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). May be involved in lateral roots and nodules formation. Shoots and roots of nodulating plants. Higher levels in roots, flowers and stems, lower in nodules, leaves, petioles and shoot apices. In primary roots, mostly localized in tips and to a lower extent in vasculature of older regions. In root tips, mostly expressed in the central tissues of the elongating zone (developing vasculature), in the starch-filled cells of the root cap and in some cortical cells. During lateral root development, striking expression in the proximal region of the primordium, close to the primary root central cylinder, and then in elongating cells of the developing vasculature and in developing root cap. During nodule formation, expressed in young elongated primordium, mostly in cells close to the root vasculature. In later stages, confined in small cells rich in amyloplasts with small nuclei. Near the periphery of developing nodules strong expression at the base that tapers off toward the apex. Not expressed in mature nodules. Transient induction in roots by S.meliloti. Belongs to the amino acid/polyamine transporter 2 family. Amino acid/auxin permease (AAAP) (TC 2.A.18.1) subfamily. Because of the similarity in sequence, the probe used to describe the developmental stages did not discriminate among the various MtLAX mRNAs.