BACTERIAL CHARACTERISTICS IMPORTANT TO RHIZOSPHERE COMPETENCE

Ten bacteria isolated from the soybean rhizosphere and 10 from soil were compared with respect to several characteristics that have been suggested as important to successful root colonization. Rhizosphere and soil isolates differed significantly in their ability to move along soybean roots. Rhizosphere bacteria were able to colonize root segments farther from the point of inoculation in greater numbers than soil bacteria. Rhizosphere and soil bacteria did not differ significantly in rates and extents of growth in either exudates collected from germinating seeds or inorganic salts solution. However, 9 of 10 rhizosphere isolates but only 5 of 10 soil isolates grew quickly or had high cell yields or both when grown in exudates. The extent of adherence to root tips did not differ significantly between rhizosphere and soil bacteria. A significant difference was not evident between rhizosphere and soil isolates in chemotactic response to exudates, tolerance of low osmotic potentials or the agglutination of cells by exudates. The results suggest that mobility along the root is important to successful rhizosphere colonization, but none of the other characteristics individually is sufficient to ensure rhizosphere competence.