51
|
Ohkama-Ohtsu N, Radwan S, Peterson A, Zhao P, Badr AF, Xiang C, Oliver DJ. Characterization of the extracellular gamma-glutamyl transpeptidases, GGT1 and GGT2, in Arabidopsis. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2007; 49:865-77. [PMID: 17316175 DOI: 10.1111/j.1365-313x.2006.03004.x] [Citation(s) in RCA: 85] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/14/2023]
Abstract
gamma-Glutamyl transpeptidase (GGT) is the only enzyme known that can cleave the gamma-peptide bond between glutamate and cysteine in glutathione, and is therefore a key step in glutathione degradation. There are three functional GGT genes in Arabidopsis, two of which are considered here. GGT1 and GGT2 are apoplastic, associated with the plasma membrane and/or cell wall. RNA blots and analysis of enzyme activity in knockout mutants suggest that GGT1 is expressed most strongly in leaves but is found throughout the plant. A GGT1::GUS fusion construct showed expression only in vascular tissue, specifically the phloem of the mid-rib and minor veins of leaves, roots and flowers. This localization was confirmed in leaves by laser microdissection. GGT2 expression is limited to embryo, endosperm, outer integument, and a small portion of the funiculus in developing siliques. The ggt2 mutants had no detectable phenotype, while the ggt1 knockouts were smaller and flowered sooner than wild-type. In ggt1 plants, the cotyledons and older leaves yellowed early, and GSSG, the oxidized form of glutathione, accumulated in the apoplastic space. These observations suggest that GGT1 is important in preventing oxidative stress by metabolizing extracellular GSSG, while GGT2 might be important in transporting glutathione into developing seeds.
Collapse
Affiliation(s)
- Naoko Ohkama-Ohtsu
- Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA 50011, USA
| | | | | | | | | | | | | |
Collapse
|
52
|
|
53
|
Mansouri-Bauly H, Kruse J, Sýkorová Z, Scheerer U, Kopriva S. Sulfur uptake in the ectomycorrhizal fungus Laccaria bicolor S238N. MYCORRHIZA 2006; 16:421-427. [PMID: 16596384 DOI: 10.1007/s00572-006-0052-y] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/27/2005] [Accepted: 03/06/2006] [Indexed: 05/08/2023]
Abstract
The importance of the ectomycorrhiza symbiosis for plant acquisition of phosphorus and nitrogen is well established whereas its contribution to sulfur nutrition is only marginally understood. In a first step to investigate the role of ectomycorrhiza in plant sulfur nutrition, we characterized sulfate and glutathione uptake in Laccaria bicolor. By studying the regulation of sulfate uptake in this ectomycorrhizal fungus, we found that in contrast to bacteria, yeast, and plants, sulfate uptake in L. bicolor was not feedback-inhibited by glutathione. On the other hand, sulfate uptake was increased by sulfur starvation as in other organisms. The activity of 3'-phosphoadenosine 5'-phosphosulfate reductase, the key enzyme of the assimilatory sulfate reduction pathway in fungi, was increased by sulfur starvation and decreased after treatment with glutathione revealing an uncoupling of sulfate uptake and reduction in the presence of reduced sulfur compounds. These results support the hypothesis that L. bicolor increases sulfate supply to the plant by extended sulfate uptake and the plant provides the ectomycorrhizal fungus with reduced sulfur.
Collapse
Affiliation(s)
- Hounayda Mansouri-Bauly
- Albert-Ludwigs-University of Freiburg, Institute of Forest Botany and Tree Physiology, Georges-Köhler-Allee 053, 79110, Freiburg, Germany
| | - Jörg Kruse
- Albert-Ludwigs-University of Freiburg, Institute of Forest Botany and Tree Physiology, Georges-Köhler-Allee 053, 79110, Freiburg, Germany
- The School of Forest and Ecosystem Science, University of Melbourne, Melbourne, Australia
| | - Zuzana Sýkorová
- Albert-Ludwigs-University of Freiburg, Institute of Forest Botany and Tree Physiology, Georges-Köhler-Allee 053, 79110, Freiburg, Germany
- Institute of Botany, University of Basel, Basel, Switzerland
| | - Ursula Scheerer
- Albert-Ludwigs-University of Freiburg, Institute of Forest Botany and Tree Physiology, Georges-Köhler-Allee 053, 79110, Freiburg, Germany
| | - Stanislav Kopriva
- Albert-Ludwigs-University of Freiburg, Institute of Forest Botany and Tree Physiology, Georges-Köhler-Allee 053, 79110, Freiburg, Germany.
