Proteinases secreted by the oomycete Phytophthora infestans (Mont.) de Bary, Rhizoctonia solani, and Fusarium culmorum belonging to different families of fungi have been studied to determine if the exoenzyme secretion depends on the environmental conditions and the phylogenetic position of the pathogen. The substrate specificity of the extracellular proteinases of F. culmorum, R. solani, and P. infestans and their sensitivity to the action of synthetic and protein inhibitors suggest that they contain trypsin-like and subtilisin-like enzymes regardless of culture medium composition. The relation of trypsin-like and subtilisin-like enzymes is dependent on the culture medium composition, especially on the form of nitrogen nutrition, particularly in the case of the exoenzymes secreted by R. solani. Phylogenetic analyses have shown that the exoproteinase set of ascomycetes and oomycetes has more similarities than basidiomycetes although they are more distant relatives. Our data suggests that the multiple proteinases secreted by pathogenic fungi could play different roles in pathogenesis, increasing the adaptability and host range, or could have different functions in survival in various ecological habitats outside the host. 1. Introduction Fungi and oomycetes are responsible for many of the most devastating plant diseases that lead to very significant losses in the agricultural sector worldwide. Approximately 100,000 species of fungi and oomycetes have been described, but only a very small proportion of these are pathogenic [1]. However, phylogenetic studies have shown that disease-causing pathogens are not necessarily closely related to each other. In fact, they are spread throughout all taxonomic groups of fungi, often showing a close evolutionary relationship to nonpathogenic species [2]. It therefore seems likely that phytopathogenicity has evolved as a trait many times during fungal and oomycete evolution [2]. A significant effort has been made into the identification of pathogenicity determinants such as individual genes that are essential for a pathogen to invade a host plant successfully but that are dispensable for saprophytic growth [3]. Despite the different origin and different site on the phylogenetic tree of the true fungi and oomycetes [4], it has been shown that a range of secreted proteins known as effectors are important for establishing infection of the host plant [2]. These secreted proteins can suppress plant defenses and subvert cellular processes to suit the needs of invading pathogens. They include a number of secreted proteinases,
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