Pyoverdine Plays Only a Minor, Strain-Specific Role in the Inhibition of Phytophthora infestans by Pseudomonas...

The oomycete Phytophthora infestans is a detrimental pathogen currently controlled by massive applications of synthetic pesticides. In view of the pesticides' toxicity, biological control represents an attractive alternative to fight this pathogen. Pseudomonas strains are known to produce diverse specialized metabolites conferring protection against several crop diseases. Next to molecules causing direct pathogen inhibition, such as antibiotics and toxins, siderophores are considered important mediators of competitive inhibition of pathogens by Pseudomonas. However, whether siderophore production plays any role in the biocontrol of P. infestans has not yet been investigated. In this study, we focused on two Pseudomonas strains, Pseudomonas donghuensis R32 and Pseudomonas chlororaphis R47, both producing the siderophore pyoverdine, and previously characterized as inhibitors of P. infestans. The aim of this study was to evaluate the role of pyoverdine in the inhibition of P. infestans by both strains. For this purpose, we created pyoverdine-deficient mutants by knocking-out pvdE, the periplasmic ferribactin exporter, in both wild-types and HCN-deletion mutants. These mutants were then tested for activity against P. infestans in different in vitro assays as well as in leaf discs. Our results indicate that pyoverdine plays no role in the inhibition of P. infestans by P. donghuensis R32 and only a minor role in P. chlororaphis R47, which was observed solely in leaf disc assays. Nevertheless, our findings demonstrate that iron competition plays an important role in the antagonistic effect of P. donghuensis R32 on P. infestans and suggest the involvement of another siderophore in the competition for iron.