EFFECTS OF A SEED COLOR MUTATION ON RHIZOBIAL NOD-GENE-INDUCING FLAVONOIDS AND NODULATION IN COMMON BEAN

A white-seeded mutant of common bean (Phaseolus vulgaris PI 165426WS) differed from its isogenic black-seeded parent, PI 165426CS, in the amounts and types of flavonoid nod-gene inducers released from the seed and in the pattern of root nodules formed by Rhizobium tropici CIAT 899 and by R. leguminosarum bv. phaseoli CNPAF 512. Comparisons of 14-day-old plants developing from black (CS) and white (WS) seeds showed that although there were no differences in any plant growth parameter measured, CS plants had at least 80% more nodules on the primary root. Rinsing seeds before planting decreased nodulation at the top of the primary root in CS, but not WS, seedlings. A direct role for seed compounds in the nodulation differences was supported by the fact that rinses from CS seeds induced 10-fold higher beta-galactosidase activity from a nodA::lacZ fusion in R. leguminosarum bv. phaseoli than WS seed rinses. Analytical chemistry techniques showed that WS seeds lacked five nod-gene-inducing anthocyanins previously identified on CS seeds. WS seed rinses contained five nod-inducing flavonol glycosides released by CS seeds, but only 45% as much of those compounds was present. The nod-gene-inducing activity of WS root exudates, however, was much more similar to that from CS roots both quantitatively and qualitatively. Adding 20 mumoles of malvidin-3-O-glucoside or quercetin-3-O-glucoside to WS seeds inoculated either with CIAT 899 or with CNPAF 512 increased nodulation by at least 40%, but malvidin and quercetin aglycones had no effect on nodulation. No flavonoids tested altered nodulation on CS plants. These data indicate that initial root nodulation of WS, but not CS, beans was limited by availability of nod-gene inducers released from the seed coat.