Mechanics of circadian pulvini movements in Phaseolus coccineus L.

The circadian movement of the lamina of primary leaves of Phaseolus coccineus L. is mediated by antagonistic changes in the length of the extensor and flexor cells of the laminar pulvinus. The cortex of the pulvinus is a concentric structure composed of hexagonal disc-like cells, arranged in longitudinal rows around the central stele. Observations with polarization optics indicate that the cellulose microfibrils are oriented in a hoop-like fashion in the longitudinal walls of the motor cells. This micellation is the structural basis of the anisotropic properties of the cells: tangential sections of the extensor and flexor placed in hypotonic mannitol solutions showed changes only in length. As a consequence a linear correlation between length and volume was found in these sections. Based on the relationship between the water potential (which is changed by different concentrations of mannitol) and the relative volume of the sections and on the osmotic pressure at 50% incipient plasmolysis, osmotic diagrams were constructed for extensor and flexor tissues (cut during night position of the pulvinus). The bulk moduli of extensibility, , were estimated from these diagrams. Under physiological conditions the values were rather low (in extensor tissue below 10 bar, in flexor tissue between 10 to 15 bar), indicating a high extensibility of the longitudinal walls of the motor cells. They are strongly dependent on the turgor pressure at the limits of the physiological pressure range.In well-watered plants, the water potentials of the extensor and flexor tissues were surprisingly low,-12 bar and-8 bar, respectively. This means that the cells in situ are by no means fully turgid. On the contrary, the cell volume in situ is similar to the volume at the point of incipient plasmolysis: the cell volumes of extensor and flexor cells in situ were only 1.01 times and 1.1 times larger, respectively, than at the point of incipient plasmolysis, whereas at full turgidity (cells in water) the corresponding factors were 1.8 and 1.5. It is suggested that the high elasticity of the longitudinal walls, the anisotropy of the cell walls, and the low water potential of the sections which is correlated with slightly stretched cell walls in situ, are favourable and effective for converting osmotic work in changes in length of the pulvinus cells, and thus for the up and down movement of the leaf.Symbols volumetric elastic modulus - _i instantaneous volumetric elastic modulus - _i stationary volumetric elastic modulus - weight-averaged stationary bulk modulus of extensibility - ₀ osmotic pressure of the vacuole of a cell at the point of incipient plasmolysis - weight-averaged osmotic pressure of the vacuoles of the tissue at 50% incipient plasmolysis - water potential     

Improved preservation of the form and contents of wall vesicles and the golgi apparatus in freeze substituted hyphae ofSaprolegnia

Summary Secretory vesicles involved in cell wall synthesis (wall vesicles) and the Golgi apparatus have been compared in conventionally fixed and freeze substituted hyphae of the oomycete fungusSaprolegnia ferax. Wall vesicles freeze substituted in various fluids range from spherical to tubular and contain an intensely staining, phosphorous rich matrix. In contrast diverse conventional fixations cause artefactual constrictions in most tubular vesicles and loss of their intensely staining contents. These data are interpreted to show the existence of an intravesicular skeletal system, with cellular regulation, to determine vesicle morphology and intravesicular synthesis of a hypothetical phosphorylated glycolipid cell wall precursor. Whilst freeze substitution gives superior preservation of wall vesicle morphology, it does not demonstrate any preferential association between wall vesicles and microtubules thus suggesting that microtubules are only indirectly involved in wall vesicle transport. Freeze substitution is superior to conventional fixation for analysis of the Golgi apparatus because it uniquely reveals both differentiation of a specific single cisterna in each Golgi body and greater differences in membrane thicknesses throughout the endomembrane system.     

