A Pressure-Jump Method as a New Tool in Growth Physiology for Monitoring Physiological Wall Extensibility and Effective Turgor

A new method has been devised in order to allow the rapid determinationof the physiological extensibility () of the cell wall togetherwith the apparent effective turgor (P — Y'). The methodconsists of determining the elongation growth rate of a segmentof pea hypocotyl with an auxanometer and applying a very smallincrease (around 10 kPa) in xylem pressure (jumps) by the xylemperfusion method. The cell turgor pressure is increased by thewater surge from the xylem that is due to the pressure jumpand reaches a new level within a short time. The elongationrate is also increased asymptotically to a new steady-statelevel within a short time. is estimated by simply dividingthe increase in the steady-state growth rate by the increasein perfusion pressure, which is monitored by a pressure transduceror by the increase in turgor pressure measured by a pressureprobe. Minimal length of the time period required to get a newsteady elongation rate is 1.5 min. Therefore, it is possibleto scan continuously the change in by the intermittent applicationof the brief, small pulses of xylem pressure during any experimentaltreatment. The apparent effective turgor P — Y' can becalculated by dividing the initial elongation growth rate by Using this method, we have demonstrated experimentally thenon-linearity of the relationship between elongation rate andturgor (Lockhart mechanical equation). (Received April 26, 1989; Accepted July 17, 1989)     

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.”     

Salinity Stress Inhibits Bean Leaf Expansion by Reducing Turgor, Not Wall Extensibility

Treatment of bean (Phaseolus vulgaris L.) seedlings with low levels of salinity (50 or 100 millimolar NaCl) decreased the rate of light-induced leaf cell expansion in the primary leaves over a 3 day period. This decrease could be due to a reduction in one or both of the primary cellular growth parameters: wall extensibility and cell turgor. Wall extensibility was assessed by the Instron technique. Salinity did not decrease extensibility and caused small increases relative to the controls after 72 hours. On the other hand, 50 millimolar NaCl caused a significant reduction in leaf bulk turgor at 24 hours; adaptive decreases in leaf osmotic potential (osmotic adjustment) were more than compensated by parallel decreases in the xylem tension potential and the leaf apoplastic solute potential, resulting in a decreased leaf water potential. It is concluded that in bean seedlings, mild salinity initially affects leaf growth rate by a decrease in turgor rather than by a reduction in wall extensibility. Moreover, longterm salinization (10 days) resulted in an apparent mechanical adjustment, i.e. an increase in wall extensibility, which may help counteract reductions in turgor and maintain leaf growth rates.     

Control of Seed Germination by Abscisic Acid : III. Effect on Embryo Growth Potential (Minimum Turgor Pressure) and Growth Coefficient (Cell Wall Extensibility) in Brassica napus L

The physical mechanism of seed germination and its inhibition by abscisic acid (ABA) in Brassica napus L. was investigated, using volumetric growth (= water uptake) rate (dV/dt), water conductance (L), cell wall extensibility coefficient (m), osmotic pressure (_i), water potential (Ψ_i), turgor pressure (P), and minimum turgor for cell expansion (Y) of the intact embryo as experimental parameters. dV/dt, _i, and Ψ_i were measured directly, while m, P, and Y were derived by calculation. Based on the general equation of hydraulic cell growth [dV/dt = Lm/(L + m) (Δ - Y), where Δ = _i - of the external medium], the terms (Lm/(L + m) and _i - Y were defined as growth coefficient (k_G) and growth potential (GP), respectively. Both k_G and GP were estimated from curves relating dV/dt (steady state) to of osmotic test solutions (polyethylene glycol 6000).

During the imbibition phase (0-12 hours after sowing), k_G remains very small while GP approaches a stable level of about 10 bar. During the subsequent growth phase of the embryo, k_G increases about 10-fold. ABA, added before the onset of the growth phase, prevents the rise of k_G and lowers GP. These effects are rapidly abolished when germination is induced by removal of ABA. Neither L (as judged from the kinetics of osmotic water efflux) nor the amount of extractable solutes are affected by these changes. _i and Ψ_i remain at a high level in the ABA-treated seed but drop upon induction of germination, and this adds up to a large decrease of P, indicating that water uptake of the germinating embryo is controlled by cell wall loosening rather than by changes of _i or L. ABA inhibits water uptake by preventing cell wall loosening. By calculating Y and m from the growth equation, it is further shown that cell wall loosening during germination comprises both a decrease of Y from about 10 to 0 bar and an at least 10-fold increase of m. ABA-mediated embryo dormancy is caused by a reversible inhibition of both of these changes in cell wall stability.

