Comparison of systemic resistance induced by avirulent and nonpathogenic Pseudomonas species

The plant growth-promoting rhizobacterium (PGPR) Pseudomonas fluorescens strain WCS417 has been shown to induce systemic resistance against Fusarium oxysporum in several plant species without inducing synthesis of pathogenesis-related proteins (PR). The aim of this study was to establish the spectrum of protection achieved by P. fluorescens-mediated induced resistance and to compare its effectiveness with pathogen-mediated systemic acquired resistance, which is associated with necrosis and induction of PR synthesis. We demonstrated that pretreatment with P. fluorescens protects radish (Raphanus sativus) through induction of systemic resistance not only against the fungal root pathogen F. oxysporum f. sp. raphani, but also against the avirulent bacterial leaf pathogen P. syringae pv. tomato and the fungal leaf pathogens Alternaria brassicicola and F. oxysporum. We thus demonstrated, for the first time, that one PGPR strain can induce resistance against multiple pathogens. The level of protection was at least as high as that achieved by the necrotizing, PR-inducing P. syringae pv. tomato, and the spectrum was even broader indicating that i) necrosis is not a prerequisite for effective, biological induction of resistance and ii) the absence of PR after induction by P. fluorescens does not lower the level of protection.