1. Introduction
Amino acids are the currency of nitrogen exchange in plants [1]. Although inorganic salts of nitrogen are initially acquired from the soil solution, these com- pounds are rapidly incorporated into amino acids in root or mature leaf tissue. While some of the newly synthesized amino nitrogen is used in protein biosyn- thesis or as the precursor of other essential nitrogen containing molecules in these tissues, most is trans- ported in the plants vascular system from the sites of primary assimilation to satisfy the nutritional needs of other organs that do not play a major role in nitrogen assimilation. Those tissues, which include developing leaves, meristems and reproductive or- gans, must import amino acids to support growth and development (Fig. 1). Amino acid transport