In soil, arbuscular mycorrhizal fungi (AMF) meet the roots of both host and presumed nonhost plants, but the interactional mechanisms of AMF with and functional relevance for nonhost plants is little known. Here we show AMF can colonize an individually grown nonhost plant, Arabidopsis thaliana, and suppress the growth of Arabidopsis and two nonhost Brassica crops. This inhibitory effect increased with increasing AMF inoculum density, and was independent of AMF species or nutrient availability. 13C isotope labeling and physiological analyses revealed no significant carbon‒phosphorus exchange between Arabidopsis and AMF, indicating a lack of nutritional function in this interaction. AMF colonization activated the danger-associated peptide Pep-PEPR signaling pathway, and caused clear defense responses in Arabidopsis. The impairment of Pep-PEPR signaling in nonhost plants greatly compromised AMF-triggered defensive responses and photosynthesis suppression, leading to higher colonization rates and reduced growth suppression upon AMF inoculation. Pretreatment with Pep peptide decreased AMF colonization, and largely substituted for AMF-induced growth suppression in nonhosts, confirming that the Pep-PEPR pathway is a key participant in resistance to AMF colonization and in mediating growth suppression of nonhost plants. This work greatly increases our knowledge about the functional relevance of AMF and their mechanisms of interactions with nonhost plants.
