High-affinity transporter for external inorganic phosphate (Pi). Probably involved in Pi uptake, translocation and internal transport throughout the plant. Measured in yeast cells deficient in Pi uptake. Optimum pH is 6.0. Highly expressed in leaves and at low levels in roots. Expressed in leaf xylem parenchyma cells. In roots and leaves by phosphate starvation. Down-regulated in roots when colonized by the mycorrhizal fungus G. intraradices. Down-regulated by high phosphate in suspension cell culture. Although related to the sugar transporter family, it does not transport sugars. Belongs to the major facilitator superfamily. phosphate:H(+) symporter (TC 2.A.1.9) family.