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

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(Brant & Budak, 2018; Hou et al., 2019; Wang et al., 2017). Long ncRNAs

(lncRNAs) having transcripts length of > 200 nucleotides. They are involved

in chromatin remodeling, transcriptional, post-transcriptional regulation,

as well as interact with various RNA molecules via numerous chromatin-

mediated processes, rather than being translated into protein.

miRNAs are single-stranded, ncRNAs, which bind to the mRNAs to

prevent translation or promote mRNA degradation. miRNA in plants gener­

ally has 20–24 nucleotides. They control a wide range of cellular activities,

such as root, stem, leaf, and flower development, the transition of vegetative

to reproductive phase, as well as adaptive responses to a variety of abiotic

and biotic stimuli. RNA polymerase-II transcribes miRNA coding genes

to produce pri-miRNAs that are subsequently splitted and transformed by

DICER-LIKE (DCLs) proteins to mature miRNAs. In Arabidopsis, ARGO­

NAUTE (AGO) protein groups like AGO1 attach to emerging miRNAs then

directs target-oriented post-transcriptional gene silencing (PTGS) (Ravi­

chandran et al., 2019). siRNAs, although resembling miRNA but in both,

RNA polymerase IV (RNAPIV) synthesizes 20–24 nt long straight chain,

dsRNAs, through transposons as well as repetitive sequences. Numerous

types of siRNAs were discovered to date, many are employed in PTGS,

while some are used for TGS (Sun et al., 2013; Yu et al., 2019).

12.4.4 RNA DIRECTED METHYLATION OF DNA

RNA-directed methylation DNA (RdDM) is a cellular mechanism that

involves the insertion of a methyl group on the DNA that takes place in

particular DNA sequences that are guided by non-coding RNA molecules.

RdDM plays an important role in a variety of cellular activities within plants,

comprising stress response, intercellular connections, as well as genomic

stability is maintained via transposable element (TE) silencing. RdDM

improves plant adaptability towards a range of abiotic stressors, including

heat, drought, shortage in phosphate, salt, etc. (Fortes & Gallusci, 2017).

The RdDM pathway is specific for plants, which is distinct from the fungi

and mammals. The pathway is similar to other sRNAs, especially the RNAi

pathway is well-conserved among fungi, plants, and mammals. RdDM and

RNAi both generate sRNAs and include the preserved Dicer Argonaute

proteins and RNA-dependent RNA polymerase. The addition of methyl

group on the DNA by RdDM is typically linked to the transcription repres­

sion of the gene. The DNA methylation supplemented by RdDM is typically