- John Innes Institute, Norwich, UK.
| |
Collapse
|
54
|
WATERWORTH WANDAM, BRAY CLIFFORDM. Enigma variations for peptides and their transporters in higher plants. ANNALS OF BOTANY 2006; 98:1-8. [PMID: 16735405 PMCID: PMC2803549 DOI: 10.1093/aob/mcl099] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
BACKGROUND Two families of proteins that transport small peptides, the oligopeptide transporters (OPTs) and the peptide transporters (PTRs), have been recognized in eukaryotes. Higher plants contain a far greater number of genes for these transporters than do other eukaryotes. This may be indicative of the relative importance of (oligo)peptides and their transport to plant growth and metabolism. RECENT PROGRESS Recent studies are now allowing us to assign functions to these transporters and are starting to identify their in-planta substrates, revealing unexpected and important contributions of the transporters to plant growth and developmental processes. This Botanical Briefing appraises recent findings that PTRs and OPTs have key roles to play in the control of plant cell growth and development. Evidence is presented that some of these transporters have functions outside that of nitrogen nutrition and that these carriers can also surprise us with their totally unexpected choice of substrates.
Collapse
|
55
|
Schmidt S, Handley LL, Sangtiean T. Effects of nitrogen source and ectomycorrhizal association on growth and δ 15N of two subtropical Eucalyptus species from contrasting ecosystems. FUNCTIONAL PLANT BIOLOGY : FPB 2006; 33:367-379. [PMID: 32689243 DOI: 10.1071/fp05260] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2005] [Accepted: 12/22/2005] [Indexed: 06/11/2023]
Abstract
Ectomycorrhizal (EM) associations facilitate plant nitrogen (N) acquisition, but the contribution of EM associations to tree N nutrition is difficult to ascertain in ecosystems. We studied the abilities of subtropical EM fungi and nutritionally contrasting Eucalyptus species, Eucalyptus grandis W.Hill ex Maiden and Eucalyptus racemosa Cav, to use N sources in axenic and soil cultures, and determined the effect of EM fungi on plant N use and plant 15N natural abundance (δ15N). As measured by seedling growth, both species showed little dependence on EM when growing in the N-rich minerotrophic soil from E. grandis rainforest habitat or in axenic culture with inorganic N sources. Both species were heavily dependent on EM associations when growing in the N-poor, organotrophic soil from the E. racemosa wallum habitat or in axenic culture with organic N sources. In axenic culture, EM associations enabled both species to use organic N when supplied with amide-, peptide- or protein-N. Grown axenically with glutamine- or protein-N, δ15N of almost all seedlings was lower than source N. The δ15N of all studied organisms was higher than the N source when grown on glutathione. This unexpected 15N enrichment was perhaps due to preferential uptake of an N moiety more 15N-enriched than the bulk molecular average. Grown with ammonium-N, the δ15N of non-EM seedlings was mostly higher than that of source N. In contrast, the δ15N of EM seedlings was mostly lower than that of source N, except at the lowest ammonium concentration. Discrimination against 15N was strongest when external ammonium concentration was high. We suggest that ammonium assimilation via EM fungi may be the cause of the often observed distinct foliar δ15N of EM and non-EM species, rather than use of different N sources by species with different root specialisations. In support of this notion, δ15N of soil and leaves in the rainforest were similar for E. grandis and co-occurring non-mycorrhizal Proteaceae. In contrast, in wallum forest, E. racemosa leaves and roots were strongly 15N-depleted relative to wallum soil and Proteaceae leaves. We conclude that foliar δ15N may be used in conjunction with other ecosystem information as a rapid indicator of plant dependency on EM associations for N acquisition.
