Genome-wide analysis reveals diversity of rice intronic miRNAs in sequence structure, biogenesis and function

Intronic microRNAs (in-miRNAs) as a class of miRNA family that regulates gene expression are still poorly understood in plants. In this study, we systematically identified rice in-miRNAs by re-mining eight published small RNA-sequencing datasets of rice. Furthermore, based on the collected expression, annotation, and putative target data, we investigated the structures, potential functions, and expression features of these in-miRNAs and the expression patterns of their host genes. A total of 153 in-miRNAs, which account for over 1/4 of the total rice miRNAs, were identified. In silico expression analysis showed that most of them (∼63%) are tissue or stage-specific. However, a majority of their host genes, especially those containing clustered in-miRNAs, exhibit stable high-level expressions among 513 microarray datasets. Although in-miRNAs show diversity in function and mechanism, the DNA methylation directed by 24 nt in-miRNAs may be the main pathway that controls the expressions of target genes, host genes, and even themselves. These findings may enhance our understanding on special functions of in-miRNAs, especially in mediating DNA methylation that was concluded to affect the stability of expression and structure of host and target genes.