Control of the rate of cell enlargement: Excision,wall relaxation,and growth-induced water potentials

A new guillotine thermocouple psychrometer was used to make continuous measurements of water potential before and after the excision of elongating and mature regions of darkgrown soybean (Glycine max L. Merr.) stems. Transpiration could not occur, but growth took place during the measurement if the tissue was intact. Tests showed that the instrument measured the average water potential of the sampled tissue and responded rapidly to changes in water potential. By measuring tissue osmotic potential ( _s ), turgor pressure ( _p ) could be calculated. In the intact plant, _s and _p were essentially constant for the entire 22 h measurement, but _s was lower and _p higher in the elongating region than in the mature region. This caused the water potential in the elongating region to be lower than in the mature region. The mature tissue equilibrated with the water potential of the xylem. Therefore, the difference in water potential between mature and elongating tissue represented a difference between the xylem and the elongating region, reflecting a water potential gradient from the xylem to the epidermis that was involved in supplying water for elongation. When mature tissue was excised with the guillotine, _s and _p did not change. However, when elongating tissue was excised, water was absorbed from the xylem, whose water potential decreased. This collapsed the gradient and prevented further water uptake. Tissue _p then decreased rapidly (5 min) by about 0.1 MPa in the elongating tissue. The _p decreased because the cell walls relaxed as extension, caused by _p , continued briefly without water uptake. The _p decreased until the minimum for wall extension (Y) was reached, whereupon elongation ceased. This was followed by a slow further decrease in Y but no additional elongation. In elongating tissue excised with mature tissue attached, there was almost no effect on water potential or _p for several hours. Nevertheless, growth was reduced immediately and continued at a decreasing rate. In this case, the mature tissue supplied water to the elongating tissue and the cell walls did not relax. Based on these measurements, a theory is presented for simultaneously evaluating the effects of water supply and water demand associated with growth. Because wall relaxation measured with the psychrometer provided a new method for determining Y and wall extensibility, all the factors required by the theory could be evaluated for the first time in a single sample. The analysis showed that water uptake and wall extension co-limited elongation in soybean stems under our conditions. This co-limitation explains why elongation responded immediately to a decrease in the water potential of the xylem and why excision with attached mature tissue caused an immediate decrease in growth rate without an immediate change in _p Abbreviations and symbols L tissue conductance for water - m wall extensibility - Y average yield threshold (MPa) - _o water potential of the xylem - _p turgor pressure - _s osmotic potential - _w water potential of the elon gating tissue     

Neither an enhancement of autolytic wall degradation nor an inhibition of the incorporation of cell wall material are pre-requisites for penicillin-induced bacteriolysis in staphylococci

In contrast to what has been postulated, penicillin G at its optimal lytic concentration of 0.1 g per ml did not lead to a detectable activation of autolytic wall processes in staphylococci in terms of the release of uniformly labelled wall fragments from cells pretreated with the drug for 1 h. Rather a considerable inhibition of this release was observed. A similarly profound inhibition of the release of peptidoglycan fragments occurred when staphylococci pretreated for 1 h with 0.1 g penicillin per ml acted as a source of crude autolysins on peptidoglycan isolated from labelled normal cells of the same strain. This clearly demonstrated that the overall inhibition of autolytic wall processes caused by penicillin was mainly due to a decreased total autolysin action rather than to an altered wall structure. Furthermore, no substantial penicillin-induced inhibition of the incorporation of ¹⁴C-N-acetylglucosamine into the staphylococcal wall could be observed before bacteriolysis started, i. e., approximately during the first 80 min of penicillin action. These results are not consistent with any of the models hitherto proposed for the action of penicillin.Dedicated to Prof. Dr. Gerhart Drews on the occasion of his 60th birthday     

A new cell wall structure in a symbiotic and a free-living strain of the blue-green alga genus Chroococcidiopsis (Pleurocapsales)

Freeze etching studies in a symbiotic and a freeliving strain of Chroococcidiopsis revealed a specific layer in the outer cell wall not described so far from Cyanophyta. The layer showed a complex organisation: The main unit are ribbons, 2–3 nm thick, striated at right angle to the longitudinal axis. They are interwoven to a patchwork-like leaflet. The ribbons are virtually composed of globular particles associated in parallel rows. The cytoplasmic membrane and the cell walls of the symbiotic and the free-living strain were compared.Abbreviations cm cytoplasmic membrane - CW 1,2,3 cell wall layer 1,2,3 - EF exoplasmic fracture face - PF protoplasmic fracture face     

