Together with STR2, required for arbuscule development in arbuscular mycorrhizal (AM) symbiosis. Heterodimerizes with STR2; the resulting transporter is located in the peri-arbuscular membrane. Located in the peri-arbuscular membrane of arbuscular mycorrhiza (AM). Expressed constitutively in the vascular tissue of roots. Accumulates in roots cortical cells containing arbuscules, in a RAM1-dependent manner, upon arbuscular mycorrhizal (AM) symbiosis with Glomus versiforme (PubMed:20453115, PubMed:26511916). Triggered by RAM1 (PubMed:26511916). Stunted arbuscule phenotype due to an impaired arbuscule development during arbuscular mycorrhizal (AM) symbiosis with Glomus versiforme; normal initial phases of the symbiosis, including hyphopodia formation and fungal entry into the cortex, as well as normal arbuscule development initiation, including arbuscule-associated genes expression, but strongly delayed and impaired arbuscule development (PubMed:20453115). Normal nodulation in the presence of Sinorhizobium meliloti (PubMed:20453115). Belongs to the ABC transporter superfamily. ABCG family. Stunted arbuscule (STR) subfamily.