Collapse
Affiliation(s)
- Susanne Schmidt
- School of Integrative Biology, University of Queensland, Brisbane, Qld 4072, Australia
| | - Linda L Handley
- School of Integrative Biology, University of Queensland, Brisbane, Qld 4072, Australia
| | | |
Collapse
|
56
|
Wiles AM, Naider F, Becker JM. Transmembrane domain prediction and consensus sequence identification of the oligopeptide transport family. Res Microbiol 2006; 157:395-406. [PMID: 16364604 DOI: 10.1016/j.resmic.2005.10.004] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2005] [Revised: 10/06/2005] [Accepted: 10/07/2005] [Indexed: 11/25/2022]
Abstract
Few polytopic membrane proteins have had their topology determined experimentally. Often, researchers turn to an algorithm to predict where the transmembrane domains might lie. Here we use a consensus method, using six different transmembrane domain prediction algorithms on six members of the oligopeptide transport family, all of which have been experimentally characterized. PSI-BLAST results indicate that the six chosen oligopeptide transport family members are distributed throughout most branches of the phylogram, suggesting that these members represent a broad view of the oligopeptide transport family. We combined the prediction algorithms with a multiple sequence alignment, and consensus transmembrane domains were assigned not only based on algorithmic output, but also based on conserved familial motifs found by analysis of the PSI-BLAST results. The consensus method combined with the "charge-difference rule" yields a model topology for the family containing 12 transmembrane domains with the N- and C-termini facing extracellular.
Collapse
Affiliation(s)
- Amy M Wiles
- Department of Microbiology, University of Tennessee, Knoxville, TN 37996, USA
| | | | | |
Collapse
|
57
|
Osawa H, Stacey G, Gassmann W. ScOPT1 and AtOPT4 function as proton-coupled oligopeptide transporters with broad but distinct substrate specificities. Biochem J 2006; 393:267-75. [PMID: 16149917 PMCID: PMC1383685 DOI: 10.1042/bj20050920] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
A group of OPTs (oligopeptide transporters) exclusively identified in plants and fungi are proposed to transport oligopeptides and derivatives of three to six amino acids in length, but their transport mechanisms and biological functions are poorly understood. We expressed the Saccharomyces cerevisiae (yeast) OPT ScOPT1 and five Arabidopsis thaliana AtOPTs in Xenopus laevis oocytes for two-electrode voltage-clamp studies. ScOPT1 produced inward currents in response to GSH or GSSG, the phytochelatin (PC) PC2 and oligopeptides including the tetrapeptide GGFL, but not KLGL. Inward currents were dependent on the external proton and substrate concentrations, with high affinity for both. This and the inward currents evoked by substrates with net negative charges showed that ScOPT1 is a proton-coupled transporter. ScOPT1 displayed highest apparent affinity for PC2, with small differences in the maximal current among substrates. Glutathione transport by any of the tested AtOPTs, including AtOPT6, was not detected in yeast growth complementation assays. With AtOPT4, initially only small KLGL-dependent currents were recorded in batches of oocytes showing high ScOPT1 expression. AtOPT4 expression was optimized by swapping the 5'-untranslated region with that of ScOPT1. AtOPT4 displayed a higher affinity for KLGL than ScOPT1 did for any peptide. AtOPT4-mediated KLGL transport was detectable at pH 5.0, but not at pH 6.0 or 7.0. Taken together, our results demonstrate that ScOPT1 and AtOPT4 are proton-coupled OPTs with broad but distinct substrate specificities and affinities.
