Plasma Membrane Of Plant Cells Research Articles

FURTHER OBSERVATIONS ON THE PHENOMENON OF SECONDARY VACUOLATION IN LIVING CELLS

Invagination of the plasma membrane in plant cells forms peripheral or endocytic structures which often contain a complement of membrane‐bound vesicles. These structures, or secondary vacuoles, move with the streaming cytoplasm although their velocities are somewhat slower than that for the various organelles within the cytoplasm. They glide over the nucleus or flow from the peripheral cytoplasm onto a transvacuolar strand and continue unabated along the length of a strand. These structures may detach from the plasma membrane as sacs to become positioned in the cytoplasm directly under the tonoplast and project into the primary vacuole. Some endocytic vacuoles may separate from the peripheral cytoplasm and remain free within the primary vacuole; subsequently they can re‐associate with the cytoplasm. While the content and function of these vacuoles are yet to be determined, indirect evidence indicates that they are pinocytic in character since the content of an invagination is confined to the sac upon its detachment from the plasma membrane and is subsequently transported throughout the cell by cyclosis.

Phosphotungstic acid-chromic acid as a selective electron-dense stain for plasma membranes of plant cells.

A mixture consisting of 1% phosphotungstic acid (PTA) in 10% chromic acid (CrO3) selectively stains the plasma membrane of plant cells. Whole tissue or pelleted cell fractions are prepared for electron microscopy using conventional methods including glutaraldehyde fixation and OsO4 postfixation, dehydration in acetone and embedding in Epon. To stain the plasma membrane, thin sections are transferred with a plastic loop to the surface of a 1% aqueous solution of periodic acid for 30 min for destaining. Following transfer through 5 distilled water rinses, the sections are exposed to the PTA-CrO3 mixture for 5 min, rinsed and mounted on grids for viewing with the electron microscope. The selectivity of the stain is retained in homogenates and serves to identify the plant plasma membrane in cell fractions.

N-1-napthylphthalamic-acid-binding activity of a plasma membrane-rich fraction from maize coleoptiles

Plasma membrane-rich fractions were prepared from maize coleoptiles by low-shear homogenization and differential and sucrose-gradient centrifugation. Plasma membrane fragments were identified using a specific cytochemical stain based on phosphotungstic acid prepared in chromic acid. In a comparison of 10 different cell fractions of varying plasma membrane content, the N-1-napthylphthalamic-acid (NPA)-binding activity of the fractions was directly proportional to the content of plasma membrane. The NPA binding appears to be strong K M between 10(-8) and 10(-7) M) but non-covalent. NPA is known to inhibit auxin transport efficiently and quickly. Thus, the results are consistent with the localization of auxin transport sites at the plasma membrane of plant cells.

Mechanism of cis- and trans-stimulation of anion fluxes across the plasma membrane of plant cells

Trans-stimulation by salts of the passive efflux of Cl⊖ across the plasma membrane of plant cells was established previously. In this paper the trans-effect of salts is compared with the effect of nystatin on ion efflux. It is further shown that the influx of anions is also stimulated by external salts. Influx of Cl⊖ was stimulated by K2SO4 (>~1 mM), influx of SO4 2⊖ was stimulated by KCl (>~lmM). This suggests that with increasing external salt concentration not only the electrical potential across the plasmalemma is lowered (due to preferential permeability to monovalent cations) but alsoth e permeability (i. e. the permeability coefficient) of the plasmalemma to anions is increased. According to the model proposed for the salt-stimulated decrease in the resistance to passive anion permeation the plasmalemma may be considered a lipid lattice-electrofilter. The nature of the coupling of the counter fluxes is discussed.

Isolation and properties of the plasmalemma in yeast.

A method is described for the isolation of fragments of the plasmalemma based on differential and density gradient centrifugation using cell free extracts from anaerobically grown Saccharomyces cerevisiae. Electron microscopically investigated frozen-etched specimens of isolated plasmalemma revealed the presence of globular particles attached to the outer surface of the membrane; these particles correspond to those observed in situ. In isolated plasmalemma a high specific activity of Mg++-dependent ATPase, which is not sensitive to Oligomycin, is present. Yeast plasmalemma contains protein, lipids (including phospholipids) and an appreciable amount of polysaccharide. Hydrolysis of this polysacharide yields only mannose. The treatment of the isolated plasmalemma with detergents liberates the globular particles which can be isolated by density gradient centrifugation. Protein and polysaccharide occur in the respective fraction; therefore the globular particle represents a mannan-protein. It is concluded that the particles, which cover the plasma-membrane of plant cells, represent glycoproteins, that is, building stones to be incorporated into the fibrillar network of the cell walls.