Responsible for the transport of sucrose into the cell, with the concomitant uptake of protons (symport system). Can also transport other glucosides such as maltose, salicin, helicin, and alpha-phenylglucoside. Probably required for apoplastic phloem sucrose loading in source tissues (e.g. leaves) in order to transport it to sink tissues (e.g. roots, flowers). Probably not involved in transport of sugars across the symplastic discontinuity between the endosperm and the embryo. Essential for normal pollen germination, but not for pollen maturation or starch accumulation in pollen. Optimum pH is 5. Glycan biosynthesis; sucrose metabolism. Homodimer. Widely expressed. Highly expressed in embryo, endosperm, germinating seeds, source leaf sheaths and panicles, but at very low levels in roots. Expressed in phloem companion cells. Expressed in developing caryopses from 3 days after flowering (DAF), increases to a maximal level at 5 to 7 DAF, and then gradually declines to a barely detectable level by 20 DAF when grain filling stage nearly terminates. Expressed in developing pollen and anther walls from 5 to 1 day before anthesis. By exogenous sugar supply. Down-regulated by sugar starvation and light in dark-grown seedlings. Male sterility probably due to impaired pollen germination. Belongs to the glycoside-pentoside-hexuronide (GPH) cation symporter transporter (TC 2.A.2.4) family.