SummaryAmino acids are available to plants in some soils in significant amounts, and plants frequently make use of these nitrogen sources. The goal of this study was to identify transporters involved in the uptake of amino acids into root cells. Based on the fact that high concentrations of amino acids inhibit plant growth, we hypothesized that mutants tolerating toxic levels of amino acids might be deficient in the uptake of amino acids from the environment. To test this hypothesis, we employed a forward genetic screen for Arabidopsis thaliana mutants tolerating toxic concentrations of amino acids in the media. We identified an Arabidopsis mutant that is deficient in the amino acid permease 1 (AAP1, At1g58360) and resistant to 10 mM phenylalanine and a range of other amino acids. The transporter was localized to the plasma membrane of root epidermal cells, root hairs, and throughout the root tip of Arabidopsis. Feeding experiments with [ 14 C]-labeled neutral, acidic and basic amino acids showed significantly reduced uptake of amino acids in the mutant, underscoring that increased tolerance of aap1 to high levels of amino acids is coupled with reduced uptake by the root. The growth and uptake studies identified glutamate, histidine and neutral amino acids, including phenylalanine, as physiological substrates for AAP1, whereas aspartate, lysine and arginine are not. We also demonstrate that AAP1 imports amino acids into root cells when these are supplied at ecologically relevant concentrations. Together, our data indicate an important role of AAP1 for efficient use of nitrogen sources present in the rhizosphere.

Seed development and nitrogen (N) storage depend on delivery of amino acids to seed sinks. For efficient translocation to seeds, amino acids are loaded into the phloem in source leaves and along the long distance transport pathway through xylem-phloem transfer. We demonstrate that Arabidopsis thaliana AMINO ACID PERMEASE2 (AAP2) localizes to the phloem throughout the plant. AAP2 T-DNA insertion lines showed changes in source-sink translocation of amino acids and a decrease in the amount of seed total N and storage proteins, supporting AAP2 function in phloem loading and amino acid distribution to the embryo. Interestingly, in aap2 seeds, total carbon (C) levels were unchanged, while fatty acid levels were elevated. Moreover, branch and silique numbers per plant and seed yield were strongly increased. This suggests changes in N and C delivery to sinks and subsequent modulations of sink development and seed metabolism. This is supported by tracer experiments, expression studies of genes of N/C transport and metabolism in source and sink, and by phenotypic and metabolite analyses of aap2 plants. Thus, AAP2 is key for xylem to phloem transfer and sink N and C supply; moreover, modifications of N allocation can positively affect C assimilation and source-sink transport and benefit sink development and oil yield.