Endobacteria or bacterial endosymbionts? To be or not to be

Symbiotic associations between endocellular bacteria and eukaryotic cells are widespread among animals (e.g. Buchnera and Wolbachia spp. in insects) and plants (e.g. Nostoc spp. with Gunnera; Burkholderia spp. with Rubiaceae; rhizobia with legumes), but relatively little is known about associations between bacteria and fungi. In the fungal kingdom, the presence of endocellular bacteria has been reported in some Glomeromycota species, both in arbuscular mycorrhizal (AM) fungi and Geosiphon pyriforme, as well as in the ectomycorrhizal basidiomycete Laccaria bicolor andTuber borchii. A summary of the main biological features of these fungal/endobacterial couples and a comment of the new association between the phytopathogenic fungus Rhizopus microsporus and an endobacterium belonging to the genus Burkholderia (Partida-Martinez & Hertweck, Nature 2005) are given in this commentary.