Our aim was to resolve a major problem in developmental biology: explaining the most fundamental cell growth process in fungi -- hyphal tip growth. Hyphae are long tubular cells which elongate by a mechanism involving movement of secretory vesicles. These vesicles and other small cell components assemble into a complex called the Spitzenkorper. The Spitzenkorper is suspected of playing a central role in apical growth of fungal cells, but its exact function and the mechanisms controlling its behavior are unknown. We have used laser microbeam irradiation to manipulate the Spitzenkorper thereby causing predictable changes in cell shape, growth, and the initiation of subapical hyphal branches in locations where they would not ordinarily occur. When we changed the position of the Spitzenkorper, the site of localized cell expansion likewise changed almost immediately, so that it became clear that the position of the Spitzenkorper was the determinant of the site of cell growth and of new cell wall formation. In future experiments, we will attempt to organize new Spitzenkorper by laser trapping in regions of the cell where they do not normally occur and attempt to generate growth sites where the cell has stopped growing.
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