BACILLUS-POLYMYXA STIMULATES INCREASED RHIZOBIUM-ETLI POPULATIONS ANDNODULATION WHEN CO-RESIDENT IN THE RHIZOSPHERE OF PHASEOLUS-VULGARIS

Microbial competition for carbon sources is a primary determinant of r hizosphere ecology. We employed the PCR to examine the population fluc tuations of a symbiotic nitrogen-fixing bacterium (Rhizobium etli) dur ing the first 11 days following inoculation of Phaseolus vulgaris seed lings grown in the presence or absence of a common asymbiotic rhizosph ere resident (Bacillus polymyxa). When B. polymyxa was applied as a co -inoculant, increases in both early rhizobial root populations and fin al root population densities were observed as compared to single inocu lation with R. etli. Modifications to host plant growth (including inc reased lateral root formation and nodules number) were found concomita nt with elevations in R. etli populations on plants co-inoculated with both bacterial genera. In contrast to the in planta results, populati on enhancements were not observed when R. etli and B. polymyxa were co -cultured in vitro using minimal media in the absence of the seedling. Addition of seed exudate to the growth media also failed to stimulate the population increases observed during co-release in planta. These results suggest that B. polymyxa acts indirectly (i.e., via the plant host) to increase R. etli populations. Our observed synergism among co -resident bacteria supports the hypothesis that microbial communities which colonize the spermosphere may play a significant role in plant d evelopment and rhizosphere ecology.