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Symbiotic mutants of Rhizobium meliloti that uncouple plant from bacterial differentiation

T M Finan, A M Hirsch, J A Leigh

    Cell
    |April 1, 1985
    PubMed

    Insights

    Spontaneous Rhizobium meliloti mutants lacking exopolysaccharide form nodules on alfalfa without infection threads. These nitrogen-fixing deficient (Fix-) nodules demonstrate that infection threads are not essential for nodule formation.

    Area of Science:

    • Microbiology
    • Plant-Bacterial Symbiosis
    • Molecular Genetics

    Background:

    • Rhizobium meliloti SU47 forms symbiotic relationships with alfalfa, crucial for nitrogen fixation.
    • Exopolysaccharides (EPS) play a role in the early stages of symbiosis, including root hair curling and infection thread formation.
    • The precise role of EPS and infection threads in nodule development and function is not fully understood.

    Purpose of the Study:

    • To investigate the symbiotic phenotype of spontaneous Rhizobium meliloti mutants deficient in exopolysaccharide.
    • To determine the necessity of infection threads for nodule formation and development in alfalfa-Rhizobium symbiosis.
    • To explore the role of bacterial signaling in initiating nodule development independently of infection.

    Main Methods:

    • Isolation and characterization of spontaneous Rhizobium meliloti SU47 mutants exhibiting altered surface properties (phage resistance, antibody insensitivity).
    • Phenotypic analysis of alfalfa root nodule formation and development upon inoculation with mutant strains.
    • Microscopic examination of nodule structure, including infection threads and bacterial localization.
    • Assessment of nitrogen fixation capacity in nodules formed by mutant strains.

    Main Results:

    • Mutants were deficient in wild-type exopolysaccharide, leading to altered surface characteristics.
    • Mutant strains initiated nodule formation on alfalfa without root hair curling or visible infection threads.
    • Nodules formed by mutants contained bacteria in superficial intercellular spaces, lacked bacteroids, and were nitrogen-fixing deficient (Fix-).
    • Evidence suggests bacterial signaling can induce nodule formation independently of infection threads and bacterial differentiation.

    Conclusions:

    • Infection threads are not essential for the initiation of cell proliferation and nodule formation in alfalfa-Rhizobium symbiosis.
    • A bacterial signal can trigger nodule development at a distance, uncoupled from the normal process of bacterial differentiation.
    • Exopolysaccharide deficiency in Rhizobium meliloti alters symbiotic interactions, highlighting its complex role beyond initial infection.

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