Siderophores and outer membrane proteins of antagonistic, plant-growth-stimulating, root-colonizing Pseudomonas spp.

As an approach to understanding the molecular basis of the reduction in plant yield depression by root-colonizing Pseudomonas spp. and especially of the role of the bacterial cell surfaces in this process, we characterized 30 plant-root-colonizing Pseudomonas spp. with respect to siderophore production, antagonistic activity, plasmid content, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis patterns of their cell envelope proteins. The results showed that all strains produce hydroxamate-type siderophores which, because of the correlation with Fe3+ limitation, are thought to be the major factor responsible for antagonistic activity. Siderophore-negative mutants of two strains had a strongly decreased antagonistic activity. Five strains maintained their antagonistic activity under conditions of iron excess. Analysis of cell envelope protein patterns of cells grown in excess Fe3+ showed that most strains differed from each other, although two classes of similar or identical strains were found. In one case such a class was subdivided on the basis of the patterns of proteins derepressed by iron limitation. Small plasmids were not detected in any of the strains, and only one of the four tested strains contained a large plasmid. Therefore, it is unlikely that the Fe3+ uptake system of the antagonistic strains is usually plasmid encoded.