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

complementary or nearly perfect complementary but in both cases gene

regulation is perfectly executed through cleavage, repressed translation or

often de-adenylation and in both cases miRNA downregulates the levels

of mRNA produced from the target gene and thus, the proteins encoded by

the target gene are also reduced (Parmer & Shaw, 2018).

In plants, some miRNAs are highly conserved, whereas some are lineage

and species-specific immature miRNA (Sun, 2012). Some of them exist as a

single copy and some are as multiple copies or gene clusters in the genome

(Si et al., 2014). In the majority of cases, plant miRNA targets transcriptional

factors that are involved in all developmental processes, from seed germina­

tion to seed production via the formation of a mature plant (Jones-Rhoades et

al., 2006). However, miRNAs indirectly regulate such processes by forming

RISCs that act on mRNAs produced from target genes, and also control

expression through adenylation, translational inhibition, or degradation of

target mRNA (Lu et al., 2008; Phillips et al., 2007). Various studies have

indicated that stress responses of a miRNA may differ with the variation of

the genus or species, while in the case of some miRNAs patterns of stress

responses are more or less similar in all plants (Zhang, 2015).

Previously, it was assumed that miRNAs were conserved throughout a

wide variety of plant species, including flowering plants and non-flowering

plants such as bryophytes and pteridophytes, but later it was proved that few

miRNAs are always restricted to some specific genera (Floyd & Bowman,

2004). In plants biding of miRNAs with their target mRNA are based on

extensive sequence complementarity, however, there are few exceptions

where five or more mismatches have been found between the target mRNA

and miRNA (Axtell, 2013). Some researchers have pointed out a few

examples to confirm about restricted miRNAs, such as few miRNAs of

Arabidopsis are not reported in the rice genome, and similarly, miRNAs

reported from poplar are absent in Arabidopsis (Lu et al., 2005). Allen

et al. (2004) conceptualized a model to explain the evolutionary aspect

of miRNA genes in plants and this model miRNAs were distinguished

into “old miRNA” and “young miRNA” and opined that the former type

evolved prior to the origin and evolution of many plant taxa and that is

why are conserved, while the latter type originated after the emergence of

taxons of the present era and hence are not conserved. This model satisfied

the fact that miRNAs present in one plant are not present in other plants

(Lu et al., 2005; Sunkar & Zhu, 2004). However, numerous miRNAs have

been identified, and their targets are conserved across various plant species

(Montes et al., 2014).