Plasma membrane “Rosettes” in carrot and sycamore suspension culture cells

Suspension culture cells of carrot,Daucus carota L., and sycamore,Acer pseudoplatanus L., were freeze-fractured after ultrarapid freezing without fixation or cryoprotection in a propane-jet freezer. Infrequently, rosettes (ca. 24 nm diameter) of six (occasionally five) subunits (ca. 8 nm diameter) were observed in P-face views of the plasma membrane of both taxa. When present, rosette density was approximately 1/μm². Generally, rosettes were less frequently seen on plasma membranes exhibiting numerous vesicle fusion figures. Due to the high quality of the freezing, cellulose microfibril impressions were rarely seen on either PF or EF views of the plasma membrane, thus precluding correlations between microfibrils on the one hand and rosettes (and terminal globules) on the other. The presence of rosettes in suspension culture cells of these two species supports the putative role of rosettes in cellulose biosynthesis in higher plants.