Cell Wall Assembly in Fungal Morphogenesis

Abstract

Fungal cells are encased in a rigid wall which determines their direction of growth and ultimate form. With respect to growth of the wall three distinctive mechanisms can be observed: (i) Filamentous fungi grow by apical extension (Fig. 1A) and several studies have shown that cell wall deposition is concentrated at the extreme apex of individual hyphae (Bartnicki-Garcia and Lippman, 1969; Gooday, 1971; Wessels et al., 1983). A new growthpoint can arise by evagination of existing wall, usually behind the latest formed septum. This evagination also elongates by polar deposition of new wall material. In this way a branched mycelium arises which can grow exponentially (Trinci, 1979). (ii) Unicellular fungi (yeasts) also grow by polar extension of their walls (Fig. 1B). In the fission yeast, Schizosaccharomyces pombe, wall material is deposited at the “old end”, that is the pole not generated by the division. Later in the cell-cycle wall material is also laid down at the “new end” and during a short period both ends expands until elongation stops, a septum is formed and fission initiated (Biely et al., 1973; Mitchison and Nurse, 1985). Budding yeasts such as Saccharomyces cerevisiae or Candida albicans, however, produce polar evaginations that excrete wall material over their entire surface until a certain size of the buds is obtained, then a septum is formed and the bud separates from the mother cell (Biely et al., 1973; Cabib et al.,1982). (iii) Diffuse or intercalary growth of existing wall is rare in fungi and seems to be restricted to hyphal structures designed to lift reproductive cells above the substrate for dispersal (Fig. 1C). Well studied examples are the sporangiophores of Phycomyces blakesleeanus (Roelofsen, 1950) and the carpophores of members of the Basidiomycetes (Gooday, 1979; Mol and Wessels, 1990; Mol et al., 1990). In these structures hyphal expansion occurs in the cylindrical part of the hyphae, usually in longitudinal direction, while cell wall deposition occurs in the expanding area. In all cases the driving force for wall expansion is the turgor pressure. In non-growing regions the wall is apparently rigid enough to resist the turgor pressure, in the growing region, however, the wall must be viscoelastic to allow for expansion At the same time new wall material has to be laid down in this region to prevent thinning of the wall.