The pH-dependent yield threshold of the cell wall in a glycerinated hollow cylinder (in vitro system) of cowpea hypocotyl

In order to determine whether the pH-dependent yield threshold of the cell wall still exists in an in vitro system, an extensometer was devised to enable the perfusion of any experimental solution through the hollow cylinder of a hypocotyl segment excised from a cowpea seedling. Stress-strain experiments on glycerinated hollow cylinders revealed the existence of a definite yield threshold (y) of the cell wall in this in vitro system. The y value decreased reversibly with acidification (pH 4) to the same extent as the decrease of the yield threshold obtained in vivo (Y) with auxin-induced growth acceleration of hypocotyl segments. Heat treatment of the glycerinated hollow cylinder completely inhibited the decrease in y with acidification. The increase in the extensibility of the cell wall with acidification was inhibited significantly but not completely by heat treatment. These results support strongly the ‘acid growth’ theory and provide evidence that the acid-induced decrement of the yield threshold is mediated by an enzymatic reaction of a wall-binding protein. The combination of in vitro and in vivo studies presented here provides a basis for the establishment of a molecular theory on the nature of the growth parameters Y and Ф which control the yielding of the cell wall.     

Wall Relaxation and the Driving Forces for Cell Expansive Growth

When water uptake by growing cells is prevented, the turgor pressure and the tensile stress in the cell wall are reduced by continued wall loosening. This process, termed in vivo stress relaxation, provides a new way to study the dynamics of wall loosening and to measure the wall yield threshold and the physiological wall extensibility. Stress relaxation experiments indicate that wall stress supplies the mechanical driving force for wall yielding. Cell expansion also requires water absorption. The driving force for water uptake during growth is created by wall relaxation, which lowers the water potential of the expanding cells. New techniques for measuring this driving force show that it is smaller than believed previously; in elongating stems it is only 0.3 to 0.5 bar. This means that the hydraulic resistance of the water transport pathway is small and that rate of cell expansion is controlled primarily by wall loosening and yielding.     

Stress relaxation of cell walls and the yield threshold for growth

Theory predicts that, for growing plant cells isolated from a supply of water, stress relaxation of the cell wall should decrease cell turgor pressure (P) until the yield threshold for cell expansion is reached. This prediction was tested by direct P measurements of pea (Pisum sativum L.) stem cortical cells before and after excision of the growing region and isolation of the growing tissue from an external water supply. Cell P was measured with the micro-pressure probe under conditions which eliminated transpiration. Psychrometric measurements of water potential confirmed the pressureprobe measurements. Following excision, P of the growing cells decreased in 1 h by an average of 1.8 bar to a mean plateau value of 2.8 bar, and remained constant thereafter. Treatment with 10^-5 M indole-3-acetic acid or 10^-5 M fusicoccin (known growth stimulants) accelerated the rate of P relaxation, whereas various treatments which inhibit growth slowed down or completely stopped P relaxation in apical segments. In contrast, P of basal (nongrowing) segments gradually increased because of absorption of solutes from the cell-wall free space of the tissue. Such solute absorption also occurred in apical segments, but wall relaxation held P at the yield threshold in those segments which were isolated from an external water supply. These results provide a new and rapid method for measuring the yield threshold and they show that P in intact growing pea stems exceeds the yield threshold by about 2 bar. Wall relaxation is shown here to affect the water potential and turgor pressure of excised growing segments. In addition, solute release and absorption upon excision may influence the water potential and turgor pressure of nongrowing excised plant tissues.Abbreviations and symbols IAA indole-3-acetic acid - P turgor pressure - SE standard error of the mean - water potential     

Regulation of plant elongation growth by surface and xylem proton pumps

The roles of plasmalemma electrogenic proton pumps in elongation growth of plant stems are discussed on the basis of growth-electrophysiological studies on hypocotyl segments ofVigna unguiculata. Plant stems usually have two spatially separated electrogenic proton pumps: the surface proton pump which is located on the surface membrane of the symplast and the xylem proton pump, on the cell membrane of the symplast/xylem apoplast boundary. The surface proton pump excretes protons into the surface cell wall layer and causes the loosening of the cell wall. The xylem proton pump excretes protons into the xylem apoplast and drives the uptake of solute and water into the symplastvia secondary and/or tertiary active mechanisms: the proton cotransport system and the apoplast canal system. Both the surface and the xylem proton pumps are active during elongation growth because both the yielding of cell wall loosening and the uptake of water are necessary for continued elongation growth.     

