In the first part of this thesis the metabolism of the phytotoxins destruxin B and sirodesmin PL in crucifers and non-crucifers was studied using HPLC-ESI-MSn. Destruxin B and sirodesmin PL are phytotoxins produced by the phytopathogenic fungi Alternaria brassicae (Berk.) Sacc. (causative agent of blackspot disease) and Leptosphaeria maculans (Desm) Ces. et de Not.[asexual stage Phoma lingam (Tode ex Fr) Desm.] (causative agent of blackleg disease). Five cruciferous species were used in this study: Arabidopsis thaliana L., Brassica rapa L., B. napus L., Thellungiella salsuginea Pallas and Erucastrum gallicum O.E. Schulz. In addition, the cereals Avena sativa L. and Triticum aestivum L. were studied similarly. Destruxin B was metabolized by all crucifers to hydroxydestruxin B, a transformation similar to previously reported reactions in other crucifers. In addition, destruxin B elicited production of phytoalexins in A. thaliana, T. salsuginea and E. gallicum, while no phytoalexins were detected in case of B. rapa and B. napus. In cereals destruxin B was transformed differently. Several metabolites were detected and identified by HPLC-ESI-MSn analyses: hydroxydestruxin B, two isomers of dehydrodestruxin B and desmethyldestruxin B. On the other hand, no metabolites related to transformation of sirodesmin PL were detected in crucifers; however, in cereals sirodesmin PL was transformed to deacetylsirodesmin PL. In all crucifers sirodesmin PL was found to be a stronger elicitor of phytoalexin production than destruxin B. In the second part of this thesis, mycelia from different pathogenic fungi were screened for indolyl-3-acetaldoxime dehydratase. L. maculans isolate Laird 2 was chosen for isolation, characterization and substrate specificity of aldoxime dehydratase, as it showed the highest specific activity among the tested pathogens. The enzyme was partially purified using three chromatographic steps. It showed Michaelis–Menten kinetics and an apparent molecular mass of about 40 kDa. Based on its substrate specificity, the enzyme appears to be an indolyl-3-acetaldoxime dehydratase