NOD FACTORS OF BRADYRHIZOBIUM-JAPONICUM AND RHIZOBIUM SP NGR234 INDUCE FLAVONOID ACCUMULATION IN SOYBEAN ROOT EXUDATE

Flavonoids play an important role as signal molecules in the early sta ges of symbiosis between legumes and Brady-rhizobium or Rhizobium. The flavonoid content of root exudates of individual seedlings of Glycine max cv. Preston was analyzed by a sensitive method that combines the adsorption of flavonoids on cellulose acetate filter strips and their separation by nano thin-layer chromatography plates, with their identi fication and quantification by an absorption scanner. B. japonicum 110 spc4 which is able to form nitrogen-fixing nodules with this legume, c aused a significant increase in daidzein, coumestrol, and genistein le vels in root exudates of infected seedlings. The flavonoid accumulatio n can be attributed to the presence of suitable rhizobial lipooligosac charide signals, called Nod factors. This conclusion is based on the f ollowing results: 1) B. japonicum Delta 1240, a Nod(-) mutant, lacking nodD(1)D(2)YABCS and therefore unable to produce Nod factors, failed to induce flavonoid accumulation. 2) Enhanced levels of daidzein, coum estrol, and genistein were also observed after G. max seedlings were t reated with pure Nod factor of B. japonicum in nanomolar concentration s. 3) A similar flavonoid accumulation was induced by a nonsulfated No d factor of Rhizobium sp. NGR234, another soybean-nodulating bacterium . Comparative assays with three Nod factors of NGR234, differing in th e substitution of the 2-O-methylfucose residue, suggest that in the sy mbiosis with G. max the absence of the sulfate group is essential for the specific induction of flavonoid accumulation. Our data further dev elop the interaction model involving flavonoids and Nod factors as com ponents of the signal chain between microsymbionts and legume host pla nts.