The genome of the obligate endobacterium of an AM fungus reveals an interphylum network of nutritional interactions

Many AMF host endobacteria in their cytoplasm. To elucidate the role of the Gram negative endobacterium Candidatus Glomeribacter gigasporarum (CaGg), we sequenced its genome using a metagenomic approach which combined Sanger sequencing of fosmid clones from a Gigaspora margarita (the host) library with a 454 pyrosequencing of an enriched- endobacterial fraction. The final assembly led to 35 contigs, totalling 1.72 Mb, consisting of a chromosome and three plasmids. The genome features of CaCg place it in the Burkolderiaceae group, while metabolic networks analysis clustered CaCg with insect endobacteria, mirroring its obligate intracellular life-style. CaCg resembles an aerobic microbe, with no fermentative and limited energy-production capabilities via glycolysis and phosphate-pentose pathways. It depends upon its fungal host for C, P and N supply; its ability to synthesize amino acids is limited suggesting that most amino acids are imported from its host. In summary, the bacterial genome data of the first endosymbiont's endosymbiont revealed a novel context of intimate symbiosis between bacteria and fungi. Since the CaGg fungal host is itself an obligate biotroph dependent on its photosynthetic host, our work uncovers a network of nutritional/evolutionary interactions between plants, AM fungi and endobacteria.