A model of cell wall expansion based on thermodynamics of polymer networks.

A theory of cell wall extension is proposed. It is shown that macroscopic properties of cell walls can be explained through the microscopic properties of interpenetrating networks of cellulose and hemicellulose. The qualitative conclusions of the theory agree with the existing experimental data. The dependence of the cell wall yield threshold on the secretion of the wall components is discussed.     

A Study on the Synthetic Characteristics of the Extracellular Polysaccharide (EPS) of Ganoderma lucidum Cultured in Batch Fermentation Using a Kinetic Model

The synthetic characteristics of the extracellular polysaccharide (EPS) of Ganoderma lucidum in batch fermentation were studied. The result showed that the production of EPS was partially growth-associated. The cell dry weight (CDW) and EPS reached 15.56 g·L⁻¹ and 3.02 g·L⁻¹, respectively. The yield of EPS to cell dry weight (Y_p/x) was 0.19. On the basis of the test results of batch fermentation, a kinetic model was proposed by using the Logistic equation for cell growth, the Luedeking–Piret equation for EPS production, and the Luedeking-piret-like equation for the consumption of glucose as substrate. The calculated results using these models were satisfactorily compared with the experimental data under various concentrations of glucose, and the average of relative errors was found to be not more than 5%. The kinetic model had practical guidance interesting in producing PES by Ganoderma lucidum.     

Does indoleacetic acid promote growth via cell wall acidification?

Growth of Triticum aestivum L. cv. Cappelle Desprez coleoptiles is promoted by 5.7×10^–5 M indole acetic acid (IAA) as effectively in pH 3.4 buffer as in water, but IAA is not effective in the presence of buffer at pH 3.0 or 3.2 A combination of 5.7×10^–5 M IAA and pH 3.4 buffer promotes growth to a greater extent than pH 3.2 buffer alone, which is optimal for acid-induced growth. IAA employed at 10^–7 M is still effective at promoting growth in the presence of pH 3.4 buffer, moreover, IAA at 10^–7 M interacts synergistically with the acidic buffer to promote growth. It is concluded that IAA and acid promote growth via separate mechanisms, and that IAA does not promote cell wall loosening by rendering the cell wall more acid.Abbreviation IAA Indoleacetic acid     

The cellular pathway of sucrose transport in developing cotyledons ofVicia faba L. andPhaseolus vulgaris L.: a physiological assessment

The cellular pathway of sugar uptake in developing cotyledons of Vicia faba L. and Phaseolus vulgaris L. seed was evaluated using a physiological approach. The cotyledon interface with the seed coat is characterised by a specialised dermal cell complex. In the case of Vicia faba cotyledons, the epidermal component of the dermal cell complex is composed of transfer cells. Sucrose is the major sugar presented to the outer surface of both cotyledons and it is taken up from the apoplasm unaltered. Estimated sucrose concentrations within the apparent free space of Vicia and Phaseolus cotyledons were 105 and 113 mM respectively. Rates of in-vitro uptake of [¹⁴C]sucrose by cotyledon segments or by whole cotyledons following physical removal or porter inactivation of the outer cells demonstrated that, for both Vicia and Phaseolus cotyledons, the dermal cell complexes are the most intense sites of sucrose uptake. Accumulation of [¹⁴C]sucrose in the storage parenchyma of whole cotyledons was directly affected by experimental manipulation of uptake by the outer cell layers and plasmolytic disruption of the interconnecting plasmodesmata. These findings indicated that sucrose accumulated by the dermal cell complexes is transported symplasmically to the storage parenchyma. Overall, it is concluded that the dermal cell complexes of the developing legume embryo, irrespective of the presence or absence of wall ingrowths, are the major sites for the uptake of sucrose released from the maternal tissues to the seed apoplasm. Thereafter, the accumulated sucrose is transported radially inward through the symplast to the storage parenchyma.Abbreviations AFS apparent free space - CF 5-(6)-carboxyfluorescein - CFDA 5-(6)-carboxyfluorescein diacetate - Mes 2-(N-morpholino)ethanesulfonic acid - PCMBS p-chloromercuribenzenesulfonic acid - SRG sulphorhodamine G The investigation was supported by funds from the Research Management Committee, The University of Newcastle and the Australian Research Council. One of us, R. McDonald, gratefully acknowledges the support of an Australian Postgraduate Research Award. We are grateful to Stella Savoury for preparing the photomicrographs.     

