Protease inhibitor involved in the control of mycorrhiza establishment and arbuscule development during root colonization by arbuscular mycorrhizal (AM) fungi (e.g. Rhizophagus irregularis). Interacts with CP. Accumulates in roots during colonization by arbuscular mycorrhizal (AM) fungi (e.g. Rhizophagus irregularis and Glomus intraradices). Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.