Abstract
MicroRNAs (miRNAs) are small RNAs acting as regulators of gene expression at the post-transcriptional level. In plants, most miRNAs are generated from independent transcriptional units, and only a few polycistronic miRNAs have been described. miR166 is a conserved miRNA in plants targeting the HD-ZIP III transcription factor genes. Here, we show that a polycistronic miRNA comprising two miR166 family members, miR166k and miR166h, functions as a positive regulator of rice immunity. Rice plants with activated MIR166k-166h expression showed enhanced resistance to infection by the fungal pathogens Magnaporthe oryzae and Fusarium fujikuroi, the causal agents of the rice blast and bakanae disease, respectively. Disease resistance in rice plants with activated MIR166k-166h expression was associated with a stronger expression of defense responses during pathogen infection. Stronger induction of MIR166k-166h expression occurred in resistant but not susceptible rice cultivars. Notably, the ethylene-insensitive 2 (EIN2) gene was identified as a novel target gene for miR166k. The regulatory role of the miR166h-166k polycistron on the newly identified target gene results from the activity of the miR166k-5p specie generated from the miR166k-166h precursor. Collectively, our findings support a role for miR166k-5p in rice immunity by controlling EIN2 expression. Because rice blast is one of the most destructive diseases of cultivated rice worldwide, unraveling miR166k-166h-mediated mechanisms underlying blast resistance could ultimately help in designing appropriate strategies for rice protection.
Highlights
MicroRNAs are endogenous, small non-coding RNAs that mediate post-transcriptional gene silencing in eukaryotes (Jones-Rhoades et al, 2006)
Polycistronic miR166 Mediates Rice Immunity to the cytoplasm, where the functional miRNA strand is incorporated into an ARGONAUTE1 (AGO1)-containing RNAinduced silencing complex (RISC) (Baumberger and Baulcombe, 2005; Jones-Rhoades et al, 2006; Rogers and Chen, 2013). miRNAs guide post-transcriptional gene silencing via sequencespecific cleavage or translational repression of target transcripts (Llave et al, 2002; Brodersen et al, 2008)
Knowing that MIR166k-166h activation has an impact on blast resistance, we considered the possibility that this phenotype might be caused by the activity of miR166 species encoded in the miR166k-166h precursor on novel, non-conserved target genes
Summary
MicroRNAs (miRNAs) are endogenous, small non-coding RNAs that mediate post-transcriptional gene silencing in eukaryotes (Jones-Rhoades et al, 2006). They are transcribed as long primary transcripts (pri-miRNAs), forming an imperfect fold-back structure, and are sequentially processed by a DICER-like ribonuclease (typically DCL1) to produce a pre-miRNA and a doublestranded miRNA duplex, the miRNA-5p/miRNA-3p duplex (previously named miRNA/miRNA∗ duplex) (Kurihara and Watanabe, 2004). The crucial role of miRNAs in controlling plant developmental processes and response to abiotic stress is well documented (De Lima et al, 2012). Our current knowledge of the biological roles of pathogen-regulated miRNAs in plant immunity is still limited, and most comes from studies in the interaction of Arabidopsis thaliana with the bacterial pathogen Pseudomonas syringae (Staiger et al, 2013; Weiberg et al, 2014; Fei et al, 2016; Kuan et al, 2016)
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