AAP1 functions in uptake of neutral amino acids and glutamate by Arabidopsis roots

Recent studies indicate that amino acids are available to plants in the soil in significant amounts and that plants frequently make use of these organic nitrogen sources. However, little is known about the transport systems involved in the uptake of amino acids from the rhizosphere. Our lab has identified an amino acid permease from Arabidopsis thaliana that is localized to the plasma membrane of root hairs and epidermal cells as well as to the root tip. This localization pattern points to a role of AtAAP1 in amino acid uptake from the soil. Based on the fact that high concentrations of amino acids in the growth media inhibit germination and growth of plants, we hypothesized that ataap1 mutants tolerating toxic concentrations of external amino acids might be deficient in the uptake of amino acids from the environment. A toxicity screen was developed and it was found that ataap1 mutants are resistant to neutral amino acids. Feeding experiments with [14C]-labeled amino acids showed significantly reduced amino acid uptake in ataap1 and identified glutamate, histidine, and neutral amino acids as physiological AAP1 substrates; aspartate, lysine, and arginine are not. Further, the regulation of AtAAP1 expression was examined using Arabidopsis wild type and AtAAP1-promoter-GUS seedlings grown in tissue and hydroponic culture. Northern blot analysis and GUS staining patterns revealed that AtAAP1 expression is induced by a variety of nitrogen sources. Overall, our studies showed that AtAAP1 is a mechanism involved in uptake of neutral, uncharged amino acids and glutamate from soil.