Iron is crucial for bacterial growth and virulence. Under iron-deficiency bacteria produce siderophores, iron chelators that facilitate the iron uptake into the cell via
specific receptors. Erwinia amylovora, the causative agent of fire blight, produces hydroxa-mate-type desferrioxamine siderophores (DFO). The presented study reassesses the impact of DFO as a virulence factor of E. amylovora during its epiphytic phase on the apple
flower. When inoculated in semisterile Golden Delicious flowers no difference in replication and induction of calyx necrosis could be observed between E. amylovora CFBP1430 siderophore synthesis (DfoA) or uptake (FoxR receptor) mutants and the parental strain. In
addition, mutant strains only weakly induced a foxR promoter-gfpmut2 reporter construct
in the flowers. When analyzing the replication of the receptor mutant in apple flowers
harboring an established microbiome, either naturally, in case of orchard flowers, or by
pre-inoculation of semisterile greenhouse flowers, it became evident that the mutant
strain had a significantly reduced replication compared to the parental strain. The results
suggest that apple flowers per se are not an iron-limiting environment for E. amylovora
and that DFO is an important competition factor for the pathogen in precolonized
flowers.