Reactive oxygen intermediates and oxalic acid in the pathogenesis of the necrotrophic fungus Sclerotinia sclerotiorum

An effective colonization of the host plant tissue by the necrotrophic fungus Sclerotinia sclerotiorum requires the secretion of the non-host specific toxin oxalic acid (OA), which is known to suppress the generation of reactive oxygen intermediates (ROI). A full-length cDNA coding for an oxalate decarboxylase (TOXDC), which converts OA into CO2 and formate, was isolated from the basidiomycete Trametes versicolor. It was overexpressed in tobacco plants to study the role of ROI and OA in the interaction between tobacco and S. sclerotiorum. The transgenic plants contained less OA and showed a delayed colonization of S. sclerotiorum; furthermore a strong ROI accumulation and nearly no catalase activity compared to the wild type (WT) plants could be detected. In addition, inoculation experiments with transgenic catalase-deficient plants (CAT1AS) and in vitro studies showed that S. sclerotiorum copes with strong ROI stress. Our results indicate that OA supports the infection process caused by S. sclerotiorum and the fungus itself is able to tolerate high ROI concentrations.