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, the involvement of lipopolysaccharides (LPSs) in induction of systemic resistance was studied. Induction of systemic resistance by selected plant growth-promoting rhizobacteria (PGPR) strains of P. fluorescens was 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 separate locations on the plant root and were confined to these locations throughout the experiments. PGPR strains WCS374 and WCS417 of P. fluorescens and their crude cell wall extracts, which contained the LPS or purified LPS (consisting of lipid A/innercore/O-antigen side chain), induced systemic resistance, whereas P. putida WCS358 or its crude or purified 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-minus mutant, also induced systemic resistance when applied on the cotyledon of radish on an agar disk cut from a plate culture. The pathogen was delivered on the root in peat 2 days later. Thus, the resistance-inducing O-antigen of strain WCS374 was effective not only onto the root, but also on the cotyledon. In a bioassay with greenhouse soil naturally 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 commercial greenhouse trials.