Collapse
Affiliation(s)
- Hiroki Osawa
- *Division of Plant Sciences, Life Sciences Center, University of Missouri, Columbia, MO 65211, U.S.A
| | - Gary Stacey
- *Division of Plant Sciences, Life Sciences Center, University of Missouri, Columbia, MO 65211, U.S.A
- †Division of Biochemistry, Life Sciences Center, University of Missouri, Columbia, MO 65211, U.S.A
| | - Walter Gassmann
- *Division of Plant Sciences, Life Sciences Center, University of Missouri, Columbia, MO 65211, U.S.A
- To whom correspondence should be addressed (email )
| |
Collapse
|
58
|
Abstract
Resolution and analysis of genes encoding components of the pathways of primary sulphur assimilation have provided the potential to elucidate how sulphur is managed by plants. Individual roles for members of gene families and regulatory mechanisms operating at gene, cellular and whole plant levels have been recognized. Sulphur is taken up and transported around the plant principally as sulphate, catalysed for the most part by a single gene family of highly regulated transporters. Additional regulation occurs in the pathway of reduction of sulphate to sulphide and its incorporation into cysteine, which occurs principally within the plastid. Cellular and whole-plant regulation of uptake, and the assimilatory pathway attempt to balance supply with demand for growth and include mechanisms for re-mobilization and redistribution of sulphur. Furthermore, optimization of sulphur assimilation requires coordination with carbon and nitrogen pathways, and multiple processes have been proposed to contribute to this balance. Present studies on cis and trans elements are focusing on transcriptional regulation, but this regulation still needs to be linked to apparent metabolite sensing. Whilst the components of the assimilatory pathways have been resolved after many years of controversy, uncertainties remain concerning roles of individual genes in gene families, their sub-cellular localization and their significance in balancing sulphur flux to sulphur demand of the plant for growth under variable environmental conditions.
Collapse
Affiliation(s)
- Malcolm J Hawkesford
- Crop Performance and Improvement Division, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.
| | | |
Collapse
|
59
|
Stacey MG, Osawa H, Patel A, Gassmann W, Stacey G. Expression analyses of Arabidopsis oligopeptide transporters during seed germination, vegetative growth and reproduction. PLANTA 2006; 223:291-305. [PMID: 16151844 DOI: 10.1007/s00425-005-0087-x] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/13/2005] [Accepted: 07/14/2005] [Indexed: 05/04/2023]
Abstract
AtOPT promoter-GUS fusions were constructed for six of the nine known, putative oligopeptide transporters (OPTs) in Arabidopsis thaliana and used to examine AtOPT expression at various stages of plant development. AtOPT1, AtOPT3, AtOPT4, AtOPT6 and AtOPT7 were expressed in the embryonic cotyledons prior to root radicle emergence. Except for AtOPT8, which gave weak expression, all AtOPTs were strongly expressed in post-germinative seedlings with strongest expression in vascular tissues of cotyledons and hypocotyls. Preferential expression of AtOPTs in vascular tissues was also observed in cotyledons, leaves, hypocotyls, roots, flowers, siliques, and seed funiculi of seedlings and adult plants. Differential tissue-specific expression was observed for specific AtOPTs. For example, AtOPT1, AtOPT3 and AtOPT8 were uniquely expressed in pollen. Only AtOPT1 was expressed in growing pollen tubes, while only AtOPT6 was observed in ovules. AtOPT8 was transiently expressed in seeds during early stages of embryogenesis. Iron limitation was found to enhance expression of AtOPT3. These data suggest distinct cellular roles for specific AtOPTs including nitrogen mobilization during germination and senescence, pollen tube growth, pollen and ovule development, seed formation and metal transport.
Collapse
Affiliation(s)
- Minviluz G Stacey
- Division of Plant Sciences, The University of Missouri, Columbia, MO 65211, USA
| | | | | | | | | |
Collapse
|
60
|
Lubkowitz M. The OPT family functions in long-distance peptide and metal transport in plants. GENETIC ENGINEERING 2006; 27:35-55. [PMID: 16382870 DOI: 10.1007/0-387-25856-6_3] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
The OPT family was first described six years ago, and much progress has been made in understanding the role these transporters play in their respective organisms. Plants are the only organisms in which both YS- and PT-type transporters have been characterized, and all of these OPTs appear to be plasma membrane-bound proteins, suggesting that they import substrates from the apoplasm or the environment. YS1 is the only OPT known to translocate substrates from the rhizosphere, whereas all the other OPTs seem to function in long-distance transport of peptides or metals. The sum of all the studies covered in this review suggest the model for OPT function in plants depicted in Figure 4. Peptides, metal-NA, and metal-MAs complexes (Strategy II plants only) are loaded into the xylem stream in the root for long-distance transport. OPTs unload the xylem by importing substrates into sink tissues such as leaves and by transloading the phloem. Peptides and metal-NA complexes exit the leaf symplasmically or by importation into the phloem from the apoplasm by OPTs. The filial tissues (endosperm and embryo) are apoplasmically separated from the maternal tissues, and OPTs may also function in loading the developing seed. Similarly, seedlings are symplasmically disconnected from the endosperm and OPTs may help move nutrients to the growing plant. Much progress has been made in the last two years toward understanding OPTs in plants, although several fundamental questions remain unanswered. Namely, what is the level of redundancy? Is there any substrate overlap between YS and PT OPTs? How crucial are their respective roles? Are there additional functions beyond peptide and metal transport? Given the recent pace of discovery, we may not have to wait long to find out the answers.
