植物细胞的形态建成

从控制细胞形态建成的通用机制,影响形态建成的因素和形态建成的调节3个方面介绍近年来植物细胞形态建成的进展。细胞壁组分和结构的修饰改变是细胞形态建成的关键;细胞骨架的组装和活性,以及膨压的变化对于细胞的形态建成有着重要的作用。     

Force and compliance: rethinking morphogenesis in walled cells

In the turgid cells of plants, protists, fungi, and bacteria, walls resist swelling; they also confer shape on the cell. These two functions are not unrelated: cell physiologists have generally agreed that morphogenesis turns on the deformation of existing wall and the deposition of new wall, while turgor pressure produces the work of expansion. In 1990, I summed up consensus in a phrase: "localized compliance with the global force of turgor pressure." My purpose here is to survey the impact of recent discoveries on the traditional conceptual framework. Topics include the recognition of a cytoskeleton in bacteria; the tide of information and insight about budding in yeast; the role of the Spitzenk?rper in hyphal extension; calcium ions and actin dynamics in shaping a tip; and the interplay of protons, expansins and cellulose fibrils in cells of higher plants.     

A computational approach for inferring the cell wall properties that govern guard cell dynamics

Guard cells dynamically adjust their shape in order to regulate photosynthetic gas exchange, respiration rates and defend against pathogen entry. Cell shape changes are determined by the interplay of cell wall material properties and turgor pressure. To investigate this relationship between turgor pressure, cell wall properties and cell shape, we focused on kidney‐shaped stomata and developed a biomechanical model of a guard cell pair. Treating the cell wall as a composite of the pectin‐rich cell wall matrix embedded with cellulose microfibrils, we show that strong, circumferentially oriented fibres are critical for opening. We find that the opening dynamics are dictated by the mechanical stress response of the cell wall matrix, and as the turgor rises, the pectinaceous matrix stiffens. We validate these predictions with stomatal opening experiments in selected Arabidopsis cell wall mutants. Thus, using a computational framework that combines a 3D biomechanical model with parameter optimization, we demonstrate how to exploit subtle shape changes to infer cell wall material properties. Our findings reveal that proper stomatal dynamics are built on two key properties of the cell wall, namely anisotropy in the form of hoop reinforcement and strain stiffening.     

Water uptake by growing cells: an assessment of the controlling roles of wall relaxation, solute uptake, and hydraulic conductance

Growing plant cells increase in volume principally by water uptake into the vacuole. There are only three general mechanisms by which a cell can modulate the process of water uptake: (a) by relaxing wall stress to reduce cell turgor pressure (thereby reducing cell water potential), (b) by modifying the solute content of the cell or its surroundings (likewise affecting water potential), and (c) by changing the hydraulic conductance of the water uptake pathway (this works only for cells remote from water potential equilibrium). Recent studies supporting each of these potential mechanisms are reviewed and critically assessed. The importance of solute uptake and hydraulic conductance is advocated by some recent studies, but the evidence is indirect and conclusions remain controversial. For most growing plant cells with substantial turgor pressure, it appears that reduction in cell turgor pressure, as a consequence of wall relaxation, serves as the major initiator and control point for plant cell enlargement. Two views of wall relaxation as a viscoelastic or a chemorheological process are compared and distinguished.     

Distinct fluorescent pattern of KAT1::GFP in the plasma membrane of Vicia faba guard cells

The organisation of membrane proteins into certain domains of the plasma membrane (PM) has been proposed to be important for signalling in yeast and animal cells. Here we describe the formation of a very distinct pattern of the K(+) channel KAT1 fused to the green fluorescent protein (KAT1::GFP) when transiently expressed in guard cells of Vicia faba. Using confocal laser scanning microscopy we observed a radially striped pattern of KAT1::GFP fluorescence in the PM in about 70% of all transfected guard cells. This characteristic pattern was found to be cell type and protein specific and independent of the stomatal aperture and the cytoskeleton. Staining of the cell wall of guard cells with Calcofluor White revealed a great similarity between the arrangement of cellulose microfibrils and the KAT1::GFP pattern. Furthermore, the radial pattern of KAT1::GFP immediately disappeared when turgor pressure was strongly decreased by changing from hypotonic to hypertonic conditions. The pattern reappeared within 15 min upon reestablishment of high turgor pressure in hypotonic solution. Evaluation of the staining pattern by a mathematical algorithm further confirmed this reversible abolishment of the radial pattern during hypertonic treatment. We therefore conclude that the radial organisation of KAT1::GFP depends on the close contact between the PM and cell wall in turgid guard cells. These results offer the first indication for a role of the cell wall in the localisation of ion channels. We propose a model in which KAT1 is located in the cellulose fibrils intermediate areas of the PM and discuss the physiological role of this phenomenon.     

