Cell-bound cellulase and polygalacturonase production by Rhizobium and Bradyrhizobium species

Carboxymethyl cellulase and polygalacturonase activities were evaluated in wild-type strains from different taxonomic groups of rhizobia which nodulate a specific range of legume hosts that do not normally overlap (R. meliloti, R. loti, R. leguminosarum biovar trifolii, R. leguminosarum biovar phaseoli, R. leguminosarum biovar viceae, and Bradyrhizobium japonicum). To detect these enzymatic activities we used various methods, a double-layer plate assay, quantitation of reducing sugar products with a bicinchoninate reagent, and viscometric assay. CM-cellulase activity was found in extracts from all the rhizobia tested. In contrast, polygalacturonase activity was only found in R. leguminosarum biovar trifolii and R. meliloti. CM-cellulase and polygalacturonase enzymes were cell-bound and not detected in the extracellular culture medium. CM-cellulase isozymes were examined using activity gel electrophoresis. The results obtained shown a heterogeneity of CM-cellulase isozymes among the strains tested. These findings are consistent with the hypothesis of an involvement of rhizobial cellulolytic enzymes in the invasion of legumes by Rhizobium.