Relationship between substrate concentration,growth rate,and respiration rate of Escherichia coli in continuous culture

Summary Using a continuous flow technique the relationship between growth rate and substrate concentration was investigated with glucose as the limiting factor of a culture of Escherichia coli. Graphical and numerical analysis of the experimental data demonstrated that the application of the Michaelis-Menten equation produced erroneous results, whereas, the constants obtained from the Teissier equation were in agreement with the experimental data. On this basis, new equations defining the steady state cell and substrate concentration in continuous flow cultures were developed and tested against experimental data.Comparison of the specific growth rates, substrate uptake rates and oxygen consumption rates demonstrated that all were directly proportional to each other and could be related to each other by mathematical equations. Specifically it was shown that as the growth rate increased from 0.06 to k _m =0.76 the substrate uptake rate increased from 134 to 1420 mg glucose per gram cell weight per hour and the oxygen consumption rate increased from 48.6 to 505 mg O₂ per gram cell weight per hour. Independent of the growth rate 37% of the carbohydrate consumed were oxidized. The yield factor varied from 0.44 at low growth rates to 0.54 at high growth rates. Analysis of the growth rate-substrate uptake rate relationship indicated that a minimum substrate uptake rate of 55 mg glucose per gram cell weight per hour existed below which cell reproduction would cease. This was supported by the fact that steady state conditions could not be maintained in the culture at D values below 0.02 when the substrate supply rate decreased below 45 mg glucose per gram cell weight per hour.Material contained in this paper was submitted as a thesis in partial fulfillment of the requirements for the Ph. D. degree of Dr. R. S. Lipe.     

A model for predicting growth responses in plants to changes in external water potential: Zea mays primary roots

In response to osmotic step changes, three distinct phases have been noted in the growth response of Zea mays primary roots. They are cessation or slowing of growth over a period of 15–20 minutes, tissue contraction, and a damped oscillatory return to nearly normal growth rate, all within a period of about one hour. A system model of the tissue response is presented to explain such behavior and to serve in a predictive capacity to govern future experiments.It is supposed that for turgor pressure in excess of a cell wall yield threshold, plastic flow is the major component of wall deformation, and that when turgor falls below yield threshold, elastic deformation is dominant. The equations of the model describe growth rate as a function of time in terms of the following properties; plastic flow, elastic deformation, permeability to water, and solute uptake. They are derived from basic equations of feedback interactions between internal osmotic pressure and growth rate, and between wall softening, turgor and growth rate.The model predicts oscillatory growth rate regulation, and phase and amplitude relationships between turgor pressure and growth rate. The simplest model which accounts for all observations is that of biphasic deformation, two modes of wall softening, and a dual feedback system involving osmotic and yield threshold control of growth rate.It should be noted that to predict the time course of turgor pressure, osmotic pressure, yield pressure, and growth rate, two initial conditions and six system parameter values are sufficient. So far only the initial values of growth rate and its derivative can be obtained for Zea mays primary roots. However, values for wall softening and hardening coefficients (including the strain and turgor independent component), plastic extensibility, water permeability and dilution rate coefficients have not been obtained as yet for Zea roots. Values for some of these parameters have been obtained for other roots, coleoptiles, and giant algal cells.Lest the reader despair, it should be pointed out that experimental observations coupled with simulation studies will help establish restricted ranges of values that the system parameters might assume. These can then be compared with known values in the literature and values experimentally obtained in the future.     

