Iron availability affects induction of systemic resistance to Fusarium wilt of radish by Pseudomonas fluorescens

A special bioassay on rock wool was used to study the influence of iron availability on the induction of systemic resistance in radish (Raphanus sativus L.) against Fusarium wilt mediated by Pseudomonas fluorescens. In this bioassay, the pathogen (Fusarium oxysporum f. sp. raphani) and a strain of Pseudomonas, salicylic acid (SA), or a pseudobactin were applied at separate locations on the plant root. Strain WCS374 of P. fluorescens and its pseudobactin-minus Tn5 mutant gave greater disease control in the induced systemic resistance bioassay when iron availability in the radish nutrient solution was low than when it was high. Mutants of P fluorescens strains WCS374 and WCS417 lacking the O-antigenic side chain of the lipopolysaccharide induced resistance at low but not at high iron availability. The purified pseudobactin of strain WCS374, but not the pseudobactins of strains WCS358 and WCS417, induced resistance. Gas chromatography and spectrophotometry were used to detect and measure production of SA by these strains. Strains WCS374 and WCS417 produced 47 and 8 mu g of SA per milliliter, respectively, at low iron availability in vitro; the production of SA decreased rapidly with increasing iron availability. P. putida WCS358 did not induce resistance, either at low or at high iron availability, and did not produce SA in vitro. Commercial SA induced resistance at concentrations as low as 100 fg per root. High concentrations (> 1 mg/ml) of SA reduced growth of hyphae from germinated conidia of F. oxysporum f. sp, raphani. We hypothesize that the Fe3+-chelating SA, produced by selected P. fluorescens strains at low iron availability, is involved in the induction of systemic resistance to Fusarium wilt of radish. The pseudobactin produced by WCS374 may be involved as well. Given these results, it seems appropriate to reevaluate the role of siderophoremediated competition for iron in the suppression of disease by fluorescent Pseudomonas spp.