Abstract
Pseudomonas aureofaciens are soil-borne root-colonizing bacteria that produce phenazine antibiotics. Populations of P. aureofaciens strain 30–84 increase over time on wheat roots in response to the presence of a specific fungal pathogen. Phenazine production is the primary mechanism responsible for the competitive fitness of P. aureofaciens in the rhizosphere. Analysis of phenazine gene ( phz ) expression demonstrates that phenazine biosynthesis in P. aureofaciens is regulated analogously to other bacterial genes involved in host-microbe interactions. Two genes, phzR and phzI , the products of which belong to the LuxR/LuxI family of cell density-dependent regulatory proteins, are required for phz expression. PhzR encodes a transcriptional activator that induces phz expression in response to the accumulation of a diffusible signal produced by PhzI. The requirement for phzI can be bypassed by exogenous diffusible signals produced by several other rhizosphere bacteria. We discuss phz regulation in the context of the ecology of P. aureofaciens on the plant root. We integrate both molecular and field observations to propose a model to explain these interactions between the plant, the pathogen and the bacterium. In addition, we discuss signaling among different bacterial populations in the rhizosphere and hypothesize that this signaling may influence phz expression and thus biological control by P. aureofaciens.
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