Salicylic acid (SA) is involved in mediating defense against biotrophic pathogens. The current knowledge of the SA-mediated signaling pathway and its effect on the transcriptional regulation of... Show moreSalicylic acid (SA) is involved in mediating defense against biotrophic pathogens. The current knowledge of the SA-mediated signaling pathway and its effect on the transcriptional regulation of defense responses are reviewed in this thesis. PR-1 is a marker gene for systemic acquired resistance (SAR) and we have shown that the plant-specific transcription factor WRKY50 binds to the Arabidopsis PR-1 promoter at two positions close to binding sites of TGA proteins, bZIP class transcription factors previously shown to be important for PR-1 expression. We found that WRKY50 interacts with TGAs and together they synergistically activate PR-1 expression. In addition, a number of W-boxes in the PR-1 promoter have been shown to be also important for expression. We have shown that WRKY28 is able to bind to these W-boxes and to activate PR-1 gene expression in protoplasts. We also describe the effects of overexpression of the WRKY50 and WRKY28 proteins in transgenic plants. High constitutive expression of the WRKY50 gene resulted in accumulation of PR-1 mRNA when the plants were treated with SA. The overexpression of WRKY50 and WRKY28 had no clear effect on the plants__ resistance to the biotrophic pathogens, and neither was this the case with WRKY50 T-DNA knockout plants. To investigate the WRKYs__ influence on metabolite composition, a 1H NMR spectroscopy-based metabolomic approach was applied. The WRKY50 overexpression plants contained two- to three-fold more sinapic acid derivatives. This indicates a possible involvement of WRKY50 on metabolite production, in particular of hydroxycinnamates such as sinapic acid. As these compounds are components of lignin, this may point to a role of WRKY50 in stress-induced lignin modification. Show less
