Priming of plant innate immunity by rhizobacteria and β-aminobutyric acid: differences and similarities in regulation

Pseudomonas fluorescens WCS417r bacteria and β-aminobutyric acid can induce disease resistance in Arabidopsis, which is based on priming of defence. In this study, we examined the differences and similarities of WCS417r- and β-aminobutyric acid-induced priming. Both WCS417r and β-aminobutyric acid prime for enhanced deposition of callose-rich papillae after infection by the oomycete Hyaloperonospora arabidopsis. This priming is regulated by convergent pathways, which depend on phosphoinositide and ABA-dependent signalling components. Conversely, induced resistance by WCS417r and β-aminobutyric acid against the bacterial pathogen Pseudomonas syringae are controlled by distinct NPR1-dependent signalling pathways. As WCS417r and β-aminobutyric acid prime jasmonate- and salicylate-inducible genes, respectively, we subsequently investigated the role of transcription factors. A quantitative PCR-based genome-wide screen for putative WCS417r- and β- aminobutyric acid-responsive transcription factor genes revealed distinct sets of priming-responsive genes. Transcriptional analysis of a selection of these genes showed that they can serve as specific markers for priming. Promoter analysis of WRKY genes identified a putative cis-element that is strongly over-represented in promoters of 21 NPR1-dependent, β-aminobutyric acid-inducible WRKY genes. Our study shows that priming of defence is regulated by different pathways, depending on the inducing agent and the challenging pathogen. Furthermore, we demonstrated that priming is associated with the enhanced expression of transcription factors