Axenic growth of the arbuscular mycorrhizal fungus Rhizophagus irregularis and growth stimulation by coculture with plant growth-promoting rhizobacteria.

Arbuscular mycorrhizal (AM) fungi are ecologically important for the growth and survival of most vascular plants. These fungi are known as obligate biotrophs that acquire carbon solely from host plants. A 13C-labeling experiment revealed the ability of axenically grown Rhizophagus irregularis DAOM 197198 to derive carbon from axenic culture on a relatively novel medium containing two sources of palmitic acid developed by Ishii (designated IH medium). In a separate experiment, this model fungus grew larger mycelia and produced more daughter spores on the IH medium in the presence of two Variovorax paradoxus strains than in axenic culture. In contrast, a strain of Mycobacterium sp. did not influence the growth of the AM fungus. Rhizophagus irregularis produced branched absorbing structures on the IH medium and, in monoxenic culture with V. paradoxus, sometimes formed densely packed hyphal coils. In this study, we report for the first time the formation of coarse terminal pelotons and of terminal and intercalary very fine (≈ 1 μm diameter) hyphal elongations, which could form daughter spores in the presence of V. paradoxus. This study shows the value of IH medium and certain rhizobacteria in the culture of R. irregularis DAOM 197198 in vitro.