Cortical F-actin underlies cellulose microfibril patterning in brown algal cells

Abstract

The organization of actin filaments (AFs) was studied using rhodamine-phalloidin staining of F-actin in four different cell types of brown algae, namely: (1) differentiating cells of Sphacelaria rigidula; (2) the apical cells of Dictyota dichotoma; (3) the subapical cells of Choristocarpus tenellus; and (4) the meristematic epidermal cells of D. dichotoma. In all cases, it was revealed that, apart from the perinuclear and cytoplasmic AFs, intensely fluorescent AF bundles are present, forming a cortical system with a definite orientation in each cell type. In (1) and (4) the cortical AFs showed a transverse orientation, in (2) they were randomly distributed, and in (3) a principally axial orientation was combined with a reticulate one. Examination of thin sections under the transmission electron microscope revealed that cell wall microfibrils (MFs) of the innermost wall layer were always oriented parallel to the underlying cortical AFs. Cytochalasin B treatment of developing thalli of D. dichotoma for 24-36 h destroys cortical AFs and disturbs the cellulose MF orientation in the innermost wall layer of the meristematic cells near the apex. It is suggested that the mutual arrangement of cortical AFs and MFs is a phenomenon of general appearance in brown algal cells and that the AFs are probably involved in the orientation of the MFs. The differences observed in the orientation of both cortical AFs and MFs were in accordance with the growth pattern of the cells. This supports the hypothesis that, in brown algae. F-actin is involved in cell morphogenesis.