Transporters for amino acids in plant cells: some functions and many unknowns

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Transporters for amino acids in plant cells: some functions and many unknowns Mechthild Tegeder Membrane proteins are essential to move amino acids in or out of plant cells as well as between organelles. While many putative amino acid transporters have been identified, function in nitrogen movement in plants has only been shown for a few proteins. Those studies demonstrate that import systems are fundamental in partitioning of amino acids at cellular and whole plant level. Physiological data further suggest that amino acid transporters are key-regulators in plant metabolism and that their activities affect growth and development. By contrast, knowledge on the molecular mechanisms of cellular export processes as well as on intracellular transport of amino acids is scarce. Similarly, little is known about the regulation of amino acid transporter function and involvement of the transporters in amino acid signaling. Future studies need to identify the missing components to elucidate the importance of amino acid transport processes for whole plant physiology and productivity. Address School of Biological Sciences, Washington State University, Pullman, WA 99164-4236, USA Corresponding author: Tegeder, Mechthild ([email protected])

Current Opinion in Plant Biology 2012, 15:315–321 This review comes from a themed issue on Physiology and metabolism Edited by Julian M Hibberd and Andreas PM Weber Available online 25th February 2012 1369-5266/$ – see front matter # 2012 Elsevier Ltd. All rights reserved. DOI 10.1016/j.pbi.2012.02.001

Introduction Amino acids have highly diverse and essential roles in plants. By being the building blocks for enzymes and proteins, they provide important components for plant metabolism and structure. Furthermore, they serve as precursors or nitrogen [N] donors for the synthesis of a large variety of compounds critical to plant development including nucleotides, chlorophyll, hormones and secondary metabolites. Plants can take up amino acids directly from the soil or assimilate inorganic N (i.e. nitrate and ammonium) into amino acids [1]. Many of the 20 proteinamino acids are produced in the plastids of roots or leaves but they are also synthesized in other cellular compartments including cytosol, mitochondria and peroxisomes www.sciencedirect.com

[2,3]. Following synthesis, amino acids are immediately used for metabolism, transiently stored (e.g. in the vacuole) or transported in the phloem to developing vegetative or reproductive sink tissues. Partitioning of amino acids within and between cells, and from source to sink organs requires proteins functioning as importers and/or exporters in cellular or subcellular membranes.

Different families of transporters have been identified for import of amino acids into plant cells The first plant amino acid transporter AAP1/NAT2 (amino acid permease 1) was identified more than 18 years ago in Arabidopsis [4,5]. AtAAP1 belongs to a family of eight members (AtAAP1-8) that transport acidic, neutral, and basic amino acids, dependent on the transporter [6,7,8]. Based on heterologous complementation experiments and sequence homology, up to date more than 60 putative amino acid transporters have been identified in Arabidopsis (see http://aramemnon.botanik.uni-koeln.de/). Transporters that have been further characterized by functional analysis in heterologous systems, and by expression and localization studies, are mostly involved in amino acid uptake into plant cells and belong to the AAP (amino acid permease), LHT (lysine/histidine-type transporter), ProT (proline/compatible solute transporter), ANT1-like (aromatic–neutral amino acid transporter), GAT (g-aminobutyric acid transporter) and CAT (cationic amino acid transporter) families (for review see [1,3,9]). The transporters generally differ in substrate selectivity and affinity when analyzed in yeast or Xenopus oocytes, and in their tissue or cellular localization. However, research demonstrating their physiological importance is restricted to root uptake, xylem–phloem transfer and import into mesophyll cells and seeds (Figure 1).

What is known about the function of amino acid importers? Amino acid uptake by the root

In recent years, four amino acid transporters have been shown to play a role in amino acid uptake by the root, that are, Arabidopsis LHT1, AAP1, AAP5 and ProT2 [8,10,11,12,13]. AtProT2 is expressed in the root epidermis and cortex and is involved in proline (compatible solute) acquisition by roots [13,14]. AtAAP1 is localized to the root tip and epidermis cells including root hairs and important in uptake of neutral and acidic amino acids when soil solution concentrations are higher than 50 mM [11,15]. At lower concentrations, AtLHT1 and AtAAP5 play a role in uptake of neutral and acidic Current Opinion in Plant Biology 2012, 15:315–321

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amino acids, and basic amino acids, respectively [8,15]. At early plant development, expression of AtAAP5 in the root cortex, and AtLHT1 in the root epidermis, cortex and endodermis of seedlings [16] support a direct role of the transporters in amino acid import into root cells. However, at later developmental stages, AtAAP5 and AtLHT1 function in amino acid uptake might rather be indirect, as AtAAP5 transcript levels in roots are low but high in leaves, stem and flowers [6,17], and AtLHT1 expression is only found in the root tips and throughout the mesophyll cells [10]. Further, in atlht1 leaves, apoplastic amino acids are strongly increased potentially feedback regulating amino acid uptake from the soil [[18,19]; see also below]. To clearly resolve if the transporters are directly or indirectly functioning in root import of amino acids, uptake studies need to be performed using plants, in which transporter repression is restricted to roots (cf. [10]). Import into the phloem

