Synthesis and turnover of cell-wall polysaccharides and starch in photosynthetic soybean suspension cultures

Soybean (Glycine max [L.] Merr.) suspension cultures grown under photoautotrophic and photomixotrophic (1% sucrose) culture conditions were used in (CO2)-C-14 pulse-chase experiments to follow cell-wall polysaccharide and starch biosynthesis and turnover. Following a 30-min pulse with (CO2)-C-14, about one-fourth of the C-14 of the photoautotrophic cells was incorporated into the cell wall; this increased to about 80% during a 96-h chase in unlabeled CO2. Cells early in the cell culture cycle (3 d) incorporated more C-14 per sample and also exhibited greater turnover of the pectin and hemicellulose fractions as shown by loss of C-14 during the 96-h chase than did 10- and 16-d cells. When the chase occurred in the dark, less C-14 was incorporated into the cell wall because of the cessation of growth and higher respiratory loss. The dark effect was much less pronounced with the photomixotrophic cells. Even though the cell starch levels were much lower than in leaves, high C-14 incorporation was found during the pulse, especially in older cells. The label was largely lost during the chase, indicating that starch is involved in the short-term storage of photosynthate. Thus, these easily labeled and manipulated photosynthetic cells demonstrated extensive turnover of the cell-wall pectin and hemicellulose fractions and starch during the normal growth process.