Studies on the Secretion of Maize Root Cap Slime: IV. Evidence for the Involvement of Dictyosomes

The involvement of dictyosomes and their vesicles in secretion of slime by maize root cap cells is demonstrated by kinetic and organelle fractionation experiments using l-fucose as a specific marker for the secreted slime. Pulse-chase experiments show that l-1-(3)H]fucose is incorporated into two distinct fractions of root cap cells. Incorporation into a water-soluble, ethyl alcohol-insoluble fraction of the homogenate has a peak at 20 minutes of chasing followed by rapid loss of label. Seventy per cent of the radioactivity in this fraction is secreted from the tissue during a 2-hour chase period. Incorporation of label from (3)H]fucose into a water-insoluble fraction is kinetically different suggesting that in situ incorporation of label is occurring into the cell wall. Labeling of the water-soluble, ethyl alcohol-insoluble fraction with an (14)C-amino acid mixture differs from that of (3)H]fucose. Thus, while release of the (3)H]fucose-containing polymer begins after 10 to 15 minutes of chasing, the release of the (14)C-amino acid polymer is delayed an additional 5 to 10 minutes and occurs at a lower rate. Cesium chloride density gradient centrifugation of secreted material labeled with radioactivity from (3)H]fucose indicates the presence of only one major component having a buoyant density similar to that of purified root cap slime (1.63 g cm(-3)). Sucrose density gradient centrifugation of homogenates of (3)H]fucose-labeled root cap tissue shows that radioactivity in nondialyzable material occurs as a broad band between densities 1.12 and 1.18 g cm(-3) with a peak at density 1.15 g cm(-3), the same density at which dictyosomes were localized by electron microscopy. Autoradiography of organelle fractions shows that radioactivity was associated almost exclusively with dictyosomes.