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The plant cell wall is a dynamic structure that plays important roles i n growth and development and in the interactions of plants with their environment and other organisms. We have used monoclonal antibodies that recognize different carbohydrate epitopes present i n plant cell-wall polysaccharides to locate these epitopes in roots of developing Arabidopsis fhaliana seedlings. An epitope i n the pectic polysaccharide rhamnogalacturonan I is observed in the walls of epidermal and cortical cells in mature parts of the root. This epitope is inserted into the walls i n a developmentally regulated manner. Initially, the epitope is observed in atrichoblasts and later appears in trichoblasts and simultaneously in cortical cells. A terminal a-fucosyl-containing epitope is present i n almost all of the cell walls in the root. An arabinosylated (1 +6)-P-galactan epitope is also found in all of the cell walls of the root with the exception of lateral root-cap cell walls. It is striking that these three polysaccharide epitopes are not uniformly distributed (or accessible) within the walls of a given cell, nor are these epitopes distributed equally across the two walls laid down by adjacent cells. Our results further suggest that the biosynthesis and differentiation of primary cell walls in plants are precisely regulated i n a temporal, spatial, and developmental manner.The understanding of the role(s) of cell walls in plant biology has evolved considerably in recent years. Whereas the function of cell walls was initially viewed as primarily providing form and structure to plant cells, it has now become clear that the wall has a variety of other functions in plant growth and development (Roberts, 1990) and in the interactions of plants with their environment and other organisms (Hahn et al., 1989). It is therefore important to learn where the macromolecular components. are located within the walls of individual cells and within tissues and how the structures of the wall polymers change as a function of plant growth and development and during active defense against disease and environmental stress.Monoclonal antibodies have proven to be valuable tools for studies of the dynamics of cell surface glycoconjugates in animals (Hakomori, 1984; Feizi and Childs, 1987). Studies of the temporal, spatial, and developmental dynamics of plant cell-wall glycoconjugates have lagged behind corresponding animal studies because of the more limited collection of monoclonal antibodies against cell-wall carbohydrate epitopes that is available and because of the difficulty in characterizing the epitopes recognized by the available antibodies. Previous studies of plant cell-wall dynamics have focused on cell surface glycoproteins, since arrays of monoclonal antibodies have been generated against two types of such complex glycoconjugates, the arabinogalactan-proteins (Anderson et al., 1984;

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