Physiological and metabolic responses of different genotypes of Cymbopogon martinii and C. winterianus to water stress

The effect of water stress on growth and essential oil metabolism wasstudied in two species of aromatic grasses under induced water stressconditions. Cultivars of Cymbopogon martinii andC. winterianus differed considerably in their response towater stress. Plant growth, oil yield and water potential decreased whileproline accumulation and oil biogenesis increased in the different genotypesunder water stress. Water stress induced a wide variation in essential oilcomposition, protein content and in activities of PEP carboxylase and nitratereductase in the different genotypes in terms of increase, decrease or nochange. The physiological and biochemical basis of drought tolerance inC. martinii has been elucidated on the basis of growth andmetabolic responses.     

Long-term acid-induced wall extension in an in-vitro system

When frozen-thawed Avena sativa L. coleoptile and Cucumis sativa L. hypocotyl sections, under tension, are acid-treated, they undergo rapid elongation (acid-extension). The acid-extension response consists of two concurrent phases: a burst of extension which decays exponentially over 1–2 h (ExE), and a constant rate of extension (CE) which can persist for at least 6 h. The extension (AL) is closely represented by the equation: L = a – a · e ^kt + c · t where a is the total extension of the exponential phase, k is the rate constant for ExE, and c is the rate of linear extension (CE). Low pH and high tension increased a and c, whereas temperature influenced k. The magnitude of the CE (over 50% extension/10 h), the similarity in its time course to auxin-induced growth, and the apparent yield threshold for CE indicate that CE is more likely than ExE to be the type of extension which cell walls undergo during normal auxin-induced growth.Abbreviations and symbols CAWL capacity for acid-induced wall extension - CE linear phase of acid-extension - ExE exponential phase of acid-extension - IAA indole-3-acetic acid     

Growth Inhibition, Turgor Maintenance, and Changes in Yield Threshold after Cessation of Solute Import in Pea Epicotyls

The dependence of stem elongation on solute import was investigated in etiolated pea seedlings (Pisum sativum L. var Alaska) by excising the cotyledons. Stem elongation was inhibited by 60% within 5 hours of excision. Dry weight accumulation into the growing region stopped and osmotic pressure of the cell sap declined by 0.14 megapascal over 5 hours. Attempts to assay phloem transport via ethylenediaminetetraacetate-enhanced exudation from cut stems revealed no effect of cotyledon excision, indicating that the technique measured artifactual leakage from cells. Despite the drop in cell osmotic pressure, turgor pressure (measured directly via a pressure probe) did not decline. Turgor maintenance is postulated to occur via uptake of solutes from the free space, thereby maintaining the osmotic pressure difference across the cell membrane. Cell wall properties were measured by the pressure-block stress relaxation technique. Results indicate that growth inhibition after cotyledon excision was mediated primarily via an increase in the wall yield threshold.     

