A team of Spanish researchers has reported on the discovery of a novel microRNA in rice that promotes pathogen resistance, and shows that the miRNA underwent natural and domestication selection events during the plant’s evolution.
Plants have long been known to possess an innate immune system capable of detecting potential pathogens through the recognition of pathogen epitopes, also known as pathogen-associated molecule patterns or elicitors, by host-encoded surface receptors, the investigators wrote in New Phytologist. However, recent work indicates that plants also rely on post-transcriptional regulation of defense responses against pathogens through the use of endogenous small RNAs such as miRNAs.
To date, most miRNAs identified in plants are highly conserved across species and regulate developmental processes, and it is generally assumed that species-specific miRNAs play a role in “a time- and/or spatial-restricted manner, or in specific biological processes,” they noted.
As such, a key challenge is to identify species-specific miRNAs in plants and understand their function, particularly in crop species that undergo “major environmental stresses.”
One such plant is rice, or Oryza sativa, which is important to agriculture worldwide and represents the model plant for research into monocotyledonous species with a sequenced genome, according to the paper. Additionally, rice has experienced extensive natural selection and selective breeding, making it an “excellent system for studies on the molecular evolution and selection of plant miRNAs.”
The investigators sequenced small RNA libraries from rice leaves and roots that had been treated with elicitors from the rice blast fungus Magnaporthe oryzae, which causes rice blast disease, and found that a “diverse set” of known miRNAs, both conserved and nonconserved, responded to the elicitors.
Additionally, a number of novel miRNAs were identified, including osa-miR7695, which negatively regulates an alternatively spliced transcript of the natural resistance-associated macrophage protein 6 metal transporter gene, or Nramp6, the team wrote.
Notably, rice plants that overexpressed the miRNA were found to have enhanced resistance to a fungal pathogen compared with control plants, although the exact mechanism behind this requires further study, the researchers stated.
The investigators also determined that osa-miR7695 recently evolved, in part because it is detected in rice but not in any other related monocotyledonous or dicotyledonous species that they analyzed. This finding suggests that the miRNA “evolved either after the divergence of the monocotyledonous and dicotyledonous lineages and/or during rice domestication.”
Overall, the research uncovers miRNA-mediated regulation of rice Nramp6 in disease resistance and, more broadly, illustrates the “existence of a novel regulatory network that integrated miRNA function and mRNA processing in plant immunity,” the paper concluded.