Fachbereich Biologie, Fg Angewandte Botanik und Zellbiologie, Philipps-Universität Marburg, Karl v. Frisch-Strasse, 35032 Marburg, Germany
Flavonoids play an important role as signal molecules in the early stages of the legume-Rhizobium symbiosis. The flavonoid content in root exudates of individual seedlings of Phaseolus vulgaris cv. Rab39 was determined by a sensitive method that combines absorption onto cellulose acetate filter strips with separation, identification, and quantification of individual compounds by high-performance liquid chromatography/diode-array detector and gas chromatography-mass spectroscopy analyses. We identified six flavonoids, daidzein, coumestrol, naringenin, genistein, liquiritigenin, and isoliquiritigenin, the latter two of which had not previously been found in bean root exudate. Biological activities were demonstrated by induction of β-galactosidase activity in a Rhizobium leguminosarum bv. phaseoli strain containing a nodC::lacZ fusion controlled by the nodD1 gene. Root exudate of common bean that had been inoculated with symbiotic R. etli, R. leguminosarum bv. phaseoli, or R. tropici contained more of the isoflavonoid daidzein, and the flavonoids naringenin, liquiritigenin, and isoliquiritigenin, than did exudate of plants that had not been inoculated. The relative concentrations of the signals depended on the nature of the bacterium. While daidzein, coumestrol, and naringenin were the dominant compounds in exudates of infected as well as uninfected seedlings, the daidzein and naringenin concentrations in particular were markedly enhanced after inoculation by all strains.