Abstract
The non-pathogenic Pseudomonas fluorescens strain WCS417r has been shown to induce systemic resistance in radish against Fusarium oxysporum f.sp. raphani. In this paper we investigate the involvement of pathogenesis-related (PR) proteins in this Pseudomonas-induced resistance. For comparison, salicylic acid (SA) and isonicotinic acid (INA) were used as inducers. It appeared that in our rock wool artificial soil system both strain WCS417r and SA could induce resistance, whereas strain WCS417r-B4, a lipopolysaccharide-mutant, and INA could not. The induced resistance was expressed as a reduction in the percentage of diseased plants, but not in the disease severity. Western blot analyses revealed that in the control plants PR-2 is constitutively present in the intercellular fluid of leaves and PR-3 in the roots. Treatment with either of the inducing agents, with or without pathogen-inoculation, induced no further accumulation of cross-reacting homologues of PR-1-5. We concluded that in our system induction of systemic resistance by strain WCS417r and SA is not associated with accumulation of PRs proteins. This demonstrates that accumulation of PR proteins is not a prerequisite for the expression of induced systemic resistance.
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