MicroRNAs are endogenous RNAs of 20–24 nucleotides that target a single spot in their target mRNA and are processed by Dicer-like (DCL) proteins from poorly paired hairpin precursor RNAs. While miRNAs have a role in the biotic and abiotic stress response, signal transduction, protein degradation, and growth and development in plants. However, miRNAs also play critical roles in plant–virus interactions.
Nowadays, miRNA-mediated gene silencing has been applied to protect several agricultural crop species against infection by diverse viruses. miRNAs are derived from single-stranded RNA transcripts (MIR genes) that can fold back onto themselves to produce imperfectly double-stranded stem-loop precursor structures. The MIR genes are RNA polymerase II (Pol II) transcription units that produce the primary miRNA transcript (pri-miRNA), which is then cleaved by DCL1 in the nucleus, leading to the production of the shorter precursor-miRNA (pre-miRNA, partially duplex molecule with a single-stranded loop, mismatches, and a single-stranded extension) with the assistance of the dsRNA-binding protein 1 (DRB1) and HYPONASTIC LEAVES1 (HYL1).
Plant genomes have a significant number of LRR and NB-LRR immune receptors encoded by resistance (R) genes, which identify particular pathogen effectors and initiate resistance responses. To a great extent, the siRNA-mediated gene silencing involved in antiviral defense occurs through the regulation of these R genes. Studies have shown that plant miRNAs target and negatively regulate plant R genes by prompting the production of phased, trans-acting siRNAs (tasiRNAs) against these R genes, and this miRNA mediated gene regulation is suppressed on bacterial or viral infection.
Fig: A pathway of plant miRNA482-mediated resistance against the virus by inducing the production of siRNAs responsible for regulating R genes. The expression of MIR482 triggers the simultaneous silencing of multiple R genes through tasiRNAs produced from dsRNA derived from a primary miR482-targeted R gene. Virus infection may trigger silencing suppression involved in this process at several steps, resulting in increased accumulation of multiple R proteins and enhanced resistance. AGO1, Argonaute 1; DCL4, Dicer-like 4; RDR6, RNA-dependent RNA polymerase 6; SGS3, Suppressor of gene silencing 3; DRB4, Double-stranded RNA binding protein 4 (Source: Liu S-R, Zhou J-J, Hu C-G, Wei C-L and Zhang J-Z (2017) MicroRNA-Mediated Gene Silencing in Plant Defense and Viral Counter-Defense. Front. Microbiol. 8:1801).
Dr. Md. Monirul Islam
ASRBC, ACI Seed