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Articles

Decreased Exopolysaccharide Synthesis by Anaerobic and Symbiotic Cells of Bradyrhizobium japonicum¹

Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803

Experiments were conducted to determine whether symbiotic bacteroids of Bradyrhizobium japonicum produce exopolysaccharide within soybean (Glycine max [L.] Merr. cv `Lee 74') nodules. B. japonicum strains RT2, a derivative of USDA 110 with resistance to streptomycin and rifampicin, and RT176-1, a mutant deficient in exopolysaccharide synthesis, were used. Although aerobically cultured RT2 produced 1550 micrograms of exopolysaccharide per 10¹⁰ cells, root nodules formed by RT2 contained only 55.7 micrograms of polysaccharide per 10¹⁰ bacteroids, indicating that little exopolysaccharide synthesis occurred within the nodules. The polysaccharide level of RT2 nodules was about equal to that of nodules containing the exopolysaccharide mutant RT176-1 (61.0 micrograms per 10¹⁰ bacteroids). Gas chromatographic analysis showed that the sugar composition of polysaccharide from nodules of RT2 or RT176-1 was almost the same as that of polysaccharide from unnodulated root tissue, but differed strikingly from that of rhizobial exopolysaccharide from aerobic cultures. Thus, the host plant and not the bacteroids was probably the source of most or all of the polysaccharide in the nodule extracts. Also, bacteroids from nodules failed to bind soybean lectin, confirming the absence of an exopolysaccharide capsule.

To test the hypothesis that this reduced synthesis of exopolysaccharide by bacteroids is related to the low free O₂ concentration within nodules, strain RT2 was grown on L-arabinose/succinate/glutamate/nitrate medium both aerobically and anaerobically. Anaerobiosis caused a 92% reduction in total exopolysaccharide synthesis, with amounts averaging only 123 micrograms per 10¹⁰ cells. Anaerobically cultured cells also failed to bind soybean lectin. These results suggest that the low free O₂ content of the nodules may be responsible for the reduced exopolysaccharide synthesis by the bacteroids.