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Institut Sophia Agrobiotech

UMR INRA - Univ. Nice Sophia Antipolis - Cnrs

Arbuscular mycorrhizal fungi and their endobacteria: an endless story

Sophia Antipolis - Inra PACA - Room A010 - Monday, July 6, 2015 - 11:00

Séminaire scientifique ISA
© Inra
As part of the scientific activities of the Institut Sophia Agrobiotech, the 'Symbiose' team invites Paola BONFANTE - Department of Life Science and Systems Biology University of Torino, Italy: "Arbuscular mycorrhizal fungi and their endobacteria: an endless story"


Being present in the rhizosphere and in plant tissues as obligate symbionts, Arbuscular Mycorrhizal (AM) fungi are important members of the plant microbiota. As beneficial microbes, they play a key role in nutrient cycling, and  boost plant growth, improving water and mineral nutrient uptake. They also provide protection against biotic and abiotic stresses. For these reasons, AM fungi are currently acknowledged as a driving force for plant evolution. By contrast, the fact that many arbuscular mycorrhizal fungi contain endobacteria inside their cytoplasm is much less known.
 By using a combination of molecular, phylogenetic and cell analysis we have found that distinct types of endobacteria may coexist in a single fungal spore, showing that AM fungi host a so far poorly known intracellular bacterial microbiota. The genome sequencing of one endobacterial type (Candidatus Glomeribacter gigasporarum) has revealed a strong genome reduction when compared to the free-living related taxa: the endobacterium depends on its host for carbon, phosphorus and nitrogen supply.
 To understand the impact of the bacterium on the fungus, a cured line of the fungus Gigaspora margarita was created, where a decrease on the spore number production is observed. Due to the limited number of genomic data on AM fungi, which at the moment are limited to Rhizophagus irregularis, a transcriptomic RNA-seq analysis was performed to get a gene catalogue for G. margarita, and to compare its transcriptomic profile with and without the endobacteria. We find that the endobacteria and the fungus cooperate, raising the fungal bioenergetic capacity, controlling oxidative stress, and regulating the intracellular calcium concentration. We hypothesise that such mechanisms have important consequences for AM fungi, which produce more spores, leading to more events of plant colonization.
 The discovery of  such a tripartite symbiosis unravels a complex network of interdomain interactions, which are expected to have a previously unrecognized ecological impact.