Detoxification of poisonous xenobiotics in animals is typically performed by multi-gene enzyme families. Within arthropods, only insect genomes have been studied in detail where these families are well characterized. I recently uncovered a new enzyme family in the genome of a non-insect arthropod, the extremely polyphagous plant-feeding mite Tetranychus urticae and showed that this proliferated family was acquired via horizontal gene transfer from a fungal donor. The family codes for intradiol ring cleaving dioxygenases which cleave a particular set of aromatic structures, commonly found in pesticides and plant metabolites. Here, I propose to functionally characterize this exciting new gene family and to elucidate its role in xenobiotic detoxification, with a focus on plant secondary metabolites. First, to create a general picture, I will map the in situ expression of dioxygenases and time their responses to plant-derived secondary defense metabolites. Second, guided by preliminary results, I plan to study how these new herbivore enzymes counteract polyphenol oxidases, well-known plant defense enzymes that act against herbivores and target the same class of substrates. State-of-the-art plant transformation experiments will be performed in order to meticulously dissect their counterplay. Finally, by means of an unbiased multi-layered strategy, I will functionally characterize these new dioxygenases. Dioxygenases will be introduced into biological systems by functional expression in E. coli or insect cells, and by genetically transforming Drosophila. Cutting-edge differential metabolomics will identify the substrates and reaction products. By means of this project, I expect to unravel the selective advantage of this new family for phytophagous mites and open up avenues to exciting biotechnological applications which I expect to extend well beyond agriculture.