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Studies on the Secretion of Maize Root Cap Slime: IV. Evidence for the Involvement of Dictyosomes
Abstract:
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.
Insights
Dictyosomes and their vesicles secrete maize root cap slime. L-fucose, a slime marker, is incorporated into dictyosomes and secreted, confirming their role in slime production.
Area of Science:
- Plant Cell Biology
- Biochemistry
- Molecular Biology
Background:
- Maize root cap slime is crucial for root growth and protection.
- The cellular origin and secretion mechanism of this slime remain incompletely understood.
Purpose of the Study:
- To investigate the role of dictyosomes and their vesicles in the secretion of maize root cap slime.
- To identify the specific cellular components involved in slime biogenesis and release.
Main Methods:
- Utilized kinetic and organelle fractionation experiments.
- Employed l-fucose as a specific marker for secreted slime.
- Performed pulse-chase experiments with radiolabeled l-fucose and (14)C-amino acid mixture.
- Conducted cesium chloride and sucrose density gradient centrifugation.
- Applied autoradiography to analyze organelle fractions.
Main Results:
- L-[1-(3)H]fucose was incorporated into a water-soluble, ethyl alcohol-insoluble fraction, with peak labeling at 20 minutes and significant secretion.
- Kinetically distinct incorporation of [(3)H]fucose into a water-insoluble fraction suggested cell wall involvement.
- Secretion of the [(3)H]fucose-containing polymer began after 10-15 minutes, preceding the release of the (14)C-amino acid polymer.
- Cesium chloride density gradient centrifugation of secreted material revealed a single major component with a buoyant density of 1.63 g cm(-3).
- Sucrose density gradient centrifugation localized radioactivity in homogenates to a density band (1.12-1.18 g cm(-3), peak at 1.15 g cm(-3)) corresponding to dictyosomes.
- Autoradiography confirmed radioactivity association primarily with dictyosomes.
Conclusions:
- Dictyosomes and their vesicles are directly involved in the synthesis and secretion of maize root cap slime.
- L-fucose serves as an effective marker for tracing slime biogenesis and secretion pathways.
- The findings elucidate the cellular machinery responsible for producing a critical component of the root cap environment.
