**2. miRNA biosynthesis and their conservation in plants**

miRNA is ranges from 20 to 24 nucleotide base pair in length and non-coding RNA molecule [6]. MIR gene (miRNA genes) found in intergenic area of genome but some miRNA gene also found within intronic region in both sense or antisense direction. miRNA biomolecules are grouped in the genome and transcribed collectively as long polycistronic RNAs. miRNA synthesis occurs in the nucleus (**Figure 1**).

MIR genes transcribed into a specific primary transcript by the RNA polymerase II [6]. These are distinctive and specific primary transcripts modified at 5′ end by capping with a modified nucleotide and polyadenylated tail at the 3′ end with some adenosines [7]. The miRNAs are further modified by DICER-LIKE (DCL1), serrate (SE) and Hyponastic leaves 1 (HYL1) into precursors miRNA (pre-miRNAs). PremiRNA is a hairpin like structure. miRNA: miRNA duplexes produce in nucleus by further modification of pre-miRNAs by DCL1 [7].

S-adenosyl-L-methionine dependent RNA methyltransferase enzyme responsible for duplex methylation and HUA enhancer 1 at their 3′ end (HEN1) [8].

#### *microRNA Utilization as a Potential Tool for Stress Tolerance in Plants DOI: http://dx.doi.org/10.5772/intechopen.97480*

Plant homologous of EXPORTIN 5 and HASTY transport this modified duplex into cytoplasm [9]. After transportation duplex separated by RNA-induced silencing complex (RISC) along with ARGONAUTE (AGO) protein [10, 11]. One strand of the miRNA is inserted into AGO protein comprised RISC complex, while other stand degraded by exosome along with small-RNA degrading nuclease [8]. This miRNA promotes the RISC loading to complementary mRNAs targets. Mature miRNAs showed resemblance with the target mRNA encourages site-specific cleavage of target mRNA while miRNAs along with some damaged base pairing with target mRNA leads inhibition of translation [8].

Some previous studies have shown that few miRNA bio-molecules are evolutionarily conserved among all significant plant lineages, including bryophytes, pteridophytes, gymnosperms, and angiosperms [12, 13]. miRNA families further divided into two different classes on the basis of miRNA diversification and conservation. The young miRNAs are expressed only in specific condition and at very low level although old miRNA is more evolutionarily conserved and highly expressed. In *Arabidopsis* some old studies reported the sign of regular synthesis and degradation of MIRNA genes. MIRNA gene producing loci either by aberrant transposition or replication/recombination from expressed gene sequences. Furthermore, it also showed that few miRNAs are lost during evolution [14].
