Transformed root cultures were established using Agrobacterium rhizogenes inoculation of pea mutants, altered in their interaction with Rhizobium. They were tested in an in vitro model for sensitivity to the vesicular-arbuscular mycorrhizal (VAM) fungus, Gigaspora margarita. VAM development was assessed using light and electron microscopy. Two non-nodulating, non-nitrogen fixing (Nod(-), Fix(-)) pea mutants were resistant to VAM colonization in vitro: Mycelium developed on the root surface but failed to colonize the interior. A nodulating (Nod(+)) genotype, which was unable to fix nitrogen (Fix(-)) in association with Rhizobium and a parental line, Lincoln (Nod(+), Fix(+)), interacted normally with the fungus, showing extensive internal colonization. These results confirm, under axenic conditions, previous reports showing that defective nodulation is correlated with defective mycorrhization. We propose using this in vitro model to identify factors necessary to initiating and maintaining the VAM/plant symbiosis.
