Rhizobacteria-induced systemic resistance

Rhizobacteria-induced systemic resistance (ISR) is a type of systemically enhanced resistance against a broad spectrum of pathogens that is triggered upon root colonization by selected strains of non-pathogenic bacteria. Over the past decade, a myriad of bacterial traits operative in triggering ISR have been identified, including flagella, cell envelope components such as lipopolysaccharides, and secreted metabolites like siderophores, cyclic lipopeptides, volatiles, antibiotics, phenolic compounds, and quorum sensing molecules. This review provides an in-depth overview of these determinants, thereby focusing specifically on the molecular recognition processes in the plant. The putative mechanisms involved in microbial perception include high- and low-affinity membrane receptors, membrane bilayer perturbation, and siderophore-mediated alterations in cellular iron homeostasis. In addition, details about the various defence signalling pathways reported to underpin rhizobacteria-mediated ISR are presented. Evidence is accumulating that there is not one definitive resistance pathway to ISR but that various hormone-dependent signalling conduits may govern the induced resistance phenotype depending on the rhizobacterium and the plant-pathogen system used