1. Three soluble polysaccharides and a soluble protein containing hydroxyproline were secreted by sycamore suspension cultures. l-[1-3H]Fucose was incorporated solely into the fucose of fucoxyloglucan and l-[1-14C]arabinose mainly into the arabinose of arabino-galactan. [U-14C]Glucose was a general precursor for soluble protein and polysaccharides. 2. The steady-state rate of secretion of all the polymers was increased within seconds of adding various electrolytes and polyelectrolytes to the growth medium. The increased secretion was induced by cations at the outer surface of the plasma membrane. It was brought about by a stimulation of the normal mechanisms of cell-wall polysaccharide secretion. It was partly inhibited by anaerobiosis or sodium arsenate and was unaffected by temperature changes in the range 0–35°C. 3. The precursor pool from which secretion was induced contained completely synthesized polysaccharides and was probably located in the Golgi-derived vesicles. The results indicated that the endoplasmic reticulum did not secrete polysaccharide directly to the cell exterior. 4. The various cations probably induced secretion by causing a depolarization of the negative electric potential of the cell surface, and this resulted in the fusion of vesicles with the plasma membrane. 5. Analogy with exocytosis and pinocytosis in various animal tissues suggested that the decreased surface potential brought about membrane fusion by causing an increase in plasma-membrane permeability to Ca2+. 6. The results showed that the fusion of vesicles with the plasma membrane was rate-limiting and a potential control point. Auxin-stimulated cell-wall deposition could be a result of a stimulated influx of Ca2+ causing vesicle fusion with the plasma membrane.

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