Fig. 1

Hypothetical microRNA-mediated feedback regulation of rice immunity against Magnaporthe oryzae. Infection by M. oryzae activates pathogen-associated molecular pattern- (PAMP-) triggered immunity (PTI) and/ or effector-triggered immunity (ETI). During PTI/ETI, a subset of miRNA genes (MIRs) can be induced or suppressed, depending on their roles as positive or negative regulators, respectively. The MIRs are transcribed and processed by the RNase III family endoribonucleases such as Dicer-like1s (DCL1s) and DCL3s. While DCL3s mainly produce 24-nt (nucleotide) miRNAs, DCL1s produce 21-nt miRNAs. After modification by a series of regulators, the mature single strand 21-nt and 24-nt miRNAs are loaded into ARGONAUTE1 (AGO1) and ARGONAUTE 4 (AGO4), respectively, forming miRNA-induced gene silence complex (miRISCs). The miRISC could then hypothetically mount feedback regulation on PTI/ETI at several layers. First, the 24-nt-AGO4 miRISC re-enters the nucleus to mediate methylation of the DNA sequences complementary to the 24-nt miRNAs, resulting in transcriptional gene silencing of target genes that act in the regulation of PTI/ETI. Second, the 21-nt-AGO1 miRISC mediates the cleavage or translational inhibition of the target mRNAs, leading to post-transcriptional gene silencing of target genes that act in the regulation of PTI/ETI. Third, some target genes of miRNAs are transcription factors that can regulate the transcription of MIR genes and/or genes acting in the regulation of PTI/ETI, forming feedback regulatory networks