RADIAL F-ACTIN ARRAYS PRECEDE NEW HYPHA FORMATION IN SAPROLEGNIA - IMPLICATIONS FOR ESTABLISHING POLAR GROWTH AND REGULATING TIP MORPHOGENESIS

The roles of cortical F-actin in initiating and regulating polarized c ell expansion in the form of hyphal tip morphogenesis were investigate d by analyzing long term effects of F-actin disruption by latrunculin B in the oomycete Saprolegnia ferax, and detecting localized changes i n the cortical F-actin organization preceding hyphal formation. Tubula r hyphal morphology was dependent on proper F-actin organization, sinc e latrunculin induced dose-dependent actin disruption and correspondin g changes in hyphal morphology and wall deposition. With long incubati on times (1 to 3 hours), abundant subapical expansion occurred, the po lar form of which was increasingly lost with increasing actin disrupti on, culminating in diffuse subapical expansion, These extreme effects were accompanied by disorganized cytoplasm, and novel reorganization o f microtubules, characterized by star-burst asters, Upon removing latr unculin, hyperbranching produced abundant polar branches with normal F -actin organization throughout the colony, The results are consistent with F-actin regulating polar vesicle delivery and controlling vesicle fusion at the plasma membrane, and suggest that F-actin participates in establishing polar growth, To test this idea further, we utilized t he hyperbranching growth form of Saprolegnia. Early during the recover y time, prior to multiple branch formation, radial arrays of filamento us F-actin were observed in regions with no detectable surface protrus ion. Their locations were consistent with those of the numerous branch es that formed with longer recovery times, Similar radial arrays prece ded germ tube formation in asexual spores. The arrays were important f or initiating polar growth since the spores lost their ability to pola rize when the F-actin was disrupted with latrunculin, and increased is ometrically in size rather than producing germ tubes. Therefore, F-act in participates in initiating tip formation in addition to its previou sly demonstrated participation in maintenance of hyphal tip growth. Th e cortical location and radial organization of the arrays suggest that they recruit and stabilize membrane-bound and cytosolic factors requi red to build a new tip.