Characterization of negative signaling between wheat rhizosphere bacteria and the biological control agent Pseudomonas aureofaciens strain 30-84

Abstract

The biological control bacterium Pseudomonas aureofaciens strain 30-84 produces three phenazine antibiotics. Phenazines are responsible for pathogen inhibition by strain 30-84 as well as its ability to persist in the rhizosphere. Although this bacterium can suppress take-all of wheat disease when applied as a seed inoculum, performance of this agent, as with many biological control agents, can be variable in the field. A factor in establishment and pathogen inhibition may be the indigenous microbial community that competes with strain 30-84 and may interfere with phenazine production as a competitive mechanism. In this study, a wheat rhizosphere microbial community library was screened and ca. 4% of the isolates were found to inhibit phenazine production by strain 30-84. A sub-group of these isolates was characterized and found to produce extracellular signals that suppressed phenazine gene expression. The signal from isolate PU-15 was initially characterized and appeared to be chemically and mechanistically unlike other known negative-acting signals. A genetic region was cloned from this isolate that decreased phenazine gene expression and production in strain 30-84. Negative communication also affected the ability of strain 30-84 to inhibit the pathogenic fungus Gaeuman-nomyces graminis pv. tritici in vitro. Therefore, negative communication may contribute to the inconsistencies of biological control in the field.