EVIDENCE THAT SPITZENKORPER BEHAVIOR DETERMINES THE SHAPE OF A FUNGALHYPHA - A TEST OF THE HYPHOID MODEL

Hyphae of the fungus Rhizoctonia solani have a characteristic Spitzenk orper in their growing tips and a cell shape described by the mathemat ical hyphoid equation. A mild disturbance of hyphae growing in a slide culture chamber on a microscope stage caused the Spitzenkorper to mov e away from its usual position next to the apical pole and wander brie fly inside the apical dome. Hyphal elongation rate declined abruptly, and the apex became rounded and increased in diameter. As the Spitzenk orper migrated back to its polar position, rapid cell elongation resum ed, and the contour of the growing hyphal tip returned to the typical hyphoid shape. The brief dislocation of the Spitzenkorper left a perma nent bulge in the hyphal profile. This morphogenetic sequence was mimi cked by computer simulation, based on the hyphoid equation which relat es the generation of hyphal shape to the linear displacement of a vesi cle supply center (VSC). The VSC was programmed to retrace the observe d movements of the Spitzenkorper during the above sequence. The result ing similarity of shape between real and computer-simulated cells rein forces the mathematical prediction that the Spitzenkorper acts as a VS C and that its continuous linear advancement generates a typical hypha l tube with the characteristic hyphoid shape. Accordingly, the hyphoid model and its VSC concept provide a plausible hypothesis to explain t he cellular basis of polarized growth of fungal hyphae. (C) 1995 Acade mic Press, Inc.