Mediates both low-affinity uptake and efflux of sugar across the plasma membrane. Involved in phloem loading by mediating export from parenchyma cells feeding H(+)-coupled import into the sieve element/companion cell complex, thus contributing to the sucrose migration from sites of synthesis in the mesophyll to the phloem (PubMed:22157085, PubMed:25988582). Contributes to seed filling by triggering sucrose efflux involved in the transfer of sugars from seed coat to embryos (PubMed:25988582). Forms homooligomers and heterooligomers with SWEET5, SWEET11 and SWEET17. Present in the plasma membrane of the phloem. Expressed in leaves, especially in phloem (PubMed:22157085). Expressed in developing seeds (PubMed:25794936). In developing seeds, accumulates specifically at different stages. At globular stage, present in micropylar end of seed coat, in the endosperm and in the embryo suspensor. At the heart stage, confined to the micropylar end of seed coat. From the linear mature cotyledon stage until mature seed, present in the micropylar end of seed coat as well as in the endosperm. Induced by the powdery mildew fungus G.cichoracearum and the pathogenic bacteria P.syringae pv. tomato. Under high-light conditions, plants lacking both SWEET11 and SWEET12 are defective in phloem loading and display slower growth, mild chlorosis, and high levels of starch and sugar accumulation in leaves (PubMed:22157085). In plants lacking SWEET11, SWEET12 and SWEET15, severe seed defects, which include retarded embryo development, reduced seed weight, and reduced starch and lipid content, causing a wrinkled seed phenotype. Altered sucrose efflux involved in the transfer of sugars from seed coat to embryos thus leading to starch accumulation in the seed coat but not in the embryo (PubMed:25794936). Belongs to the SWEET sugar transporter family.