Auxin increases elastic wall-properties in rye coleoptiles: implications for the mechanism of wall loosening

Auxin-induced changes of wall-rheological properties during different growth rates of rye coleoptile segments (Secale cereale L.) were investigated. In addition, changes of osmotic concentration and turgor pressure were measured. Decrease of turgor and of osmotic concentration followed a synchronous time course. Auxin-incubated segments exhibited a faster decrease and eventually lower values of both parameters. Creep test extensibility measurements demonstrate that apparent plastic as well as elastic extensibility of distilled-water-incubated segments strongly decreased during 24 h. In auxin-incubated segments apparent plastic as well as elastic extensibilities were strongly increased, even in the absence of growth due to insufficient turgor pressure. The increasing effect of auxin on elastic wall properties is also reflected by an increase in relative reversible length (part of segment length by which segments shrink after freezing/thawing as referred to total length) and a complementary decrease of relative irreversible length (remaining length after turgor elimination as referred to turgid length); again the effects were independent of growth rate and turgor pressure. Cellulose synthesis inhibition of approx. 80% by dichlorobenzonitrile (DCB) had no significant effect either on growth or on wall-rheological properties. Independent of whether the changed rheological wall behaviour of auxin-incubated segments is causally related to the mechanism of auxin-induced wall loosening, it indicates changes of wall polymer properties and/or interactions which are conserved when no actual length increase occurs due to insufficient turgor pressure. The results suggest that IAA-induced wall loosening may be primarily mediated by cell wall changes other than cleavage of covalent, load-bearing bonds as hypothesized in various wall loosening models.     

Turgor pressure sensing in plant cell membranes

Experimental evidence is reviewed which shows that the cell membrane is compressible by both mechanical and electrical forces. Calculations are given which show that significant changes in the thickness of cell membranes can occur as a result of (a) direct compression due to the turgor pressure; (b) indirect effects due to the stretching of the cell wall; and (c) the stresses induced by the electric field in the membrane.     

Aluminium effects on mechanical properties of cell wall analogues

Aluminium (Al) toxicity adversely impacts plant productivity in acid soils by restricting root growth and although several mechanisms are involved the physiological basis of decreased root elongation remains unclear. Understanding the primary mechanisms of Al rhizotoxicity is hindered due to the rapid effects of soluble Al on root growth and the close proximity of many cellular components within the cell wall, plasma membrane, cytosol and nucleus with which Al may react. To overcome some of these difficulties, we report on a novel method for investigating Al interactions with Komagataeibacter xylinus bacterial cellulose (BC)‐pectin composites as cell wall analogues. The growth of K. xylinus in the presence of various plant cell wall polysaccharides, such as pectin, has provided a unique in vitro model system with which to investigate the interactions of Al with plant cell wall polysaccharides. The BC‐pectin composites reacted in a similar way with Al as do plant cell walls, providing insights into the effects of Al on the mechanical properties of the BC‐pectin composites as cell wall analogues. Our findings indicated that there were no significant effects of Al (4–160 μM) on the tensile stress, tensile strain or Young's modulus of the composites. This finding was consistent with cellulose, not pectin, being the major load bearing component in BC‐pectin composites, as is also the case in plant cell walls.     

Effects of glucanase treatment on the structure and mechanical properties of cell walls in the giant‐celled xanthophycean alga Vaucheria frigida

The cell wall of the tip‐growing cells of the giant‐cellular xanthophycean alga Vaucheria frigida is mainly composed of cellulose microfibrils (CMFs) arranged in random directions and the major matrix component into which the CMFs are embedded throughout the cell. The mechanical properties of a cell‐wall fragment isolated from the tip‐growing region, which was inflated by artificially applied pressure, were measured after enzymatic removal of the matrix component by using a protease; the results showed that the matrix component is involved in the maintenance of cell wall strength. Since glucose and uronic acid are present in the matrix component of Vaucheria cell walls, we measured the mechanical properties of the cell wall after treatment with endo‐1,3‐ß‐glucanase and observed the fine structures of its surfaces by atomic force microscopy. The major matrix component was partially removed from the cell wall by glucanase, and the enzyme treatment significantly weakened the cell wall strength without affecting the pH dependence of cell wall extensibility. The enzymatic removal of the major matrix component by using a protease released polysaccharide containing glucose and glucuronic acid. This suggests that the major matrix component of the algal cell walls contains both proteins (or polypeptides) and polysaccharides consisting of glucose and glucuronic acid as the main constituents.     

