PISUM-SATIVUM MUTANTS INSENSITIVE TO NODULATION ARE ALSO INSENSITIVE TO INVASION IN-VITRO BY THE MYCORRHIZAL FUNGUS, GIGASPORA-MARGARITA

Transformed root cultures were established using Agrobacterium rhizoge nes inoculation of pea mutants, altered in their interaction with Rhiz obium. They were tested in an in vitro model for sensitivity to the ve sicular-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 resi stant to VAM colonization in vitro: Mycelium developed on the root sur face but failed to colonize the interior. A nodulating (Nod(+)) genoty pe, which was unable to fix nitrogen (Fix(-)) in association with Rhiz obium and a parental line, Lincoln (Nod(+), Fix(+)), interacted normal ly with the fungus, showing extensive internal colonization. These res ults confirm, under axenic conditions, previous reports showing that d efective nodulation is correlated with defective mycorrhization. We pr opose using this in vitro model to identify factors necessary to initi ating and maintaining the VAM/plant symbiosis.