Mediates both low-affinity uptake and efflux of sugar across the plasma membrane. Required, in pollen, for microspore cell integrity and primexine pattern formation (PubMed:18434608, PubMed:25988582). Forms homooligomers and heterooligomers with SWEET4, SWEET5, SWEET6, SWEET7, SWEET9, SWEET10, SWEET11, SWEET13, SWEET15, SWEET16 and SWEET17. Expressed in inflorescences, embryo sacs and pollen, and at a lower level in stems. Barely detected in roots, leaves and seedlings. Expressed during anther development in tapetal cells and microsporocytes during meiosis. When the tapetum degenerates, detected only in pollen grains. Induced by the pathogenic bacteria P.syringae pv. tomato. Dramatically reduced fertility due to a postmeiotic rupture of microspores. Not detected in inflorescence (PubMed:18434608). Belongs to the SWEET sugar transporter family.