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

disassembly, alterations in nucleosome accessibility, nucleosome content,

and DNA-protein associations also contribute to transcription regulation as

well as other nucleosome and DNA-related alterations (Pikaard & Scheid,

2014). ATP-dependent protein complexes known as chromatin remodeling

factors mediate chromatin changes by dissociating or relocating nucleo­

somes. Three types of chromatin remodeling ATPases have been identified:

(i) SWF (switch)/SNF (sucrose non-fermenting) ATPases (Tang et al., 2010);

(ii) CHD (chromodomain and helicase-like domain ATPases) (Marfella

& Imbalzano, 2007); and (iii) ISWI (imitation switch) ATPases. In yeast,

the SWF1/SNF complex is an initial chromatin modification ATPase to be

found, they are responsible for mating-type switching (SW1) and sucrose

non-fermenting (SNF) deficiencies (Sudarsanam & Winston, 2000). Histone

chaperones get involved in histone accumulation as well as elimination in

nucleosome formation and dissolution.

12.6.1 TRANSGENERATIONALLY INHERITED EPIGENETIC STRESS

MEMORY

In addition to plant genome sequences, epigenome has now been proved to

have a role in stress responses. The epigenome is defined as a set of biochem­

ical alterations to DNA sequences or associated proteins that regulate the

genome (Bernstein et al., 2007; Park et al., 2016). Epigenetic memory has

become a plant’s adaptation strategy to a constantly changing world, particu­

larly to severe biotic and abiotic stressors. Not only under stress conditions

but also in normal or under mild stress conditions, these epigenetic changes

take place in the plants as creating memory these changes, that continues

from one generation to the next (Ashapkin et al., 2020). As discussed earlier

in the epigenetic regulation, there are three main components of gene regula­

tions such as DNA methylation, modifications of histone, and the expression

of small RNAs (sRNAs). Plants often develop a memory of stress exposures

at the somatic level, allowing them to better withstand future encounters

with similar and different stresses. These mechanisms are known as stress

acclimation in the case of abiotic and systemic acquired resistance (SAR) in

the case of biotic stress exposure. The acquired memory of stress exposure

is also seen in the progenies of plants. This memory can be stored in the

chromatin conformation and at the distinct level of sRNAs in the cytoplasm

of gametes and growing embryos, priming an organism to cope better with

future situations (Kachroo & Robin, 2013). The phenomena of stress-induced