DIFFERENTIATION AND CELL-WALL DEGRADING ENZYMES IN THE OBLIGATELY BIOTROPHIC RUST FUNGUS UROMYCES-VICIAE-FABAE

Rust fungi differentiate a series of complex infection structures to i nfect their host plants. Artificial membranes providing a signal for t he induction of infection structure differentiation have been used to study events taking place during early stages of host-pathogen interac tion. During the prepenetration phase, serine esterases, one of which shows cutinase activity, appear to be important for adhesion of uredos pores of Uromyces viciae-fabae to the plant cuticle. When the fungus g rows through the stomatal opening, chitin deacetylase activity increas es drastically. The role of this enzyme in masking and preventing degr adation of fungal structures by plant chitinases is discussed. Differe nt isoforms of protease, cellulase, and pectin methylesterases (PME) a re formed when the fungus enters the intercellular space, and synthesi s of polygalacturonate lyase (PL) coincides with formation of haustori al mother cells. Based on the physicochemical and catalytic properties of these cell wall degrading enzymes a model is presented that explai ns highly localized breaching of plant cell walls by obligate biotroph s. cDNAs corresponding to genes activated during late stages of infect ion structure differentiation of Uromyces viciae-fabae have been isola ted by differential hybridization. The transcripts of the genes design ated rif16 and rif21 occur when haustorial mother cells are formed, an d the corresponding gene products may thus be important for successful infection.