TIP CELL-GROWTH AND THE FREQUENCY AND DISTRIBUTION OF CELLULOSE MICROFIBRIL-SYNTHESIZING COMPLEXES IN THE PLASMA-MEMBRANE OF APICAL SHOOT CELLS OF THE RED ALGA PORPHYRA-YEZOENSIS

The supramolecular organization of the plasma membrane of apical cells in shoot filaments of the marine red alga Porphyra yezoensis Ueda (co nchocelis stage) was studied in replicas of rapidly frozen and fractur ed cells. The protoplasmic fracture (PF) face of the plasma membrane e xhibited both randomly distributed single particles (with a mean diame ter of 9.2 +/- 0.2 nm) and distinct linear cellulose microfibril-synth esizing terminal complexes (TCs) consisting of two or three rows of li nearly arranged particles (average diameter of TC particles 9.4 +/- 0. 3 nm). The density of the single particles of the PF face of the plasm a membrane was 3000 mum-2, whereas that of the exoplasmic fracture fac e was 325 mum-2 . TCs were observed only on the PF face. The highest d ensity of TCs was at the apex of the cell (mean density 23.0 +/- 7.4 T Cs mum-2 within 5 mum from the tip) and decreased rapidly from the ape x to the more basal regions of the cell, dropping to near zero at 20 m um. The number of particle subunits of TCs per mum2 of the plasma memb rane also decreased from the tip to the basal regions following the sa me gradient as that of the TC density. The length of TCs increased gra dually from the tip (mean length 46.0 +/- 1.4 nm in the area at 0.5 mu m from the tip) to the cell base (mean length 60.0 +/- 7.0 mum in the area at 15-20 mum). In the very tip region (0-4 mum from the apex), ra ndomly distributed TCs but no microfibril imprints were observed, whil e in the region 4-9 mum from the tip microfibril imprints and TCs, bot h randomly distributed, occurred. Many TCs involved in the synthesis o f cellulose microfibrils were associated with the ends of microfibril imprints. Our results indicate that TCs are involved in the biosynthes is, assembly, and orientation of cellulose microfibrils and that the f requency and distribution of TCs reflect tip growth (polar growth) in the apical shoot cell of Porphyra yezoensis Polar distribution of line ar TCs as ''cellulose synthase' complexes within the plasma membrane o f a tip cell was recorded for the first time in plants.