Collapse
Affiliation(s)
- Mark Lubkowitz
- Biology Department, Saint Michael's College, Colchester, Vermont 05477, USA
| |
Collapse
|
61
|
Wachter A, Wolf S, Steininger H, Bogs J, Rausch T. Differential targeting of GSH1 and GSH2 is achieved by multiple transcription initiation: implications for the compartmentation of glutathione biosynthesis in the Brassicaceae. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2005; 41:15-30. [PMID: 15610346 DOI: 10.1111/j.1365-313x.2004.02269.x] [Citation(s) in RCA: 152] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
The genome of Arabidopsis thaliana reveals that in this species the enzymes of glutathione biosynthesis, GSH1 and GSH2, are encoded by single genes. In silico analysis predicts proteins with putative plastidic transit peptides (TP) for both genes, but this has not been experimentally verified. Here we report a detailed analysis of the 5'ends of GSH1 and GSH2 mRNAs and demonstrate the subcellular targeting of the proteins encoded by different transcript types. GSH1 transcript analysis revealed two mRNA populations with short and long 5'-UTRs, respectively, both including the entire TP sequence. The ratio of long/total GSH1 transcripts was subject to developmental regulation. Transient transformation experiments with reporter gene fusions, bearing long or short 5'-UTRs, indicated an exclusive targeting of GSH1 to the plastids. Corroborating these results, endogenous and ectopically expressed GSH1 proteins were always present as a single polypeptide species with the size expected for correctly processed GSH1. Finally, the plastidic GSH1 localization was confirmed by immunocytochemistry. Similar to GSH1, multiple transcript populations were found for GSH2. However, here the prevalent shorter transcripts lacked a complete TP sequence. As expected, the large (but less abundant) transcript encoded a plastidic GSH2 protein, whereas GSH2 synthesized from the shorter transcript was targeted to the cytosol. The implications of the results for the compartmentation and regulation of GSH synthesis are discussed.
Collapse
Affiliation(s)
- Andreas Wachter
- Heidelberg Institute of Plant Sciences (HIP), INF 360, D-69120-Heidelberg, Germany
| | | | | | | | | |
Collapse
|
62
|
Dietrich D, Hammes U, Thor K, Suter-Grotemeyer M, Flückiger R, Slusarenko AJ, Ward JM, Rentsch D. AtPTR1, a plasma membrane peptide transporter expressed during seed germination and in vascular tissue of Arabidopsis. THE PLANT JOURNAL : FOR CELL AND MOLECULAR BIOLOGY 2004; 40:488-99. [PMID: 15500465 DOI: 10.1111/j.1365-313x.2004.02224.x] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
For the efficient translocation of organic nitrogen, small peptides of two to three amino acids are posited as an important alternative to amino acids. A new transporter mediating the uptake of di- and tripeptides was isolated from Arabidopsis thaliana by heterologous complementation of a peptide transport-deficient Saccharomyces cerevisiae mutant. AtPTR1 mediated growth of S. cerevisiae cells on different di- and tripeptides and caused sensitivity to the phytotoxin phaseolotoxin. The spectrum of substrates recognized by AtPTR1 was determined in Xenopus laevis oocytes injected with AtPTR1 cRNA under voltage clamp conditions. AtPTR1 not only recognized a broad spectrum of di- and tripeptides, but also substrates lacking a peptide bond. However, amino acids, omega-amino fatty acids or peptides with more than three amino acid residues did not interact with AtPTR1. At pH 5.5 AtPTR1 had an apparent lower affinity (K(0.5) = 416 microm) for Ala-Asp compared with Ala-Ala (K(0.5) = 54 microm) and Ala-Lys (K(0.5) = 112 microm). Transient expression of AtPTR1/GFP fusion proteins in tobacco protoplasts showed that AtPTR1 is localized at the plasma membrane. In addition, transgenic plants expressing the beta-glucuronidase (uidA) gene under control of the AtPTR1 promoter demonstrated expression in the vascular tissue throughout the plant, indicative of a role in long-distance transport of di- and tripeptides.
