Determinants of fungal cell wall morphology: the vesicle supply center

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Abstract
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 formation required for cell morphogenesis ensue from the pattern of vesicle discharge. A vesicle-based, mathematical model of fungal morphogenesis revealed that a relatively simple process could establish these gradients. The model and its underlying hyphoid equation assumes that wall-building vesicles emanate from a vesicle supply center (VSC). The VSC serves as an organizing center from which vesicles would move radially to the cell surface in all directions at random. VSC displacement is then an immediate cause of morphogenesis: a sustained linear displacement of the VSC would generate a polarized pattern of exocytosis required to make a hypha. The model predicts that the Spitzenkörper functions as a VSC. We have tested this prediction by analyzing instances where dislocations in Spitzenkörper position result in hyphal deformations. When the VSC was programmed to duplicate the movements of the Spitzenkörper before, during, and after a deformation, the resulting shapes mimicked closely the observed deformations. These correlations support the contention that the position and movement of the VSC determines the morphology of the fungal cell wall. The computer model has been refined to incorporate transverse random oscillations of the VSC to simulate more realistically the meandering shape of fungal hyphae. The model predicts that hyphal ring formation results from a sustained directional bias to the transverse oscillations of the Spitzenkörper. Key words: hyphal morphogenesis, Spitzenkörper, vesicle supply center, mathematical model, hyphoid equation, hyphal curvature.