Many of the plant microRNA (miRNA) genes contain introns. The miRNA-containing hairpin loop structure is predominantly located within the first exon of such pri-miRNAs. We have shown that the downstream intron and its splicing are important for the regulation of the processing of these pri-miRNAs. The 5' splice site in MIR genes is essential in the process of miRNA biogenesis. We postulate that the presence of yet undefined interactions between U1 snRNP and the pri-miRNA processing machinery leads to an increase in the efficiency of miRNA biogenesis. The 5' splice site also decreases the accessibility of the polyadenylation machinery to the pri-miRNA polyA signal located within the same intron. It is likely that the spliceosome assembly controls the length and structure of MIR primary transcripts, and regulates mature miRNA levels. The emerging picture shows that introns, splicing, and/or alternative splicing have highly relevant roles in regulating the miRNA levels in very specific conditions that contribute to proper plant response to stress conditions.
Keywords: intron-containing pri-miRNAs; plant microRNA biogenesis; polyadenylation; splicing.