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

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involve 2 molecules of each histone, i.e., H2A, H2B, H3, and H4 (Luger et

al., 1997; McGinty & Tan, 2015). Protruding unstructured N-terminal tail of

histone is exposed towards diverse post-transcriptional changes which include

methylation, acetylation, phosphorylation, ubiquitinylation, glycosylation,

ADP‐ribosylation, and sumoylation (Figure 12.2) (Sadakierska-Chudy &

Filip, 2015). By changing the interface among histones and the adjacent

DNA, as well as the interaction of other transcription factors to DNA, can

modify chromatin structure and gene transcription (Allis & Jenuwein, 2016;

Chang et al., 2020; Pikaard & Scheid, 2014). Methylation and acetylation

of histones have been well-described. They play diverse functions of plants,

i.e., evolution, development, and reproduction.

Histone acetylation and methylation are two epigenetic markers that are

now acknowledged as crucial and ubiquitous in gene regulation (Xu et al.,

2017). The enzymes involved in histone modifications are fascinating since

some operate as “writers” some are “erasers” while others act as “readers”

also known as “effector proteins.” Writer enzymes which include HATs,

methyltransferases, kinases, and ubiquitinates catalyze the attachment

of chemicals to the histone tail or core domain. On the other hand, eraser

enzymes which comprise deacetylases (HDACs), phosphatases, demethyl­

ases (HDMs), as well as deubiquitinases remove those alterations caused by

writers (Figure 12.3). “Readers” proteins usually offer an exposed surface

(such as a crater or surface groove) for a changed histone residue and detect

the alteration (acetylation versus methylation) or even the specific state

(such as mono-, di- or tri-methylation of lysine). In order to discriminate

sequence context, readers contact the neighboring sequence of the changed

amino acid. Amidst methyl-lysine-binding motifs discovered in the “reader”

proteins are PHD, chromo, Tudor, PWWP, WD40, BAH, ADD, ankyrin

repeat, MBT, and zn-CW domains, which may distinguish desired methyl­

lysines based on its methylation state and adjacent amino-acid sequence (Xu

et al., 2017). Histone acetylation imprints (particularly H3 and H4) improve

DNA accessibility by neutralizing the basic charge on histones, which

weakens histone and DNA methylation (Allis & Jenuwein, 2016; Onufriev

& Schiessel, 2019). In Arabidopsis, histone alterations – methylation have

both suppressive (symmetric H4R3me2, H3K9me2/3, and H3K27me3) as

well as operative (asymmetric H4R3me2, H3K4me3, and H3K36me2/3)

mark (Liu et al., 2010; Wang et al., 2016). Histone methylation, unlike

acetylation, preserves the electron charge of Lys and does not affect histone

proteins’ electrostatic properties. The occupancy or lack of methyl group

either on Lys or Arg, or both Lys and Arg amino acids on histones cause