Presymbiotic growth and sporal morphology are affected in the arbuscular mycorrhizal fungus Gigaspora margarita cured of its endobacteria

Authors

  • Erica Lumini,

    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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    • These authors contributed equally to the work.

  • Valeria Bianciotto,

    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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    • These authors contributed equally to the work.

  • Patricia Jargeat,

    1. UMR 5174 University of Toulouse 3/CNRS/ENFA, 118, route de Narbonne, 31062 Toulouse, France.
    2. UMR 5546 University of Toulouse 3/CNRS, 24 chemin de Borde-Rouge, 31326 Castanet-Tolosan, Toulouse, France.
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    • These authors contributed equally to the work.

  • Mara Novero,

    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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  • Alessandra Salvioli,

    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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  • Antonella Faccio,

    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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  • Guillaume Bécard,

    1. UMR 5546 University of Toulouse 3/CNRS, 24 chemin de Borde-Rouge, 31326 Castanet-Tolosan, Toulouse, France.
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  • Paola Bonfante

    Corresponding author
    1. Dipartimento di Biologia Vegetale dell'Università and Istituto per la Protezione delle Piante – CNR, Viale Mattioli 25, 10125-I, Torino, Italy.
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*E-mail p.bonfante@ipp.cnr.it; Tel. (+39) 0116502927; Fax (+39) 0116705962.

Summary

Some arbuscular mycorrhizal fungi contain endocellular bacteria. In Gigaspora margarita BEG 34, a homogenous population of β-Proteobacteria is hosted inside the fungal spore. The bacteria, named Candidatus Glomeribacter gigasporarum, are vertically transmitted through fungal spore generations. Here we report how a protocol based on repeated passages through single-spore inocula caused dilution of the initial bacterial population eventually leading to cured spores. Spores of this line had a distinct phenotype regarding cytoplasm organization, vacuole morphology, cell wall organization, lipid bodies and pigment granules. The absence of bacteria severely affected presymbiotic fungal growth such as hyphal elongation and branching after root exudate treatment, suggesting that Ca. Glomeribacter gigasporarum is important for optimal development of its fungal host. Under laboratory conditions, the cured fungus could be propagated, i.e. could form mycorrhizae and sporulate, and can therefore be considered as a stable variant of the wild type. The results demonstrated that – at least for the G. margarita BEG 34 isolate – the absence of endobacteria affects the spore phenotype of the fungal host, and causes delays in the growth of germinating mycelium, possibly affecting its ecological fitness. This cured line is the first manipulated and stable isolate of an arbuscular mycorrhizal fungus.

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