Cytoplasmic contractions in growing fungal hyphae and their morphogenetic consequences

Cristina G. Reynaga-Peña, Salomón Bartnicki-García

Video-enhanced light microscopy of the apical and subapical regions of growing hyphae of several fungal species revealed the existence of momentary synchronized motions of subcellular organelles. First discovered in a temperature-sensitive morphological mutant (ramosa-1) of Aspergillus niger, these seemingly spontaneous cytoplasmic contractions were also detected in wild-type hyphae of A. niger, Neurospora crassa, and Trichoderma atroviride. Cytoplasmic contractions in all fungi lasted about 1 s. Although the cytoplasm recovered its motility and appearance, the contraction usually led to drastic changes in Spitzenkörper (apical body) behavior and hyphal morphology, often both. Within 10 s after the contraction, the Spitzenkörper commonly became dislodged from its polar position; sometimes it disassembled into phase-dark and phase-light components; more commonly, it disappeared completely. Whether partial or complete, the dislocation of the Spitzenkörper was always accompanied by a sharp reduction or cessation of growth, and was usually followed by marked morphological changes that included bulbous hyphal tips, bulges in the hyphal profile, and formation of subapical and apical branches. The cytoplasmic contractions are vivid evidence that the most conspicuous cell organelles (membrane-bound) in living hyphae are interconnected via a contractile cytoskeletal network.