Fitness in soil and rhizosphere of Pseudomonas fluorescens C7R12 compared with a C7R12 mutant affected in pyoverdine synthesis and uptake

Fluorescent pseudomonads have evolved an efficient strategy of iron uptake based on the synthesis of the siderophore pyoverdine and its relevant outer membrane receptor. The possible implication of pyoverdine synthesis and up take on the ecological competence of a model strain (Pseudomonas fluorescen s C7R12) ill soil habitats was evaluated using a pyoverdine minus mutant (P L1) obtained by random insertion of the transposon Tn5. The Tn5 flanking DN A was amplified by inverse PCR and sequenced. The nucleotide sequence was f ound to show a high level of identity with pvsB, a pyoverdine synthetase. A s expected, the mutant PL1 was significantly more susceptible to iron starv ation than the wild-type strain despite its ability to produce another unkn own siderophore. As with the wild-type strain, the mutant PL1 was able to i ncorporate the wild-type pyoverdine and five pyoverdines of foreign origin, but at a significantly lower rate despite the similarity of the outer memb rane protein patterns of the two strains. The survival kinetics of the wild -type and of the pyoverdine minus mutant. in bulk and rhizosphere soil, wer e compared under gnotobiotic and non-gnotobiotic conditions. In gnotobiotic model systems, both strains, when inoculated separately, showed a similar survival in soil and rhizosphere, suggesting that iron was not a limiting f actor. In contrast, when inoculated together, the bacterial competition was favorable to the pyoverdine producer C7R12. The efficient fitness of PL1 i n the presence of the indigenous microflora, even when coinoculated with C7 R12, is assumed to be related to its ability to uptake heterologous pyoverd ines. Altogether, these results suggest that pyoverdine-mediated iron uptak e is involved in the ecological competence of the strain P. fluorescens C7R 12. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.