Introns targeted by plant microRNAs: a possible novel mechanism of gene regulation

In plant cells, most microRNAs (miRNAs) perform cleavages of target mature mRNAs in the cytoplasm. A recent report of a miRNA pathway involved in DNA methylation in the rice nucleus raises the possibility that plant miRNAs could cleave intron-containing pre-mRNAs (the precursor of messenger RNAs) located in the nucleus.

In this study, we searched for the miRNA binding sites present within the introns of Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) genes. All miRNA-intron interactions predicted to result in cleavages were validated by using the public degradome sequencing data. As a result, 40 miRNA-intron pairs involving 25 miRNAs in Arabidopsis and 1912 pairs involving 91 miRNAs in rice were identified. For several rice genes, not all transcription forms (alternative splicing variants) were under similar regulation by specific miRNAs. Certain transcripts could escape cleavages due to the absence of intronic miRNA binding sites within these sequences. In some instances, specific cleaved intron remnants could be converted to double-stranded RNAs (dsRNAs) by RNA-dependent RNA polymerase 2. These dsRNAs could then be processed into 21- and 24-nt phased sRNAs by the activity of Dicer-like 1 and 3, respectively. The resultant siRNAs have the potential to be incorporated into Argonaute (AGO)-associated silencing complexes and result in cleavages of target pre-mRNA sequences.

A regulatory model, miRNA-targeting of intron-containing pre-mRNAs-phased sRNAs-targeting of mature mRNAs is proposed, which further expands the potential modes of action of plant miRNAs.