Visualization of elicitor-binding loci at the plant cell surface

We describe a method which allows the visualization of elicitor-binding loci at the surface of plant protoplasts. Prerequisites for this method are the preparation of protoplasts under conditions of minimal proteolysis, and the availability of antibodies against either the elicitor itself or against the fluorescein portion of elicitor-fluorescein conjugates. Silver enhancement is used to amplify the visibility of 5-nm gold particles which are attached to an appropriate secondary antibody. Bound elicitor can then be visualized by epipolarization microscopy. This method, designated SEIG-EPOM (for silver enhanced immunogold as viewed by epipolarization microscopy), has been applied to protoplasts of parsley (Petroselineum cirspum L.), using the Phytophthora megasperma elicitor, and soybean (Glycine max Merr.) using polygalacturonic acid elicitor isolated from citrus pectin. We have been able to estimate the number of specific binding loci as being less than 100 per protoplast. Such loci possibly represent clusters of individual elicitor-receptor complexes. Structurally related elicitors have been shown to compete effectively for binding sites. The latter are sensitive to proteolysis, as is the elicitation response of protoplasts.