Amino acid uptake in Arabidopsis

Nitrogen (N) is essential for all living organisms and is considered to be the limiting factor for plant growth in many ecosystems. Although generally believed to rely on mineral N to fulfill their N needs, plants have also been found to access organic N such as amino acids. Despite extensive research, the importance of amino acids as N sources for plants still remains unclear. The work presented in this thesis has focused on identifying the transporters responsible for amino acid uptake in plants and to characterize mutants lacking these transporters. Two transporters important for Arabidopsis thaliana amino acid uptake were identified, the lysine histidine transporter 1 (LHT1) and amino acid permease 5 (AAP5). These two transporters were found to have complementary, non-overlapping affinity spectra, i.e. LHT1 displayed affinity for neutral- and acidic amino acids and for L-Histidine, whereas AAP5 exhibited affinity for L-Arginine and L-Lysine only. Mutants lacking both LHT1 and AAP5 were found to have little residual uptake of the amino acids tested, suggesting these transporters to be the most important for Arabidopsis root amino acid uptake. Mutants lacking LHT1 or AAP5 displayed much reduced uptake rates in the low !M range suggesting these transporters mediate efficient uptake at field relevant concentrations. LHT1 mutants did not only have impaired uptake capacity, but also grew less than wild type when grown on for example L-Glutamine as the sole N source. In contrast, by over-expressing LHT1, plants grew larger on amino acids, suggesting a connection between uptake capacity and growth. Growth experiments using labeled amino acids in a mixture with nitrate revealed that a substantial amount of plant N was amino acid derived, suggesting that Arabidopsis has the ability to efficiently use amino acids as a source of N. The results presented in this thesis provide a mechanistic understanding to the process of root amino acid uptake in plants. This knowledge is important for future research within the field of plant organic N nutrition and Arabidopsis genotypes with altered amino acid uptake capacities can be used as tools to further elucidate the ecological benefit plants may have by taking up amino acids.