COMPETITION BETWEEN PSEUDOMONAS-FLUORESCENS AG1 AND ALCALIGENES-EUTROPHUS JMP134 (PJP4) DURING COLONIZATION OF BARLEY ROOTS

To use deliberately released beneficial microorganisms in the rhizosph ere, we need a better understanding of the process of root colonizatio n by seed-borne or soil-borne inocula. In this study, we determine the survival of Pseudomonas fluorescens Ag1 and Alcaligenes eutrophus JMP 134, their colonization ability as affected by substrates, and the rel ative importance of migration versus competition for colonization of t he root. Ag1 and the 2,4-dichlorophenoxy-acetic acid (2,4-D) degrader JMP134 were inoculated in sterile barley rhizosphere systems. After in oculation of seeds with individual strains, comparable population size s were established in the rhizosphere as determined by immunofluoresce nce microscopic total cell counts. Both strains were motile and able t o colonize the entire root system without percolating water to stimula te passive transport. Comparing immunofluorescence microscopic cell co unts with colony-forming units demonstrated that a subpopulation of A. eutrophus JMP134 closely associated with the root was non-culturable in contrast to the population in rhizosphere soil. Hence, the sole use of culture-dependent methods may give misleading information about th e distribution of bacteria in the rhizosphere. Colonization studies wi th both strains showed that co-inoculation of Ag1 and JMP134 caused a decrease of the population size of JMP134 if 2,4-D was not added to th e soil as a specific carbon source for this strain. Thus, competition for limited carbon sources might influence the composition of the bact erial community in the rhizosphere. We also found that the presence of an established inoculum in the soil reduced subsequent root colonizat ion by a seed-inoculated strain, probably by filling available niches, also indicating that competition from other bacteria may be an import ant factor determining the distribution of seed-borne inocula. This fa ctor may be just as important for the distribution of bacteria as migr ation.