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Using the nodY::lacZ fusion system in Bradyrhizobium japonicum USDA110, 22 flavonoids, which have structurally different features, were tested to define the role of the substituted functional groups as an inducer or inhibitor for the nod gene expression. A functional group of 4’-OH on the B-ring and the double bond between 2-C and 3-C on the C ring were required to induce the nod gene expression in B. japonicum USDA110. In the case of isoflavones, the 4’- methoxyl group, which blocks the open 4’-OH functional group, did not significantly lower inducing activity, as compared with isoflavones with 4’-OH. However, all flavonols tested, which have a 3-OH functional group on the C-ring, did not induce, but inhibited the nod gene expression. Flavone, 7- hydroxyflavone, and kaempferol (5,7,4’-trihydroxyflavonol) at 1 μM concentration significantly inhibited the nod gene expression induced by 7,4’-dihydroxyflavone. However, 7- hydroxy-4’-methoxyflavone at 1 μM concentration showed a synergistic effect with genistein and 7,4’-dihydroxyflavone on the induction activity.
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