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 f or the survival of microorganisms in diverse ecological niches, such a s the rhizosphere, To investigate whether the phytopathogen Pythium ul timum could alter gene expression in the biocontrol strain Pseudomonas fluorescens F113, which protects the roots of sugar beet from the fun gus, a screening system was developed to detect differential expressio n 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 th is screening procedure, five P. fluorescens F113 gene clusters were id entified and shown to be repressed in the presence of P, ultimum. The ecological fitness of three of the five reporter mutants in the rhizos phere of seed-inoculated sugar beet was lower than that of the wild ty pe. Furthermore, all five mutants were impaired in their ability to su bsequently colonize the rhizosphere of uninoculated sugar beet sown re peatedly in the same soil, With the exception of reporter mutant SF10, which was impaired in nitrogen metabolism, the reporter mutants had g rowth requirements and biocontrol abilities similar to those of the wi ld type, This is the first reported case of a fungus repressing the ex pression of bacterial genes.