Rhizobacteria-mediated induced systemic resistance (ISR) in Arabidopsis requires sensitivity to jasmonate and ethylene but is not accompanied by an increase in their production

Plants develop an enhanced defensive capacity against a broad spectrum of p lant pathogens after colonization of the roots by selected strains of nonpa thogenic biocontrol bacteria. In Arabidopsis thaliana, this induced systemi c resistance (ISR) functions independently of salicylic acid but requires a n intact response to the plant hormones jasmonic acid (JA) and ethylene. To further investigate the roles of JA and ethylene in the ISR signalling pat hway, the levels of these signalling molecules were determined in A. thalia na upon induction of ISR by Pseudomonas fluorescens WCS417r and subsequent challenge inoculation with Pseudomonas syringae pv. tomato DC3000. Upon tre atment of the roots with ISR-inducing WCS417r bacteria, neither the JA cont ent, nor the level of ethylene evolution was altered in systemically resist ant leaves. Infiltration of leaves with WCS417r triggered the JA- and ethyl ene-dependent ISR pathway, but did not cause local changes in the productio n of either of these signalling molecules. These results indicate that rhiz obacteria mediated ISR is not based on the induction of changes in the bios ynthesis of either JA or ethylene. However, in ISR-expressing plants the ca pacity to convert 1-aminocyclopropane-1-carboxylate (ACC) to ethylene was s ignificantly enhanced, providing a greater potential to produce ethylene up on pathogen attack. (C) 2000 Academic Press.