Transcription factors in roots and shoots of Arabidopsis involved in rhizobacteria-induced systemic resistance

Plants possess inducible resistance mechanisms through which they can regulate their defense response to pathogen attack. Colonization of Arobidopsis thaliana roots by non-pathogenic Pseudomonas fluorescens WCS417r bacteria triggers a jasmonate- and ethylene-dependent induced systemic resistance (ISR) that is effective against a broad range of foliar pathogens. In the roots, the transcriptional activity of a large number of genes is altered upon colonization by WCS417r. To investigate the role of WCS417r-responsive, root specific genes in ISR signaling we screened T- DNA insertion lines of a subset of these genes. Bioassays revealed that AtMYB72, a transcription factor gene specifically induced in the roots upon colonization by WCS417r, is essential for activation of ISR. The myb72 knockout mutant was incapable of mounting WCS417r-mediated ISR against the challenging pathogens Pseudomonas syringae pv. tomoto DC3000 and Hyaloperonospora parasitica. Analysis of AtMYB72 gene expression revealed that ethylene is an important regulator of AtMYB72. This was supported by the finding that AtMYB72 was found to physically interact with the ethylene-regulatory protein EIL3 in a yeast two-hybrid assay. Transcript profiling revealed that ISR-expressing leaves are primed for augmented expression of predominantly jasmonate- and ethylene-responsive genes. Promoter analysis of these primed genes showed overrepresentation of an AtMYC2 binding motif, suggesting a regulatory role for this transcription factor in ISR. Further evidence for the involvement of AtMYC2 in ISR arose from bioassays showing that AtMYC2 knockout mutants were not able to show ISR after root colonization by WCS417r, while their level of basal resistance was comparable to that of wild-type Col-0