Evidence for signaling between the phytopathogenic fungus Pythium ultimum and Pseudomonas fluorescens F113: P. ultimum represses the expression of genes in P. fluorescens F113, resulting in altered ecological fitness.

There is increasing evidence that communication between members of the same species, as well as members of different species, is important for the survival of microorganisms in diverse ecological niches, such as the rhizosphere. To investigate whether the phytopathogen Pythium ultimum could alter gene expression in the biocontrol strain Pseudomonas fluorescens F113, which protects the roots of sugar beet from the fungus, a screening system was developed to detect differential expression of bacterial genes in the presence of P. ultimum. The transposon Tn5, containing a promoterless lacZ reporter gene, was used to generate a library of transcriptional gene fusions in P. fluorescens F113. By this screening procedure, five P. fluorescens F113 gene clusters were identified and shown to be repressed in the presence of P. ultimum. The ecological fitness of three of the five reporter mutants in the rhizosphere of seed-inoculated sugar beet was lower than that of the wild type. Furthermore, all five mutants were impaired in their ability to subsequently colonize the rhizosphere of uninoculated sugar beet sown repeatedly in the same soil. With the exception of reporter mutant SF10, which was impaired in nitrogen metabolism, the reporter mutants had growth requirements and biocontrol abilities similar to those of the wild type. This is the first reported case of a fungus repressing the expressing of bacterial genes.