Regulation of Synthesis and Secretion of Fucose-containing Polysaccharides in Cultured Sycamore Cells

Abstract

Regulation of the synthesis and secretion of sycamore extracellular polysaccharides (SEPS) was studied in cultured cells of Acer pseudoplatanus L. cv. Pope by monitoring the incorporation of L-[3H]fucose, a specific marker for the polysaccharide, into cell wall, soluble intracellular polysaccharide and secreted fractions. Actively growing cultures of sycamore cells produce SEPS, and it reaches a concentration in the incubation medium of up to 0.7 mg ml-¹ after 25 days of growth. Addition of SEPS at 10 mg ml-¹ to cells during the phase of active growth causes a marked and rapid inhibition of secretion of the polysaccharide into which [3H]fucose is incorporated without markedly affecting uptake of the label. Added SEPS also inhibits the incorporation of [3H]fucose into the homogenate fraction although this inhibition is less marked and requires higher concentrations of added SEPS. This inhibitory effect of SEPS can be mimicked by other charged polymers, for example polygalacturonic acid, polylysine and polyglycine, while uncharged polymers like polyethylene glycol and dextran are without effect. Furthermore, the incorporation of [3H]fucose into secreted polysaccharides of isolated protoplasts is relatively unaffected by added SEPS. We propose that SEPS and other charged polymers interact with the cell wall causing it to become impermeable to synthesized SEPS, which then accumulates between the plasmalemma and the cell wall and eventually inhibits polysaccharide synthesis.