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

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acquired memory are also known as transgenerational memory, transgenera­

tional consequences, transgenerational inheritance is described as the ability

to pass on information from one generation to the next, the ability of an

organism to “remember” its surroundings at the molecular level, resulting

in a change in phenotype of the progeny (Tricker, 2015). The term priming

was coined to describe how mild stress could induce plant stress responses,

allowing them to respond faster and stronger when the stress recurs. In

general, being able to recall previous molecular experiences and use this

preserved information to adjust to new environments is life-saving when

repeated biotic and abiotic challenges occur. The phenomenon of priming

of organismal stress responses explains how a plant is modified and gets

ready for subsequent stress exposure (a ‘triggering stress cue’) by a tempo­

rally restricted environmental stimulation (a “priming stress cue”) (Lämke

& Bäurle, 2017). Epigenetic regulation is closely linked to the formation

of stress memory (Friedrich et al., 2019; Lämke & Bäurle, 2017). Stress-

induced chromatin alterations might influence particular regions or occur

genome-wide and are frequently linked to transcriptional regulation. Most of

these alterations occur only during the stress exposure, both the expression

as well as chromatin states usually return to pre-stress levels quickly. While

other modifications cause novel chromatin structures and altered expression

of stress-responsive genes, to last longer post-stress exposure prepares an

organism for developmental options or even more efficient defense. Plants’

future responses towards stress may be altered by prior exposure to stress,

resulting in quicker and/or extreme responses, suggesting the plants have a

type of “stress memory.” These epigenetic changes persist or are even trans­

mitted in the next progeny (Pecinka & Mittelsten Scheid, 2012). Arabidopsis

show transcriptional stress memory after several exposures to drought stress,

as evidenced by the rise in transcription rate and increased transcript amount

in a group comprising stress-response genes (trainable genes). Accumula­

tion of H3K4me3 on “trainable genes” is more than “non-trainable genes,”

implying H3K4me3 can operate like a persistent epigenetic impression

linked to transcriptional memory. Trainable genes generate transcripts to the

baseline during recovery, i.e., watered condition, however they are linked

with unusually high amounts of H3K4me3 along with Ser5P polymerase II,

representing the RNA polymerase II is paused (Ding et al., 2012). H3K4me3

and H3K27me3 co-occur but functions independently in the transcription

of memory genes of dehydration stress-response. Whereas H3K27me3

does not work as an epigenetic memory imprint of the dehydration stress-

responsive genes (Liu et al., 2014). Heat Shock Proteins (HSPs) like HSP21,