Ontogeny and distribution of cell‐wall glycans during aerenchyma formation in roots of Pistia stratiotes (Araceae)

Aerenchyma is widely known to be lysigenous, schizogenous or, more recently, expansigenous. The interpretation and understanding of its function is questionable, given the lack of extensive knowledge on the development and cellular changes of this tissue. The aerenchyma of Pistia stratiotes roots reportedly originates from packet lysigeny. However, our observations suggest schizogenous development. Our objective was to analyse ontogeny of aerenchyma in P. stratiotes roots and evaluate the morphological and chemical changes in the cell wall during the formation of aerenchyma. The aerenchymatous inner cortex of schizogenous origin was observed under light and electron microscopy. Lacunae are formed by the separation, division and stretching of cells, which remain alive until maturity. Analyses using monoclonal anti‐glycan antibodies show that formation of that type of aerenchyma apparently proceeds through the same mechanisms as the genesis of intercellular spaces. However, the greatest changes occur when cells undergo stretching, including the loss of methyl‐esterification and detection of arabinans, which are not directly involved in cell separation. Thus, other factors may account for the formation of schizogenous aerenchyma.     

Glycerinated catch apparatus of sea urchin spines: the effects of cations on its mechanical properties and ultrastructure

Sea urchin spinal ligaments (the catch apparatus) were extracted with glycerin, and electron microscopic observations comfirmed that no cell membranes remained intact after glycerination. We studied the effects of cations (Na(+), K(+), Ca(2+), Mg(2+)) on the mechanical properties of the glycerinated ligaments. Monovalent cations decreased whereas divalent cations increased the viscosity of the ligaments. The ion dependencies were similar to previous results with detergent-extracted holothurian dermis, which suggests that the echinoid ligament shares a similar mechanism for changes in mechanical properties with other catch connective tissues. This provides evidence against the hypothesis of that muscles in the catch apparatus are responsible for the changes in mechanical properties of the ligament. Fine projections cross-bridging collagen fibrils were observed in the glycerin-extracted ligaments as well as in the intact ligaments. They were found in all the ionic conditions studied.     

Force extension analysis of Avena coleoptile cell walls

Summary A method is described for measuring the cell wall mechanical properties of Avena coleoptiles in the absence of turgor stress or influences of a living protoplast. Forceextension curves obtained with a constant-rate-of-extension instrument and standard fiber-testing techniques demonstrate the permanence of cell wall loosening effects of prior indoleacetic acid (IAA) treatment of living tissue and provide evidence that these changes involve interactions between cell wall polymers. By this method various chemical and enzymatic modifications of cell walls can be evaluated in terms of altered mechanical properties. Thus, it was possible to remove over 97% of the cell nitrogen (including some hydroxyproline-containing protein) by hot methanol followed by enzymatic treatment and not change the extensibility properties of the tissue. In contrast, coleoptile mechanical properties were markedly influenced by chemical acetylation procedures or cellulase treatment.With 3 Figures in the Text     

STRUCTURAL CHANGES OCCURRING IN NUCLEI OF BARLEY ROOT CELLS IN RESPONSE TO A COMBINED EFFECT OF SALINITY AND AGEING

Electron microscopic observations of epidermal and cortical cells of the root tips of barley (Hordeum vulgare L.) grown in 192 mm NaCl and aged in 192 mm NaCl + 0.2 mm CaSO₄ revealed marked condensation of chromatin in the nuclei which was not observed in freshly cut tissue grown in the presence of 192 mM NaCl. Other changes due to salinity were observed, such as the increase of the number of ribosomes and of mitochondria and the appearance of translucent areas in slightly swollen mitochondria. The mechanism by which the nuclear changes occurred or their meaning for cell function are not understood.     

