Characterization and expression pattern analysis of microRNAs in wheat under drought stress

Plant microRNAs (miRNAs) play important roles in regulating plant growth, development, and responses to abiotic stresses. In this study, 38 miRNAs (TaMIRs) from wheat (Triticum aestivum L.), 36 from the miRBase database, and two from our previous work were characterized and subjected to an expression pattern analysis under normal conditions and a drought stress. A semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR), real-time quantitative PCR (qPCR), and small RNA blot analyses revealed that two TaMIRs (TaMIR1120 and TaMIR1123) were root-predominant and two TaMIRs (TaMIR1121 and TaMIR1134) were leaf-predominant. Seven TaMIR precursors showed altered expressions after the drought; of these, TaMIR1136 was upregulated, whereas TaMIR156, TaMIR408, TaMIR1119, TaMIR1129, TaMIR1133, and TaMIR1139 were downregulated. These seven drought-responsive TaMIRs showed dose-dependent and typical temporal expression patterns during drought induction, and they gradually returned back under the normal growth conditions. The drought-responsive and the tissue-predominant TaMIRs had varying numbers of target genes. Randomly selected target genes exhibited opposite expression patterns to their corresponding TaMIRs suggesting that they were regulated by distinct TaMIRs through a post-transcriptional cleavage. The target genes regulated by drought-responsive and tissue-predominant TaMIRs are involved in various cellular processes, such as signal transduction, transcriptional regulation, primary and secondary metabolisms, development, and defense responses. These results provide a novel insight into the miRNA-mediated responses of wheat to drought stress.