DUAL ROLE OF DESFERRIOXAMINE IN ERWINIA-AMYLOVORA PATHOGENICITY

To investigate the role of iron in Erwinia amylovora pathogenicity, vi rulence properties of two mutants of strain CFBP 1430 isolated by inse rtional mutagenesis and affected in the iron transport pathway mediate d by desferrioxamine (DFO) were analyzed. One mutation (dfoA::MudIIpR1 3) disrupts DFO biosynthesis. The present analysis shows that this mut ation affects an open reading frame that belongs to a biosynthetic gen e cluster and shares identity with the alcA gene required for synthesi s of the siderophore alcaligin in Bordetella spp. A second mutation (f oxR::MudIIpR13) affects the synthesis of the ferrioxamine receptor Fox R, encoded by the foxR gene, and was shown to be transcribed into a mo nocistronic message. Accordingly, the foxR mutant accumulates DFO in t he external medium. The growth of the mutants when supplied with vario us iron sources was examined; it indicates that the production of DFO and the specific transport of the DFO ferric complex are required only when iron is strongly liganded. Pathogenicity was scored after inocul ation of apple seedlings and after infection of apple flowers. On seed lings, the DFO biosynthetic mutant behaved like the wild-type strain w hile the frequency of necrotic plants caused by the receptor mutant de creased by a factor of two to five, depending on the initial inoculum. On flowers, both mutants were strongly affected in their ability to i nitiate a necrotic symptom and their growth was reduced by two orders of magnitude relative to the wild-type strain. However, the virulence of the dfoA mutant varied with the inoculum concentration. Unlike the foxR mutant, the dfoA mutant only weakly induced plant cell electrolyt e leakage in tobacco leaf disks. The supply with exogenous DFO, only w hen iron free, restored the ability to induce electrolyte leakage to t he dfoA mutant and increased the leakage induced by other strains. DFO alone was not an inducer. Iron-free DFO was able to protect E. amylov ora cells against lethal doses of hydrogen peroxide. The main conclusi on was that production of DFO in E. amylovora during pathogenesis is n ot only a critical function for iron acquisition, but can play a role in the oxidative burst elicited by the bacteria.