Source to sink translocation of amino acids generally occurs in the phloem and requires in most herbaceous plants, including Arabidopsis, loading of the organic N into the collection phloem of leaf minor veins [20]. However, transporters involved in this step have not been identified up to date, but potential candidates might belong to the AAP family [21]. Evidence for the importance of amino acid phloem loaders for whole plant physiology was obtained by expressing a yeast transporter for S-methyl methionine, a sulfur-containing non-protein amino acid [22], as well as pea PsAAP1 (Zhang and Tegeder, unpublished) in the phloem of pea plants. Both approaches lead to increased S and/or N translocation to sinks and elevated protein levels in seeds. Phloem loading also occurs along the translocation pathway when root-derived amino acids are transferred from xylem to phloem [23]. A transporter responsible for this loading step is Arabidopsis AAP2 [24]. In ataap2 plants, amino acid transport to sinks is decreased leading to reduced total elemental N and protein levels in seeds. Although not directly involved in phloem uptake, expression of AtAAP6 in xylem parenchyma cells [25] and reduced amino acid levels in the phloem of ataap6 plants [26] further underlines the importance of transporters in amino acid transfer from xylem to phloem. Import into seeds

During the reproductive phase, Arabidopsis AAP8 and AAP1 are involved in amino acid delivery to seeds. AtAAP8 mediates amino acid uptake into the endosperm at early embryo stage, and its function is essential as about 50% of fertilized ovules abort in ataap8 siliques [27]. However, the transporter is also expressed in the vasculature, presumably the phloem [21] of siliques, stem and probably other organs [17,25] and the observed effects on seed development in ataap8 plants might also partly be caused by reduced phloem loading and amino acid delivery to sinks. Uptake of amino acids into the embryo is mediated by AtAAP1 [28]. While in ataap1 plants amino Current Opinion in Plant Biology 2012, 15:315–321

acid import into the embryo and subsequently seed N and protein levels are reduced, other embryo-expressed amino acid transporters such as AtCAT6 [29] seem to some extent compensate for AtAAP1 loss of function, as ataap1 seed development is not affected [28]. Cotyledon storage parenchyma cells also express AAP transporters to import apoplastic amino acids for storage protein accumulation [28,30,31], and overexpression of Vicia faba VfAAP1 in the cotyledon parenchyma of Faba bean and Vicia narbonensis resulted in increased seed protein levels as well as seed weight and size [32].

Regulation of amino acid transporters While there is currently no experimental evidence that plant amino acid transporters are post-transcriptionally regulated or controlled by post-translational modifications, regulation does occur at transcriptional level. Expression of amino acid transporters is cell or tissuespecific, developmentally regulated and influenced by environmental challenges such as water and salt stress [1,33–35]. Interestingly, pathogen attack seems to affect the expression of amino acid transporter AtLHT1 in Arabidopsis, probably as a means to increase cytosolic glutamine level. Glutamine, in turn, influences a salicylic acid-mediated signaling pathway leading to decreased disease resistance [36]. Similarly, in Lotus japonicus, mycorrhizal fungi induce expression of LjLHT1 in root cells [37]. Further, amino acid transporter expression is influenced by N availability and the presence of both inorganic and organic N [33,35,38]. In addition, cellular changes in amino acid levels, as they occur during senescence when amino acids are remobilized, and in plants with altered amino acid transport processes (mutants or overexpressors), affect expression of specific amino acid transporters [22,24,28,39,40] (see also below).

What is known about amino acid exporters? Physiological data support that export of amino acids from plant cells requires transporters, but little is known at the molecular level (Figure 1). Potential exporter candidates have been discussed based on sequence homology to proteins of bacteria, mammals and other organisms facilitating efflux of amino acids [3,41,42] but their function in plants remains to be elucidated. This includes Arabidopsis BAT1 (Bidirectional Amino acid Transport 1) that in yeast mediates import of arginine and alanine, and export of lysine and glutamate [43]. On the contrary, studies with mutants of the aromatic-neutral amino acid transporter AtANT1 showed an increased sieve-tube amino acid level, suggesting that ANT1 might move amino acids out of the Arabidopsis phloem, provided that the transporter is localized to the phloem [44,45]. Promising results were recently obtained describing a new class of membrane proteins that seem to function as facilitators for amino acid export [46] (Koch, personal communication). SiAR1 (Siliques Are Red 1) is a bidirectional amino acid transporter involved in cellular efflux of www.sciencedirect.com

Amino acid transport in plants Tegeder 317

Figure 1

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AAP1 ?

AAP8 AAP6 ? ? ?

