Cell-substratum adhesive protein involved in surface contact responses of the bean rust fungus

Uredospore germlings of the bean rust fungus Uromyces appendiculatus display two contact-sensitive responses on leaves or certain synthetic surfaces: a specific orientation of germ tubes and an induction of appressoria at leaf stomates or scratched surfaces. Germ tube orientation, nuclear division (used as a marker for appressorium formation) and germling adhesion on scratched “Parafilm” was reduced by the proteolytic enzyme, pronasc E (ED₅₀ = 20–50 μg ml⁻¹), but not by the heat denatured proteolytic enzyme. Forty micrograms of pronase E per millilitre significantly reduced nuclear division of germlings incubated on either a hydrophilic or a hydrophobic surface. Pronase E reduced adhesion regardless of whether the enzyme was applied during growth or after attachment had occurred, although higher concentrations were required after attachment had occurred. The data suggest that extracellular protein is required for germling adhesion to a substratum and that adhesion is required for germ tube orientation and thigmodifferentiation.We partially characterized the composition of the bean rust extracellular material and analyzed for proteins on SDS-polyacrylamide gels. Intact germlings have six predominant extracellular peptides detected by ¹²⁵I labelling. Spent culture fluid contains several different peptides; four of the five major peptides in the spent culture fluid are glycosylated.