Rust fungi differentiate a series of complex infection structures to infect their host plants. Artificial membranes providing a signal for the induction of infection structure differentiation have been used to study events taking place during early stages of host–pathogen interaction. During the prepenetration phase, serine esterases, one of which shows cutinase activity, appear to be important for adhesion of uredospores of Uromyces viciae-fabae to the plant cuticle. When the fungus grows through the stomatal opening, chitin deacetylase activity increases drastically. The role of this enzyme in masking and preventing degradation of fungal structures by plant chitinases is discussed. Different isoforms of protease, cellulase, and pectin methylesterases (PME) are formed when the fungus enters the intercellular space, and synthesis of polygalacturonate lyase (PL) coincides with formation of haustorial mother cells. Based on the physicochemical and catalytic properties of these cell wall degrading enzymes a model is presented that explains highly localized breaching of plant cell walls by obligate biotrophs. cDNAs corresponding to genes activated during late stages of infection structure differentiation of Uromyces viciae-fabae have been isolated by differential hybridization. The transcripts of the genes designated rif16 and rif21 occur when haustorial mother cells are formed, and the corresponding gene products may thus be important for successful infection. Key words: adhesion, cell wall degrading enzymes, chitin deacetylase, infection structure differentiation, penetration process, rif genes.
Uredospores of the obUgately biofrophic broad bean rust fungus Uromyces viciae-fabae form infection structures on artificial membranes providing a thigmotropic signal. In nalure these are essential for invasion of the host plant through the stomata. '[his experimental system was used to analyse the production of cellulolytic enzymes duting the differentiation of rust infection stru~lures.Low .cellulase activity was detected in dormant spores and in germ tubes. Enzyme activity increased duting appressorium development and reached a maximum when infection hyphae and haustorial mother cells were formed. Seven cellulolytic enzymes were separated by chromatofocusing on DEAE = Si500 and PBE 94 exchangers. At least two cellulases, characterized by isoelectric points of 7' I and 7'3, were identified as endo-eellulases. The neutral enzyme forms increased from 3'3 % of the total activity at appressorium formation to 36'6% when infection hyphae were formed (18 h p.i.), and to 45'4% in haustorial mother cells (24 h p.i.). Cellulase activity of the rust' fungus is neither substrate-inducible nor catabolite repressible. The regulation of these enzymes appears to be strictly controlled by the differentiation of infection struclures, and is therefore distinct from cellulases of necrotrophs and saprophytes investigated thus far.
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