Extracellular Proteins Associated with Induction of Differentiation in Bean Rust Uredospore Germlings

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Abstract
The bean rust fungus (Uromyces appendiculatus) has a contact-sensitive response to the stoma, the fungal penetration site on a leaf. Stomatal recognition is followed by mitotic nuclear division and differentiation of the first infection structure, the appressorium. Our evidence suggests that proteins in the extracellular matrix of the germ tube may be involved in the thigmotropic response. Nuclear division of germlings incubated in vitro on a normally inductive scratched surface was significantly reduced by 500 .mu.g of either proonase E or trypsin per milliliter but not by heat-denatured enzymes, trypsin mixed with trypsin inhibitor, or 500 .mu.g of either .alpha.- or .beta.-glucosidase, .alpha.-mannosidase, or lipase per milliliter. Incubating germlings with pronase E at any time prior to nuclear division inhibited germling response to the scratched surface. Pronase E at 200 .mu.g/ml did not decrease rates of germination or germ tube elongation, but adhesion of the germlings to the substrate was significantly reduced. The data suggest that extracellular proteins may bind the germ tube to an inductive surface, and that binding may be necessary for induction of infection structures.