In vivo extraction of Arabidopsis cell turgor pressure using nanoindentation in conjunction with finite element modeling

Turgor pressure in plant cells is involved in many important processes. Stable and normal turgor pressure is required for healthy growth of a plant, and changes in turgor pressure are indicative of changes taking place within the plant tissue. The ability to quantify the turgor pressure of plant cells in vivo would provide opportunities to understand better the process of pressure regulation within plants, especially when plant stress is considered, and to understand the role of turgor pressure in cellular signaling. Current experimental methods do not separate the influence of the turgor pressure from the effects associated with deformation of the cell wall when estimates of turgor pressure are made. In this paper, nanoindentation measurements are combined with finite element simulations to determine the turgor pressure of cells in vivo while explicitly separating the cell‐wall properties from the turgor pressure effects. Quasi‐static cyclic tests with variable depth form the basis of the measurements, while relaxation tests at low depth are used to determine the viscoelastic material properties of the cell wall. Turgor pressure is quantified using measurements on Arabidopsis thaliana under three pressure states (control, turgid and plasmolyzed) and at various stages of plant development. These measurements are performed on cells in vivo without causing damage to the cells, such that pressure changes may be studied for a variety of conditions to provide new insights into the biological response to plant stress conditions.     

The role of cell wall in plant embryogenesis

This review presents recent data about cell wall involvement in plant embryogenesis. During plant development, the cell wall is subjected to precise regulation. During this process a bidirectional information exchange between the cell wall and the protoplast is observed. The cell wall also mediates in the cell-cell (apoplastic) and cell to cell (symplastic) information flow. Especially some products derived from the hydrolysis of specific cell wall compounds can act as short distance signal transduction molecules during the development. Oligosaccharins are a group of such products. Their activity and sources focused the researchers' attention on the biochemical composition of the cell wall and the activity of some cell wall enzymes. The dramatic influence on the embryo body shape has also the cell wall synthesis machinery, including vesicular secretion pathways. Moreover, the interplay between the turgor pressure and counteracting cell walls and neighbouring cells (in higher organisms) creates the specific mechanical forces influencing the development of the whole plant. We conclude that discovering factors which can influence cell wall physiology and architecture is crucial for a better understanding of plant embryogenesis. In this review we summarize some recent experimental data reporting plant cell wall involvement in embryogenesis, putting special emphasis on somatic embryogenesis.     

Wirkung der Turgorreduktion auf den Golgi-Apparat und die Bildung der Zellwand bei Wurzelhaaren

Zusammenfassung Die Reduktion des Turgordrucks hemmt bei Wurzelhaaren das Flächenwachstum der Zell-wand. Nach Turgorreduktion werden jedoch noch Zellwandstoffe sezerniert, so daß apikale Wandverdickungen gebildet werden. Ihre Dimension und ihr Feinbau sind abhängig vom Grad des Turgordrucks. 3 Stunden nach Turgorreduktion ist die für Zellen mit Spitzen-wachstum typische polare Organisation des Cytoplasmas zugunsten einer gleichmäßigen Ver-teilung der Zellorganellen verändert. Die Anzahl der Zisternen in den Dictyosomen ist erhöht. Besonders die Zisterne auf der Sekretionsseite ist stark gekrümmt. Golgi-Vesikel werden wahr-scheinlich noch produziert, aber ihre Inkorporation in das Plasmalemma wurde nicht be-obachtet.

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Effect of turgor reduction on the golgi apparatus and the cell wall formation in root hairs

Summary In root hairs the reduction of turgor pressure inhibits growth in area of the cell wall. After turgor reduction, however, the cell wall substances are still secreted so that apical wall thickenings are formed. Their dimension and ultrastructure depend on the degree of turgor pressure. 3 hours after turgor reduction the polar organisation of the cytoplasm, which is typical for cells with tip growth, has changed in favour of an uniform distribution of cell organelles. The number of the dictyosome cisternae is increased. Especially the cisterna at the secreting side is strongly curved. Probably Golgi vesicles are still produced, but their incorporation into the plasma membrane was not observed.

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Der Deutschen Forschungsgemeinschaft danken wir für Sachbeihilfen. Die Stiftung Volks-wagenwerk stellte das Siemens-Elmiskop I A zur Verfügung.     

