Pyoverdin production by the plant growth beneficial Pseudomonas strain 7NSK₂: Ecological significance in soil

The P. aeruginosa-related plant growth beneficial strain 7NSK₂ was able to increase the yield of barley, wheat, 4 varieties of maize, cucumber, spinach and corn salad from 10 to 24%. Siderophore deficient mutants of 7NSK₂, obtained by Tn5-mutagenesis were never able to stimulate plant growth, although their root colonizing capacities were not impaired. The effect of 7NSK₂ inoculation on cucumber dry weight was more pronounced in EDDHA-treated soil (+18%) than in the control soil (+10%). Soil inoculation with 7NSK₂ in a FeEDDHA-treated soil had no effect on plant growth. The 7NSK₂ strain and a sid^- mutant MPFM1 could make use of a variety of Fe(III) siderophore complexes, while the pyoverdin produced by 7NSK₂ was only used by a limited number of other fluorescent Pseudomonas strains. Many more root than soil bacteria were able to use Fe-desferal or Fe-pyoverdin to overcome iron-shortage. It is postulated that pyoverdin production plays an important role in plant growth stimulation, but is not the only factor involved. Pyoverdin production might be very strategic, especially in the rhizosphere, not only to supply the cell with iron, but also to keep other microorganisms, which might be harmful to 7NSK₂ and to the plant, at bay.