     

An Analysis of Turgor and Turgor Pressure

Turgor depends on the excess pressure inside a cell, not upona reaction between the wall and contents. It is independentof the environmental pressure whereas the total reaction isnot. Turgor pressure is not identical with the reaction, or,unless the effects of the protoplasmic membranes be ignored,with the wall pressure. Changes in turgor pressure can be saidto cause changes in volume, but only when both are actuallysimultaneous results of differential diffusion. The effectsof environmental pressures are considered. Some misconceptionsare discussed and some terms more fully defined.     

An Increase in Mechanical Extensibility during the Period of Light-stimulated Growth

The sporangiophore of Phycomyces responds to a temporary increase in light intensity with a transient increase in growth rate that begins 2 to 3 minutes after the initiation of the stimulus and continues until approximately the 12th minute. Tensile tests conducted on the stage IVb sporangiophore demonstrate that an increase in mechanical extensibility of the cell wall occurs 2 minutes after the initiation of a light stimulus and continues until approximately the 15th minute. This finding supports the theory that light-stimulated plant cell expansion and rate of expansion is a function of the mechanical extensibility of the cell wall.     

Shoot Turgor Does Not Limit Shoot Growth of NaCl-Affected Wheat and Barley

The aim of this work was to test the hypothesis that the reduced growth rate of wheat and barley that results when the roots are exposed to NaCl is due to inadequate turgor in the expanding cells of the leaves. The hypothesis was tested by exposing plants to 100 millimolar NaCl (which reduced their growth rates by about 20%), growing them for 7 to 10 days with their roots in pressure chambers, and applying sufficient pneumatic pressure in the chambers to offset the osmotic pressure of the NaCl, namely, 0.48 megapascals. The results showed that applying the pressure had no sustained effect (relative to unpressurized controls) on growth rates, transpiration rates, or osmotic pressures of the cell sap, in either the fully expanded or currently expanding leaf tissue, of both wheat and barley. The results indicate that the applied pressure correspondingly increased turgor in the shoot although this was not directly measured. We conclude that shoot turgor alone was not regulating the growth of these NaCl-affected plants, and, after discussing other possible influences, argue that a message arising in the roots may be regulating the growth of the shoot.     

Turgor, Growth and Rheological Gradients of Wheat Roots Following Osmotic Stress

The growth rate of hydroponically grown wheat roots was reducedby mannitol solutions of various osmotic pressures. For example,following 24 h exposure to 0·96 MPa mannitol root elongationwas reduced from 1· mm h^–1 to 0·1 mm h^–1 Mature cell length was reduced from 290 µm in unstressedroots to 100 µm in 0·96 MPa mannitol. This indicatesa reduction in cell production rate from about 4 per h in theunstressed roots to 1 per h in the highest stress treatment. The growing zone extended over the apical 4·5 mm in unstressedroots but became shorter as growth ceased in the proximal regionsat higher levels of osmotic stress. The turgor pressure along the apical 5·0 mm of unstressedroots was between 0·5 and 0·6 MPa but declinedto 0·41 MPa over the next 50 mm. Following 24 h in 0·48(200 mol m^–3) or 0·72 MPa (300 mol m^–) mannitol,turgor along the apical 50 mm was indistinguishable from thatof unstressed roots but turgor declined more steeply in theregion 5·10 mm from the tip. At the highest level ofstress (0·96 MPa or 400 mol m^–3 mannitol) turgordeclined steeply within the apical 20 mm. Key words: Growth, turgor pressure, wall rheology, osmotic stress, osmotic adjustment     

Growth, Turgor, Water Potential, and Young's Modulus in Pea Internodes

The relations between longitudinal growth, Young's modulus, turgor, water potential, and tissue tensions have been studied on growing internodes of etiolated pea seedlings in an attempt to apply some physical concepts to the growth of a well-known plant material. The modulus has been determined by the resonance frequency method and expressed as E_tissue It increases nearly proportional to the turgor pressure and is at water saturation more than 50 times higher than at plasmolysis. E_tissue is higher in the epidermis than in the ground parenchyma. Indoleacetic acid causes a decrease in Etissue Other properties have been studied on intact and split segments of internodes in solutions of graded mannitol additions. — The following tentative picture of the normal course of the growth has been obtained. Auxin induces growth both in the periphery (epidermis) and in the central core (parenchyma) under a decrease in E_tissue This is followed by an increase of E_tissue which is independent of auxin but depending upon the turgor pressure. It is assumed to involve internal structural changes of the cell walls of the type of creep. The rapid growth takes place in a dynamic system with a low water potential despite favourable water conditions. Epidermis and parenchyma grow equally rapid without tissue tensions. — Such can be produced artificially by splitting of segments and water uptake. The parenchyma thereby loses its sensitivity to auxin. This is the background of the split stem test for auxin. — E_tissue increases when growth is slowing down, probably owing to both synthesis of wall substance and structural changes within the wall. The cells attain a more static condition with E_tissue higher in epidermis than in parenchyma. This leads to the normal tissue tensions. — The result agrees with growth according to the multi-net-principle. The cause of the low water potential and low turgor is discussed with reference to the dynamic nature of both growth and water transport and a probably low matric potential of the streaming water. The decrease in E_tissue following auxin addition is small but is the net difference between an auxin-induced decrease and an increase through the assumed creep.     

