THE EFFECT OF SYMBIOTIC MICROORGANISMS ON PHYTOALEXIN CONTENTS OF SOYBEAN ROOTS

Plant defence-like responses to arbuscular mycorrhizal (AM) fungi have generally been reported to be weak and transient, but it is unknown w hether there is a relationship between the observed levels and the fun ctional compatibility between plant and AM fungus. Root accumulation o f the phytoalexin glyceollin I in soybean (Glycine max L.) was measure d over a 6-week growth period, as influenced by AM fungal isolates of contrasting P transport capacity and plant growth promoting ability. T wo AM fungal inocula were used: spores of Scutellospora calospora (Nic ol. and Gerd.) Walker and Sanders and a mixture of spores of Glomus ca ledonium (Nicol. and Gerd.) Trappe and Gerdemann and Glomus claroideum Schenck and Smith. Root glyceollin I levels in plants inoculated with the pathogen Rhizoctonia solani Kuhn served as reference. Two control treatments comprised plants receiving no inoculum and plants receivin g sterilized R. solani inoculum. All plant-fungus combinations were co mpatible with respect to root colonization; however, the rate of root colonization was lower with S. calospora than with G. caledonium/G. cl aroideum, and R. solani infected less than 7% of the root length. Shoo t P contents and plant biomass were much higher in plants colonized by G. caledonium/G. claroideum than in plants colonized by S. calospora or in uninoculated control plants. These differences in effectiveness were nor reflected in root phytoalexin concentrations, as glyceollin I levels were low and similar in the two mycorrhizal treatments and in the non-mycorrhizal control plants. In contrast, glyceollin I had accu mulated in R. solani-infected roots, which accords with phytoalexin pr oduction as a part of the active, inducible plant defence system. We c onclude that low functional compatibility of AM, with respect to P sup ply and plant growth stimulation by the fungus, is not associated with a defence-like response (glyceollin I) by the plant.