Collapse
Affiliation(s)
- Daniela Dietrich
- Molecular Plant Physiology, Institute of Plant Sciences, University of Berne, 3013 Berne, Switzerland
| | | | | | | | | | | | | | | |
Collapse
|
63
|
Saito K. Sulfur assimilatory metabolism. The long and smelling road. PLANT PHYSIOLOGY 2004; 136:2443-50. [PMID: 15375200 PMCID: PMC523311 DOI: 10.1104/pp.104.046755] [Citation(s) in RCA: 209] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/19/2004] [Revised: 06/22/2004] [Accepted: 06/23/2004] [Indexed: 05/18/2023]
Affiliation(s)
- Kazuki Saito
- Graduate School of Pharmaceutical Sciences, Chiba University, Inage-ku, Chiba 263-8522, Japan.
| |
Collapse
|
64
|
Cagnac O, Bourbouloux A, Chakrabarty D, Zhang MY, Delrot S. AtOPT6 transports glutathione derivatives and is induced by primisulfuron. PLANT PHYSIOLOGY 2004; 135:1378-87. [PMID: 15247401 PMCID: PMC519055 DOI: 10.1104/pp.104.039859] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/28/2004] [Revised: 04/08/2004] [Accepted: 04/13/2004] [Indexed: 05/18/2023]
Abstract
The oligopeptide transporter (OPT) family contains nine members in Arabidopsis. While there is some evidence that AtOPTs mediate the uptake of tetra- and pentapeptides, OPT homologs in rice (Oryza sativa; OsGT1) and Indian mustard (Brassica juncea; BjGT1) have been described as transporters of glutathione derivatives. This study investigates the possibility that two members of the AtOPT family, AtOPT6 and AtOPT7, may also transport glutathione and its conjugates. Complementation of the hgt1met1 yeast double mutant by plant homologs of the yeast glutathione transporter HGT1 (AtOPT6, AtOPT7, OsGT1, BjGT1) did not restore the growth phenotype, unlike complementation by HGT1. By contrast, complementation by AtOPT6 restored growth of the hgt1 yeast mutant on a medium containing reduced (GSH) or oxidized glutathione as the sole sulfur source and induced uptake of [3H]GSH, whereas complementation by AtOPT7 did not. In these conditions, AtOPT6-dependent GSH uptake in yeast was mediated by a high affinity (Km = 400 microm) and a low affinity (Km = 5 mm) phase. It was strongly competed for by an excess oxidized glutathione and glutathione-N-ethylmaleimide conjugate. Growth assays of yeasts in the presence of cadmium (Cd) suggested that AtOPT6 may transport Cd and Cd/GSH conjugate. Reporter gene experiments showed that AtOPT6 is mainly expressed in dividing areas of the plant (cambium, areas of lateral root initiation). RNA blots on cell suspensions and real-time reverse transcription-PCR on Arabidopsis plants indicated that AtOPT6 expression is strongly induced by primisulfuron and, to a lesser extent, by abscisic acid but not by Cd. Altogether, the data show that the substrate specificity and the physiological functions of AtOPT members may be diverse. In addition to peptide transport, AtOPT6 is able to transport glutathione derivatives and metal complexes, and may be involved in stress resistance.
Collapse
Affiliation(s)
- Olivier Cagnac
- Unité Mixte de Recherches, Centre National de la Recherche Scientifique 6161, Transport des Assimilats, Laboratoire de Physiologie, Biochimie et Biologie Moléculaire Végétales, Bâtiment Botanique, Poitiers Cédex, France
| | | | | | | | | |
Collapse
|