RHIZOBIAL NODULATION FACTORS STIMULATE MYCORRHIZAL COLONIZATION OF NODULATING AND NONNODULATING SOYBEANS

Legumes form tripartite symbiotic associations with nodule-inducing rh izobia and vesicular-arbuscular mycorrhizal fungi. Co-inoculation of s oybean (Glycine max [L.] Merr.) roots with Bradyrhizobium japonicum 61 -A-101 considerably enhanced colonization by the mycorrhizal fungus Gl omus mosseae. A similar stimulatory effect on mycorrhizal colonization was also observed in nonnodulating soybean mutants when inoculated wi th Bradyrhizobium japonicum and in wild-type soybean plants when inocu lated with ineffective rhizobial strains, indicating that a functional rhizobial symbiosis is not necessary for enhanced mycorrhiza formatio n. inoculation with the mutant Rhizobium sp. NCR Delta nodABC, unable to produce nodulation (Nod) factors, did not show any effect on mycorr hiza. Highly purified Nod factors also increased the degree of mycorrh izal colonization. Nod factors from Rhizobium sp. NCR234 differed in t heir potential to promote fungal colonization. The acetylated factor N odNCR-V (MeFuc, Ac), added at concentrations as low as 10(-9) M, was a ctive, whereas the sulfated factor, NodNCR-V (MeFuc, S), was inactive. Several soybean flavonoids known to accumulate in response to the ace tylated Nod factor showed a similar promoting effect on mycorrhiza. Th ese results suggest that plant flavonoids mediate the Nod factor-induc ed stimulation of mycorrhizal colonization in soybean roots.