SiAR1

AAP2

DiT2.1 LHT1

Xylem

Phloem

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AAP1 ProT2 AAP5 LHT1 ? Current Opinion in Plant Biology

Overview of amino acid transporters with demonstrated functions in plants (arrow with black circle and two black arrows). This includes transporters involved in (i) amino acid uptake into root cells [LHT1, AAP1, AAP5, ProT2 and CAT6], (ii) import into mesophyll cells [LHT1], the endosperm [AAP8], embryo [AAP1], xylem parenchyma [AAP6] and transport phloem [AAP2], (iii) cellular efflux of glutamine and histidine, and uptake of aspartate and glutamate [SiAR1] and in (iv) glutamate/malate exchange across chloroplast membranes [DiT2.1]. However, many amino acid transport systems still need to be discovered and their physiological function elucidated (arrow with white circle). Examples for these are transporters functioning in phloem loading of leaf minor veins for N source–sink partitioning, or in import of amino acids into floral cells/tissue in support of reproduction. In addition, cellular export systems for specific amino acids or with broad substrate specificity as well as intracellular transporters (e.g. vacuole, chloroplast, peroxisome and mitochondria) remain to be identified and analyzed with respect to their physiological significance.

glutamine and histidine, and uptake of aspartate and glutamate. It is expressed in cells of roots and seeds as well as in the vascular tissue of different plant organs. SiAR1 is important since its knockout causes reduced leaf growth and leads to red siliques (anthocyanin accumulation) due to altered N partitioning. While not directly involved in amino acid export, AtGDU1, a protein with one predicted transmembrane domain, seems to regulate the cellular release of glutamine potentially by activating non-selective amino acid facilitators [47,48].

What is known about intracellular amino acid transporters? While many protein amino acids are produced in plastids, synthesis (or degradation) of amino acids and further www.sciencedirect.com

organic compounds relying on amino acids takes also place in the cytosol and other organelles. This requires the presence of import and export proteins in subcellular membranes that direct the amino acids to the specific metabolic pathways. Based on biochemical, proteome, localization and other studies, Arabidopsis proteins have been identified that might play roles in amino acid transport across the mitochondria [49–52], vacuole [44,53–56], plastid [57–59] and ER or Golgi [29,54,60] membranes, but their physiological functions remain to be demonstrated (Figure 1). The only transporter characterized in plants so far is AtDiT2.1, a glutamate/malate exchanger exporting glutamate from the plastid to the cytosol [61]. Mutants of AtDiT2.1 support a function of the translocator in ammonia assimilation as they show a photorespiratory phenotype. Current Opinion in Plant Biology 2012, 15:315–321

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Are amino acid transporters key-players in plant metabolism and development? Transcriptome and metabolome studies have revealed regulatory interactions between N/carbon/sulfur/phosphorus metabolisms, and primary and secondary metabolism as well as system responses to N [62–68]. Amino acids are not only critical for the synthesis of proteins, enzymes, and many compounds essential for plant metabolism, growth and defense, they also affect biological processes by being signal molecules or by influencing hormone action via amino acid conjugation [69–73]. Consequently, transporters are located in a central position within a complex metabolic network regulating the supply of amino acids for the different pathways in source and sink (see for example [74–76]). Recent studies support that physiological processes, both upstream and downstream of amino acid transporter function are strongly affected by the activity of the specific organic N transporters. For example, changes in phloem loading, source–sink translocation and seed import of amino acids affect source leaf development as well as source N, carbon, and/or sulfur metabolism, and feedback regulates uptake of N and sulfur from the soil [22,24,28] (Zhang and Tegeder, unpublished). Similarly, sink development (i.e. number of flowers, branches, pods/siliques and seeds, and seed weight) as well as seed metabolism are influenced by the activity of amino acid transporters [22,24,28] (Zhang and Tegeder, unpublished). In the transporter mutants and overexpressors, specific amino acids present most probably the signal that leads to the observed changes by inducing complex signal transduction cascades resulting in alteration of gene expression [18,77]. However, the mechanisms through which changes in amino acid levels are sensed or signals are transduced are unknown, but could involve amino acid transporters or kinases as shown in yeast for amino acid permeases Ssy1 and Gap1, and GCN2 kinase [78–81].

posttranslationally regulated and requires protein modifications or protein-protein interactions [82]? Membrane transport proteins are the gatekeepers for amino acid movement in and out of cells or cellular compartments. Once protein functions are assigned, the amino acid transporters need to be integrated into a complex metabolic and signaling network to fully understand their importance for whole plant physiology and their contribution to plant productivity. Efforts need also to include their interaction with the environment to allow prediction on plant performance under stress conditions and nutrient limitations.

Acknowledgements Our research on organic nitrogen transport is supported by grants from the National Science Foundation Grant IOS 1021286 and the Agricultural and Food Research Initiative Competitive Grant no. 2010-65115-20382 from the USDA National Institute of Food and Agriculture. I thank Doris Rentsch for comments on the manuscript.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Tegeder M, Rentsch D: Uptake and partitioning of amino acids and peptides. Mol Plant 2010, 3:997-1011.

2.

Tegeder M, Weber APM: Metabolite transporters in the control of plant primary metabolism. In Control of Primary Metabolism in Plants, vol 22. Edited by Plaxton WC, McManus MT. Oxford: Blackwell Publishing; 2006:85-120.

3.

Rentsch D, Schmidt S, Tegeder M: Transporters for uptake and allocation of organic nitrogen compounds in plants. FEBS Lett 2007, 581:2281-2289.

4.

