INHIBITION OF RHIZOBIUM-ETLI POLYSACCHARIDE MUTANTS BY PHASEOLUS-VULGARIS ROOT COMPOUNDS

Crude bean root extracts of Phaseolus vulgaris were tested for inhibit ion of the growth of several polysaccharide mutants of Rhizobium etli biovar phaseoli CE3. Mutants deficient only in exopolysaccharide and s ome mutants deficient only in the O-antigen of the lipopolysaccharide were no more sensitive than the wild type strain to the extracts, wher eas mutants defective in both lipopolysaccharide and exopolysaccharide were much more sensitive. The inhibitory activity was found at much h igher levels in roots and nodules than in stems or leaves. Inoculation with either wild-type or polysaccharide-deficient R. etli did not app ear to affect the level of activity. Sequential extractions of the cru de root material with petroleum ether, ethyl acetate, methanol, and wa ter partitioned inhibitory activity into each solvent except methanol. The major inhibitors in the petroleum ether and ethyl acetate extract s were purified by C-18 high-performance liquid chromatography. These compounds all migrated very similarly in both liquid and thin-layer ch romatography but were distinguished by their mass spectra. Absorbance spectra and fluorescence properties suggested that they were coumestan s, one of which had the mass spectrum and nuclear magnetic resonances of coumestrol. These results are discussed with regard to the hypothes is that one role of rhizobial polysaccharides is to protect against pl ant toxins encountered during nodule development.