Transcription factor acting as a regulator of arbuscular mycorrhiza (AM)-related genes (e.g. PT4, STR and RAM2) (PubMed:25560877). Required for the morphogenesis of arbuscules upon symbiosis with AM fungi (e.g. Rhizophagus irregularis) (PubMed:25560877). Also involved in restricting mycorrhizal colonization of the root meristem (PubMed:25560877). Interacts with RAM1 and NSP2. Expressed in roots under low phosphate (Pi) conditions. Accumulates in well-developed arbuscule-containing cells during arbuscule development upon arbuscular mycorrhizal (AM) symbiosis. Induced in root epidermal cells and outer cortical cells upon root colonization by arbuscular mycorrhiza (AM) fungi (e.g. Rhizophagus irregularis), especially in arbuscule-containing cells under low phosphate (Pi) conditions. Strongly reduced number of arbuscules undergoing accelerated degeneration during root colonization by arbuscular mycorrhiza (AM) fungi (e.g. Rhizophagus irregularis). Belongs to the GRAS family.