MECHANICAL BEHAVIOR OF PLANT TISSUES AS INFERRED FROM THE THEORY OF PRESSURIZED CELLULAR SOLIDS

The mechanical behavior of plant tissues and its dependency on tissue geometry and turgor pressure are analytically dealt with in terms of the theory of cellular solids. A cellular solid is any material whose matter is distributed in the form of beamlike struts or complete “cell” walls. Therefore, its relative density is less than one and typically less than 0.3. Relative density is the ratio of the density of the cellular solid to the density of its constitutive (“cell wall”) material. Relative density depends upon cell shape and the density of cell wall material. It largely influences the mechanical behavior of cellular solids. Additional important parameters to mechanical behavior are the elastic modulus of “cell walls” and the magnitude of internal “cell” pressure. Analyses indicate that two “stiffening” agents operate in natural cellular solids (plant tissues): 1) cell wall infrastructure and 2) the hydrostatic influence of the protoplasm within each cellular compartment. The elastic modulus measured from a living tissue sample is the consequence of both agents. Therefore, the mechanical properties of living tissues are dependent upon the magnitude of turgor pressure. High turgor pressure places cell walls into axial tension, reduces the magnitude of cell wall deformations under an applied stress, and hence increases the apparent elastic modulus of the tissue. In the absence of turgid protoplasts or in the case of dead tissues, the cell wall infrastructure will respond as a linear elastic, nonlinear elastic, or “densifying” material (under compression) dependent upon the magnitude of externally applied stress. Accordingly, it is proposed that no single tangent (elastic) modulus from a stress-strain curve of a plant tissue is sufficient to characterize the material properties of a sample. It is also suggested that when a modulus is calculated that it be referred to as the tissue composite modulus to distinguish it from the elastic modulus of a noncellular solid material.     

Biomechanics of plant growth

Growth of turgid cells, defined as an irreversible increase in cell volume and surface area, can be regarded as a physical process governed by the mechanical properties of the cell wall and the osmotic properties of the protoplast. Irreversible cell expansion is produced by creating a driving force for water uptake by decreasing the turgor through stress relaxation in the cell wall. This mechano-hydraulic process thus depends on and can be controlled by the mechanical properties of the wall, which in turn are subject to modification by wall loosening and wall stiffening reactions. The biochemical mechanisms of these changes in mechanical wall properties and their regulation by internal signals (e.g., hormones) or external signals (e.g., light, drought stress) are at present incompletely understood and subject to intensive research. These signals act on walls that have the properties of composite materials in which the molecular structure and spatial organization of polymers rather than the distribution of mechanical stresses dictate the allometry of cell and organ growth and thus cell and organ shape. The significance of cell wall architecture for allometric growth can be demonstrated by disturbing the oriented deposition of wall polymers with microtubule-interfering drugs such as colchicine. Elongating organs (e.g., cylindrical stems or coleoptiles) composed of different tissues with different mechanical properties exhibit longitudinal tissue tensions resulting in the transfer of wall stress from inner to peripheral cell layers that adopt control over organ growth. For physically analyzing the growth process leading to seed germination, the same mechanical and hydraulic parameters as in normal growth are principally appropriate. However, for covering the influences of the tissues that restrain embryo expansion (seed coat, endosperm), an additional force and a water permeability term must be considered.     

ULTRASTRUCTURE OF CEPHALEUROS VIRESCENS (CHROOLEPIDACEAE; CHLOROPHYTA). I. SCANNING ELECTRON MICROSCOPY OF ZOOSPORANGIA

Scanning election microscopic examination of Cephaleuros virescens growing on leaves of Magnolia grandiflora has provided detailed observations which parallel, extend, and, in general, confirm previous light microscopic studies. The present study has revealed that in the ontogeny of terminal zoosporangia, apical zones form in the surface of an enlarged pyriform cell and that these zones, in some cases, are surrounded by a circumferential ridge, an external indication of an internal septum. A similar circumferential ridge is seen at the base of developing terminal zoosporangia. Contrary to some published accounts, the abscission of terminal sporangia does not commence with tearing of the pedicel-sporangium septal wall, but rather with an internal separation of the septum which is followed by a circumscissile tearing of the septal wall. The completion of abscission entails the emergence of a septal protuberance from the pedicel and/or the terminal sporangium in a process reminiscent of filament fragmentation in zygnematacean algae containing “replicate end walls.” Zoospore exit pores form in a lateral position on the terminal sporangia and are not coincident with the septal protuberances as has been reported in some recent accounts. Although both biflagellate and quadriflagellate zoospores have been seen in the light microscope, only the former have been observed with SEM. The SEM observations recorded in this study have provided a basis for comparison not only with light microscopic data, but also with transmission electron microscopic data which are now being recorded. Observations reported will be useful for interspecific and intergeneric comparisons.     