Frommer WB, Hummel S, Riesmeier JW: Expression cloning in yeast of a cDNA encoding a broad specificity amino acid permease from Arabidopsis thaliana. Proc Natl Acad Sci USA 1993, 90:5944-5948.

5.

Hsu LC, Chiou TJ, Chen L, Bush DR: Cloning a plant amino acid transporter by functional complementation of a yeast amino acid transport mutant. Proc Natl Acad Sci USA 1993, 90:7441-7445.

6.

Fischer WN, Kwart M, Hummel S, Frommer WB: Substrate specificity and expression profile of amino acid transporters (AAPs) in Arabidopsis. J Biol Chem 1995, 270:16315-16320.

Conclusions and future challenges Recent amino acid transporter work demonstrates the importance of cellular import processes for plant performance. However, current knowledge is restricted to only a few transporters and cell-types or tissues, and mostly concerns Arabidopsis rather than monocotyledonous species or crop plants. In addition, many physiological functions such as the role of amino acid loading into the collection phloem or reproductive floral tissues for growth and reproduction, respectively, remain to be addressed. We also still lack basic information on transporter function in amino acid export or movement across subcellular membranes, a challenge that needs to be tackled. Other missing pieces of a highly complicated puzzle relate to transporter regulation. What are the signals controlling transporter expression that lead to changes in nutrient uptake, metabolism and development? How do cells sense the amino acid status? Are amino acid transporters involved? Similarly, is amino acid transporter activity Current Opinion in Plant Biology 2012, 15:315–321

7. 

Fischer WN, Loo DDF, Koch W, Ludewig U, Boorer KJ, Tegeder M, Rentsch D, Wright EM, Frommer WB: Low and high affinity amino acid H+-cotransporters for cellular import of neutral and charged amino acids. Plant J 2002, 29:717-731. This paper provides a detailed electrophysiological analysis of AAP amino acid transporters expressed in Xenopus oocytes demonstrating that they recognize a broad spectrum of amino acids, including neutral, acidic and basic amino acids. However, the substrate specificities and affinities vary among the individual amino acid transporters suggesting different functions in plants. Svennerstam H, Ganeteg U, Na¨sholm T: Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease. New Phytol 2008, 180:620-630. The authors show that AtAAP5 plays a role in amino acid acquisition from the soil. The work confirms in planta that AtAAP5 transports basic amino acid as previously demonstrated in Xenopus oocytes [7].

8. 

9.

Tegeder M, Rentsch D, Patrick JW: Organic carbon and nitrogen transporters. In Plant Plasma Membrane: Plant Cell Monographs. Edited by Murphy A, Peer W, Schulz B. Berlin: Springer; 2011:331-352. www.sciencedirect.com

Amino acid transport in plants Tegeder 319

10. Hirner A, Ladwig F, Stransky H, Okumoto S, Keinath M, Harms A,  Frommer WB, Koch W: Arabidopsis LHT1 is a high-affinity transporter for cellular amino acid uptake in both root epidermis and leaf mesophyll. Plant Cell 2006, 18:1931-1946. The first demonstration of the function for an amino acid transporter in plant cells. Mutants of LHT1 show a striking growth phenotype consistent with a function of the transporter in uptake of amino acids into root and mesophyll cells. 11. Lee YH, Foster J, Chen J, Voll LM, Weber APM, Tegeder M: AAP1  transports uncharged amino acids into roots of Arabidopsis. Plant J 2007, 50:305-319. This work resolved the function of AtAAP1 in direct uptake of amino acids into the root epidermis, root hairs and root tip. It further shows that substrate selectivity of AtAAP1 in planta is consistent with the electrophysiological analysis of the transporter in oocytes [7]. 12. Svennerstam H, Ganeteg U, Bellini C, Na¨sholm T: Comprehensive screening of Arabidopsis mutants suggests the lysine histidine transporter 1 to be involved in plant uptake of amino acids. Plant Physiol 2007, 143:1853-1860. 13. Lehmann S, Gumy C, Blatter E, Boeffel S, Fricke W, Rentsch D: In  planta function of compatible solute transporters of the AtProT family. J Exp Bot 2011, 62:787-796. This work addresses the role of AtProTs in Arabidopsis. Presented data support a function of AtProT1 and AtProT2 in proline/compatible solute uptake into root cells and germinating pollen, respectively. 14. Grallath S, Weimar T, Meyer A, Gumy C, Suter-Grotemeyer M, Neuhaus JM, Rentsch D: The AtProT family: compatible solute transporters with similar substrate specificity but differential expression patterns. Plant Physiol 2005, 137:117-126. 15. Svennerstam H, Ja¨mtga˚rd S, Ahmad I, Huss-Danell K, Na¨sholm T,  Ganeteg U: Transporters in Arabidopsis roots mediating uptake of amino acids at naturally occurring concentrations. New Phytol 2011, 191:459-467. Uptake studies were performed with Arabidopsis atlht1, ataap1 and ataap5 amino acid transporter mutants at Gln, Ala, Glu, Asp, Arg and Lys concentrations ranging from 2 to 50 mM. The results show that in contrast to AtAAP1, AtLHT1 and AtAAP5 are directly or indirectly involved in root uptake of amino acids when amino acid levels in the nutrient (soil) solution are low. 16. Brady SM, Orlando DA, Lee JY, Wang JY, Koch J, Dinneny JR, Mace D, Ohler U, Benfey PN: A high-resolution root spatiotemporal map reveals dominant expression patterns. Science 2007, 318:801-806. 17. Winter D, Vinegar B, Nahal H, Ammar R, Wilson G, Provart N: An ‘electronic fluorescent pictograph’ browser for exploring and analyzing large-scale biological data sets. PLoS ONE 2007, 2:e718. 18. Walch-Liu P, Filleur S, Gan Y, Forde BG: Signaling mechanisms integrating root and shoot responses to changes in the nitrogen supply. Photosynth Res 2005, 83:239-250. 19. Miller AJ, Shen Q, Xu G: Freeways in the plant: transporters for N,P and S and their regulation. Curr Opin Plant Biol 2009, 12:284-290. 20. Rennie EA, Turgeon R: A comprehensive picture of  phloem loading strategies. Proc Natl Acad Sci USA 2009, 106:14162-14167. This is an interesting study examining the mechanisms of phloem loading of 45 eudicotyledonous species by for example leaf uptake of 14C-labeled compounds and subsequent autoradiography, and counting of plasmodesmata. The authors conclude that strategies for phloem loading of individual species correlate with their physiology, development and with the environment they live in. 21. Tegeder M, Ward JM: Molecular evolution of plant AAP and LHT amino acid transporters. Front Plant Physiol 2012, 3:21 doi: 10.3389/fpls.2012.00021. 22. Tan Q, Zhang L, Grant J, Cooper P, Tegeder M: Increased phloem  transport of S-methylmethionine positively affects sulfur and nitrogen metabolism, and seed development in pea plants. Plant Physiol 2010, 154:1886-1896. This study provides clues on the importance of phloem loading of amino acids/sulfur for whole plant physiology. A yeast transporter for S-methylmethionine (SMM) was expressed in the phloem of pea minor veins positively affecting S/N uptake, metabolism and source–sink translocawww.sciencedirect.com

