Abstract
The plant growth-promoting rhizobacterium Pseudomonas fluorescens WCS374r effectively suppresses fusarium wilt in radish by induced systemic resistance (ISR). In radish, WCS374r-mediated ISR depends partly on iron-regulated metabolites. Under iron-limiting conditions, P. fluorescens WCS374r produces the fluorescent siderophore pseudobactin (Psb), salicylic acid (SA), and pseudomonine (Psm), a siderophore that contains a
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SA moiety. Applying femtogram quantities of SA to radish roots was sufficient to trigger ISR against Fusarium. In Arabidopsis WCS374r was not able to elicit ISR against Pseudomonas syringae pv. tomato DC3000 (Pst), when applied at high inoculum density, whereas SA does induce resistance against this pathogen. The inability of WCS374r to induce resistance in Arabidopsis suggested that SA is not released into the rhizosphere. In this study the role of iron-regulated metabolites of WCS374r in ISR is further unraveled. Effects of iron availability, pH, and growth temperature on production of Psb, SA and Psm were evaluated. Ferric chloride suppressed biosynthesis of Psb, SA and Psm. Production of Psb was higher at lower pH, whereas biosynthesis of Psm was stimulated at pH above neutral. Biosynthesis of Psb and Psm was strongly influenced by temperature with the highest production at 20 C and hardly any production at 35 C. For SA production an optimum was observed at 30 C. Supplementing the medium with the Psm precursor histamine increased Psm production by WCS374r, whereas it led to a decrease in the production of the precursor SA. Mutants defective in production of at least one of the iron-regulated metabolites were generated. SA/Psm and SA/Psm/Psb mutants colonized Arabidopsis roots to the same extent as the wild-type strain. Therefore, production of these iron-regulated metabolites did not seem necessary for effective colonization of the Arabidopsis rhizosphere. Whereas P. fluorescens WCS374r did not elicit ISR against Pst in Arabidopsis when introduced at high densities it did when only 103 cfu.g-1 was applied. At this density, it also elicited ISR against the fungi Alternaria brassicicola and Botrytis cinerea, the oomycete Hyaloperonospora parasitica, and against turnip crinkle virus (TCV). WCS374r-mediated ISR against Pst did not rely on any of the iron-regulated metabolites. In contrast, biosynthesis of both SA and Psb appeared a prerequisite to effectively elicit ISR against TCV. The differential requirement for bacterial determinants of WCS374r in ISR against Pst and TCV indicated that WCS374r activates different defence signalling pathways. Signal transduction pathways leading to ISR by WCS374r against Pst and TCV were investigated. Using Arabidopsis transformant NahG, that degrades SA, the sid1 mutant that is unable to synthesize SA, and jasmonate response (jar1) and ethylene response (etr1) mutants, we demonstrated that induced resistance by WCS374r against Pst is independent of SA but requires plant responsiveness to jasmonate and ethylene. In contrast, WCS374r-mediated ISR against TCV requires SA-signalling. This differential requirement for SA-independent ISR against Pst and SA-dependent induced resistance against TCV indicates that WCS374r can activate both defence signalling pathways through different determinants. The wide-spectrum effectiveness against pathogens through activation of both SA-dependent and –independent defences make WCS374r a good candidate for application in crop protection.
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