The transition of Rhizobium fredii lipopolysaccharides induced by soybean root exudation

The chemical compounds contained in leguminous plant root exudation induce structural and compositional transitions of the lipopolysaccharides of Rhizobium that occur during rhizobia-legume symbiotic infection processes. The root exudate was extracted from the seed germinated solution and seedling roots of soybean. We added the exudation to bacteria cultured medium. Rhizobium and Bradyrhizobium strains were purified through gel filtration chromatography followed by SDS-PAGE analysis. Different chromatographic and SDS-PAGE patterns among the different rhizobia strains were obtained. We determined by means of the gel filtration chromatography that soybean root exudations induced a molecular weight range change in the lipopolysaccharides of these strains. The band of this LPS-II region exhibited diffused patterns in SDS-PAGE as a result of soybean root secretion. The LPS-I region of four strains cultured in TY medium with root exudates exhibited clearer and denser bands than those of rhizobia from TY medium without added soybean root exudates. Based on the composition analysis, we found that the KDO ratio increased in R. fredii TU6, R. meliloti ATCC 9930, R. leguminosarum bv. viciae ATCC 10004 and B. japonicum USDA 110, but decreased in R. fredii KR23, R. loti Lu7, and R. loti ATCC 33669 when soybean root exudation was added to the medium. The hexouronate ratio decreased except in R. loti Lu7. This result was consistent with the SDS-PAGE profile. The LPS-I region of R. loti LU7 showed little variation. It was therefore ascertained that soybean root secretion induces a variation in the structure of rhizobial lipopolysaccharides.