Acid-induced wall loosening is confined to the accelerating region of the root growing zone

The aims of this study were to quantify developmental differences in acid growth along the root axis and to determine whether these differences were due to alterations in cell turgor or cell wall properties. The apoplast pH of maize roots growing in hydroponics was altered from pH 7.0 to pH 3.4 using 2 mol m^-3 citrate-phosphate buffer or unbuffered solutions. Whole root elongation rate rapidly increased and measurement of the local growth profile indicated that this increase in growth occurred in young cells in the accelerating zone (apical 0-4 mm) while more proximal growing cells were unaffected. Unbuffered solutions of identical pH produced qualitatively similar results. Single cell turgor pressures were unchanged between pH treatments both longitudinally and radially in the root tip. This suggests that the rapid acid-induced changes in growth rate were due to an increase in cell wall loosening. Single cell osmotic pressure and water potential were not significantly different between pH treatments. Acid pH caused net solute import at the root tip to increase 3- to 4-fold, which, coupled with the maintenance of turgor and osmotic pressure, indicated that solute import was not limiting expansion. Thus, acidic solutions cause an increase in growth in accelerating but not decelerating regions. It has been shown for the first time that acid growth in intact, growing roots is not due to differences in turgor, assigning these changes to cell wall properties. Possible cell wall biochemical alterations are discussed.     

Mechanical Response of Single Plant Cells to Cell Poking： A Numerical Simulation Model

Cell poking is an experimental technique that is widely used to study the mechanical properties of plant cells. A full understanding of the mechanical responses of plant cells to poking force Is helpful for experimental work. The aim of this study was to numerically investigate the stress distribution of the cell wall, cell turgor, and deformation of plant cells in response to applied poking force. Furthermore, the locations damaged during poking were analyzed. The model simulates cell poking, with the cell treated as a spherical, homogeneous, isotropic elastic membrane, filled with incompressible, highly viscous liquid. Equilibrium equations for the contact region and the non-contact regions were determined by using membrane theory. The boundary conditions and continuity conditions for the solution of the problem were found. The forcedeformation curve, turgor pressure and tension of the cell wall under cell poking conditions were obtained. The tension of the cell wall circumference was larger than that of the meridian. In general, maximal stress occurred at the equator around. When cell deformation increased to a certain level, the tension at the poker tip exceeded that of the equator. Breakage of the cell wall may start from the equator or the poker tip, depending on the deformation. A nonlinear model is suitable for estimating turgor, stress, and stiffness, and numerical simulation is a powerful method for determining plant cell mechanical properties.     

Loss of stability: a new look at the physics of cell wall behavior during plant cell growth

In this article we investigate aspects of turgor-driven plant cell growth within the framework of a model derived from the Eulerian concept of instability. In particular we explore the relationship between cell geometry and cell turgor pressure by extending loss of stability theory to encompass cylindrical cells. Beginning with an analysis of the three-dimensional stress and strain of a cylindrical pressure vessel, we demonstrate that loss of stability is the inevitable result of gradually increasing internal pressure in a cylindrical cell. The turgor pressure predictions based on this model differ from the more traditional viscoelastic or creep-based models in that they incorporate both cell geometry and wall mechanical properties in a single term. To confirm our predicted working turgor pressures, we obtained wall dimensions, elastic moduli, and turgor pressures of sequential internodal cells of intact Chara corallina plants by direct measurement. The results show that turgor pressure predictions based on loss of stability theory fall within the expected physiological range of turgor pressures for this plant. We also studied the effect of varying wall Poisson's ratio nu on extension growth in living cells, showing that while increasing elastic modulus has an understandably negative effect on wall expansion, increasing Poisson's ratio would be expected to accelerate wall expansion.     

Chemically mediated mechanical expansion of the pollen tube cell wall

Morphogenesis of plant cells is tantamount to the shaping of the stiff cell wall that surrounds them. To this end, these cells integrate two concomitant processes: 1), deposition of new material into the existing wall, and 2), mechanical deformation of this material by the turgor pressure. However, due to uncertainty regarding the mechanisms that coordinate these processes, existing models typically adopt a limiting case in which either one or the other dictates morphogenesis. In this report, we formulate a simple mechanism in pollen tubes by which deposition causes turnover of cell wall cross-links, thereby facilitating mechanical deformation. Accordingly, deposition and mechanics are coupled and are both integral aspects of the morphogenetic process. Among the key experimental qualifications of this model are: its ability to precisely reproduce the morphologies of pollen tubes; its prediction of the growth oscillations exhibited by rapidly growing pollen tubes; and its prediction of the observed phase relationships between variables such as wall thickness, cell morphology, and growth rate within oscillatory cells. In short, the model captures the rich phenomenology of pollen tube morphogenesis and has implications for other plant cell types.     

