Rapid wound responses of Saprolegnia ferax hyphae depend upon actin and Ca2+-involving deposition of callose plugs

Growing hyphae of the oomycete Saprolegnia ferax wounded by impalement with a ca. 0.2 mum diameter glass microelectrode normally respond within second s with an apically directed cytoplasmic contraction followed by production of a plug which encases the electrode and occludes its recording of transme mbrane potentials. This plug contains callose and Ca2+-associated membranes . To characterize the rapid wounding response, we disrupted specific filame ntous (F) actin populations and Ca2+ regulation. Plug formation is inhibite d by disruption of F-actin populations and low exogenous Ca2+ but not by in hibition of stretch-activated Ca2+ channels with Gd3+. Therefore, stretch-a ctivated channels are not the immediate sensor. Instead, sensing may involv e strain on the actin cytoskeleton which triggers the occlusion response. T his wound response is qualitatively similar to the production of septa whic h isol;lte developing sporangia and seal severed hyphae, indicating the use of a normal basic cellular developmental system as a protective mechanism against environmental damage. The wound response is essential, since an ina bility to seal sites of mechanical damage is potentially catastrophic in ac ellular coenocytic organisms.