Policing the legume-Rhizobium symbiosis: a critical test of partner choice

In legume- Rhizobium symbioses, specialised soil bacteria fix atmospheric nitrogen in return for carbon. However, ineffective strains can arise, making discrimination essential. Discrimination can occur via partner choice, where legumes prevent ineffective strains from entering, or via sanctioning,...

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Bibliographic Details
Published in:Scientific reports 2017-05, Vol.7 (1), p.1419-10, Article 1419
Main Authors: Westhoek, Annet, Field, Elsa, Rehling, Finn, Mulley, Geraldine, Webb, Isabel, Poole, Philip S., Turnbull, Lindsay A.
Format: Article
Language:English
Subjects:
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Bacteria
Diffusion models
Humanities and Social Sciences
Legumes
multidisciplinary
NifH gene
Nitrogen
Nitrogen Fixation
Nodules
Pisum sativum - microbiology
Pisum sativum - physiology
Rhizobium leguminosarum - physiology
Root Nodules, Plant - microbiology
Root Nodules, Plant - physiology
Science
Science (multidisciplinary)
Soil microorganisms
Strains (organisms)
Symbiosis
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Summary:In legume- Rhizobium symbioses, specialised soil bacteria fix atmospheric nitrogen in return for carbon. However, ineffective strains can arise, making discrimination essential. Discrimination can occur via partner choice, where legumes prevent ineffective strains from entering, or via sanctioning, where plants provide fewer resources. Several studies have inferred that legumes exercise partner choice, but the rhizobia compared were not otherwise isogenic. To test when and how plants discriminate ineffective strains we developed sets of fixing and non-fixing strains that differed only in the expression of nifH – essential for nitrogen fixation – and could be visualised using marker genes. We show that the plant is unable to select against the non-fixing strain at the point of entry, but that non-fixing nodules are sanctioned. We also used the technique to characterise mixed nodules (containing both a fixing and a non-fixing strain), whose frequency could be predicted using a simple diffusion model. We discuss that sanctioning is likely to evolve in preference to partner choice in any symbiosis where partner quality cannot be adequately assessed until goods or services are actively exchanged.
ISSN:2045-2322
2045-2322
DOI:10.1038/s41598-017-01634-2