Ion exchange properties of plant root cell walls

Acid-base properties and the swelling capacity of wheat, lupin and pea root cell walls were investigated. Roots of seedlings and green plants of different age were analysed by the potentiometric method. The ion exchange capacity (S _i) and the swelling coefficient (K ^cw) of root cell walls were estimated at various pH values (from 2 to 12) and at different ionic strength (between 0.3 and 1000 mM). To analyse the polysigmoid titration curves pH_i = f (S _i), the Gregor's equation was employed. It was shown that the Gregor's model fits well the experimental data. The total number of the cation exchange (S _t ^cat) and the anion exchange (S _t ^an) groups were determined in the root cell walls. The number of the functional group of each type (S ^j) was estimated, and the corresponding values of pK _a ^j were calculated. It was shown that for all types of cation exchangeable groups arranged in the cell wall structure the acid properties are enhanced by the increasing concentration of electrolyte. For each ionogenic group the coefficients of Helfferich's equation [pK _a ^j = f (C ^K+)] were determined. It was found that the swelling of root cell walls changes with pH, C ^K+ and strongly depends on plant species. Within the experimental pH and C ^K+ range the swelling coefficient changes as follows: lupin > pea > wheat. The obtained results show that for the plant species under investigation the differences in the swelling coefficients originate from (a) the differences in the cross-linking degrees of polymeric chains arranged in the cell wall structure, (b) the differences in the number of carboxyl groups and (c) the differences in the total number of functional groups. Based on the estimated swelling coefficients in water it could be inferred that for wheat the cross-linking degree of the polymeric chains in the root cell walls is higher than those for lupin or pea. It has been emphasized that the calculated parameters (S ^j, pK _a ^j, K ^cw), the equation {pK _a ^j = f (C^K+)} and the dependencies {K ^cw = f (C^K+, pH)} allow to estimate quantitatively the changes in the ion exchange capacity of the root cell walls in response to the changes in an ionic composition of an outer solution. The results of these estimations allow to suggest that (a) the root apoplast is a compartment where the accumulation of cations takes place during the first stage of cation uptake from an outer medium, and (b) the accumulation degree is defined by pH and ionic composition of an outer solution. On the basis of the literature review and the results of the present experimental study it was proposed that the changes in the cell wall swelling in response to variances of environmental or experimental conditions could lead to a change of the water flow through a root apoplast. It has been supported that there is direct relationship between the swelling of root cell walls and the water flow within the plant root apoplast.     

油菜素内酯（BR）促进植物生长机理研究进展

介绍了油菜素内酯促进植物生长、提高作物产量的作用,并简述了促进生长的生理代谢基础。通过比较油菜素内酯与生长素、赤霉素促进生长作用方式的异同,提出油菜素内酯促进生长的信号传导路径不同于其它植物激素。另外从细胞的形态发生、细胞壁扩展的机制和细胞骨架在细胞伸长中的作用等几个方面对油菜素内酯促进植物生长的细胞及分子生物学机制进行了详尽的论述。     

油菜素内酯（BR）促进植物生长机理研究进展

介绍了油菜素内酯促进植物生长,提高作物产量的作用,并简述了促进生长的生理代谢基础,通过比较油菜素内酯与生长素,赤霉素促进生长作用方式的异同,提出油菜素内酯促进生长的信号传导路径不同于其它植物激素,另外从细胞的形态发生,细胞壁扩展的机制和细胞骨架在细胞伸长中的作用等几个方面对油菜素内酯促进植物生长的细胞及分子生物学机制进行了详尽的论述。     

Control of light-induced bean leaf expansion: Role of osmotic potential,wall yield stress,and hydraulic conductivity

The role of three-turgor-related cellular parameters, the osmotic potential ( _s), the wall yield stress (Y) and the apparent hydraulic conductivity (L'p), in the initiation of ligh-induced expansion of bean (Phaseolus vulgaris L.) leaves has been determined. Although light causes an increase in the total solute content of leaf cells, the water uptake accompanying growth results in a slight increase in _s. Y is about 4 bar; and is unaffected by light. L'p, as calculated from growth rates and isopiestic measurements of leaf water potential, is only slightly greater in rapidly-growing leaves. The turgor pressure of growing cells is lower than that of the controls by about 35%. We conclude that light does not induce cell enlargement in the leaf by altering any of the above parameters, but does so primarily by increasing wall extensibility.Abbreviations and symbols RL red light - WL white light - L'p apparent hydraulic conductivity - OC osmotic concentration - Y wall yield stress - _s osmotic potential     

