The ectomycorrhizal fungus Pisolithus microcarpus encodes a microRNA involved in cross-kingdom gene silencing during symbiosis

Significance Plant genomes encode hundreds of genes controlling the detection, signaling pathways, and immune responses necessary to defend against pathogens. Pathogens, in turn, continually evolve to evade these defenses. Small RNAs, such as microRNAs (miRNAs), are one mechanism used by pathogens to overcome plant defenses and facilitate plant colonization. Mounting evidence would suggest that beneficial microbes, likewise, use miRNAs to facilitate symbiosis. Here, we demonstrate that the beneficial fungusPisolithus microcarpus encodes a miRNA that enters plant cells and stabilizes the symbiotic interaction. These results demonstrate that beneficial fungi may regulate host gene expression through the use of miRNAs and sheds light on how beneficial microbes have evolved mechanisms to colonize plant tissues.