Actin microfilaments are involved in scale formation of the chrysomonad cellSynura

SummaryIn the first step of scale formation several small vesicles originating from the Golgi fuse to form a large flattened primary vesicle associated with the surface of one plastid. In a second step this vesicle undergoes several morphogenetic events to form the mold of ornamented scales elaborated in the lumen of the scale forming vesicle (SFV). Thick filaments composed of a stack of actin microfilaments (MFs) settle on the membrane of the SFV turned toward the plastid. They are laterally cross-linked to each other and to the SFV membrane and assembled in a horseshoe figure at the place of the future base-plate of the scale. On the center of the vesicle free of actin MF, where the periplastidial endoplasmic reticulum (PER) and the SFV membranes are glued, a protrusion occurs to form a diverticulum which is to become the future hull or the spine of the finished scale. On the external side of the SFV a microfibrillar network covers the surface of the vesicle. Microtubules (MTs) originating near the kinetosomes change their rectilinear course to follow the two longitudinal margins and the diverticulum of the SFV. MTs are not directly attached to the membrane of the SFV but rather through the microfibrillar network. A set of observations suggest that actin MFs have a structural function in maintaining the shape of the vesicle rather than a role in the migration of the SFV on the surface of the plastid. MTs probably play a role in the migration and in the morphogenesis of the SFV in conjunction with microfibrils. Also, the formation of SFVs and silica deposition are compared to that in diatoms.