Mechanisms of rhizobacteria-mediated induced systemic resistance

Some of non-pathogenic rhizosphere bacteria reduce disease by activating a resistance mechanism in the plant called rhizobacteria-mediated induced systemic resistance (ISR). Rhizobacteria-mediated ISR resembles classic pathogen-induced systemic acquired resistance (SAR) in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. In contrast to SAR, ISR induced by Pseudomonas fluorescens WCS417r functions independently of salicylic acid accumulation and pathogenesis-related gene activation. Mutant analyses showed that this ISR follows a novel signalling pathway in which components from the plant hormones jasmonic acid (JA) and ethylene response are successively engaged to trigger a defensive state that, like SAR, is controlled by the regulatory factor NPR1. To investigate the role of JA and ethylene in ISR, their production was monitored in ISR-expressing plants. Neither local nor systemic levels of JA and ethylene production changed upon induction of ISR. Therefore, we postulate that ISR is mediated via an increase in the plant's sensitivity to JA and ethylene. Interestingly, ISR-expressing plants show an increase in ACC-oxidase activity, providing a greater potential to produce ethylene after pathogen infection. In addition, ISR is associated with the potentiation of specific JA-inducible genes, resulting in an enhanced level of their expression after pathogen infection. These results suggest that ISR-expressing plants are primed to express ethylene- and JA-dependent defenses more efficiently upon pathogen infection.