Mapping the growth of fungal hyphae: Orthogonal cell wall expansion duringtip growth and the role of turgor

By computer-enhanced videomicroscopy, we mapped the trajectory of external and internal cell surface markers in growing fungal hyphae to determine the pattern of cell wall expansion during apical growth, Carbon particles (Ind ia ink) were chosen as external markers for tip expansion of Rhizoctonia so lani hyphae. Irregularities in the growing apical walls of R. solani sewed as internal markers. Marker movement was traced in captured frames from the videotaped sequences. External and internal markers both followed orthogon al trajectories; i.e., they moved perpendicular to the cell surface regardl ess of their initial position in the hyphal apex. We found no evidence that the tip rotates during elongation. The discovery that the cell wall of a g rowing hypha expands orthogonally has major repercussions on two fronts: 1) It supports the long-held view that turgor pressure is the main force driv ing cell wall expansion. 2) It provides crucial information to complete the mathematical derivation of a three-dimensional model of hyphal morphogenes is based on the vesicle supply center concept. In three dimensions, the ves icle gradient generated by the vesicle supply center is insufficient to exp lain shape; it is also necessary to know the manner in which the existing s urface is displaced during wall expansion.