Studies on the Secretion of Maize Root Cap Slime: IV. Evidence for the Involvement of Dictyosomes 1

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RESUMO

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-[1-3H]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 [3H]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 14C-amino acid mixture differs from that of [3H]fucose. Thus, while release of the [3H]fucose-containing polymer begins after 10 to 15 minutes of chasing, the release of the 14C-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 [3H]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 [3H]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.

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