The model legume Medicago truncatula A17 is poorly matched for N2 fixation with the sequenced microsymbiont Sinorhizobium meliloti 1021

Authors

  • Jason J. Terpolilli,

    1. Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology, Murdoch University, South St, Murdoch, WA 6150, Australia;
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  • Graham W. O’Hara,

    1. Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology, Murdoch University, South St, Murdoch, WA 6150, Australia;
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  • Ravi P. Tiwari,

    1. Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology, Murdoch University, South St, Murdoch, WA 6150, Australia;
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  • Michael J. Dilworth,

    1. Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology, Murdoch University, South St, Murdoch, WA 6150, Australia;
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  • John G. Howieson

    1. Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology, Murdoch University, South St, Murdoch, WA 6150, Australia;
    2. Department of Agriculture and Food Western Australia, Baron-Hay Court, South Perth, WA 6015, Australia
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Author for correspondence: J. Terpolilli Tel:+61 8 9360 2372 Fax:+61 8 9360 6303 Email: j.terpolilli@murdoch.edu.au

Summary

  • • Medicago truncatula (barrel medic) A17 is currently being sequenced as a model legume, complementing the sequenced root nodule bacterial strain Sinorhizobium meliloti 1021 (Sm1021). In this study, the effectiveness of the Sm1021–M. truncatula symbiosis at fixing N2 was evaluated.
  • • N2 fixation effectiveness was examined with eight Medicago species and three accessions of M. truncatula with Sm1021 and two other Sinorhizobium strains. Plant shoot dry weights, plant nitrogen content and nodule distribution, morphology and number were analysed.
  • • Compared with nitrogen-fed controls, Sm1021 was ineffective or partially effective on all hosts tested (excluding M. sativa), as measured by reduced dry weights and shoot N content. Against an effective strain, Sm1021 on M. truncatula accessions produced more nodules, which were small, pale, more widely distributed on the root system and with fewer infected cells.
  • • The Sm1021–M. truncatula symbiosis is poorly matched for N2 fixation and the strain could possess broader N2 fixation deficiencies. A possible origin for this reduction in effectiveness is discussed. An alternative sequenced strain, effective at N2 fixation on M. truncatula A17, is Sinorhizobium medicae WSM419.

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