tion, leading to increased plant growth, seed development and seed protein levels. The results support that amino acid transporter function in the phloem is central for plant performance. 23. Pate JS, Sharkey PJ, Lewis OAM: Xylem to phloem transfer of solutes in fruiting shoots of legumes, studied by a phloem bleeding technique. Planta 1975, 122:11-26. 24. Zhang L, Tan Q, Lee R, Trethewy A, Lee Y-H, Tegeder M: Altered  xylem-phloem transfer of amino acids affects metabolism and leads to increased seed yield and oil content in Arabidopsis. Plant Cell 2010, 22:3603-3620. This work addresses the significance of AtAAP2 in xylem to phloem transfer of amino acids and its role in source and sink physiology. Ataap2 mutants show reduced amino acid translocation to sinks, resulting in decreased seed protein levels. In addition, allocation of nitrogen to leaves was increased, positively affecting carbon metabolism and carbon source–sink translocation, leading to increased silique set, seed yield and seed oil levels. 25. Okumoto S, Schmidt R, Tegeder M, Fischer WN, Rentsch D, Frommer WB, Koch W: High affinity amino acid transporters specifically expressed in xylem parenchyma and developing seeds of Arabidopsis. J Biol Chem 2002, 277:45338-45346. 26. Hunt E, Gattolin S, Newbury HJ, Bale JS, Tseng HM, Barrett DA,  Pritchard J: A mutation in amino acid permease AAP6 reduces the amino acid content of the Arabidopsis sieve elements but leaves aphid herbivores unaffected. J Exp Bot 2010, 61:55-64. The authors provide evidence that AtAAP6, localized to xylem parenchyma cells, contributes to xylem–phloem transfer of amino acids. 27. Schmidt R, Stransky H, Koch W: The amino acid permease AAP8  is important for early seed development in Arabidopsis thaliana. Planta 2007, 226:805-813. It is reported that ataap8 plants show a 50% abortion of fertilized ovules supporting that AtAAP8 function in amino acid transport is essential for embryo development. 28. Sanders A, Collier R, Trethewy A, Gould G, Sieker R, Tegeder M:  AAP1 regulates import of amino acids into developing Arabidopsis embryos. Plant J 2009, 59:540-552. This study resolves that AtAAP1 transporter function in the outer epidermal cell layer of embryos is important for amino acid import into seeds and affects seed protein levels. 29. Hammes UZ, Nielsen E, Honaas LA, Taylor CG, Schachtman DP:  AtCAT6, a sink-tissue-localized transporter for essential amino acids in Arabidopsis. Plant J 2006, 48:414-426. AtCAT6 expression, localization and mutant growth studies support that the transporter functions in uptake of essential amino acids into sink cells of flowers, seeds and root primordia. 30. Tegeder M, Offler CE, Frommer WB, Patrick JW: Amino acid transporters are localized to transfer cells of developing pea seeds. Plant Physiol 2000, 122:319-325. 31. Miranda M, Borisjuk L, Tewes A, Heim U, Sauer N, Wobus U, Weber H: Amino acid permeases in developing seeds of Vicia faba L: expression precedes storage protein synthesis and is regulated by amino acid supply. Plant J 2001, 28:61-71. 32. Rolletschek H, Hosein F, Miranda M, Heim U, Gotz KP,  Schlereth A, Borisjuk L, Saalbach I, Wobus U, Weber H: Ectopic expression of an amino acid transporter (VfAAP1) in seeds of Vicia narbonensis and pea increases storage proteins. Plant Physiol 2005, 137:1236-1249. The paper shows that expression of an amino acid transporter in the storage parenchyma cells of legume seeds is important for import of organic nitrogen into the storage tissue affecting seed protein levels. 33. Delrot S, Atanassova R, Maurousset L: Regulation of sugar, amino acid and peptide plant membrane transporters. Biochim Biophys Acta 2000, 1465:281-306. 34. Liu X, Bush DR: Expression and transcriptional regulation of amino acid transporters in plants. Amino Acids 2006, 30:113-120. 35. Rentsch D, Hirner B, Schmelzer E, Frommer WB: Salt stressinduced proline transporters and salt stress-repressed broad specificity amino acid permeases identified by suppression of a yeast amino acid permease-targeting mutant. Plant Cell 1996, 8:1437-1446. 36. Liu G, Ji Y, Bhuiyan NH, Pilot G, Selvaraj G, Zou J, Wei Y: Amino  acid homeostasis modulates salicylic acid-associated redox Current Opinion in Plant Biology 2012, 15:315–321

