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