STRAIN DISTRIBUTION AND IN PLANTA PRODUCTION OF AN EXTRACELLULAR POLYSACCHARIDE DEPOLYMERASE FROM BRADYRHIZOBIUM-JAPONICUM

Thirty-four strains of Bradyrhizobium japonicum were screened for the in vitro production of an extracellular polysaccharide depolymerase active against the B. japonicum acidic extracellular polysaccharide that contains mannose, glucose, galactose, and 4-O-methylgalactose as neutral sugar components. Over 90% of tested strains producing this type of extracellular polysaccharide also produced the extracellular polysaccharide depolymerase, whereas strains producing a compositionally different extracellular polysaccharide did not. In addition, representatives of species related to B. japonicum by extracellular polysaccharide composition or host range were also phenotypically depolymerase negative. Depolymerase was also present in soybean root nodules formed by B. japonicum strain 2143. In contrast to the cell-associated depolymerase activity found in free-living cells of this strain, most of the depolymerase activity present in nodules is free of the bacteroids. The widespread occurrence of the depolymerase among B. japonicum strains and the spatiotemporal distribution of its activity in planta are consistent with the enzyme playing a role in the removal of surface extracellular polysaccharide from the microorganism during the infection of nodulation process.