9514074 Pierson The rhizosphere, the zone of soil surrounding plant roots, contains greater amounts of nutrients in the form of root exudate than the surrounding soil. Therefore, microbial competition for these root exudates is intense. In response to this competition, bacteria have evolved diverse competitive mechanisms to allow them to compete for these nutrients. One benefit of the expression of these competitive mechanisms is the inhibition of plant pathogens, commonly termed biological control. However, commercial use of biological control as been limited by inconsistent performance. This is the result of our lack of understanding of the factors, including the plant host and the natural microbial community, that affect the expression of these competitive genes in the rhizosphere. Pseudomonas aureofaciens 30-84, one of the best genetically defined biological control organisms, produces phenazine antibiotics. Phenazine production is the primary mechanism responsible for the competitive fitness of this species in the rhizosphere and for its ability to suppress plant pathogens. We have shown that this bacterium regulates the expression of the genes encoding phenazine production through the recognition of a diffusible signal encoded by the gene phzI. Recently, it has been shown that populations of diverse bacteria rely on the accumulation of structurally similar signals to trigger the expression of host-associated genes. We have shown that the expression of genes involved in phenazine production in strain 30-84 is influenced by the production of diffusible signals produced by other rhizosphere bacteria. The ecological significance of these interactions is that competing strains may be able to influence the outcome of competition by affecting the regulation of a competitive mechanism at a genetic level. The proposed research will directly test the effect of diffusible signals produced by other populations of bacteria on phenazine gene expression in P. aureofaciens, including the effect of interactions on population densities in situ on plant roots.
