Evidence that Spitzenkörper behavior determines the shape of a fungal hypha: a test of the hyphoid model.

Hyphae of the fungus Rhizoctonia solani have a characteristic Spitzenkörper in their growing tips and a cell shape described by the mathematical hyphoid equation. A mild disturbance of hyphae growing in a slide culture chamber on a microscope stage caused the Spitzenkörper to move away from its usual position next to the apical pole and wander briefly inside the apical dome. Hyphal elongation rate declined abruptly, and the apex became rounded and increased in diameter. As the Spitzenkörper migrated back to its polar position, rapid cell elongation resumed, and the contour of the growing hyphal tip returned to the typical hyphoid shape. The brief dislocation of the Spitzenkörper left a permanent bulge in the hyphal profile. This morphogenetic sequence was mimicked by computer simulation, based on the hyphoid equation which relates the generation of hyphal shape to the linear displacement of a vesicle supply center (VSC). The VSC was programmed to retrace the observed movements of the Spitzenkörper during the above sequence. The resulting similarity of shape between real and computer-simulated cells reinforces the mathematical prediction that the Spitzenkörper acts as a VSC and that its continuous linear advancement generates a typical hyphal tube with the characteristic hyphoid shape. Accordingly, the hyphoid model and its VSC concept provide a plausible hypothesis to explain the cellular basis of polarized growth of fungal hyphae.