DETERMINANTS OF FUNGAL CELL-WALL MORPHOLOGY - THE VESICLE SUPPLY CENTER

Because fungal cell walls are shaped during construction, factors that regulate wall biogenesis must play a decisive role in morphogenesis. By delivering ingredients for wall formation in vesicles, fungi have a de facto mechanism to mold their own shape. The gradients in wall for mation required for cell morphogenesis ensue from the pattern of vesic le discharge. A vesicle-based, mathematical model of fungal morphogene sis revealed that a relatively simple process could establish these gr adients. The model and its underlying hyphoid equation assumes that wa ll-building vesicles emanate from a vesicle supply center (VSC). The V SC serves as an organizing center from which vesicles would move radia lly to the cell surface in all directions at random. VSC displacement is then an immediate cause of morphogenesis: a sustained linear displa cement of the VSC would generate a polarized pattern of exocytosis req uired to make a hypha. The model predicts that the Spitzenkorper funct ions as a VSC. We have tested this prediction by analyzing instances w here dislocations in Spitzenkorper position result in hyphal deformati ons. When the VSC was programmed to duplicate the movements of the Spi tzenkorper before, during, and after a deformation, the resulting shap es mimicked closely the observed deformations. These correlations supp ort the contention that the position and movement of the VSC determine s the morphology of the fungal cell wall. The computer model has been refined to incorporate transverse random oscillations of the VSC to si mulate more realistically the meandering shape of fungal hyphae. The m odel predicts that hyphal ring formation results from a sustained dire ctional bias to the transverse oscillations of the Spitzenkorper.