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. Unloads auxin from the mature phloem to deliver the hormone to the root meristem via the protophloem cell files. Coordinated subcellular localization of AUX1 is regulated by a brefeldin A-sensitive (BFA) vesicle trafficking process. Involved in lateral root formation, trichoblast polarization and root hair elongation. Required for gravitropism and thigmotropism, especially in roots, by modulating responses to auxin, ethylene and cytokinins such as benzyladenine (BA). Needed for ammonium-mediated root-growth inhibition. Confers sensitivity to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D, auxin analog), and to polar auxin transport inhibitors such as N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA). Auxin uptake mediated by AUX1 is inhibited by chromosaponin-1 (CSI), 1-naphthoxyacetic acid (1-NOA) and 3-chloro-4-hydroxyphenylacetic acid (CHPAA). In S2 columella cells, a dynamic cytoplasmic to membrane localization seems to occur during early stage of gravity signal transduction. In roots protophloem cells, asymmetric repartition in the upper plasma membrane. Expressed in root and shoot apical tissues. In root apex, confined to stele initials, protophloem poles, statolith-containing S2 columella cells, lateral root cap cells (LRC), and in epidermal cells from the distal elongation zone (DEZ) up to central elongation zone (CEZ). In seedlings, first expressed in primary root, followed by shoot apical meristem and then in lateral roots. During lateral root formation, expression in primordia starts at a late phase of stage I, before the first periclinal division. In later stages II and III, localized in the apical tip, extending to the elongation zone, and in the vascular cylinder extending to the junction with the primary root. Belongs to the amino acid/polyamine transporter 2 family. Amino acid/auxin permease (AAAP) (TC 2.A.18.1) subfamily.