Plasma membrane-adjacent actin filaments, but not microtubules, are essential for both polarization and hyphal tip morphogenesis in Saprolegnia feraxand Neurospora crassa

The organization and roles of F-actin and microtubules in the maintenance a nd initiation of hyphal tip growth have been analyzed in Saprolegnia ferax and Neurospora crassa, In hyphae of both species, the apex is depleted of m icrotubules relative to subapical regions and near-normal morphogenesis occ urs in concentrations of nocodazole or MBC which remove microtubules, slow growth, and disrupt nuclear positioning. In contrast, each species contains characteristic tip-high arrays of plasma membrane-adjacent F-actin, whose organization is largely unaltered by the loss of microtubules but disruptio n of which by latrunculin B disrupts tip morphology, Hyphal initiation and subsequent normal morphogenesis from protoplasts of both species and spores of S. ferax are independent of microtubules, but at least in S. ferax obli gatorily involve the formation of F-actin caps adjacent to the hyphal tip p lasma membrane. These observations indicate an obligatory role for F-actin in hyphal polarization and tip morphogenesis and only an indirect role for microtubules, (C) 2000 Academic Press.