Induced systemic resistance by plant growth-promoting rhizobacteria

Abstract

Rhizobacteria are present in large numbers on the root surface, where plant exudates and lysates provide nutrients. Selected strains of beneficial, plant growth-promoting rhizobacteria (PGPR) trigger a plant-mediated induced systemic resistance (ISR) response that is effective against a broad spectrum of plant pathogens. To study the molecular basis of ISR, an Arabidopsis thaliana-based model was developed, using PGPR strain Pseudomonas fluorescens WCS417r as the inducing agent. Genetic dissection of the ISR signalling pathway revealed that ISR is regulated by a defence pathway in which the phytohormones jasmonic acid and ethylene play key roles. Interestingly, the state of ISR is not associated with major changes in gene expression. Instead, ISR-expressing plants are primed to activate specific sets of defense-related genes faster or to a higher level upon pathogen attack. Here we review the current state of knowledge of the signal transduction steps involved in the ISR pathway in Arabidopsis that leads from recognition of the rhizobacteria by the roots to systemic expression of broad-spectrum disease resistance in above-ground foliar tissues.