Quantifying whole bladder biomechanics using the novel pentaplanar reflected image macroscopy system

Optimal bladder compliance is essential to urinary bladder storage and voiding functions. Calculated as the change in filling volume per change in pressure, bladder compliance is used clinically to characterize changes in bladder wall biomechanical properties that associate with lower urinary tract dysfunction. But because this method calculates compliance without regard to wall structure or wall volume, it gives little insight into the mechanical properties of the bladder wall during filling. Thus, we developed Pentaplanar Reflected Image Macroscopy (PRIM): a novel ex vivo imaging method to accurately calculate bladder wall stress and stretch in real time during bladder filling. The PRIM system simultaneously records intravesical pressure, infused volume, and an image of the bladder in five distinct visual planes. Wall thickness and volume were then measured and used to calculate stress and stretch during filling. As predicted, wall stress was nonlinear; only when intravesical pressure exceeded ~ 15 mmHg did bladder wall stress rapidly increase with respect to stretch. This method of calculating compliance as stress vs stretch also showed that the mechanical properties of the bladder wall remain similar in bladders of varying capacity. This study demonstrates how wall tension, stress and stretch can be measured, quantified, and used to accurately define bladder wall biomechanics in terms of actual material properties and not pressure/volume changes. This method is especially useful for determining how changes in bladder biomechanics are altered in pathologies where profound bladder wall remodeling occurs, such as diabetes and spinal cord injury.

     

A continuum model for root growth

This paper presents a study of a simple one-dimensional continuum model for growth of the plant root. A fundamental constitutive equation is derived. The model is studied by means of various special cases of increasing complexity. Asymptotic expansions are used to derive approximate solutions to the equation of the model under the fundamental assumption that cell wall thickness is small in comparison with the diameter of the cell. The basic results of the study may be summarized as follows. The observed growth pattern of the root cannot be modelled by a mechanical system whose properties are independent of position on the root. The observed pattern can be modelled by a simple mechanical system in which, for example, cell wall yield stress first decreases and then increases. Two fundamental observations are made based on the modelling study. The first is that any mechanical model must take into account the convective displacement from the tip of points along the root. The second is that in describing growth, data on cell wall mechanical properties are meaningless without corresponding data on cell water potential, and vice versa.     

Cell Growth and the Structure and Mechanical Properties of the Wall in Internodal Cells of Nitella opaca: II. MECHANICAL PROPERTIES OF THE WALLS

The internodal cells of Nitella opaca L. have been used in anattempt to assess the part which mechanical properties of thewall may play in the control of cell growth. It is shown thatthe wall is mechanically anisotropic in both its plastic andelastic properties, and evidence is presented which indicatesthat this arises from its anisotropy of structure. The degreeof anisotropy is greater in cells with a high growth-rate thanin those with a low growth-rate. Evidence is presented thatthis variation in properties with growth-rate is due wholly,or in part, to changes in the orientation of the crystallinecomponent, in the relative proportion of wall constituents,and in the condition of active groups of the wall components.The findings are in harmony with the theory that extension growthof the cell wall is due to ‘creep’, i.e. disturbancesof the molecular forces within the wall leading to a slow plasticyielding to turgor pressure.     

Histochemistry of Tobacco Mesophyll Cell Walls Pretreated with Bacterial Protein-lipopolysaccharides

Prevention of hypersensitive confluent-necrosis in tobacco mesophyll, caused by Pseudomonas syringae pv. aptata, and tolerance towards the incompatible bacterium are induced 48 h after intercellular injection of protein-lipopolysaccharide complexes. Histochemical analysis and ultrastructural observations were carried out to determine whether there was an association between this tolerance state and cell wall alterations due to callose, lignin and/or suberin deposition either before or after challenge with the incompatible bacterium. No evidence was obtained for such wall alterations.     

