Survival and root colonization of mutants of plant growth‐promoting pseudomonads affected in siderophore biosynthesis or regulation of siderophore production

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Abstract
The importance of the high affinity iron‐uptake system for survival and root colonization was studied via mutants of plant growth‐promoting pseudomonads affected in siderophore biosynthesis or regulation of siderophore production. In a nutrient‐enriched soil, but not in a normal soil, survival of regulatory and biosynthetic siderophore mutants was impaired in comparison with the wild type strain. Especially a mutation in an apparent “stress”; regulation of siderophore production had a deleterious effect on survival. Biosynthetic side mutants colonized maize roots as well as the parent strains. However, the root colonization capabilities of a mutant affected in the iron regulation of siderophore production were significantly impaired, probably because this mutant is unable to induce ironregulated outer membrane proteins (IROMPs) for its own siderophores and those produced by other microorganisms under iron‐limiting conditions. The results presented suggest that competition for iron is more important in the rhizosphere than in bulk soil and that the ability to use siderophores produced by others increases the competitiveness of rhizosphere bacteria. In a soil with a high microbial activity, pyoverdin production appears to have a defense function rather than an iron‐supply function.