A CYTOCHALASIN-SENSITIVE ACTIN FILAMENT MESHWORK IS A PREREQUISITE FOR LOCAL WOUND WALL DEPOSITION IN NITELLA INTERNODAL CELLS

Reorganization of the actin cytoskeleton following cell wall puncturin g of characean internodal cells was studied by immunofluorescence and confocal laser scanning microscopy. Injury locally destroyed the paral lel subcortical actin filament bundles and cortical actin strands that are characteristic of unwounded regions. At wounds, a delicate three- dimensional interlaced structure of actin strands, with meshes up to 5 mu m wide, formed by de novo assembly of isolated filaments and by th e elongation of residual subcortical actin bundles and cortical actin strands. The actin meshwork persist ed for up to 2 h, corresponding to the duration of intense wound wall secretion. Actin filament bundles continuous with the subcortical bundles outside the wound then regener ated, their parallel alignment probably assisted by endoplasmic flow. Cytochalasin D concentrations that arrested cytoplasmic streaming comp letely inhibited the formation of the actin meshwork, wound wall depos ition and recovery of actin bundles. Concentrations that only reduced streaming velocity delayed meshwork formation and wound walls were thi nner than ill controls. The actual amount of F-actin within the meshwo rk, however, was clearly greater in the presence of low cytochalasin c oncentrations. In late stages of recovery, the actin bundles became ve ry thick and intervening spaces became wider thereby forming a conspic uous, three-dimensional lattice that was continuous with interwebbing subcortical bundles and cortical actin around the periphery of the wou nd. Our experiments suggest that actin meshwork formation is a prerequ isite for plasma membrane-directed transport of vesicles involved in w ounding-induced exocytosis in characean internodes. Stabilization of t he meshwork by subinhibitory concentrations of cytochalasin D is proba bly caused by actin-binding properties of the drug that either induce bundling or impede function of associated proteins.