Systemic resistance in Arabidopsis thaliana induced by biocontrol bacteria

Systemic acquired resistance (SAR) is a pathogen-inducible defense mechanism in plants effective against a broad spectrum of plant pathogens. The resistant state is dependent on endogenous accumulation of salicylic acid (SA) and is associated with the activation of a specific set of genes encoding pathogenesis-related (PR) proteins. Recently, selected nonpathogenic, root-colonizing bacteria with disease suppressive properties have been shown to trigger a systemic resistance response as well, without provoking any symptoms themselves. To study the molecular and mechanistic basis underlying this type of induced resistance, we developed a model system using Arabidopsis thaliana as the host plant and the fungal root pathogen Fusarium oxysporum f. sp. raphani and the bacterial leaf pathogen Pseudomonas syringae pv. tomato as challenging pathogens. Colonization of the rhizosphere of A. thaliana ecotypes Columbia and Landsberg erecta by biocontrol strain Pseudomonas fluorescens WCS417r resulted in a plant-mediated resistance response that significantly reduced symptoms elicited by both F. oxysporum and P. syringae. Moreover, proliferation of P. syringae in infected leaves was strongly inhibited in WCS417r-treated plants. Treatments with cell wall preparations of WCS417r and a WCS417r-mutant lacking the 0-antigenic side chain of the extracellular lipopolysaccharide (LPS), revealed that LPS is one of the bacterial determinants involved in the elicitation of the induced systemic resistance (ISR) response. Two other biocontrol strains, P. putida WCS35gr and P. fluorescens WCS374r, appeared to be intermediate and noninducers of ISR, respectively. Interestingly, A. thaliana ecotype RLD showed no resistance response upon treatment with WCS- 417r. These results suggest that elicitation of ISR involves specific recognition mechanisms between Arabidopsis and inducing rhizobacteria. Transgenic Arabidopsis NahG plants, unable to accumulate SA, and wild-type plants were equally responsive to WCS417r-induction. Furthermore, WCS417r-mediated induction of systemic resistance did not coincide with the accumulation of PR mRNAs prior to challenge inoculation. These results demonstrate that WCS417r induces a signalling pathway different from that controlling classical SAR, leading to a form of systemic resistance that is independent of SA accumulation and PR gene expression