Carrier protein involved in proton-driven auxin influx. Mediates the formation of auxin gradient from developing leaves (site of auxin biosynthesis) to tips by contributing to the loading of auxin in vascular tissues and facilitating acropetal (base to tip) auxin transport within inner tissues of the root apex, and basipetal (tip to base) auxin transport within outer tissues of the root apex (By similarity). Belongs to the amino acid/polyamine transporter 2 family. Amino acid/auxin permease (AAAP) (TC 2.A.18.1) subfamily.