Pseudomonas orientalis F9 Pyoverdine, Safracin, and Phenazine Mutants Remain Effective Antagonists against Erwinia amylovora in Apple Flowers.

The recently characterized strain F9, an isolate from apple flowers in a Swiss orchard, exhibits antagonistic traits against phytopathogens. At high colonization densities, it exhibits phytotoxicity against apple flowers. F9 harbors biosynthesis genes for the siderophore pyoverdine as well as for the antibiotics safracin and phenazine. To elucidate the role of the three compounds in biocontrol, we screened a large random knockout library of F9 strains for lack of pyoverdine production or antagonism. Transposon mutants that lacked the ability for fluorescence carried transposons in pyoverdine production genes. Mutants unable to antagonize in an double-layer assay carried transposon insertions in the safracin gene cluster. As no phenazine transposon mutant could be identified using the chosen selection criteria, we constructed a site-directed deletion mutant. Pyoverdine-, safracin-, and phenazine mutants were tested for their abilities to counteract the fire blight pathogen on apple flowers or the soilborne pathogen in a soil microcosm. In contrast to some assays, and assays did not reveal significant differences between parental and mutant strains in their antagonistic activities. This suggests that, and , other factors, such as competition for resources or space, are more important than the tested antibiotics or pyoverdine for successful antagonism of F9 against phytopathogens in the performed assays. F9 is an antagonist of the economically important phytopathogen , the causal agent of fire blight in fruit. On King's B medium, F9 produces a pyoverdine siderophore and the antibiotic safracin. F9 transposon mutants lacking these factors fail to antagonize , depending on the assay. On isolated flowers and in soil microcosms, however, pyoverdine, safracin, and phenazine mutants control phytopathogens as clearly as their parental strains.