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Amino-acid cycling drives nitrogen fixation in the legume–Rhizobium symbiosis


The biological reduction of atmospheric N2 to ammonium (nitrogen fixation) provides about 65% of the biosphere's available nitrogen. Most of this ammonium is contributed by legume–rhizobia symbioses1, which are initiated by the infection of legume hosts by bacteria (rhizobia), resulting in formation of root nodules. Within the nodules, rhizobia are found as bacteroids, which perform the nitrogen fixation: to do this, they obtain sources of carbon and energy from the plant, in the form of dicarboxylic acids2,3. It has been thought that, in return, bacteroids simply provide the plant with ammonium. But here we show that a more complex amino-acid cycle is essential for symbiotic nitrogen fixation by Rhizobium in pea nodules. The plant provides amino acids to the bacteroids, enabling them to shut down their ammonium assimilation. In return, bacteroids act like plant organelles to cycle amino acids back to the plant for asparagine synthesis. The mutual dependence of this exchange prevents the symbiosis being dominated by the plant, and provides a selective pressure for the evolution of mutualism.

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Figure 1: Effect of mutation of amino-acid uptake in R. leguminosarum on growth and nodulation of peas.
Figure 2: Growth and nodulation of peas infected by either wild type or a mutant unable to transport amino acids.
Figure 3: Nitrogen fixation and assimilation of peas infected by either wild type or a mutant unable to transport amino acids.
Figure 4: The role of amino-acid cycling in nitrogen fixation in pea nodules.


  1. Newton, W. E. in Nitrogen Fixation: From Molecules to Crop Productivity (eds Pedrosa, F. O., Hungria, M., Yates, M. G. & Newton, W. E.) 3–8 (Kluwer, Dordrecht, 2000)

    Google Scholar 

  2. Vance, C. P. in Prokaryotic Nitrogen Fixation (ed. Triplett, E.) 589–607 (Horizon Scientific, Wymondham, UK, 2000)

    Google Scholar 

  3. Poole, P. S. & Allaway, D. Carbon and nitrogen metabolism in Rhizobium. Adv. Microb. Physiol. 43, 117–163 (2000)

    CAS  Article  Google Scholar 

  4. Patriarca, E. J., Tate, R. & Iaccarino, M. Key role of bacterial NH4+ metabolism in Rhizobium-plant symbiosis. Microb. Mol. Biol. Rev. 66, 203–222 (2002)

    CAS  Article  Google Scholar 

  5. Kahn, M. L., Kraus, J. & Sommerville, J. E. in Nitrogen Fixation Research Progress (eds Evans, H. J., Bottomley, P. J. & Newton, W. E.) 193–199 (Martinus Nijhoff, Dordrecht, 1985)

    Book  Google Scholar 

  6. Rosendahl, L., Dilworth, M. J. & Glenn, A. R. Exchange of metabolites across the peribacteroid membrane in pea root nodules. J. Plant Physiol. 139, 635–638 (1992)

    CAS  Article  Google Scholar 

  7. Appels, M. A. & Haaker, H. Glutamate oxaloacetate transaminase in pea root nodules—participation in a malate/aspartate shuttle between plant and bacteroid. Plant Physiol. 95, 740–747 (1991)

    CAS  Article  Google Scholar 

  8. Hosie, A. H. F., Allaway, D., Dunsby, H. A., Galloway, C. S. & Poole, P. S. Rhizobium leguminosarum has a second general amino acid permease with unusually broad substrate specificity and high similarity to branched-chain amino acid transporters (Bra/LIV) of the ABC family. J. Bacteriol. 184, 4071–4080 (2002)

    CAS  Article  Google Scholar 

  9. Hosie, A. H. F. et al. Solute-binding protein-dependent ABC transporters are responsible for solute efflux in addition to solute uptake. Mol. Microbiol. 40, 1449–1459 (2001)

    CAS  Article  Google Scholar 

  10. Walshaw, D. L. & Poole, P. S. The general L-amino acid permease of Rhizobium leguminosarum is an ABC uptake system that influences efflux of solutes. Mol. Microbiol. 21, 1239–1252 (1996)

