The roles of rice microRNAs in rice-Magnaporthe oryzae interaction

MicroRNAs (miRNAs) are a class of small (20-24 nucleotides (nt) long) non-coding RNAs. One mature miRNA can be transcribed from one or more gene loci known as miRNA genes (MIRs). The transcript of a MIR forms a stem-loop structure that is processed into a 20-24-nt miRNA-5p/-3p duplex by RNase III family endoribonucleases such as Dicer-like1 (DCL1). In turn, the overhang ends of the duplex are methylated by HUA ENHANCER 1 (HEN1), generating stabilized mature miRNAs. The mature miRNAs are loaded onto ARGONAUTE (AGO) proteins, forming a miRNA-induced gene silencing complex (miRISC). Then, the miRISC binds to target sites with sequences complementary to the miRNAs, leading to either cleavage or translational inhibition of the target mRNAs, or methylation of the target sequences, resulting in post-transcriptional and transcriptional gene silencing, respectively. In the past decade, more than 700 miRNAs have been identified in rice, a subset of which have been found to be responsive to the rice blast fungus, Magnaporthe oryzae, or its elicitors. Moreover, members of 10 miRNA families have been found to positively or negatively regulate rice defense against M. oryzae, namely miR160, miR164, miR166, miR167, miR169, miR319, miR396, miR398, miR444 and miR7695. This review summarizes the identification and functional characterization of the miRNAs, which respond to M. oryzae or its elicitors and describes the current understanding of the complicated but well-organized network in the context of rice-M. oryzae interaction.