Enhanced hyphal growth and spore production of the arbuscular mycorrhizal fungus Glomus intraradices in an in vitro system in the absence of host roots

Arbuscular mycorrhizal (AM) fungi are ecologically important for most vascular plants because they benefit plant growth and survival. The obligate biotrophic nature of AM fungi imposes limitations in inoculum production which could be used in the management of the symbiosis in field crops. Glomus intraradices was grown on genetically transformed Daucus carota roots in a two-compartment in vitro system. The growth of mycorrhizal roots was restricted to one compartment (proximal) containing a complete growth medium. Only the endosymbiont was permitted to grow on to the second compartment (distal) containing the same medium lacking sugar. Colonization of the distal compartment by the mycelium took place between six and eight weeks after subculturing the mycorrhizal roots in the proximal compartment. Hyphal- and spore-densities were significantly higher in the distal compartment. Up to 34000 spores with a mean of 15 000 mostly viable spores per plate were counted in the distal compartment. This opens the possibility of producing aseptic spores, not only for research purposes but also for large-scale inoculum production. The possible factors involved in the enhancement of hyphal- and spore-densities and their ecological role are discussed.