INDUCTION OF SYSTEMIC RESISTANCE AGAINST FUSARIUM-WILT OF RADISH BY LIPOPOLYSACCHARIDES OF PSEUDOMONAS-FLUORESCENS

In commercial greenhouse trials, Pseudomonas fluorescens strain WCS374 suppressed Fusarium wilt and increased radish yield. In bioassays, th e involvement of lipopolysaccharides (LPSs) in induction of systemic r esistance was studied. Induction of systemic resistance by selected pl ant growth-promoting rhizobacteria (PGPR) strains of P. fluorescens wa s involved in the suppression of Fusarium wilt of radish in a special rock-wool bioassay. In this bioassay, the pathogen, Fusarium oxysporum f. sp. raphani, and the PGPR strain were inoculated at spatially sepa rate locations on the plant root and were confined to these locations throughout the experiments. PGPR strains WCS374 and WCS417 of P. fluor escens and their crude cell wall extracts, which contained the LPS or purified LPS (consisting of lipid A/innercore/O-antigen side chain), i nduced systemic resistance, whereas P. putida WCS358 or its crude or p urified LPS did not. Neither the phage-resistant mutants of WCS374 and WCS417 lacking the O-antigenic side chain of the LPS nor the crude or purified lipid A/innercore of these mutants reduced disease incidence in this experimental design. Strain WCS374, but not its O-antigen-min us mutant, also induced systemic resistance when applied on the cotyle don of radish on an agar disk cut from a plate culture. The pathogen w as delivered on the root in peat 2 days later. Thus, the resistance-in ducing O-antigen of strain WCS374 was effective not only onto the root , but also on the cotyledon. In a bioassay with greenhouse soil natura lly infested with the Fusarium wilt pathogen of radish, strain WCS374, but not the O-antigen-minus mutant, suppressed disease. This suggests the involvement of induced resistance in natural soil bioassays and c ommercial greenhouse trials.