EXTRACELLULAR PROTEASES OF THE RUST FUNGUS UROMYCES-VICIAE-FABAE

On thigmo-inductive membranes the broad bean rust fungus Uromyces viciae-fabae differentiates complex infection structures including haustorial mother cells. Using this in vitro system, formation of extracellular proteases of the obligately biotrophic fungus was studied during infection structure differentiation. Enzyme activities occur when appressoria are formed, and extracellular washing fluids of substomatal vesicles, infection hyphae, and haustorial mother cells show complex protease patterns on polyacrylamide gels containing gelatin as substrate. The majority of the rust proteases can be classified as metallo-, including Ca2+-stabilized proteases. The presence of substrate is not required for synthesis of the enzymes. The extracellular proteases, in contrast to intracellular enzymes of this fungus, specifically degrade fibrous, hydroxyproline-rich proteins. Since such proteins are important in plants for cell wall stability and play a role in defense against fungal pathogens, the extracellular proteases of U. viciae-fabae may be involved in localized breaching of the host cell wall. (C) 1995 Academic Press, Inc.