Experimental approaches used to quantify physical parameters at cellular and subcellular levels

From a mechanical point of view, plant and hyphal cells are more complex than their animal counterparts because the variety of structural components determining cellular architecture is broader. In addition to cytoskeletal elements and the plasma membrane, the cell wall and turgor pressure equip plant and hyphal cells with structures analogous to an exoskeleton and a hydroskeleton, respectively. To quantify the physical properties of plant and hyphal cells, researchers have developed a plethora of experimental methods. This review provides an overview of experimental approaches that have been used to measure turgor pressure and to determine the mechanical properties of the plant cell wall at the subcellular level. It is completed by a glimpse into the arsenal of techniques that has been used to characterize the physical properties of cytoskeletal elements. These have mostly been used on animal cells, but we hope they will find their way into plant cell research. Finally, assays and tests to measure the generation of forces by cells and subcellular structures are discussed.     

Analysis of the dynamic and steady-state responses of growth rate and turgor pressure to changes in cell parameters

The physical analysis of plant cell enlargment is extended to show the dependence of turgor pressure and growth rate under steady-state conditions on the parameters which govern cell wall extension and water transport in growing cells and tissues, and to show the dynamic responses of turgor and growth rate to instantaneous changes in one of these parameters. The analysis is based on the fact that growth requires simultaneous water uptake and irreversible wall expansion. It shows that when a growing cell is perturbed from its steady-state growth rate, it will approach the steady-state rate with exponential kinetics. The half-time of the transient adjustment depends on the biophysical parameters governing both water transport and irreversible wall expansion. When wall extensibility is small compared to hydraulic conductance, the growth rate is controlled by the yielding properties of the cell wall, while the half-time for changes in growth rate is controlled by the water transport parameters. The reverse situation occurs when hydraulic conductance is lower than wall extensibility. The analysis also shows explicitly that turgor pressure is tightly coupled with growth rate when growth is controlled by both water transport and wall yielding parameters.     

Two water molds can grow without measurable turgor pressure

The water molds Achlya bisexualis Coker and Saprolegnia ferax (Gruithuisen) Thuret (Class: Oomycetes) normally grow in the form of slender hyphae with up to 0.8 MPa (8 bar) of internal pressure. Models of plant cell growth indicate that this turgor pressure drives the expansion of the cell wall. However, under conditions of prolonged osmotic stress, these species were able to grow in the absence of measurable turgor. Unpressurized cells of A. bisexualis grew in the form of a plasmodium-like colony on solid media, and produced a multinucleate yeast-like phase in liquid. By contrast, the morphology of S. ferax was unaffected by the loss of turgor, and the mold continued to generate tip-growing hyphae. Measurements of cell wall strength indicate that these microorganisms produce a very fluid wall in the region of surface growth, circumventing the usual requirement for turgor.Abbreviations DAPI 4,6-diamidino-2-phenylindole - PEG polyethylene glycol This work was supported by National Science Foundation grant DCB 90-17130.     

Maintenance of turgor by rapid sealing of puncture wounds in leaf epidermal cells

When leaf epidermal cells are puncture wounded with a glass microcapillary tip, a small droplet of fluid is discharged and then evaporates, leaving a solid residue on the cell surface. For puncture wounds of about 3.5 micrometers in diameter, this process is complete within 2 to 3 seconds. A second puncture wound also exhibits a similar discharge, indicating the persistence of some turgor pressure within the cell, despite damage to the cell wall. Direct measurement of turgor on the large epidermal cells of Tradescantia virginiana L. demonstrated that turgor was substantially maintained (91-96%) after puncture wounding. Anatomical and histochemical evidence suggests that the damaged portion of the cell wall was sealed with an amorphous plug of material comprised of pectinaceous polysaccharides. Rapid sealing of puncture wounds and the maintenance of turgor in epidermal cells may be an important functional component of plant adaptation to physical damage such as that caused by insect feeding.     

