Abstract
Isoflavonoid signal molecules from soybean (Glycine max (L.) Merr.) seed and root exudate induce the transcription of nodulation (nod) genes in Bradyrhizobium japonicum. In this study, a new compound with symbiotic activity was isolated from soybean root exudate. The isolated 2',4',4-trihydroxychalcone (isoliquiritigenin) is characterized by its strong inducing activity for the nod genes of B. japonicum. These genes are already induced at concentrations 1 order of magnitude below those required of the previously described isoflavonoid inducers genistein and daidzein. Isoliquiritigenin is also a potent inducer of glyceollin resistance in B. japonicum, which renders this bacterium insensitive to potentially bactericidal concentrations of glyceollin, the phytoalexin of G. max. No chemotactic effect of isoliquiritigenin was observed. The highly efficient induction of nod genes and glyceollin resistance by isoliquiritigenin suggests the ecological significance of this compound, although it is not a major flavonoid constituent of the soybean root exudate in quantitative terms.