DIFFERENTIAL INDUCTION OF SYSTEMIC RESISTANCE IN ARABIDOPSIS BY BIOCONTROL BACTERIA

Selected nonpathogenic, root-colonizing bacteria are able to elicit in duced systemic resistance (ISR) in plants, To elucidate the molecular mechanisms underlying this type of systemic resistance, an Arabinopsis -based model system was developed in which Pseudomonas syringae pv. to mato and Fusarium oxysporum f. sp. raphani were used as challenging pa thogens, In Arabidopsis thaliana ecotypes Columbia and Landsberg erect a, colonization of the rhizosphere by P. fluorescens strain WCS417r in duced systemic resistance against both pathogens, In contrast, ecotype RLD did not respond to WCS417r treatment, whereas all three ecotypes expressed systemic acquired resistance upon treatment with salicylic a cid (SA). P. fluorescens strain WCS374r, previously shown to induce IS R in radish, did not elicit ISR in Arabidopsis. The opposite was found for P. putida strain WCS358r, which induced ISR in Arabidopsis but no t in radish, These results demonstrate that rhizosphere pseudomonads a re differentially active in eliciting ISR in related plant species, Th e outer membrane lipopolysaccharide (LPS) of WCS417r is the main ISR-i nducing determinant in radish and carnation, and LPS-containing cell w alls also elicit ISR in Arabidopsis. However, mutant WCS417rOA(-), lac king the O-antigenic side chain of the LPS, induced levels of protecti on similar to those induced by wild-type WCS417r. This indicates that ISR-inducing bacteria produce more than a single factor that trigger I SR in Arabidopsis. Furthermore, WCS417r and WCS358r induced protection in both wildtype Arabidopsis and SA-nonaccumulating NahG plants witho ut activating pathogenesis-related gene expression, This suggests that elicitation of an SA-independent signaling pathway is a characteristi c feature of ISR-inducing biocontrol bacteria.