Plant isomiRs: origins, biogenesis, and biological functions

MicroRNAs (miRNAs) are small endogenous non-coding RNAs in eukaryotes which regulate the expression of numerous genes post-transcriptionally, thereby playing critical roles in cells and organismal development. The high-throughput sequencing technologies enable the effective detection and annotation of miRNAs. Several miRNA variants with heterogeneous ends, lengths, and sequences can be generated from a single miRNA locus. Discovery of these miRNA variants, also known as miRNA isoforms or isomiRs, has made our understanding of the cells' miRNome deeper than previously pictured. Despite their wide presence in multiple datasets, the different possible origins and true biological significance of isomiRs are yet to be uncovered. Several recent emerging studies suggest that isomiRs are biologically active and non-randomly formed. This review aims to provide a comprehensive insight into the origins and biological importance of isomiRs, highlighting the enormous complexity of miRNA regulatory networks which broadens our knowledge about the post-transcriptional gene regulation in plants.