The interrelationships of actin and hyphal tip growth in the ascomycete Geotrichum candidum

Geotrichum candidum is unusual among reported hyphal ascomycetes in that its hyphae readily stain with phalloidin to reveal actin concentrated in the Spitzenkörper (SPK) and plaques associated with the plasma membrane (PM). Loss of SPK actin, but not the PM plaques, following latrunculin B treatment produces tip swelling, consistent with actin restraining tip morphology or localizing vesicle exocytosis. Tip morphogenesis may also involve a spectrin-like protein which concentrates at the apical PM in plaques unassociated with the actin plaques. Branch formation occurs with growth rates initially about 20% those of leading tips, and does not involve a morphologically detectable SPK, nor SPK-like actin ensembles, indicating the dispensibility of this structure in tip growth. Surprisingly, new tubular tips can form in the continued presence of latrunculin, consistent with alternative cellular systems, such as the spectrin-like protein, substituting for actin's critical functions.