Biogenesis, conservation, and function of miRNA in liverworts

Micro RNAs (miRNAs) are small, non-coding endogenous RNA molecules of 18–24 nucleotides in length that control multiple gene regulatory pathways via posttranscriptional gene silencing in eukaryotes. To develop a comprehensive picture of the evolutionary history of miRNA biogenesis and action in land plants, new studies on bryophyte representatives are needed. Here, we review the current understanding of MIR gene structure, miRNA biogenesis, and function. In addition, we focus on representatives of two liverwort lineages: the simple thalloid Pellia endiviifolia and the complex thalloid Marchantia polymorpha. The purpose of this review is to bring readers up to date on what is known about conserved and non-conserved miRNAs, their targets, and the functional implications of miRNA action in Marchantia and Pellia. We also note that most Marchantia miRNAs are encoded within protein-coding genes and provide the data for 23 MIR gene structures recognized as independent transcriptional units. Furthermore, we identify Marchantia genes involved in miRNA biogenesis, that are homologous to those identified in higher plants. These genes include those that encode core microprocessor components as well as other auxiliary and regulatory proteins that influence the stability, folding, and processing of pri-miRNAs. We analyzed miRNA biogenesis proteins and found similar domain architecture in majority of the cases. Taken together, our data support the hypothesis that almost all miRNA biogenesis factors known in higher plants are also present in liverworts, suggesting that they emerged early during land plant evolution.