Evaluating the contribution of pyoverdine to the anti-Phytophthora activity of two potato-associated Pseudomonas strains

The oomycete Phytophthora infestans has been causing detrimental yield losses over the last 200 years and to this day, controlling measures heavily rely on synthetic pesticides. In view of their environmental toxicity, biological control represents an attractive alternative to fight this pathogen. Pseudomonas strains are known to produce a large arsenal of secondary metabolites conferring protection against several crop diseases. Next to biocontrol traits causing direct pathogen inhibition, such as antibiotics and toxins, siderophores are considered important mediators of competitive inhibition of plant pathogens by Pseudomonas . However, whether siderophore production plays any role in the biocontrol of the late blight causing agent P. infestans has not yet been investigated. In this study, we focused on two Pseudomonas strains, R32 and R47, which have been previously characterized as successful antagonists against P. infestans . Both strains produce pyoverdine, thus the aim of this study was to evaluate the role of pyoverdine in the inhibition of P. infestans in both strains. For this purpose, we created pyoverdine-deficient mutants by knocking-out pvdE , the periplasmic ferribactin exporter. We did this in both wild-type and HCN-deletion backgrounds for the two strains. These mutants were then tested for loss of antagonistic activity against P. infestans in several in vitro assays targeting different developmental stages of the pathogen life cycle, as well as in leaf disc assays to assess the relevance of pyoverdine in planta. Our results indicate that pyoverdine plays a different role in both Pseudomonas strains: in leaf disc assays, lack of pyoverdine completely suppressed the ability of R47 to restrict symptom development, but it increased the protective efficacy of R32. In this latter strain, the lack of pyoverdine alone did not diminish its ability to inhibit the pathogen‘s mycelium or spores, but when combined to the loss of HCN, it either led to a complete loss of inhibition in spore assays, or to stronger inhibition in mycelium assays. These results suggest an interplay between HCN and pyoverdine and the upregulation of a yet unknown mechanism underlying the higher in planta protective efficacy observed in R32 pyoverdine-deficient mutants.