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Funding
Introduction Plants absorb both inorganic nitrogen (ammonium and nitrate) and organic nitrogen (amino acids and peptides) from the soil [1]. The first organic nitrogenous molecule produced from inorganic nitrogen is Gln, whose terminal amino group is successively transferred to make Glu and other amino acids to synthesize all the nitrogen-containing compounds of the cell. Within the cell, amino acids are synthesized [2], used, and degraded [3] in various compartments (namely chloroplast, cytosol, peroxisomes, mitochondrion and vacuole), requiring several transport steps across membranes to link the different pathways. Intracellular and intercellular transport of amino acids is mediated by dedicated membrane proteins; amino telaprevir transporters. The first amino acid transporter identified in plants was the Arabidopsis Amino Acid Permease 1 (also called Neutral Amino acid Transport system 2, NAT2) gene, published in 1993 by two groups [4,5]. Since then, thanks to advances in yeast functional complementation and genome sequencing, many amino acid transporters from Arabidopsis and other species have been isolated and characterized. This topic is regularly reviewed and the reader is invited to read excellent past articles that summarize our previous knowledge about the roles and functional properties of amino acid transporters [2,6–10]. Briefly, studies of the past decade identified many transporters active in roots, a few transporters connected with seed filling function, and some genes with a proposed role in source to sink translocation of amino acids (Fig. 1): AAP1 and AAP5, together with Lysine and Histidine Transporter 1 (LHT1), LHT6 and Proline Transporter 2 (ProT2) have been shown to have a role in root uptake; AAP2 and AAP6 are involved in xylem-phloem transfer, while AAP2, AAP3, AAP5, ProT1, Cationic Amino Acid Transporter 1 (CAT1), CAT6 and CAT9 contribute to phloem loading and are expressed in vascular tissues; AAP1 and AAP8 have also been shown to be involved in seed loading with amino acids [8–10]. We will focus in the present review on recent advances in the study of amino acid transport. Research completed in the past few years in Arabidopsis and other plant species has increased our knowledge of amino acid translocation from source to sink, for fruit development and seed filling, and transport within the cell. While all the previously known plant amino acid transporters belong to two families, the Amino Acid/Auxin Permease family (AAAP; [11]) and the Amino acid-Polyamine-organoCation family (APC; [12]), a new family of transporters has recently been identified. This family is the Usually Multiple Amino Acids Move In and Out Transporter (UMAMIT, see [13,14]), which is part of the Drug/Metabolite Transporter (DMT) superfamily [15]. As interest in the field of amino acid transport grows, the number of organisms in which it is being studied is expanding. Genome-wide surveys of different species are leading to more direct identification of previously unknown amino acid transporters in rice [16], Selaginalla[17], poplar [18], soybean [19], potato [20] and Ricinus [21].
Amino acid uptake, transport, and partitioning
Other functional roles of amino acid transporters Some members of amino acid transporter families have been found to transport molecules other than amino acids. A forward genetic screening aiming at finding 1-aminocyclopropane-1-carboxylic acid (ACC)-resistant plants isolated the amino acid transporter LHT1 [38]. LHT1 had been previously characterized for its role in amino acid uptake from the soil and leaf mesophyll apoplasm [39,40], and the lht1 knockout mutant shows an early senescence phenotype, induced defense responses, and disturbance in amino acid homeostasis and redox status [41]. This newly discovered role of LHT1 in ACC transport suggests that some components of the pleiotropic Lht1 phenotype could result from ethylene signaling defects. Interestingly, the authors reported that at least one other member of the LHT family is able to transport ACC [38], suggesting that this property is physiologically relevant. AAC transport by an amino acid transporter is not surprising since ACC is an α-amino acid, whose α-carbon is part of a three carbon-ring. Substrate specificity of plant amino acid transporters has indeed been shown to be borne by the α-carbon amino and carboxylic groups [42–44], similar to animal amino acid transporters [45–47].