ARS | Publication request: Ferric-pyoverdine recognition by Fpv outer-membrane proteins of Pseudomonas protegens Pf-5

Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2012
Publication Date: N/A

Interpretive Summary: Biological control provides a promising strategy for managing plant diseases, but has not yet been utilized widely in agriculture due, in part, to unexplained variation in its success in managing disease. Our research goals are to identify sources of variation in biological control and devise ways to make it more reliable. We focus on Pseudomonas fluorescens, which is a species of bacteria that occurs naturally on plant surfaces such as leaves and roots. Specifically, we focus on Pseudomonas protegens strain Pf-5, which lives on plant seeds and roots, and protects the plant from infection by plant pathogens that live in the soil. Iron is a prevalent element in the soil, but it is in a form that is unavailable to bacteria like Pf-5. In this study, we identified outer membrane receptors in Pf-5 that can transport iron-chelating compounds called siderophores produced by other bacteria into the bacterial cell. These receptors allow Pf-5 to acquire iron, which is otherwise limiting to its growth, by using resources produced by other bacteria in the environment. These data fit into an emerging picture of environmental factors controlling the colonization of plants by Pf-5.

Technical Abstract: The soil bacterium Pseudomonas protegens Pf-5 (previously called P. fluorescens Pf-5) produces two siderophores, enantio-pyochelin and a compound in the large and diverse pyoverdine family. Using high-resolution mass spectroscopy, we determined the structure of the pyoverdine produced by Pf-5. In addition to producing its own siderophores, Pf-5 also utilizes ferric complexes of some pyoverdines produced by other strains of Pseudomonas spp. as sources of iron. Previously, phylogenetic analysis of the 45 TonB-dependent outer-membrane proteins in Pf-5 indicated that six are in a well-supported clade with ferric-pyoverdine receptors (Fpvs) from other Pseudomonas spp. We used a combination of phylogenetics, bioinformatics, mutagenesis, pyoverdine structural determinations, and crossfeeding bioassays to assign specific ferric pyoverdine substrates to each of the six Fpvs of Pf-5. We identified at least one ferric pyoverdine that was taken up by each of the six Fpvs of Pf-5. Functional redundancy of the Pf-5 Fpvs also was apparent, with one ferric-pyoverdine taken up by all single Fpv- mutants but not by a mutant having deletions in two of the Fpv-encoding genes. Finally, we demonstrated that phylogenetically-related Fpvs take up ferric complexes of structurally-related pyoverdines, thereby establishing structure-function relationships that can be employed in the future to predict the pyoverdine substrates of Fpvs in other Pseudomonas spp.