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

regulation on several biological processes including cellular metabolisms,

seed germination, growth, and development events, adaptive responses

against abiotic stresses (Chen et al., 2018; Llave et al., 2002). The majority

of plant sRNA comes from one of these three pathways: miRNA biogen-

esis, secondary siRNA biogenesis, or heterochromatic siRNA (hc-siRNA)

biogenesis (Borges & Martienssen, 2015). sRNA biogenesis may be regarded

as a mechanism that enables plants to reorganize gene expression pathways

needed for adjustment and survival under stressful conditions (Phillips et al.,

2007). Major characteristics of small RNAs (s-RNAs) are:

1. sRNAs are riboregulators that have pivotal roles in the majority of

the biological processes of eukaryotic organisms. They control gene

expression in several ways. In some cases, sRNAs act on DNA and

execute sequence elimination and chromatin remodeling, whereas

in some other cases, sRNAs act on RNA molecules and execute

cleavage and translation repression.

2. Usually, sRNAs are made from dsRNA few recent investigations

enlightened some new RNA-silencing pathways wherein sRNAs are

generated from long-stranded RNA (ssRNA) precursors (Martienssen

& Moazed, 2015).

3. In the context of plant science, sRNAs have an unusual convolu-

tion, and they are classified according to their biogenesis and the

conformation of genome loci from which these are generated. To

date, sRNAs are classified into two broad categories microRNAs

(miRNAs) and interfering RNAs (siRNAs). As per the knowledge

of the latest literature in this field, siRNAs are further classified

into three subgroups, namely natural antisense siRNA (natsiRNA),

trans-acting siRNA (ta-siRNA) and hc-siRNA. As a result of the

steady analysis of large-scale siRNA sequencing efforts, the number

of siRNA subgroups may expand in the near future (Phillips et al.,

2007).

4. Until now sRNAs in plants have been found to be between 20 nt and

24 nt long (Hackenberg et al., 2015).

5. All variants of characterized sRNA in plants associate with a special

class of endonucleases, known as – ARGONAUTE (AGO) family

proteins and collectively these two form a specialized ribonucleopro-

tein complex-RNA induced silencing complexes (RISCs). sRNAs

guide the complex to bind with DNA through base pairing or with

target RNA (Peters & Meister, 2007; Vaucheret, 2008).