The evolution,structure and functioning of stomata

Summary

The anatomy and ultrastructure of guard cells from a range of species varying from the primitive types, such as mosses, to the advanced grasses and orchids are described. An attempt is made to trace the lines along which stomata developed and to define what might be considered advanced stomata. Additionally, the differentiation of guard cells from guard mother cells is discussed. Of particular note is the preprophase band of microtubules which marks the zone where the future cell wall will form and the movement of the spindle and developing cell plate through 45 degrees. The structure and function of guard cells are intimately linked. Stomata are turgor regulated valves; the osmotica for absorbing water during opening are K⁺, Cl^- and malate anions which accumulate in the guard cell vacuoles. Upon stomatal closure, K⁺ and Cl^- exit from the guard cells while at least some of the carbon from malate is channelled into starch and there is a resultant loss of guard cell turgor. The Calvin cycle may be absent or of low activity in guard cell chloroplasts and under those circumstances a source of carbon and energy to sustain the guard cells is needed. Hence it is believed that sucrose is transported into the guard cells from mesophyll cells. A brief consideration of the mechanism by which the ions are transported across the plasma membrane and tonoplast is made: the driving force for the K⁺, Cl^- and malate movement across the membranes is the proton motive force set up by proton-pumping ATPases.     

Growth Physics in Nitella: a Method for Continuous in Vivo Analysis of Extensibility Based on a Micro-manometer Technique for Turgor Pressure

The view that the plant cell grows by the yielding of the cell wall to turgor pressure can be expressed in the equation: rate = cell extensibility × turgor. All growth rate responses can in principle be resolved into changes in the 2 latter variables. Extensibility will relate primarily to the yielding properties of the cell wall, turgor primarily to solute uptake or production. Use of this simple relationship in vivo requires that at least 2 of the 3 variables be measured in a growing cell. Extensibility is not amenable to direct measurement. Data on rate and turgor for single Nitella cells can, however, be continuously gathered to permit calculation of extensibility (rate/turgor). Rate is accurately obtained from measurements on time-lapse film. Turgor is estimated in the same cell, to within 0.1 atm or less, by measurement of the ability of the cell to compress gas trapped in the closed end of a capillary the open end of which is in the cell vacuole. The method is independent of osmotic equilibrium. It operates continuously for several days, over a several fold increase in cell length, and has response time of less than one minute. Rapid changes in turgor brought on by changes in tonicity of the medium, show that extensibility, as defined above, is not constant but has a value of zero unless the cell has about 80% of normal turgor. Because elastic changes are small, extensibility relates to growth. Over long periods of treatment in a variety of osmotica the threshold value for extensibility and growth is seen to fall to lower values to permit resumption of growth at reduced turgor. A brief period of rapid growth (5× normal) follows the return to normal turgor. All variables then become normal and the cycle can be repeated. The cell remains essentially at osmotic equilibrium, even while growing at 5× the normal rate. The method has potential for detailed in vivo analyses of “wall softening.”     

Contributions of turgor pressure, the contractile ring, and septum assembly to forces in cytokinesis in fission yeast

A paradigm of cytokinesis in animal cells is that the actomyosin contractile ring provides the primary force to divide the cell [1]. In the fission yeast Schizosaccharomyces pombe, cytokinesis also involves a conserved cytokinetic ring, which has been generally assumed to provide the force for cleavage [2-4] (see also [5]). However, in contrast to animal cells, cytokinesis in yeast cells also requires the assembly of a cell wall septum [6], which grows centripetally inward as the ring closes. Fission yeast, like other walled cells, also possess high (MPa) turgor pressure [7-9]. Here, we show that turgor pressure is an important factor in the mechanics of cytokinesis. Decreasing effective turgor pressure leads to an increase in cleavage rate, suggesting that the inward force generated by the division apparatus opposes turgor pressure. The contractile ring, which is predicted to provide only a tiny fraction of the mechanical stress required to overcome turgor, is largely dispensable for ingression; once septation has started, cleavage can continue in the absence of the contractile ring. Scaling arguments and modeling suggest that the large forces for cytokinesis are not produced by the contractile ring but are driven by the assembly of cell wall polymers in the growing septum.     

Regulatory T cells and control of the germinal centre response

Germinal centres (GCs) are specialised lymphoid microenvironments that form in secondary B-cell follicles upon exposure to T-dependent antigens. In the GC, clonal expansion, selection and differentiation of GC B cells result in the production of high-affinity plasma cells and memory B cells that provide protection against subsequent infection. The GC is carefully regulated to fulfil its critical role in defence against infection and to ensure that immunological tolerance is not broken in the process. The GC response can be controlled by a number of mechanisms, one of which is by forkhead box p3 expressing regulatory T (Treg) cells, a suppressive population of CD4⁺ T cells. A specialised subset of Treg cells – follicular regulatory T (Tfr) cells – form after immunisation and are able to access the GC, where they control the size and output of the response. Our knowledge of Treg cell control of the GC is expanding. In this review we will discuss recent advances in the field, with a particular emphasis on the differentiation and function of Tfr cells in the GC.