Transcriptional regulators of rhizobacteria-induced systemic resistance

To defend themselves against pathogens, plants can activate a wide array of inducible defense mechanisms. Colonization of plant roots by selective non-pathogenic rhizobacteria or fungi systemically enhances the defensive capacity of the plant. In Arabidopsis thaliana this so-called induced systemic resistance (ISR) is regulated by a defense signaling pathway in which the phytohormones jasmonic acid (JA) and ethylene (ET) play an important role. In Arabidopsis roots, colonization by the ISR-inducing strain Pseudomonas fluorescens WCS417r altered the transcriptional activity of 97 genes. One of the induced genes encodes the transcription factor (TF) MYB72. Arabidopsis myb72 mutants were not able to enhance their defensive capacity upon colonization by WCS417r or the fungal strain Trichoderma asperellum T34, demonstrating that MYB72 is required for ISR. However, transformants in which MYB72 is constitutively expressed did not show an increased level of resistance. This indicates that besides MYB72 at least one other component is required for ISR-signaling. Yeasts two-hybrid experiments demonstrated a physical interaction of MYB72 with the EIN3-like protein EIL3 in vitro. The latter has been implicated in the regulation of responses to ET and is as such a likely candidate involved in MYB72-dependent ISR signaling. In the shoot tissue no direct changes in gene expression could be observed upon colonization of the roots by WCS417r. However, after challenge inoculation, a large set of especially JA and/or ET-regulated genes showed a primed expression pattern. To investigate the molecular mechanism of this priming phenomenon, we monitored the expression of all methyl-JA-responsive genes in control and WCS417r-ISR-expressing plants. Promoter analysis of the genes that showed a WCS417r-primed response revealed a significant enrichment for the CACATG motif, which was previously demonstrated to serve as a docking site for the TF MYC2. Mutants defective in the MYC2 gene were not able to express WCS417r-ISR, indicating that MYC2 plays an important role in priming for enhanced JA-responsive gene expressing during WCS417r-ISR. To gain further insight in priming for enhanced defense we compared WCS417r-ISR with resistance induced by the non-protein amino acid ? -aminobutyric acid (BABA-IR). Both BABA-IR and WCS417r-IR against Hyaloperonospora parasitica is associated with priming for enhanced formation of callose-containing cell-wall appositions at sites of pathogen entry. Mutants with a disruption in the IBS2 or IBS3 gene were specifically blocked in this response and consequently were affected in both BABA-IR and WCS417r-ISR, indicating that the pathways share signaling components involved in priming for enhanced callose deposition. Q-PCR-based profiling of all 2300 putative Arabidopsis TFs revealed a higher activity of a subset of TF-encoding genes in the systemic tissue of WCS417r- and BABA-primed plants. The set of WCS417r-induced TF-genes was enriched for those involved in the regulation of responses to JA, such as AP2/EREBPs and MYC2. Amongst the TF genes of which the expression was induced by BABA, 21 encoded WRKYs, which have been demonstrated to regulate downstream processes in SA-signaling. Thus, induced resistance is accompanied by a systemic increase in the activity of a subset of TF genes, the composition of which depends on the nature of the priming agent. Collectively, the work described in this thesis advanced our understanding of rhizobacteria-mediated ISR and provided novel insights in the molecular mechanisms underlying priming for enhanced defense.