Antibody localization of extensin in cell walls of carrot storage roots

Abstract. The accumulation and cross-linking of hydroxyproline-rich glycoproteins (HRGPs) in cell walls of dicotyledonous plants has been correlated with a number of wall-strengthening phenomena. Polyclonal antibodies raised against glycosylated extensin-l, the most abundant HRGP in carrot (Daucus carota L.) cell walls, recognize this antigen on gel and dot blots and on thin sections of epoxy-embedded carrot-root cell walls. Since wall label-ing can be largely reduced by preincubating the antibodies with purified extensin-1, most labeling can be attributed to recognition of this antigen. The remaining label may be the result of recogni-tion of extensin-2, a second carrot HRGP, or other wall components (cellulose, hemicellulose and pec-tin are not recognized). Extensin-I label was dis-tributed quite uniformly across the cell wall but was absent from the expanded middle lamella at the intersection of three or more cells and was re-duced in the narrow middle lamella between two cells. This distribution is essentially the same as that of cellulose. Because of limitations of this la-beling technique, it is not possible to construct a complete model of the structure of the cross-linked extensin matrix. Nonetheless, hort, linear arrays of gold particles may represent small portions of the extensin matrix or of individual extensin mole-cules as they are exposed on the surface of sections. These and other results presented here indicate that: a) newly synthesized extensin is added to the wall by intussusception; b) extensin cannot cross the middle lamella separating the walls of adjacent cells; and c) incorporation of extensin is a late * Present address. " Department of Biology, Yale University