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

siRNA, the most commonly documented two are retroelements and RNA

virus replication (Khraiwesh et al., 2012). Though siRNAs are categorized

into many subclasses all are produced by the action of a specific RDL-DCL

protein complex (Kumar et al., 2018). In plants precursors of almost all

siRNAs are long and double-stranded (Bartel & Bartel, 2003; Kim, 2005b)

but the initiation of biogenesis is specific for each type of siRNA. To date,

various researchers identified few types of siRNA precursors in plants. For

example, ta-si RNAs are originated from non-coding regions, ra-siRNA

originated from the heterochromatic locus. Guleria et al. (2011) found that

single-stranded RNA (ssRNA) is utilized for particular siRNA via miRNA­

mediated cleavage or RNA pol IV activity, and dsRNA is subsequently

transcribed from single-stranded RNA precursors by RNA Dependent RNA

polymerases (RDRs). The synthesized dsRNA is then split into two siRNAs

by the action of the DCL protein, one of which is 21 nt long and the other

is 24 nt long. The length of the synthesized siRNAs is determined by the

specific enzymatic activity of the DCL proteins involved. A dsRNA molecule

is often cleaved by several DCL proteins, that is why siRNAs of varying

sizes are produced. After formation, siRNAs bind with AGO proteins to form

RISCs, just like miRNAs, and similarly regulate target genes at the transcrip­

tional and post-transcriptional levels via RNA-directed DNA methylation

(RdDM). Several studies have proven the particular function of several DCL

proteins in the production of siRNAs. In the case of ra-siRNAs dsRNAs are

cleaved by DCL3 proteins whereas DCL 4 proteins catalyze the production

of transacting siRNA. 21 nt long for ra-siRNA and 24 nt long siRNAs are

produced by DCL3, whereas DCL4 helps in the biogenesis of 21 nt long

ta-siRNA from the TAS RNA precursors (Figure 9.2), in association with

the involvement of some other proteins namely RDR6, SGS3 and DRB 4 (ds

RNA Binding Protein 4), etc. (Chowdhury, 2018). Biogenesis of siRNA and

miRNA distinguished from each other in respect of dsRNA precursors and

except this both the small RNA molecules are derived by the similar mecha­

nisms, i.e., cleavage of dsRNA by the enzymatic action of Dicer-mediated

cleavage (Khare et al., 2018).

9.5.1 MAJOR ROLES OF SMALL RNA AND SMALL RNA BASED

TACTICS IN PLANTS

Small RNAs are now widely known biomolecules that act as the riboregulator

and control different gene regulation systems. In the context of plant science

sRNAs are widely studied as they have a remarkable role in the generation