TRANSMURAL SECRETION OF A HIGHLY-EXPRESSED CELL-SURFACE ANTIGEN OF OAT ROOT CAP CELLS
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A monoclonal antibody (MAb), designated 40.1C2.8, has been produced which recognizes an antigen in the outer membrane and cell wall of oat root cap cells. Antibody binding is insensitive to periodate oxidation and sensitive to proteolytic digestion by trypsin, indicating that the epitope is protein, not carbohydrate. Western immunoblots of polypeptide extracts from oat, maize, and bean seedlings revealed that MAb 40.1C2.8 was primarily directed to an epitope on a 40 kDa protein very highly expressed in root cap cells. Root cap cells and mature sloughed cells were heavily stained with the antibody on silver-enhanced immunogold-stained thick sections of intact roots. At the ultrastructural level, the antigen was found on the cell surface of root cap meristem cells. In the root meristem proper, the antigen was only detected in the vacuole, not on the cell surface. As the root cap cells matured, the thickness of the zone containing the antigen increased. In the outer-most peripheral cells, the antigen was found within the inner layer of the wall, but not coextensive with the secreted mucilage which occurs throughout the wall, indicating that there may be a barrier to transmural secretion even in the most mature cells of the root cap. 1993 Springer-Verlag.