Responses of an Arbuscular Mycorrhizal Fungus, Gigaspora margarita, to Exudates and Volatiles from the Ri T-DNA-Transformed Roots of Nonmycorrhizal and Mycorrhizal Mutants of Pisum sativum L Sparkle

Balaji, B., Poulin, M. J., Vierheilig, H., and Piché Y. 1995. Responses of an arbuscular mycorrhizal fungus, Gigaspora margarita, to exudates and volatiles from the Ri T-DNA-transformed roots of nonmycorrhizal and mycorrhizal mutants of Pisum sativum L Sparkle. Experimental Mycology 19, 275-283. Transformed root cultures were established from the nonmycorrhizal (Myc^-) and mycorrhizal (Myc⁺) Pisum sativum L Sparkle mutants to study the biochemical factors necessary for initiating and maintaining the arbuscular mycorrhizal (AM) symbiosis. Root exudates produced by both the Myc^- and the Myc⁺ mutants inhibited the hyphal growth of Gigaspora margarita, whereas root volatiles from these mutants stimulated the hyphal growth significantly in the precolonization stage. Carbon dioxide is the principal volatile compound necessary for the elongation of hyphae from both the Myc^- and the Myc⁺ transformed roots. The addition of quercetin, a flavonol compound, to the medium with a Myc^- mutant enriched with an optimal CO₂ improved hyphal elongation and spreading as previously reported but did not cause Myc^- roots to become mycorrhizal. These results suggest that the root factors may stimulate or inhibit AM fungal growth and that they do not determine the mycorrhizal nature of P. sativum Sparkle mutants.