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.