Stalk formation in Dictyostelium discoideum involves the synthesis of a stalk tube by the prestalk cell population

Cellulose synthesis is a significant activity of the differentiating cells of the cellular slime mold Dictyostelium dis coideum. During development, cellulose is first detected biochemically just prior to the slug stage (Sussman, 1972) and cytochemically in late aggregates (Harrington and Raper, 1968). By the completion of the developmental program, all cells will have contributed cellulose to at least one of the following structures: (1) the slime sheath, which surrounds the slug (Hohl and Jehli, 1973); (2) the stalk tube, which encloses the stalk cell population (Raper and Fennell, 1952); (3) the stalk cell walls (Raper and Fennell, 1952); and (4) the inner layers of the spore walls (Erdos and West, 1989). The predominant role of cellulose is most likely structural, although there is evidence for its involvement in slug motility (Freeze and Loomis, 1977) and in cell type determination (Farnsworth, 1974). No mutant strains deficient in cellulose synthase activity have yet been isolated. However, mutants deficient in UDPG-pyrophosphorylase activity cannot synthesize UDPG, which is the substrate for the cellulose synthase, and hence do not synthesize cellulose. These strains form slugs that migrate inefficiently and are incapable of culminating (Dimond et al., 1976). Raper and Fennel (1952) observed that the stalk tube (called by them the sorophore sheath) was the product of the prestalk cell population as a whole, whereas the stalk cell wall was the product of an individual prestalk cell. They drew an analogy between stalk tube formation and Aceto bacter xylinum cellulose synthesis, and between stalk cell wall formation and plant cell wall synthesis (see also Rosness and Wright, 1974). This analogy was adopted by Brown (1978; Brown et al., 1983), and the two modes of cellulose synthesis were explained in terms of the plasma membrane-associated cellulose synthase complexes being either fixed or mobile with respect to the membrane surface. A fixed complex would tend to synthesize cellulose that moved away from the cell (as in Acetobacter); a mobile complex would tend to synthesize cellulose that formed a cell wall (Brown et al., 1983). Most cellulose-synthesizing organisms can be categorized in either the fixed or the mobile site group; Dictyostelium is the only organism that may use both modes of cellulose synthesis (Brown et al., 1983). The spatial and temporal control of cellulose synthesis in D. discoideum is poorly understood despite its developmental importance. For instance, it is not known which cells are responsible for the synthesis of the cellulose found in the slug slime trail. The precise timing of spore cellulose synthesis has been debated in the past, although there is now evidence that it occurs late in spore development (Erdos and West, 1989). The cellulose found in the stalk tube and in stalk cell walls is clearly the product of prestalk cell activity, 703 Development 119, 703-710 (1993) Printed in Great Britain © The Company of Biologists Limited 1993