Abstract Most plants form mutualistic symbioses with soil fungi, including arbuscular mycorrhizal (AM) fungi. These fungi usually transfer soil nutrients to plants and assimilate carbon from host plant photosynthesis. Recently, Mucoromycotina 'fine root endophytes' (MFRE) were identified as nutritionally mutualistic and widespread fungal symbionts of plants, establishing MFRE as a new class of mycorrhizal fungi. However, the regulatory mechanisms for MFRE symbioses are completely unknown.