T he uptake and transport of inorganic nitrogen is essential for plant growth. Following the carrier-mediated uptake of nitrate or ammonium from the soil, or of ammonium derived from symbiotic dinitrogen fixation, inorganic nitrogen is assimilated into amino acids in an energy-requiring process 1 . The assimilation of ammonium usually occurs in the roots, whereas nitrate assimilation, depending on the species and environmental conditions, occurs either in the roots or in the leaves, after trans- port via the xylem 2 . From the leaves, excess amino acids can be exported via the phloem. Thus, amino acids are present in both xylem and phloem, and can be withdrawn from the vascular sys- tem by cells that depend on an external supply, such as apices, newly developing tissues and reproductive organs. In most plants, the spectra of amino acids found in the phloem and xylem appear to be similar, the major components being amides such as glutamine and asparagine, and acidic amino acids such as glutamate and as- partate. However, the relative concentrations of these amino acids are very different: the xylem contains low concentrations (e.g. 3– 20 mM in Urtica 3 ); and the phloem contains higher concentrations (e.g. 60–140 mM in sugar beet 4 ). The phloem, as a living system with cytoplasmic continuity between cells, mediates transport intracellularly, whereas the xylem can be considered to be an extra- cellular compartment. Because of loading of amino acids from the xylem to the phloem (e.g. in the case of seeds that receive their organic nitrogen mainly via the phloem), the composition of the phloem sap can differ along its path 5 . Such complexity in compo- sition, and the multiplicity of cell types that are involved in long- distance transport during the life cycle, suggests that multiple, highly regulated transport systems for amino acids exist. There is now physiological and genetic evidence for the activity of multi- ple, carrier-mediated transport systems responsible for the uptake and transfer of amino acids 6 . A molecular dissection of amino acid transport and characterization of individual carriers is thus required. Yeast as a model for amino acid transport Yeast has been invaluable for cloning and expressing plant genes encoding amino acid transporters, and could also be a good model for investigating aspects of their regulation. A given amino acid is typically transported into yeast cells by several permeases with different specificities, affinities and regulation 7 . In addition to their role in uptake, high-affinity amino acid permeases are also important for retention of intracellular amino acids, which tend to leak out of the cell 8 (Table 1). With the ongoing functional analy- sis of additional likely amino acid permeases (found during se- quencing of the yeast genome 9 ), a complete picture of individual amino acid transport systems in this unicellular eukaryote should soon be available. All yeast amino acid permeases that have been characterized fall into the APC (‘amino acid–polyamine–choline facilitator’) superfamily 10 , which may itself be subdivided (Fig. 1a). The largest family of Gap1 (‘general amino acid permease’)-related amino acid transporters includes Gap1; ten additional, more spe- cific, permeases; and seven others found by genome sequencing (Table 1). These proteins are very similar, and are closely related to amino acid permeases found in bacteria and other fungi. Analy- sis of the predicted topological features of bacterial proteins of this family (aroP and pheP) suggests a structure of 12 membrane- spanning domains separated by hydrophilic regions with the N- and C-termini located in the cytosol 11 . The second family [- aminobutyric acid (GABA) permease-related] includes two very similar methionine permeases (Mup1 and Mup3) (Ref. 12) that are homologous to several other proteins: a Schistosoma mansoni amino acid transporter; the yeast GABA-, choline- and KAPA/ DAPA permeases 13 ; and paralogues found by genome sequencing. In addition to the APC superfamily, yeast cells contain seven highly related proteins that are similar to the ATF (‘amino acid transporter family’) superfamily in plants 9 . However, the functional role of these proteins has yet to be tested. There might also be additional classes of amino acid transport proteins, which could, for example, be involved in compartmentalization of amino acids into the vacuole. It has been suggested that these proteins are members of a large family (31 members in yeast) of putative H + - antiporters 9 . The large ABC superfamily of transport proteins 14 might also include amino acid transport systems – several members of this family from other organisms have been shown to transport amino acids 15 . Regulation of amino acid transport in yeast occurs mainly by transcriptional control of amino acid permease genes and by tar- geting and turnover of the proteins. Transcription of several of the permease genes, such as GAP1, is subject to nitrogen repression: transcription is highest on media containing nitrogen sources that support only limited growth rates (urea or proline), and is down- regulated in the presence of preferred nitrogen sources (NH 4 + , glu- tamine or asparagine 7 ). Four transcription factors of the GATA family (Gln3, Nil1/Gat1, Uga43/Dal80 and Gzf3/Nil2/Deh1) play key functions in this global regulation and seem to be part of a complex network of auto- and cross-regulation 16 . Among genes subject to nitrogen repression, some are induced in the presence of their own substrates (e.g. UGA4); expression of other amino acid permease genes (e.g. HIP1) does not appear to be influenced by the 188 trends in plant science reviews May 1998, Vol. 3, No. 5 Copyright © 1998 Elsevier Science Ltd. All rights reserved. 1360 - 1385/98/$19.00 PII: S1360-1385(98)01231-X Amino acid transport in plants Wolf-Nicolas Fischer, Bruno André, Doris Rentsch, Sylvia Krolkiewicz, Mechthild Tegeder, Kevin Breitkreuz and Wolf B. Frommer Amino acids are transported between different organs through both xylem and phloem. This redistribution of nitrogen and carbon requires the activity of amino acid transporters in the plasma membrane. In addition, amino acids can be taken up directly by the roots. Amino acid transport has been well characterized in the yeast Saccharomyces cerevisiae, and functional complementation has served as an excellent tool for identifying and characterizing amino acid transporters from plants. The transporters from yeast and plants are related and can be grouped into two large superfamilies. Based on substrate specificity and affinity, as well as expression patterns in plants, different functions have been assigned to some of the individ- ual transporters. Plant mutants for amino acid transporter genes are now being used to study the physiological functions of many of the cloned genes.