A novel defence pathway in Arabidopsis induced by biocontrol bacteria

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis thaliana, non-pathogenic, root-colonising Pseudomonas fluorescens bacteria with biological disease control activity trigger an induced systemic resistance (ISR) response against infection by P. syringae pv tomato (Pst), the causal agent of bacterial speck disease. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR is independant of salicylic acid (SA) accumulation and pathogenesis related (PR) gene activation. Using the jasmonate response mutant jar1, the ethylene response mutant etr1, and the SAR regulatory mutant npr1 of Arabidopsis, it is demonstrated that signal transduction leading to P. fluoresens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene, and is dependant on the SAR regulatory protein NPR1. In addition to WCS417r, methyl jasmonate (MeJA) and the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) were effective in inducing resistance against Pst in transgenic Arabidopsis NahG plants that are unable to accumulate SA. Furthermore, MeJA-induced protection was blocked in jar1, etr1, and npr1 plants, whereas ACC-induced protection was effected in etr1 and npr1 plants, but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signalling pathway in which components from the jasmonate and ethylene response are successively engaged to trigger a defence reaction that, like SAR, is regulated by NPR1.