Auxin influx transporter that mediates the transport of auxin in roots. Contributes to the basipetal transport in hypocotyls and root tips by establishing an auxin uptake sink in the root cap. Confers sensitivity to 1-N-naphthylphthalamic acid (NPA). Regulates the root elongation, the initiation of lateral roots and the development of root hairs. Can transport IAA, indole-3-propionic acid, NPA syringic acid, vanillic acid and some auxin metabolites, but not 2,4-D and 1-naphthaleneacetic acid. Interacts with 1-naphthylphthalamic acid (NPA). Non-polar distribution in apical cells. Apical (bottom) localization in mature root cells. Basal (top) localization in the root elongation zone. Mostly expressed in roots, especially in the root elongation zone and lateral roots. In mature portion of the root, expressed in the epidermis and cortex. In the root elongation zone, confined to epidermis. In root tips, present in the root cap, S3 columella and epidermal cells. Highly expressed at early stages of root development. By auxin, and cytokinins such as kinetin. Repressed by abscisic acid and cold treatment. Phosphorylation level varies significantly during early response to bacterial elicitor. Belongs to the ABC transporter superfamily. ABCB family. Multidrug resistance exporter (TC 3.A.1.201) subfamily.