of molecular processes like gene transcription, DNA

replication, repair, and recombination (Allis & Jenuwein, 2016; Lebedeva

et al., 2017; Pikaard & Scheid, 2014; Zhang et al., 2018). They regulate

growth and development, involving differentiation of cell, regeneration,

reproduction, flowering, and aging, as well as acclimating plants to a variety

of natural cues like pathogen infection, rainlessness, elevated salt, severe

temperature, and heavy metal stressors (Chang et al., 2020; Lebedeva et

al., 2017; Lee & Seo, 2018; Pikaard & Scheid, 2014; Ramirez-Prado et al.,

2018; Zhang et al., 2018). The various epigenetic alterations, as well as their

mechanisms, are detailed here.

12.4.1 DNA METHYLATION

DNA methylation is a post-replication alteration where certain cytosine bases

get methylated at the 5’ site forming m⁵C. Methylation takes place in plants

within the base C of “CG,” “CHG,” and “CHH” where H signifies A, C, or

T (Law & Jacobsen, 2010). The m⁵C known is as “minor base” which may

account for up to 30% of the total amount of m⁵C in plants (Grafi & Ohad,

2013; Vaniushin & Belozerskii, 1959). Along with m⁵C, N6-methyladenine

was discovered in mitochondrial DNA in several angiosperms (Burianov et

al., 1972; Vanyushin et al., 1971). The most common kind of DNA methyla­

tion is cytosine methylation, which involves adding a methyl group to the

cytosine ring’s fifth carbon (Figure 12.4). Enzymatic substitution of methyl

group takes place with S-adenosyl L methionine (SAM) donor by the enzy­

matic modification whereas non-enzymatic DNA methylation is also evident

(Vanyushin, 2014). Cytosine methylation is universally present inside CG

dinucleotide of all life forms; but, in plants, both symmetric (CG and CHG)

as well as asymmetric (CHH) DNA methylation has been reported (H stands

for A, C, or T). Levels of methylation in Arabidopsis are 24% in CG, 6.7% in

CHG, and 1.7% in CHH sites (Dhar et al., 2014). Because of their symmetry,

CG and CHG methylation can be easily repeated during DNA replication,

whereas after each replication cycle CHH must undergo de novo synthesis.

Three types of enzymes are necessary for methylation in plants. These are:

1. DNA Methyltransferase 1 (MET1): Primarily engaged in the meth­

ylation of symmetric methylation sites for the support of methylation

(Sahu et al., 2013).