Functional Roles of microRNAs in Agronomically Important Plants-Potential as Targets for Crop Improvement and Protection

Abstract

MicroRNAs (miRNAs) are a class of small non-coding RNAs that have recently emerged as important regulators of gene expression, mainly through cleavage and/or translation inhibition of the target mRNAs during or after transcription. miRNAs play important roles by regulating a multitude of biological processes in plants which include maintenance of genome integrity, development, metabolism, and adaptive responses toward environmental stresses. The increasing population of the world and their food demands requires focused efforts for the improvement of crop plants to ensure sustainable food production. Manipulation of mRNA transcript abundance via miRNA control provides a unique strategy for modulating differential plant gene expression and miRNAs are thus emerging as the next generation targets for genetic engineering for improvement of the agronomic properties of crops. However, a deeper understanding of its potential and the mechanisms involved will facilitate the design of suitable strategies to obtain the desirable traits with minimum trade-offs in the modified crops. In this regard, this review highlights the diverse roles of conserved and newly identified miRNAs in various food and industrial crops and recent advances made in the uses of miRNAs to improve plants of agronomically importance so as to significantly enhance crop yields and increase tolerance to various environmental stress agents of biotic-or abiotic origin.

Keywords: agricultural crops; crop improvement; gene expression regulation; microRNA (miRNA).

Figure 1

microRNA biogenesis and mode of action. The biogenesis of miRNAs starts in the nucleus where miRNA genes from distinct genomic loci are transcribed by RNA polymerase II into long primary transcripts (pri-miRNAs) followed by cleavage to precursor mRNA (pre-miRNA) by the nuclear RNase III-like enzyme, called DICER-LIKE (DCL1) in association with other proteins such as hyponastic leaves 1 (HYL1) and serrate (SE). The imperfect stem-loop secondary structure of the pre-miRNA hairpins are cut by DCL1 enzymes to generate the miRNA:miRNA^* duplexes. The duplexes are methylated by S-adenosyl-L-methionine-dependent RNA methyltransferase, HUA enhancer 1 (HEN1); and exported to the cytoplasm by HASTY where they undergo RNA-induced silencing complex (RISC) loading. The miR^* is released and the mature miRNA loads onto ARGONAUTE (AGO) ribonucleases in the RISC complex. Extensive base-pairing with mRNA targets is required for plant miRNA function to regulate gene expression. miRNAs guide Ago proteins to their specific targets through sequence complementarity which then leads to degrading the target mRNA transcript or by repressing its translation. On binding with perfect complementarity to a target mRNA, the Ago-miRNA complex induces its cleavage and degradation. An Ago-miRNA complex binding imperfectly to the 3′ UTR of the target mRNA induces translational inhibition, or deadenylation and subsequent decapping (DCP1,2) and degradation of the target mRNA.