Dark-stimulated calcium ion fluxes in the chloroplast stroma and cytosol

Using transgenic Nicotiana plumbaginifolia seedlings in which the calcium reporter aequorin is targeted to the chloroplast stroma, we found that darkness stimulates a considerable flux of Ca(2+) into the stroma. This Ca(2+) flux did not occur immediately after the light-to-dark transition but began approximately 5 min after lights off and increased to a peak at approximately 20 to 30 min after the onset of darkness. Imaging of aequorin emission confirmed that the dark-stimulated luminescence emanated from chloroplast-containing tissues of the seedling. The magnitude of the Ca(2+) flux was proportional to the duration of light exposure (24 to 120 h) before lights off; the longer the duration of light exposure, the larger the dark-stimulated Ca(2+) flux. On the other hand, the magnitude of the dark-stimulated Ca(2+) flux did not appear to vary as a function of circadian time. When seedlings were maintained on a 24-h light/dark cycle, there was a stromal Ca(2+) burst after lights off every day. Moreover, the waveform of the Ca(2+) spike was different during long-day versus short-day light/dark cycles. The dark-stimulated Ca(2+) flux into the chloroplastidic stroma appeared to affect transient changes in cytosolic Ca(2+) levels. DCMU, an inhibitor of photosynthetic electron transport, caused a significant increase in stromal Ca(2+) levels in the light but did not affect the magnitude of the dark-stimulated Ca(2+) flux. This robust Ca(2+) flux likely plays regulatory roles in the sensing of both light/dark transitions and photoperiod.     

Phycomyces: An Increase in Mechanical Extensibility during the Avoidance Growth Response

The sporangiophore of Phycomyces shows a transient response to a double barrier, the avoidance growth response. Tensile tests conducted on the stage IV sporangiophore demonstrate that an increase in mechanical extensibility occurs about a minute after a double barrier stimulus. This change in mechanical extensibility is similar to the one that occurs after a light stimulus. We have concluded that the avoidance stimulus occurs somewhere on the same pathway between the photoreceptor mechanism and the final growth response.     

Inhibition of RNA Synthesis and Auxin-Induced Cell Wall Extensibility and Growth by Actinomycin D

A linear stress strain analyzer was used to determine the effects of inhibitors of RNA and protein synthesis on auxin-induced increases in cell wall extensibility. With etiolated soybean hypocotyl, maize mesocotyl and Avena coleoptile sections and light-grown pea internode sections, inhibition of RNA synthesis resulted in inhibition of auxin-induced extensibility changes and cell expansion. The results with both actinomycin D and cycloheximide support an earlier conclusion that unstable cell constituents, presumably enzymes, are essential for cell wall loosening induced by auxin as well as for cell elongation.     

Interaction of Gibberellin A3 and Ancymidol in the Growth and Cell-Wall Extensibility of Dwarf Pea Roots

Effects of ancymidol (Anc) and gibberellin A₃ (GA₃) on rootgrowth, osmotic concentration and cell-wall extensibility ofthe root were investigated in the gibberellin-sensitive cultivarof dwarf pea, Little Marvel. Anc strongly suppressed elongationof both shoots and roots in darkness. Although the elongationof shoots of this dwarf cultivar was severely retarded in thelight, it was repressed still further by Anc. GA₃ promoted elongationof shoots both in the presence and in the absence of Anc, whereasit reversed suppression of root elongation by Anc. The concentrationof GA₃ required for the recovery of root elongation was lowerthan that required for the promotion of shoot elongation. Treatmentwith Anc led to increased thickening of roots with increasednumbers of cells per cross section and lateral expansion ofcells in the cortex. GA₃ had little effect on the osmotic concentration of cell sapobtained from root segments. Anc-treated roots did not respondto acid solutions by elongation, whereas GA₃-treated roots respondednormally to such solutions. Anc suppressed but GA₃ enhancedthe cell-wall extensibility of roots as measured in vivo andin vitro. These results indicate that a low concentration of gibberellinplays a role in normal elongation of roots by maintaining theextensibility of the cell wall in this gibberellin-sensitivedwarf pea. (Received January 17, 1994; Accepted July 15, 1994)     

