Interaction between the nitrogen fixing bacterium Rhizobium japonicum and soybean (Glycine max L.) involving specificity and recognition

The initial cell surface interaction between legumes and rhizobia during infection, is believed to be mediated by legume seed lectins. The specificity and host-recognition are the two important prerequisites for the successful infection of legumes by rhizobia. The present study deals with the involvement of soybean lectin in the interaction between soybean and Rhizobium japonicum. -- Using biochemical and cytological techniques, soybean lectin binding properties of a non-mucoid strain of R. japonicum under two different laboratory culture conditions were studied. The primary lectin binding sites were localized in the capsular extracellular polysaccharide (EPS) of the bacteria using ferritin and colloidal gold labeled soybean lectin at the ultrastructural level. It was also possible to show the secondary soybean lectin binding sites to be the outer cell wall of the bacterial cells. Further, the lectin specific cells were characterized as coccoid forms embedded in a capsular matrix and rod forms having a polar capsule. In experimental studies both capsulated coccoid and rod forms were found to attach to the root hair surface with the rod forms showing distinct polar attachment. -- Differential centrifugation was used to fractionate the rod and coccoid forms which were then further characterized. Both the forms were clearly distinguishable at the light and electron microscope levels. The rods were often found aggregated by their polar ends to form 'stars’. The coccoid forms were found to be either capsulated or non-capsulated. The coccoid and rod cells when allowed to grow separately in fresh nutrient medium were shown to undergo interconversion. The lectin binding kinetics of both the types of cells were studied. The nodulation capacity of these two types against soybean were carried out. -- Gas chromatographic analyses of EPS and LPS fractions of the bacteria grown in yeast extract mannitol medium did not reveal any detectable quantity of lectin specific receptors (galactose or N-acetylgalactosamine). However, when the bacteria were grown in soil extract medium, galactose, a soybean lectin specific sugar, was found to be one of the components in both EPS and LPS. This induction of galactose synthesis was accompanied by increases in cell size, percentage of coccoid forms and in the amount of lectin bound to the bacterial cells. These results indicate that more lectin binding receptors can be induced in rhizobia grown in soil extract medium. -- The study leads to the following conclusions: (1) the lectin binding sites are located in the capsular extracellular polysaccharide (primary lectin binding site) and in the lipopolysaccharide of the outer cell wall envelope (secondary binding site) of the bacteria; (2) there are distinct rods and coccoid forms of bacteria in the broth culture reflecting the different phases of growth; and (3) that the lectin receptor sites are induced in Rhizobium when culture conditions simulate the natural environment (soil) of the bacteria