SYSTEMIC RESISTANCE IN ARABIDOPSIS INDUCED BY BIOCONTROL BACTERIA IS INDEPENDENT OF SALICYLIC-ACID ACCUMULATION AND PATHOGENESIS-RELATED GENE-EXPRESSION

Systemic acquired resistance is a pathogen-inducible defense mechanism in plants, The resistant state is dependent on endogenous accumulatio n of salicylic acid (SA) and is characterized by the activation of gen es encoding pathogenesis-related (PR) proteins, Recently, selected non pathogenic, root-colonizing biocontrol bacteria have been shown to tri gger a systemic resistance response as well, To study the molecular ba sis underlying this type of systemic resistance, we developed an Arabi dopsis-based model system using Fusarium oxysporum f sp raphani and Ps eudomonas syringae pv tomato as challenging pathogens, Colonization of the rhizosphere by the biological control strain WCS417r of P. fluore scens resulted in a plant-mediated resistance response that significan tly reduced symptoms elicited by both challenging pathogens, Moreover, growth of P. syringae in infected leaves was strongly inhibited in P. fluorescens WCS417r-treated plants, Transgenic Arabidopsis NahG plant s, unable to accumulate SA, and wild-type plants were equally responsi ve to P. fluorescens WCS417r-mediated induction of resistance. Further more, P. fluorescens WCS417r-mediated systemic resistance did not coin cide with the accumulation of PR mRNAs before challenge inoculation, T hese results indicate that P. fluorescens WCS417r induces a pathway di fferent from the one that controls classic systemic acquired resistanc e and that this pathway leads to a form of systemic resistance indepen dent of SA accumulation and PR gene expression.