Different and overlapping functions of Arabidopsis LHT6 and AAP1 transporters in root amino acid uptake

Plants acquire nitrogen in the form of amino acids from the soil, and transport proteins located in the plasma membrane of root cells are required for this process. It was found that the Arabidopsis lysine-histidine-like transporter LHT6 is expressed in root cells important for amino acid uptake, including the epidermis, root hairs, and cortex. Transport studies with lht6 mutants using high levels of amino acids demonstrated that LHT6 is in fact involved in amino acid uptake. To determine if LHT6 plays a role in nitrogen acquisition at soil amino acid concentrations, growth and uptake studies were performed with low levels of toxic amino acid analogues and radiolabelled amino acids, respectively. In addition, mutants of AAP1, another root amino acid transporter, and lht6/aap1 double mutants were examined. The results showed that LHT6 is involved in uptake of acidic amino acids, glutamine and alanine, and probably phenylalanine. LHT6 seems not to transport basic or other neutral amino acids, or, alternatively, other transporters might compensate for eliminated LHT6 function. Previous studies suggested that AAP1 only takes up amino acids at high concentrations; however, here it is demonstrated that the transporter functions in acquisition of glutamate and neutral amino acids when present at soil concentrations. When comparing the characterized root uptake systems, it appears that transporters both with overlapping substrate specificity and with preference for specific substrates are required to access the soil amino acid pool.