    CAS  Article  Google Scholar 

  11. Ronson, C. W., Lyttleton, P. & Robertson, J. G. C4-dicarboxylate transport mutants of Rhizobium trifolii form ineffective nodules on Trifolium repens. Proc. Natl Acad. Sci. USA 78, 4284–4288 (1981)

    ADS  CAS  Article  Google Scholar 

  12. Finan, T. M., Wood, J. M. & Jordan, D. C. Symbiotic properties of C4-dicarboxylic acid transport mutants of Rhizobium leguminosarum. J. Bacteriol. 154, 1403–1413 (1983)

    CAS  PubMed  PubMed Central  Google Scholar 

  13. Watson, R. J. & Rastogi, V. K. Cloning and nucleotide sequencing of Rhizobium meliloti aminotransferase genes: an aspartate aminotransferase required for symbiotic nitrogen fixation is atypical. J. Bacteriol. 175, 1919–1928 (1993)

    CAS  Article  Google Scholar 

  14. Craig, J. et al. Mutations at the rug4 locus alter the carbon and nitrogen metabolism of pea plants through an effect on sucrose synthase. Plant J. 17, 353–362 (1999)

    CAS  Article  Google Scholar 

  15. Saalbach, G., Erik, P. & Wienkoop, S. Characterisation by proteomics of peribacteroid space and peribacteroid membrane preparations from pea (Pisum sativum) symbiosomes. Proteomics 2, 325–337 (2002)

    CAS  Article  Google Scholar 

  16. Rosendahl, L., Mouritzen, P. & Rudbeck, A. Nitrogen transfer in the interface between the symbionts in pea root nodules. Plant Soil 230, 31–37 (2001)

    CAS  Article  Google Scholar 

  17. Mouritzen, P. & Rosendahl, L. Identification of a transport mechanism for NH4+ in the symbiosome membrane of pea root nodules. Plant Physiol. 115, 519–526 (1997)

    CAS  Article  Google Scholar 

  18. Udvardi, M. K., Price, G. D., Gresshoff, P. M. & Day, D. A. A dicarboxylate transporter on the peribacteroid membrane of soybean nodules. FEBS Lett. 231, 36–40 (1988)

    CAS  Article  Google Scholar 

  19. Udvardi, M. K., Salom, C. L. & Day, D. A. Transport of L-glutamate across the bacteroid membrane but not the peribacteroid membrane from soybean root nodules. Mol. Plant-Microbe Interact. 1, 250–254 (1988)

    Article  Google Scholar 

  20. Indiveri, C., Krämer, R. & Palmieri, F. Reconstitution of the malate/aspartate shuttle from mitochondria. J. Biol. Chem. 262, 15979–15983 (1987)

    CAS  PubMed  Google Scholar 

  21. Allaway, D. et al. Identification of alanine dehydrogenase and its role in mixed secretion of ammonium and alanine by pea bacteroids. Mol. Microbiol. 36, 508–515 (2000)

    CAS  Article  Google Scholar 

  22. Beringer, J. E., Johnston, A. W. B. & Wells, B. The isolation of conditional ineffective mutants of Rhizobium leguminosarum. J. Gen. Microbiol. 98, 339–343 (1977)

    Article  Google Scholar 

  23. Parsons, R. & Baker, A. Cycling of amino compounds in symbiotic lupin. J. Exp. Bot. 47, 421–429 (1996)

    CAS  Article  Google Scholar 

  24. Prell, J., Boesten, B., Poole, P. & Priefer, U. B. The Rhizobium leguminosarum bv. viciae VF39 gamma aminobutyrate (GABA) aminotransferase gene (gabT) is induced by GABA and highly expressed in bacteroids. Microbiology 148, 615–623 (2002)

    CAS  Article  Google Scholar 

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We thank the BBSRC for supporting this research. We thank A. Hepburn and M. Heaps for GC-MS and nitrogen analysis and A. East for manuscript preparation.

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Correspondence to P. S. Poole.

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Lodwig, E., Hosie, A., Bourdès, A. et al. Amino-acid cycling drives nitrogen fixation in the legume–Rhizobium symbiosis. Nature 422, 722–726 (2003).

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