灵芝胞外多糖分批发酵动力学模型

利用分批发酵研究了灵芝（Ganoderma lucidum）胞外多糖的合成特性,结果表明Ganoderma lucidum多糖合成和菌体生长呈部分生长关联型。菌体干重、胞外多糖分别达到15.56g·L^-1<、3.02g·L^-1<,胞外多糖对细胞干重得率系数(Y_p/x）为0.19。根据分批发酵试验结果采用Logistic方程、Luedeking-Piret方程和类似Luedeking-Piret方程,得到了描述灵芝生长、胞外多糖以及葡萄糖底物消耗分批发酵动力学模型。同时在初始葡萄糖变化较大范围内,试验数据与模型预测值进行了比较拟合,平均相对误差小于5％,表现出很好的适用性。表明该动力学模型对指导灵芝胞外多糖的发酵生产具有实际意义。     

Cooperation of epidermis and inner tissues in auxin-mediated growth of maize coleoptiles

The function of the epidermis in auxinmediated elongation growth of maize (Zea mays L.) coleoptile segments was investigated. The following results were obtained: i) In the intact organ, there is a strong tissue tension produced by the expanding force of the inner tissues which is balanced by the contracting force of the outer epidermal wall. The compression imposed by the stretched outer epidermal wall upon the inner tissues gives rise to a wall-pressure difference which can be transformed into a water-potential difference between inner tissues and external medium (water) by removal of the outer epidermal wall. ii) Peeled segments fail to respond to auxin with normal growth. The plastic extensibility of the inner-tissue cell walls (measured with a constant-load extensiometer using living segments) is not influenced by auxin (or abscisic acid) in peeled or nonpeeled segments. It is concluded that auxin induces (and abscisic acid inhibits) elongation of the intact segment by increasing (decreasing) the extensibility specifically in the outer epidermal wall. In addition, tissue tension (and therewith the pressure acting on the outer epidermal wall) is maintained at a constant level over several hours of auxin-mediated growth, indicating that the inner cells also contribute actively to organ elongation. However, this contribution does not involve an increase of cell-wall extensibility, but a continuous shifting of the potential extension threshold (i.e., the length to which the inner tissues would extend by water uptake after peeling) ahead of the actual segment length. Thus, steady growth involves the coordinated action of wall loosening in the epidermis and regeneration of tissue tension by the inner tissues. iii) Electron micrographs show the accumulation of striking osmiophilic material (particles of approx. 0.3 m diameter) specifically at the plasma membrane/cell-wall interface of the outer epidermal wall of auxin-treated segments. iv) Peeled segments fail to respond to auxin with proton excretion. This is in contrast to fusicoccin-induced proton excretion and growth which can also be readily demonstrated in the absence of the epidermis. However, peeled and nonpeeled segments show the same sensitivity to protons with regard to the induction of acid-mediated in-vivo elongation and cell-wall extensibility. The observed threshold at pH 4.5–5.0 is too low to be compatible with a second messenger function of protons also in the growth response of the inner tissues. Organ growth is described in terms of a physical model which takes into account tissue tension and extensibility of the outer epidermal wall as the decisive growth parameters. This model states that the wall pressure increment, produced by tissue tension in the outer epidermal wall, rather than the pressure acting on the inner-tissue walls, is the driving force of growth.Abbreviations and symbols E _el, E _pl elastic and plastic in-vitro cell-wall extensibility, respectively - E _tot E _el+E _pl - FC fusicoccin - IAA indole-3-acetic acid - IT inner tissue - ITW inner-tissue walls - OEW outer epidermal wall - osmotic pressure - P wall pressure - water potential     

培养条件下发菜细胞超微结构的观察

通过观察发菜(Nostoc flagelliforme Born.et Flah.)细胞在不同生长条件的超微结构,探讨细胞的生长和发育特点以及胶质鞘的形成规律。在蒸馏水中浸泡2h后,各种细胞代谢活跃,能进行正常的生长和分裂,其中营养细胞的细胞壁以及一特殊类型的大细胞与胶质鞘的形成有关。在pH7.0的培养液中,出现与胶质鞘有关的细胞结构。在酸性培养液中未见这些结构。在不合适的pH条件下,一特殊类型的小细胞大量增殖。