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status and defense responses in Arabidopsis. Plant Cell 2010, 22:3845-3863. This study analyzes the role of Arabidopsis amino acid transporter AtLHT1 in plant-pathogen interaction. Most probably due the changes in cytosolic glutamine levels, leaves of atlht1 mutants display increased disease resistance against a broad spectrum of pathogens in a salicylic acid-dependent fashion. 37. Guether M, Volpe V, Balestrini R, Requena N, Daniel Wipf D, Bonfante P: LjLHT1.2-a mycorrhiza-inducible plant amino acid transporter from Lotus japonicus. Biol Fertil Soils 2011, 47:925-936. 38. Tegeder M, Tan Q, Grennan AK, Patrick JW: Amino acid transporter expression and localisation studies in pea (Pisum sativum). Funct Plant Biol 2007, 34:1019-1028. 39. Masclaux-Daubresse C, Reisdorf-Cren M, Orsel M: Leaf nitrogen remobilisation for plant development and grain filling. Plant Biol 2008, 10(Suppl. 1):23-36. 40. Couturier J, Doidy J, Guinet F, Wipf D, Blaudez D, Chalot M: Glutamine, arginine and the amino acid transporter Pt-CAT11 play important roles during senescence in poplar. Ann Bot 2010, 105:1159-1169. 41. Wipf D, Ludewig U, Tegeder M, Rentsch D, Koch W, Frommer WB: Conservation of amino acid transporters in fungi, plants and animals. Trends Biochem Sci 2002, 27:139-147. 42. Okumoto S, Pilot G: Amino acid export in plants: a missing link in nitrogen cycling. Mol Plant 2011, 4:453-463. 43. Du¨ndar E, Bush DR: BAT1, a bidirectional amino acid  transporter in Arabidopsis. Planta 2009, 229:1047-1056. When expressed in yeast, Arabidopsis BAT1 mediates export as well as import of amino acids. 44. Chen LS, Ortiz-Lopez A, Jung A, Bush DR: ANT1, an aromatic and neutral amino acid transporter in Arabidopsis. Plant Physiol 2001, 125:1813-1820. 45. Hunt EJ, Pritchard J, Bennett MJ, Zhu X, Barrett DA, Allen T,  Bale JS, Newbury HJ: The Arabidopsis thaliana/Myzus persicae model system demonstrates that a single gene can influence the interaction between a plant and a sap-feeding insect. Mol Ecol 2006, 15:4203-4213. The authors examined the effect of phloem amino acid levels on aphid feeding behaviour, growth and reproduction. They found that in mutants of AtAAP6, an amino acid transporter that is expressed in the xylem parenchyma, the phloem amino acid composition and concentration was altered, causing small changes in aphid feeding behavior. 46. Koch W, Ladwig F, Hirner A: A new class of membrane proteins  as potential amino acid exporters/facilitators. XIVth International workshop on Plant Membrane Biology; June 26–30, Valencia, Spain: 2007:P7-P8. This is a very interesting conference report describing the function of Arabidopsis SiAR1 (Siliques Are Red 1) as amino acid facilitators or exporter. In yeast, SiAR1 mediates efflux of glutamine and histidine, and import of aspartate and glutamate. Mutant analysis supports that SiAR1 might be important for organic nitrogen transport to developing seeds and plays a role in the radial transfer of amino acids in roots. This research has recently been submitted for publication (Koch, personal communication). 47. Pilot G, Stransky H, Bushey DF, Pratelli R, Ludewig U, Wingate VPM,  Frommer WB: Overexpression of glutamine dumper1 leads to hypersecretion of glutamine from hydathodes of Arabidopsis leaves. Plant Cell 2004, 16:1827-1840. Using activation-tagged glutamine dumper1 (gdu1) mutants, the authors provide evidence that GDU1 is affecting glutamine secretion from Arabidospis hydathodes as well as the amino acid content in the leaf apoplast and xylem sap, probably by influencing cellular export processes [48].

