Plant microRNAs: An insight into their gene structures and evolution

MicroRNAs (miRNAs) are 21–23 nucleotide (nt) non-coding RNAs that play a key role in regulating the expression of protein-coding genes at post-transcriptional levels in plants and animals. MiRNA genes, which serve as genetic buffers and regulators, are primarily located in the intergenic regions of the plant genome. The similar structure of a miRNA promoter to that of a protein-coding gene signifies the likely origin of miRNA genes from the latter. Imperfect “inverted repeats”, the hallmark of miRNA genes that defines the asymmetry of the “stem–loop” region of the miRNA precursors (pre-miRNAs), reflect the evolution of miRNA genes from the inverted duplication of their target genes over a long period of time. The deep conservation of most miRNAs and the presence of some of the non-conserved, species-specific miRNAs among various plant species demonstrate a continuous, but frequently an uneven evolutionary process of miRNA genes. Thus, duplication, inversion, mutation, amplification, and other types of genetic drift from protein-coding genes might be the primary events in the genesis and evolution of the miRNA genes. Subsequent co-evolution of the miRNA genes and their target genes ensures the maintenance and the fine-tuning nature of a dynamic gene regulatory network governed by miRNAs in plants.