The cellulose synthase 3 (CesA3) gene of oomycetes: structure, phylogeny and influence on sensitivity to carboxylic acid amide (CAA) fungicides.

Proper disease control is very important to minimize yield losses caused by oomycetes in many crops. Today, oomycete control is partially achieved by breeding for resistance, but mainly by application of single-site mode of action fungicides including the carboxylic acid amides (CAAs). Despite having mostly specific targets, fungicidal activity can differ even in species belonging to the same phylum but the underlying mechanisms are often poorly understood. In an attempt to elucidate the phylogenetic basis and underlying molecular mechanism of sensitivity and tolerance to CAAs, the cellulose synthase 3 (CesA3) gene was isolated and characterized, encoding the target site of this fungicide class. The CesA3 gene was present in all 25 species included in this study representing the orders Albuginales, Leptomitales, Peronosporales, Pythiales, Rhipidiales and Saprolegniales, and based on phylogenetic analyses, enabled good resolution of all the different taxonomic orders. Sensitivity assays using the CAA fungicide mandipropamid (MPD) demonstrated that only species belonging to the Peronosporales were inhibited by the fungicide. Molecular data provided evidence, that the observed difference in sensitivity to CAAs between Peronosporales and CAA tolerant species is most likely caused by an inherent amino acid configuration at position 1109 in CesA3 possibly affecting fungicide binding. The present study not only succeeded in linking CAA sensitivity of various oomycetes to the inherent CesA3 target site configuration, but could also relate it to the broader phylogenetic context.