Novel miRNAs in the control of arsenite levels in rice.

In a species, the miRNA repertoire comprises many lineage- or species-specific miRNAs. Using deep sequencing, the whole transcriptome of seedling roots was studied and a total of 18 novel miRNAs in indica rice Minghui 86 were identified. Among these novel miRNAs six were up-regulated and two were down-regulated under arsenite stress. Quantitative real-time RT-PCR analysis revealed that miR6254 was predominantly expressed in roots relative to other tissues, whereas miR6250 and miR169i-3p were constitutively expressed in all tissues examined, with high abundance in roots (miR6250) or leaves (miR169i-3p). The miR6250 and miR169i-3p miRNAs also exhibited distinct expression patterns in rice cultivars Minghui 86 and Nipponbare at different time points after arsenite treatment. The predicted targets for these miRNAs included some protein kinases, DNA, or ATP-binding proteins. Besides arsenite, the expression of targets of miR6250 and miR6254 was also up- or down-regulated in response to abiotic environmental stresses, indicative of their involvement in regulation of plant adaptation. Three types of cis-elements involved in hormone, light, and stress response were found to occur frequently in the promoter regions. Interestingly, miR6254 was originally characterized to be an exonic miRNA located in the exon of AK101391, supporting the notion that miRNAs may also originate from some exons in plants.