Sensitivity of Rhizobium to selected isoflavonoids.

Eleven isoflavonoids were tested in both soft agar and liquid media for inhibitory activity against eight Rhizobium strains. Results of the two bioassays showed that pisatin, coumestriol, biochanin A, formononetin, genistein, rotenon, and vestitol lacked significant inhibitory activity. Phaseolin and maackiain were moderately inhibitory (ED50 (mean effective dose) = 30-100 microgram/mL), and medicarpin and kievitone were strongly inhibitory (ED50 = 10-60 microgram/mL) towards slow-growing R. japonicum and R. lupini and also towards two fast-growing Lotus rhizobia. In contrast, R. trifolii, R. leguminosarum, and R. phaseoli were not affected by these compounds ED50 greater than 100 microgram/mL), and R. meliloti was inhibited only by kievitone (ED50 = 30-60 microgram/mL). At a concentration of 100 microgram/mL, medicarpin and kievitone were found to be bactericidal to R. japonicum, R. lupini, and the fast-growing Lotus rhizobia.