No abstract

In uniseriate ceramiacean red algae, intercalary cells grow in a longitudinal direction by the formation of new cell surface in narrow, transverse bands of the cell. This localized growth mode is known as 'band growth'. In this study, the distribution of vesicle exocytosis in the axial cells of the red alga Antithamnion nipponicum was investigated by transmission electron microscopy in specimens prepared by freeze-substitution. Two zones of band growth, which were detected as dark, horizontal bands after fluorescent cell wall staining followed by 24-h incubation in stain-free medium, were observed in the apical and basal regions of the cell. The width of the band was much greater in the basal region than in the apical region. Comparisons between the total elongation of the cells and the total width of the apical and basal bands during a 24-h incubation indicated that all of the cell elongation could be attributed to band growth. A short incubation in stain-free medium revealed that cell elongation in the basal band occurred in a narrower zone, less than 5 mm wide. Small depressions in the plasma membrane and small vesicles located beneath the plasma membrane were observed in actively growing axial cells. The distribution of these structures was limited to the narrow zones near the bases of the cells (the basal bands) where cell elongation occurred. A significant longitudinal elongation of the axial cells was still detectable after 24-h treatment of the plants with the microtubule inhibitor griseofulvin or the actin filament inhibitor cytochalasin D, but there was scarcely any discernible localization of cell growth to the band. The inhibitory treatments also caused dispersion of the plasma membrane depressions and vesicles along the plasma membrane. These results suggest that these membranous structures are related to the vesicle exocytosis that occurs during longitudinal elongation of the axial cells of Antithamnion and that cytoskeletal elements, such as microtubules and actin filaments, control the localization of vesicle exocytosis and the mode of cell growth.