Systemic Resistance in Arabidopsis lnduced bY Biocontrol Bacteria 1s lndependent of Salicylic Acid Accumulation and Pat hogenesis-Related Gene Expression

Systemic acquired resistance is a pathogen-inducible defense mechanism in plants. The resistant state is dependent on endogenous accumulation of salicylic acid (SA) and is characterized by the activation of genes encoding pathogenesisrelated (PR) proteins. Recently, selected nonpathogenic, root-colonizing biocontrol bacteria have been shown to trigger a systemic resistance response as well. To study the molecular basis underlying this type of systemic resistance, we developed an Arabidopsis-based model system using Fusarium oxysporum f sp raphani and Pseudomonas syringae pv tomato as challenging pathogens. Colonization of the rhizosphere by the biological control strain WCS417r of R fluorescens resulted in a plant-mediated resistance response that significantly reduced symptoms elicited by both challenging pathogens; Moreover, growth of R syringae in infected leaves was strongly inhibited in R fluorescens WCS417r-treated plants. Transgenic Arabidopsis NahG plants, unable to accumulate SA, and wild-type plants were equally responsive to R fluorescens WCS417r-mediated induction of resistance. Furthermore, R fluorescens WCS417r-mediated systemic resistance did not coincide with the accumulation of PR mRNAs before challenge inoculation. These results indicate that R fluorescens WCS417r induces a pathway different from the one that controls classic systemic acquired resistance and that this pathway leads to a form of systemic resistance independent of SA accumulation and PR gene expression.