Amino acid-proton symporter. Stereospecific transporter with a broad specificity for tryptophan, proline, and neutral and acidic amino acids. Has an affinity for aspartate in a physiological range. Involved in the uptake of amino acids diffusing out of the xylem tracheids into the xylem parenchyma. Expressed in roots and leaves, and at lower levels in stems and flowers. Found in the xylem parenchyma. Expressed at higher levels in sink as compared with source leaves. Down-regulated by drought and salt stress. Larger seeds and rosette leaves and increased number of cauline leaves at flowering. Decreased concentration of lysine, phenylalanine, leucine and aspartic acid in the sieve element sap, but no significant effect on aphid feeding behavior or reproduction. Belongs to the amino acid/polyamine transporter 2 family. Amino acid/auxin permease (AAAP) (TC 2.A.18.2) subfamily.