Identification of novel miRNAs and their target genes from Populus szechuanica infected with Melampsora larici-populina.

Two novel miRNAs were selected from a pre-constructed RNA library of Populus szechuanica infected with the foliar rust fungus Melampsora larici-populina in order to detect the genes regulated as targets of the miRNAs novel_mir_11 and novel_mir_357. The novel miRNAs were identified from P. szechuanica using stem-loop methods and their precursors were able to fold into a complete stem loop structure. The predicted target genes of the novel miRNAs were verified with RNA ligase-mediated 5' rapid amplification of cDNA ends (RLM-5'RACE). The full-length sequences of target genes, RPM1 and RPS2/5, in P. szechuanica were obtained through rapid amplification of cDNA ends (RACE) and officially named PsRPM1 and PsRPS2/5. These genes contain nucleotide binding site-leucine-rich repeats (NBS-LRR) domains typical of resistance genes. The expression levels of miRNAs and their target genes in different periods post infection were analysed with quantitative real-time PCR (qRT-PCR). After infection with the foliar rust fungus, the expression levels of the novel miRNAs and their target genes were dynamic. Both novel_mir_11 and novel_mir_357 negatively regulated the expression of their target genes. In this study, the regulatory effects of two novel miRNAs through their target genes were characterized to provide further mechanistic information regarding the interaction between Populus and a foliar rust fungus. Results of this study improve our understanding of the defence response mechanisms of Populus and will stimulate future work to characterize strategies to prevent and control Populus diseases.