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ISME J. 2014 Feb;8(2):257-70. doi: 10.1038/ismej.2013.151. Epub 2013 Sep 5.

Detection of a novel intracellular microbiome hosted in arbuscular mycorrhizal fungi.

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Department of Life Sciences and Systems Biology, University of Torino, Torino, Italy.
Institute of Agronomic Research for Development (IRAD), Yaoundé, Cameroon.
Department of Plant Pathology and Plant Microbe-Biology, Cornell University, Ithaca, NY, USA.
Department of Veterinary Science and Public Health, University of Milano, Milano, Italy.
Institute of Plant Protection, UOS Torino, CNR, Torino, Italy.
Center for Microscopy and Image Analysis, University of Zurich, Zurich, Switzerland.


Arbuscular mycorrhizal fungi (AMF) are important members of the plant microbiome. They are obligate biotrophs that colonize the roots of most land plants and enhance host nutrient acquisition. Many AMF themselves harbor endobacteria in their hyphae and spores. Two types of endobacteria are known in Glomeromycota: rod-shaped Gram-negative Candidatus Glomeribacter gigasporarum, CaGg, limited in distribution to members of the Gigasporaceae family, and coccoid Mollicutes-related endobacteria, Mre, widely distributed across different lineages of AMF. The goal of the present study is to investigate the patterns of distribution and coexistence of the two endosymbionts, CaGg and Mre, in spore samples of several strains of Gigaspora margarita. Based on previous observations, we hypothesized that some AMF could host populations of both endobacteria. To test this hypothesis, we performed an extensive investigation of both endosymbionts in G. margarita spores sampled from Cameroonian soils as well as in the Japanese G. margarita MAFF520054 isolate using different approaches (molecular phylotyping, electron microscopy, fluorescence in situ hybridization and quantitative real-time PCR). We found that a single AMF host can harbour both types of endobacteria, with Mre population being more abundant, variable and prone to recombination than the CaGg one. Both endosymbionts seem to retain their genetic and lifestyle peculiarities regardless of whether they colonize the host alone or together. These findings show for the first time that fungi support an intracellular bacterial microbiome, in which distinct types of endobacteria coexist in a single cell.

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