Rhizobacteria-mediated induced systemic resistance (ISR) in Arabidopsis isnot associated with a direct effect on expression of known defense-relatedgenes but stimulates the expression of the jasmonate-inducible gene Atvsp upon challenge

Selected strains of nonpathogenic rhizobacteria from the genus Pseudomonas are capable of eliciting broad-spectrum induced systemic resistance (ISR) i n plants that is phenotypically similar to pathogen-induced systemic acquir ed resistance (SAR). In Arabidopsis, the ISR pathway functions independentl y of salicylic acid (SA) but requires responsiveness to jasmonate and ethyl ene. Here, we demonstrate that known defense-related genes, i.e. the SA-res ponsive genes PR-1, PR-2, and PR-5, the ethylene-inducible gene Hel, the et hylene- and jasmonate-responsive genes ChiB and Pdf1.2, and the jasmonate-i nducible genes Atvsp, Lox1, Lox2, Pal1, and Pin2, are neither induced local ly in the roots nor systemically in the leaves upon induction of ISR by Pse udomonas fluorescens WCS417r. In contrast, plants infected with the virulen t leaf pathogen Pseudomonas syringae pv. tomato (Pst) or expressing SAR ind uced by preinfecting lower leaves with the avirulent pathogen Pst(avrRpt2) exhibit elevated expression levels of most of the defense-related genes stu died. Upon challenge inoculation with Pst, PR gene transcripts accumulated to a higher level in SAR-expressing plants than in control-treated and ISR- expressing plants, indicating that SAR involves potentiation of SA-responsi ve PR gene expression. In contrast, pathogen challenge of ISR-expressing pl ants led to an enhanced level of Atvsp transcript accumulation. The other j asmonate-responsive defense-related genes studied were not potentiated duri ng ISR, indicating that ISR is associated with the potentiation of specific jasmonate-responsive genes.