Ultrastructural analysis of siliceous cell wall regeneration in the stramenopile Triparma laevis (Parmales, Bolidophyceae)

The marine alga Triparma laevis NIES-2565, belonging to the Parmales (Bolidophyceae), possesses a siliceous cell wall that is composed of shield, ventral, dorsal and girdle plates. The cell wall is lost under silicate-deficient culture conditions. Regeneration of the cell wall occurs with the addition of sufficient silicate ions into the culture medium. In this study, the formation of the siliceous cell wall was observed by transmission electron microscopy. Silica deposition vesicles (SDVs) forming the shield and ventral plates were initially found around the chloroplast, moving toward the plasma membrane as they expanded, with siliceous plates being built within the SDV. The SDV membrane was tightly adpressed to the siliceous plate. In contrast, SDVs for dorsal and girdle plates were formed adjacent to the plasma membrane. Extracellular secretion of mature siliceous plates began with shield plates, followed in turn by ventral, dorsal and girdle plates. The dorsal and girdle plates were inserted into the space between the previously secreted shield and ventral plates. Our study demonstrates that SDVs develop in different areas of the cell, according to whether they are forming dorsal and girdle plates or shield and ventral plates.