acids in Arabidopsis thaliana by functional reconstitution into liposomes and complementation in yeast. Plant J 2003, 33:1027-1035. 51. Palmieri L, Todd CD, Arrigoni R, Hoyos ME, Santoro A, Polacco JC, Palmieri F: Arabidopsis mitochondria have two basic amino acid transporters with partially overlapping specificities and differential expression in seedling development. Biochim Biophys Acta 2006, 1757:1277-1283. 52. Palmieri F, Pierri CL, De Grassi A, Nunes-Nesi A, Fernie AR: Evolution, structure and function of mitochondrial carriers: a review with new insights. Plant J 2011, 66:161-181. 53. Carter C, Pan SQ, Jan ZH, Avila EL, Girke T, Raikhel NV: The vegetative vacuole proteome of Arabidopsis thaliana reveals predicted and unexpected proteins. Plant Cell 2004, 16:3285-3303. 54. Su YH, Frommer WB, Ludewig U: Molecular and functional characterization of a family of amino acid transporters from Arabidopsis. Plant Physiol 2004, 136:3104-3113. 55. Endler A, Meyer S, Schelbert S, Schneider T, Weschke W, Peters SW, Keller F, Baginsky S, Martinoia E, Schmidt UG: Identification of a vacuolar sucrose transporter in barley and Arabidopsis mesophyll cells by a tonoplast proteomic approach. Plant Physiol 2006, 141:196-207. 56. Yang H, Bogner M, Stierhof YD, Ludewig U: H+-independent glutamine transport in plant root tips. PLoS ONE 2010, 5:e8917. 57. Pohlmeyer K, Soll J, Steinkamp T, Hinnah S, Wagner R: Isolation and characterization of an amino acid-selective channel protein present in the chloroplastic outer envelope membrane. Proc Natl Acad Sci USA 1997, 94:9504-9509. 58. Pohlmeyer K, Soll J, Grimm R, Hill K, Wagner R: A highconductance solute channel in the chloroplastic outer envelope from pea. Plant Cell 1998, 10:1207-1216. 59. Kleffmann T, Russenberger D, von Zychlinski A, Christopher W, Sjolander K, Gruissem W, Baginsky S: The Arabidopsis thaliana chloroplast proteome reveals pathway abundance and novel protein functions. Curr Biol 2004, 14:354-362. 60. Okumoto S, Koch W, Tegeder M, Fischer WN, Biehl A, Leister D, Stierhof YD, Frommer WB: Root phloem-specific expression of the plasma membrane amino acid proton co-transporter AAP3. J Exp Bot 2004, 55:2155-2168. 61. Renne´ P, Dressen U, Hebbeker U, Hille D, Flu¨gge UI, Westhoff P,  Weber APM: The Arabidopsis mutant dct is deficient in the plastidic glutamate/malate translocator DiT2. Plant J 2003, 35:316-331. This work describes the identification and functional characterization of DiT2, a plastidic Arabidopsis glutamate-malate exchanger, and reports that the photorespiratory Arabidopsis mutant dct is deficient in DiT2. It was further resolved that the related 2-oxoglutarate/malate translocator DiT1 counter-exchanges oxaloacetate for malate. 62. Wang R, Okamoto M, Xing X, Crawford NM: Microarray analysis of the nitrate response in Arabidopsis roots and shoots reveals over one thousand rapidly responding genes and new linkages to glucose, trehalose-6-P, iron and sulfate metabolism. Plant Physiol 2003, 132:556-567. 63. Scheible W, Morcuende R, Czechowski T, Fritz C, Osuna D, Palacios-Rojas N, Schindelasch D, Thimm O, Udvardi MK, Stitt M: Genome-wide reprogramming of primary and secondary metabolism, protein synthesis, cellular growth processes, and the regulatory infrastructure of Arabidopsis in response to nitrogen. Plant Physiol 2004, 136:2483-2499.

48. Pratelli R, Voll LM, Horst RJ, Frommer WB, Pilot G: Stimulation of nonselective amino acid export by glutamine dumper proteins. Plant Physiol 2010, 152:762-773.

64. Nikiforova VJ, Daub CO, Hesse H, Willmitzer L, Hoefgen R: Integrative gene-metabolite network with implemented causality deciphers informational fluxes of sulfur stress response. J Exp Bot 2005, 56:1887-1896.

49. Catoni E, Desimone M, Hilpert M, Wipf D, Kunze R, Schneider A, Flu¨gge UI, Schumacher K, Frommer WB: Expression pattern of a nuclear encoded mitochondrial arginine-ornithine translocator gene from Arabidopsis. BMC Plant Biol 2003, 3:1.

