Anthocyanidins and Flavonols, Major nod Gene Inducers from Seeds of a Black-Seeded Common Bean (Phaseolus vulgaris L.)

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Abstract
Eleven compounds released from germinating seeds of a black-seeded bean (Phaseolus vulgaris L., cv PI165426CS) induce transcription of nod genes in Rhizobium leguminosarum biovar phaseoli. Aglycones from 10 of those compounds were identified by spectroscopic methods (ultraviolet/visible, proton nuclear magnetic resonance, and mass spectroscopy), and their biological activities were demonstrated by induction of β-galactosidase activity in R. leguminosarum strains containing nodA-lacZ or nodC-lacZ fusions controlled by R. leguminosarum biovar phaseoli nodD genes. By making comparisons with authentic standards, the chemical structures for aglycones from the 10 molecules were confirmed as being anthocyanidins (delphinidin, petunidin, and malvidin) and flavonols (myricetin, quercetin, and kaempferol). All anthocyanidins and flavonols had 3-O-glycosylation and free hydroxyl groups at the 4′, 5, and 7 positions. Hydrolysis experiments showed that the mean concentration required for half-maximum nod gene induction (I50) by the 10 glycosides was about half that of the corresponding aglycones. The mean I50 value for the three anthocyanidins (360 nanomolar) was less (P ≤ 0.05) than that of the three flavonol aglycones (980 nanomolar). Each seed released approximately 2500 nanomoles of anthocyanidin and 450 nanomoles of flavonol nod gene inducers in conjugated forms during the first 6 hours of imbibition. Based on amounts and activities of the compounds released, anthocyanins contributed approximately 10-fold more total nod-inducing activity than flavonol glycosides. These anthocyanidins from bean seeds represent the first nod-inducing compounds identified from that group of flavonoids.