Reversible and repeatable linear local cell force response under large stretches

Large stretching and un-stretching force response of adherent fibroblasts is measured by micromachined mechanical force sensors. The force sensors are composed of a probe and flexible beams. The probe, functionalized by fibronectin, is used to contact the cells. The flexible beams are the sensing element. The sensors are made of single crystal silicon and fabricated by the SCREAM process. The maximum cell stretch reached is approximately 50 microm, which is about twice of the cell initial size, and the time delay between two consecutive stretching/un-stretching steps is 75 s unless otherwise stated. We find that the force response of the cells is strongly linear, reversible, and repeatable, with a small stiffening at the initial deformation stage. Force response of single cells measured before and after cytochalasin D treatment suggests that actin filaments take almost all the cell internal forces due to stretch. These findings may shed light on the increasing understanding on the mechanical behavior of cells and provide clues for making new classes of biological materials having uncommon properties.     

Pectin and the role of the physical properties of the cell wall in pollen tube growth of<Emphasis Type="Italic"> Solanum chacoense</Emphasis>

The cell wall is one of the structural key players regulating pollen tube growth, since plant cell expansion depends on an interplay between intracellular driving forces and the controlled yielding of the cell wall. Pectin is the main cell wall component at the growing pollen tube apex. We therefore assessed its role in pollen tube growth and cytomechanics using the enzymes pectinase and pectin methyl esterase (PME). Pectinase activity was able to stimulate pollen germination and tube growth at moderate concentrations whereas higher concentrations caused apical swelling or bursting in Solanum chacoense Bitt. pollen tubes. This is consistent with a modification of the physical properties of the cell wall affecting its extensibility and thus the growth rate, as well as its capacity to withstand turgor. To prove that the enzyme-induced effects were due to the altered cell wall mechanics, we subjected pollen tubes to micro-indentation experiments. We observed that cellular stiffness was reduced and visco-elasticity increased in the presence of pectinase. These are the first mechanical data that confirm the influence of the amount of pectins in the pollen tube cell wall on the physical parameters characterizing overall cellular architecture. Cytomechanical data were also obtained to analyze the role of the degree of pectin methyl-esterification, which is known to exhibit a gradient along the pollen tube axis. This feature has frequently been suggested to result in a gradient of the physical properties characterizing the cell wall and our data provide, for the first time, mechanical support for this concept. The gradient in cell wall composition from apical esterified to distal de-esterified pectins seems to be correlated with an increase in the degree of cell wall rigidity and a decrease of visco-elasticity. Our mechanical approach provides new insights concerning the mechanics of pollen tube growth and the architecture of living plant cells.     

A close-up view of wood structure and properties across a growth ring of Norway spruce (Picea abies [L] Karst.)

A growth ring of an adult Norway spruce (Picea abies [L] Karst.) was analyzed to a high resolution at the single cell level with respect to structural and mechanical changes during the growth period. For this purpose structural characterization was performed by means of light microscopy, scanning electron microscopy and wide angle X-ray diffraction for investigating the geometry of cells, their cell wall fractions and cellulose microfibril angles (MFA). The mechanical properties were determined in microtensile tests on individual tracheids which had been taken from sequentially cut tangential slices. The results revealed pronounced differences in tensile stiffness between earlywood and latewood cells but only minor differences in tensile stiffness between the cell walls of both tissue types. These comparatively small changes in cell wall stiffness across the growth ring were caused by slight changes in MFA. The findings suggest that trees mainly vary cell size to optimize water transport and mechanical stability during the growth period and that modification of the cell wall organisation plays a minor role.     

On the morphology and ultrastructure of the cell wall of Spirulina platensis

The cell wall of the blue-green alga Spirulina platensis was studied with the electron microscope using ultra-thin sectioning, shadowing, carbon-replication or freeze-etching techniques for specimen preparation. The cell wall could be resolved into four layers, L-I through L-IV. The L-I and L-III layers contain fibrillar material. The septum is a three-layered wall: an L-II layer sandwiched between L-I layers. The shape in vitro of isolated septa might be an artifact due to the preparation technique used. Certain structural properties of the septum seem to allow tangential stretching; they might be reflected in the flexible gliding mobility of Spirulina species. The outer, L-IV layer contains material longitudinally arranged along the trichome axis.