Fungal Growth Stimulation by CO(2) and Root Exudates in Vesicular-Arbuscular Mycorrhizal Symbiosis.

Transformed roots of carrot were used to determine the effects of root metabolites on hyphal development from spores of the vesicular-arbuscular mycorrhizal fungus Gigaspora margarita. Hyphal growth of this obligately biotrophic symbiont was greatly stimulated by a synergistic interaction between volatile and exudated factors produced by roots. Root volatiles alone provided little stimulation, and root exudates alone had no effect. For the first time, carbon dioxide was demonstrated to be a critical root volatile involved in the enhancement of hyphal growth. C-labeled root volatiles were fixed by the fungus and thus strongly suggested that CO(2) served as an essential carbon source.