We investigate the manipulation of extracellular plant enzymes by pathogens. The apoplast (the extracellular space in plants) is one of the largest biotic interfaces on earth. The apoplast can be heavily colonized by microbes, mostly bacteria, but also fungi and oomycetes. The apoplast is a dynamic environment where conditions depend on the time of the day and the weather. This environment is also controlled by the host plant, which responds to microbes by secreting defence compounds and proteins. Defence proteins include a broad range of hydrolytic enzymes such as glycosidases, proteases and lipases that are potentially harmful for microbes colonizing the apoplast. We study how successful pathogens manipulate these hydrolytic enzymes to suppress their activity. The aim of our research program is to identify manipulated enzyme activities and unravel their molecular mechanism and biological relevance. The model system is the interaction between wild tobacco (Nicotiana benthamiana) and commensal and pathogenic strains of the bacterium Pseudomonas syringae. In some cases, studies are extended to include infection of other solanaceae plants by the leaf mould fungus Cladosporium fulvum and the potato blight pathogen Phytophthora infestans. Our studies on the tomato proteases and its inhibition by Avr2 from Cladosporium fulvum and EpiCs from Phytophthora infestans has revealed how important these host enzymes are in basal defence, and how the arms-race during co-evolution has shaped natural variation in host proteases.