Lipid vertical motion and related steric effects in bilayer membranes

We present a theoretical model of a lipid bilayer in its gel state which explicity couples the vertical displacements of the lipid chains to their conformational state. In this model the chains are free to move longitudinally under a potential due to the neighbouring chains. The potential is due to a restoring force with a force constant k and thus acts to keep them in the local plane as defined by their nearest neighbours. It is demonstrated that the force constant k is directly related to the internal bilayer pressure, , and that if a value =33 dynes/cm is assumed then k=17.3 dynes/cm. Steric effects are explicity included by allowing chains to twist into free volume created by the vertical displacement of neighbouring chains. The Hamiltonian is expressed in terms of the projection operators, P _ij , describing the displacement of chain i relative to a neighbour j, and G _ij describing the direction of a twist in chain i. The model is solved both analytically and via Monte Carlo simulations for a one-dimensional system. The possibility of phase-transitions in two-dimensions and the relevance to the bilayer pre-transition is discussed.This work was first presented in poster form at the Canadian Biochemical Society Conference held in Banff, Alberta, Canada, April 29–May 4, 1984Work supported in part by the Natural Sciences and Engineering Research Council of Canada, Le Fonds Formation des Chercheurs et Action a la Recherche du Quebec, and the Advisory Research Council of Queen's University     

Structural and chemical characterization of S-layers of selected strains ofBacillus stearothermophilus andDesulfotomaculum nigrificans

The structures, amino acid- and neutral sugar compositions of the crystalline surface layers (S-layers) of four selected strains each ofBacillus stearothermophilus andDesulfotomaculum nigrificans were compared. Among the four strains of each species a remarkable diversity in the molecular weights of the S-layer subunits and in the geometry and constants of the S-layer lattices was apparent. The crystalline arrays included hexagonal (p6), square (p4) and oblique (p2) lattices. In vitro self-assembly of isolated S-layer subunits (or S-layer fragments) led to the formation of flat sheets or open-ended cylindrical assembly products. The amino acid composition of the S-layers exhibited great similarities and was predominantly acidic. With the exception of the S-layers of two strains ofB. stearothermophilus (where only traces of neutral sugars could be detected), all other S-layer proteins seemed to be glycosylated. Among these strains significant differences in the amount and composition of the glycan portions were found. Based on this diversity interesting questions may be asked about the biological significance of the carbohydrate units of glycoproteins in prokaryotic organisms.     

Heliobacterium chlorum: cell organization and structure

The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.Abbreviations PF protoplasmic fracture face - EF exoplasmic fracture face     

Anatomical aspects of hormonal regulation of abscission in citrus – The shoot-peduncle abscission zone in the non-abscising stage

Although mature citrus fruits [ Citrus sinensis (L.) Osbeck cv. Shamouti] did not abscise at the peduncle-shoot abscission zone (AZ–A) when incubated in ethylene environment, abscission processes did occur in a limited number of cell layers situated in the inner bark, the starch sheath region, and in the pith of AZ–A. These processes were regulated by 2,4-D and ethylene treatments. Cells responding to the "separation processes", particularly in the ethylene treatment, underwent either (a) cell wall swelling, dissolving and breakdown, or (b) growth and expansion in a radial plane. Further away from the dissolving area, the response of some cells of the mid and outer bark took the form of divisions or growth in a circumferential plane, while other cells remained unchanged. Non-responding tissues of the outer bark formed a "sleeve" of undissolved cells, and the vascular cylinder produced no abscission in AZ–A. It is concluded that the partial cell wall dissolution in AZ–A explains the increased activity of cellulase and polygalacturonase in the non-abscising AZ–A of the mature fruit (Greenberg et al. 1975. Physiol. Plant. 37: 1–7).