nocytochem ical Localization of Wheat Germ Agglutinin in Wheat

Immunocytological techniques were developed to localize the plant lectin, wheat germ agglutinin (WGA), in the tissues and cells of wheat plants . In a previous study we demonstrated with a radioimmunoassay that the lectin is present in wheat embryos and adult plants both in the roots and at the base of the stem . We have now found, using rhodamine, peroxidase, and ferritin-labeled secondary antibodies, that WGA is located in cells and tissues that establish direct contact with the soil during germination and growth of the plant. In the embryo, WGA is found in the surface layer of the radicle, the first adventitious roots, the coleoptile, and the scutellum . Although found throughout the coleorhiza and epiblast, it is at its highest levels within the cells at the surface of these organs . In adult plants, WGA is located only in the caps and tips of adventitious roots. Reaction product for WGA was not visualized in embryonic or adult leaves or in other tissues of adult plants . At the subcellular level, WGA is located at the periphery of protein bodies, within electron-translucent regions of the cytoplasm, and at the cell wall-protoplast interface. Since WGA is found at potential infection sites and is known to have fungicidal properties, it may function in the defense against fungal pathogens. Lectins bind and cross-link specific monosaccharides and oligosaccharides . They thus serve as powerful probes in the study of the carbohydrate moieties at cell surfaces (see reference 14 for review) . Because this remarkable specificity is probably not fortuitous, the function of lectins in the organisms in which they are found should reflect their particular sugar-binding properties . In some cases the binding of a lectin to a specific carbohydrate receptor has been shown to mediate recognition events at the cellular level . Examples include influenza virus, which possesses a hemagglutinin that allows the virus to attach to sialic acid residues found on host cell plasma membranes (45), and the amoebae of Dictyostelium discoideum, which during aggregation synthesize a lectin that promotes cell-cell adhesion (1) . Although higher plants have been the richest source of lectins, the role of lectins in these organisms is poorly understood . An oft-repeated hypothesis is that lectins act as receptor sites for symbionts (3, 34) or pathogens (35). For example, there is evidence that suggests that in certain legumes lectins might be receptors for the nitrogen-fixing rhizobia that form symbiotic nodules in legume roots (3, 10, 34, 40) . Other evidence, consistent with their cytotoxic properties and their high THE JOURNAL OF CELL BIOLOGY " VOLUME 92 MARCH 1982 753-764 ©The Rockefeller University Press " 0021-9525/82/03/0753/12 $1 .00 concentration in certain seeds, suggests that they may serve as a defense against seed-eating herbivores (17) . Investigations such as these on the roles of lectins in plants have been confined largely to the legume family . Lectins, however, are a diverse group of proteins and glycoproteins that are found throughout the plant kingdom and, in all likelihood, perform diverse functions . We have sought, therefore, to broaden this focus by examining the tissue distribution of wheat germ agglutinin (WGA), a lectin from the grass family . Although a study such as this by itself will not discern the function of the lectin, it will establish criteria consistent with its role in the plant. WGA is the most thoroughly characterized of the numerous lectins that bind N-acetylglucosamine (G1cNAc) and its biologically widespread polymer, chitin (see reference 14 for review) . In addition, its toxic effects on fungal metabolism (24) suggest a role in the defense against pathogens . In our previous study (25) we quantified the levels of WGA in wheat plants with a radioimmunoassay (RIA) . We found that ungerminated wheat grains contain three orders-of-magnitude less lectin than do typical legume seeds, or 1 Frg of WGA per grain, all associated with the embryo . After one 753 on Jne 1, 2017 D ow nladed fom Published March 1, 1982