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

9.3.1 MICRORNA (miRNA)

microRNAs (miRNAs) are single-stranded RNA molecules that are short

in length and non-coding. They can bind with the non-coding region in

the poly-A tail of mRNA to inhibit the activity or promote the degradation

of mRNA (Bartel, 2004). These are abundantly present in the genomes of

plants, animals, and viruses, and the vast majority of them have distinct

genetic loci and exhibit conservation among species belonging to the

same biological kingdom (Lagos-Quintana et al., 2001). The length of

miRNAs for animals ranges from 20–22 nucleotides, whereas it is 20–24

nucleotides for plants (Bartel, 2004; Reinhart et al., 2002). miRNAs are

reported to regulate gene expression in plants that are involved in a variety

of activities, including the development of root, stem, leaf, and flower,

shifting of vegetative phase to reproductive phase and stress responses

(Zhang et al., 2006). During the biogenesis of miRNAs, the template

genes are catalyzed by RNA polymerase II and resulted in a primary

transcript that adopts a feedback structure (Yaish et al., 2015). With the

assistance of some nuclear proteins, such fold back structure is ultimately

converted into a specialized RNA duplex structure often called miR-3P/

miR-5P duplex (Kurihara et al., 2006). The complex is then divided to

produce mature miRNA. After the formation, mature miRNAs bind with

the functional endonuclease protein, i.e., Argonaute (AGO) to form the

RISC, which in turn accomplish gene regulation through the splitting

of mRNA or cleavage of mRNA or inhibition of translational activities

(Naqvi et al., 2012). Both pleiotropically and prevalently, a single miRNA

is mostly able to recognize more or less hundreds of distinct mRNA

transcripts (Li et al., 2018). To recognize target mRNA such miRNA, use

sequence complementarity which is the prerequisite to regulating the level

of target protein (Bartel, 2004; Llave et al., 2002; Shukla et al., 2011). The

microRNA can recognize multiple mRNAs and can bind with them and

in this way mRNA and microRNA jointly consist of a precise regulation

system (Zhang & Xie, 2017). Along with normal developmental events

and stress response miRNAs are found to be a key player in cell differen­

tiation, proliferation, tumorigenesis, apoptosis, etc., in plants as well as

other eukaryotes (Cai et al., 2009). Few miRNAs have been revealed that

are highly conserved across distantly related taxa, ranging from mosses to

higher flowering eudicots in the plant kingdom and worms to mammals

in the animal kingdom (Axtell et al., 2007; Zhang et al., 2006; Bartel,

2004). The binding of miRNA and their target mRNA maybe with perfect