F-actin involvement in apical cell morphogenesis of Sphacelaria rigidula (Phaeophyceae): mutual alignment between cortical actin filaments and cellulose microfibrils

The polarized apical cells of Sphacelaria rigidula display a well-organized cortical F-actin cytoskeleton. This consists of bundles of actin filaments (AFs), assuming definite patterns of organization in different regions of the cell cortex. At the tip region of the apical dome the AFs appear random ly oriented, showing a diffuse fluorescence. Immediately below, at the base of the apical hemisphere, the AFs form a ring-like band around the plasmal emma transverse to the polar cell axis. The rest of the cell cortex is trav ersed by AFs showing an axial or slightly inclined or helical orientation. Examination of the apical cells of S. rigidula in appropriate thin sections revealed that the wall has a multi-layered structure. In the tip region of the apical dome the cell wall bears randomly oriented cellulose microfibri ls (MFs), while in the basal part of the apical dome it is reinforced by a layer of densely arranged transverse MFs. As the cell grows at the apex, th e transverse MFs are continuously displaced towards the cell base. Below th e transverse MF layer, an additional layer with axial or slightly oblique M Fs starts being depositing internally, on the tubular part of the cell. Ext ernally to them, the layer of transversely oriented MFs remains visible. Th e above observations were confirmed in apical cells of S. tribuloides. MF o rientation in the innermost wall layer of the apical cells coincides with t hat of the cortical AFs observed by fluorescence. This mutual alignment bet ween AFs and MFs in a cell that lacks cortical microtubules (MTs) suggests that the AFs are involved in the oriented deposition of MFs. Experimental d isruption of AFs with cytochalasin B caused abnormal MF deposition, a fact strongly supporting the above hypothesis. The transverse MFs forming at the base of the apical dome define the diameter and consequently the cylindric al shape of the apical cells. It is suggested that in the brown algal cells examined the AFs play a morphogenetic role similar to that of cortical mic rotubules in higher plant cells.