Mediates both low-affinity uptake and efflux of sugar across the plasma membrane (By similarity). Nectary-specific sugar transporter required for nectar production by mediating the secretion of sucrose from the nectary parenchyma to the extracellular space. Forms heterooligomers with SWEET1, SWEET5, SWEET8, SWEET11, SWEET13, SWEET16 and SWEET17. Specifically expressed in nectaries, mostly in the lower half of nectary parenchyma. Accumulates progressively in floral nectaries during maturation, reaching highest levels during maximal nectar secretion. In sweet9-1, reduced nectar production accompanied by starch accumulation in nectaries. Belongs to the SWEET sugar transporter family.