65. Fritz C, Palacios-Rojas N, Feil R, Stitt M: Regulation of secondary metabolism by the carbon-nitrogen status in tobacco: nitrate inhibits large sectors of phenylpropanoid metabolism. Plant J 2006, 46:533-548.

50. Hoyos ME, Palmieri L, Wertin T, Arrigoni R, Polacco JC, Palmieri F: Identification of a mitochondrial transporter for basic amino

66. Gutie´rrez RA, Lejay LV, Dean A, Chiaromonte F, Shasha DE, Coruzzi GM: Qualitative network models and genome-wide

Current Opinion in Plant Biology 2012, 15:315–321

www.sciencedirect.com

Amino acid transport in plants Tegeder 321

expression data define carbon/nitrogen-responsive molecular machines in Arabidopsis. Genome Biol 2007, 8:R7.

between diverse phenotypes in Arabidopsis. Plant Physiol 2008, 146:1482-1500.

67. Vidal EA, Gutie´rrez RA: A systems view of nitrogen nutrient and metabolite responses in Arabidopsis. Curr Opin Plant Biol 2008, 11:521-529.

76. Gu L, Jones AD, Last RL: Broad connections in the Arabidopsis seed metabolic network revealed by metabolite profiling of an amino acid catabolism mutant. Plant J 2010, 61:579-590.

68. Nunes-Nesi A, Fernie AR, Stitt M: Metabolic and signaling aspects underpinning the regulation of plant carbon nitrogen interactions. Mol Plant 2010, 3:973-996.

77. Forde BG, Lea PJ: Glutamate in plants: metabolism, regulation, and signaling. J Exp Bot 2007, 58:2339-2358.

69. Dennison KL, Spalding EP: Glutamate gated calcium fluxes in Arabidopsis. Plant Physiol 2000, 124:1511-1514.

78. Didion T, Regenberg B, Jorgensen MU, Kielland-Brandt MC, Andersen HA: The permease homologue Ssy1p controls the expression of amino acid and peptide transporter genes in Saccharomyces cerevisiae. Mol Microbiol 1998, 27:643-650.

70. Dubos C, Huggins D, Grant GH, Knight M-R, Campbell MM: A role for glycine in the gating of plant NMDA-like receptors. Plant J 2003, 35:800-810. 71. Gutie´rrez RA, Stokes TL, Thum K, Xu X, Obertello M, Katari MS, Tanurdzic M, Dean A, Nero DC, McClung CR, Coruzzi GM: Systems approach identifies an organic nitrogen-responsive gene network that is regulated by the master clock control gene CCA1. Proc Natl Acad Sci USA 2008, 105:4939-4944. 72. Joshi V, Joung JG, Fei Z, Jander G: Interdependence of threonine, methionine and isoleucine metabolism in plants: accumulation and transcriptional regulation under abiotic stress. Amino Acids 2010, 39:933-947. 73. Westfall CS, Herrmann J, Chen Q, Wang S, Jez JM: Modulating plant hormones by enzyme action: the GH3 family of acyl acid amido synthetases. Plant Signal Behav 2010, 5:1597-1602. 74. Kliebenstein D, D’Auria J, Behere A, Kim J, Gunderson K, Breen J, Lee G, Gershenzon J, Last R, Jander G: Characterization of seed-specific benzoyloxyglucosinolate mutations in Arabidopsis thaliana. Plant J 2007, 51:1062-1076. 75. Lu Y, Savage LJ, Ajjawi I, Imre KM, Yoder DW, Benning C, DellaPenna D, Ohlrogge JB, Osteryoung KW, Weber AP et al.: New connections across pathways and cellular processes: industrialized mutant screening reveals novel associations

www.sciencedirect.com

79. Iiboshi Y, Papst PJ, Kawasome H, Hosoi H, Abraham RT, Houghton PJ, Terada N: Amino acid-dependent control of p70(s6k) Involvement of tRNA aminoacylation in the regulation. J Biol Chem 1999, 274:1092-1099. 80. Donaton MC, Holsbeeks I, Lagatie O, Van Zeebroeck G, Crauwels M, Winderickx J, Thevelein JM: The Gap1 general amino acid permease acts as an amino acid sensor for activation of protein kinase A targets in the yeast Saccharomyces cerevisiae. Mol Microbiol 2003, 50:911-929. 81. Wek RC, Jiang HY, Anthony TG: Coping with stress: eIF2 kinases and translational control. Biochem Soc Trans 2006, 34:7-11. 82. Lalonde S, Sero A, Pratelli R, Pilot G, Chen J, Sardi MI, Parsa SA,  Kim D-Y, Acharya BR, Stein EV et al.: A membrane protein/ signaling protein interaction network for Arabidopsis version AMPv2. Front Plant Physiol 2010, 1:24 doi: 10.3389/ fphys201000024. The authors describe a large-scale, robotic approach to identify potential protein-protein interactions in Arabidopsis using the open-reading frames of 3852 membrane and signaling proteins and the split ubiquitin system. They found around 350 interactions among 179 proteins, including 134 membrane proteins. 80 of these interactions involved receptor-like kinases suggesting that the interactors may represent their substrate.

Current Opinion in Plant Biology 2012, 15:315–321