Protease inhibitor that, together with SCP1, controls mycorrhiza establishment and arbuscule development during root colonization by arbuscular mycorrhizal (AM) fungi (e.g. Rhizophagus irregularis), probably by degrading SCP1 in the apoplast of the periarbuscular region. Interacts with SCP1 and CP. Expressed at low levels in non-mycorrhizal roots. Accumulates in roots during colonization by arbuscular mycorrhizal (AM) fungi (e.g. Rhizophagus irregularis). Belongs to the protease inhibitor I3 (leguminous Kunitz-type inhibitor) family.