Leaf endophytes affect mycorrhizal status and growth of co‐infected and neighbouring plants

Summary Fungal leaf endophytes and arbuscular mycorrhizal (AM) fungi are common constituents of natural grasslands. The simultaneous presence of these two grass plant symbionts is highly probable. We describe the results of a glasshouse experiment investigating the outcome of dual infection of a cool‐season grass species, Lolium multiflorum , by the fungal endophyte Neotyphodium occultans and three species of Glomus AM fungi. Mycorrhizal colonization was investigated on monocultures of plants with or without leaf endophytes, and on mixtures of endophyte‐infected and uninfected plants. In both scenarios, endophyte‐infected plants had lower levels of mycorrhizal colonization, but in the endophyte mixtures the presence of endophyte‐infected plants caused an increase in AM colonization in non‐endophyte‐infected conspecific neighbours. Host‐plant biomass, nutrient (nitrogen and phosphorus) accumulation, and competitive ability were increased by the presence of endophytes. AM fungi did not improve host performance or nutrient content (concentration or accumulation) in the presence or absence of the endophyte. Interactions between host plants and AM fungi are mediated by fungal endophyte infection. The implications of such modified interactions for ecosystem dynamics and functioning are considered.