Rhizobacteria-mediated induced systemic resistance: Triggering, signallingand expression

Selected strains of rhizosphere bacteria reduce disease by activating a res istance mechanism in the plant named rhizobacteria-mediated induced systemi c resistance (ISR). Rhizobacteria-mediated ISR resembles pathogen-induced s ystemic acquired resistance (SAR) in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of pl ant pathogens. Some rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface. In other cases, rhizobact eria trigger a different signalling pathway that does not require SA. The e xistence of a SA-independent ISR pathway has been demonstrated in Arabidops is thaliana. In contrast to pathogen-induced SAR, ISR induced by Pseudomona s fluorescens WCS417r is independent of SA accumulation and pathogenesis-re lated (PR) gene activation but, instead, requires responsiveness to the pla nt hormones jasmonic acid (JA) and ethylene. Mutant analyses showed that IS R follows a novel signalling pathway in which components from the JA and et hylene response are successively engaged to trigger a defensive state that, like SAR, is controlled by the regulatory factor NPR1. Interestingly, simu ltaneous activation of both the JA/ethylene-dependent ISR pathway and the S A-dependent SAR pathway results in an enhanced level of protection. Thus co mbining both types of induced resistance provides an attractive tool for th e improvement of disease control. This review focuses on the current status of our research on triggering, signalling, and expression of rhizobacteria -mediated ISR in Arabidopsis.