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Small RNAs – The Big Players in Developing Salt-Resistant Plants

mediated PTGS. miRNA precursors are processed by dicer like 1 (DCL1)

into mature miRNAs, whereas siRNAs from transgenes, viruses, and endog­

enous transcripts are generated by other DCLs (Fei et al., 2013; Fukudome

et al., 2011; Kurihara & Watanabe, 2004). Several investigations concluded

that target genes of miRNAs in plants are linked with many developmental

processes, as a result, any change in miRNAs might cause anomalies in plant

growth and development. In Arabidopsis, it was demonstrated that the split­

ting of miRNAs from dsRNA is directed by some other stress-responsive

miRNAs and further studies with high-throughput sequencing has screened

such stress-responsive miRNAs as well as the cleavage sites of these miRNAs

(Jagadeeswaran et al., 2009; Llave et al., 2002). For example, it was reported

that the expression level of miR398 was decreased under salt-induced stress

(Bouche, 2010). It has been concluded that PTGS is quite essential to check

the total “transcript dynamics” under stress conditions (Joshi et al., 2017). In

general salt resistance in plants is achieved by multiple genes, and the genes

are also regulated at multiple levels due to salinity. Under salinity, plants

activate all the stress-responsive genes as well as stress-responsive sRNAs

to combat stressful situations and try to maintain structural, biochemical,

and physiological activities properly (Covarrubias & Reyes, 2010). Several

salt stress-responsive novels and identified miRNAs have been identified

from many plants. In most situations, miRNAs suppress the expression of

endogenous genes by directing the cleavage of their target mRNAs (Mallory

& Vaucheret, 2006). Expression of miRNAs has been found to alter in a

dose-dependent manner by salinity. Frazier et al. (2011) showed that salinity

stress can change the level of expression of miRNA from 6.86-fold down-

regulation to 616.57-fold upregulation in Nicotiana. Extensive investigations

on miRNA and post-transcriptional gene regulation in plants have explored

the fact that none of the target genes/proteins of conserved miRNAs is

directly associated with salt tolerance; however, they have control over the

expression of genes that are associated with salt tolerance (Sunkar et al.,

2012). The impact of miRNAs on the salt responsive gene depends on their

affinity, location, and abundance. Manavella et al. (2019) opined that whole

machinery associated with the biogenesis of miRNA can be turned down

through the feedback regulatory mechanism. Many siRNAs have recently

come into the limelight as important players in plant stress responses; these

are more diverse than miRNA. Among the identified siRNAs most widely

studied subsets are ta-siRNAs, nat-siRNAs, and hc-siRNAs (Ku et al.,

2015). The mode of actions of different siRNA is quite different. Plants have

two major endogenous siRNA-mediated regulations, hc-siRNAs mediated