Turgor regulation in hyphal organisms

Turgor regulation in two saprophytic hyphal organisms was examined directly with the pressure probe technique. The ascomycete Neurospora crassa, a terrestrial fungi, regulates turgor after hyperosmotic treatments when growing in a minimal medium containing K(+), Mg(2+), Ca(2+), Cl(-), and sucrose. Turgor recovery by N. crassa after hyperosmotic treatment is concurrent with changes in ion transport: hyperpolarization of the plasma membrane potential and a decline in transmembrane ion conductance. In contrast the oomycete Achlya bisexualis, a freshwater hyphal organism, does not regulate turgor after hyperosmotic treatment, although small transient increases in turgor were occasionally observed. We also monitored turgor in both organisms during hypoosmotic treatment and did not observe a turgor increase, possibly due to turgor regulation. Both hyphal organisms grow with similar morphologies, cellular expansion rates and turgor (0.4-0.7 MPa), yet respond differently to osmotic stress. The results do not support the assumption of a universal mechanism of tip growth driven by cell turgor.     

Roles of Pectin Methylesterases in Pollen-Tube Growth

Elongation of the pollen tube in pistil is essential for delivering sperms into the female gametophyte in sexual plant reproduction. Recently, a group of cell wall enzymes, pectin methylesterases （PMEs）, have been identified as playing an important role in this process. This article reviews the new understanding of the roles of PMEs in regulating pollen tube growth.     

Transient Responses of Cell Turgor and Growth of Maize Roots as Affected by Changes in Water Potential

Transient responses of cell turgor (P) and root elongation to changes in water potential were measured in maize (Zea mays L.) to evaluate mechanisms of adaptation to water stress. Changes of water potential were induced by exposing roots to solutions of KCl and mannitol (osmotic pressure about 0.3 MPa). Prior to a treatment, root elongation was about 1.2 mm h-1 and P was about 0.67 MPa across the cortex of the expansion zone (3-10 mm behind the root tip). Upon addition of an osmoticum, P decreased rapidly and growth stopped completely at pressure below approximately 0.6 MPa, which indicated that the yield threshold (Ytrans,1) was just below the initial turgor. Turgor recovered partly within the next 30 min and reached a new steady value at about 0.53 MPa. The root continued to elongate as soon as P rose above a new threshold (Ytrans,2) of about 0.45 MPa. The time between Ytrans,1 and Ytrans,2 was about 10 min. During this transition turgor gradients of as much as 0.15 MPa were measured across the cortex. They resulted from a faster rate of turgor recovery of cells deeper inside the tissue compared with cells near the root periphery. Presumably, the phloem was the source of the compounds for the osmotic adjustment. Turgor recovery was restricted to the expansion zone, as was confirmed by measurements of pressure kinetics in mature root tissue. Withdrawal of the osmoticum caused an enormous transient increase of elongation, which was related to only a small initial increase of P. Throughout the experiment, the relationship between root elongation rate and turgor was nonlinear. Consequently, when Y were calculated from steady-state conditions of P and root elongation before and after the osmotic treatment, Yss was only 0.21 MPa and significantly smaller compared with the values obtained from direct measurements (0.42-0.64 MPa). Thus, we strongly emphasize the need for measurements of short-term responses of elongation and turgor to determine cell wall mechanics appropriately. Our results indicate that the rate of solute flow into the growth zone could become rate-limiting for cell expansion under conditions of mild water stress.     

PPR蛋白在植物生长发育中的作用

PPR蛋白在陆生植物中属于最大的蛋白家族之一,其成员种类和数量均十分庞大。PPR蛋白主要的功能是通过在多种细胞器中进行定位从而参与细胞核和细胞器中特异单链RNA的转录后修饰和编辑,在植物生长发育的多个阶段均发挥着重要的作用。多数PPR蛋白编码基因的突变体呈现异常的发育表型,如胚胎致死、发育迟缓及绿化延迟等。对近年来植物PPR蛋白的分类、定位、RNA修饰的机制及其对植物生长发育影响进行了综述,并展望了植物PPR发挥功能区域和参与的调控网络研究。     

多胺在植物生长发育过程中的生理作用

多胺在植物生长发育过程中具有广泛的生理作用,如参与植物衰老进程的调控、体细胞胚发生、花芽分化、花和果 实的发育及参与各种生理胁迫反应等。本文重点综述了多胺在植物生长发育过程中生理学功能方面的研究进展,并对有关 问题进行了讨论和展望。     

A New Turgor/Membrane Potential Probe Simultaneously Measures Turgor and Electrical Membrane Potential

Abstract: A new combined turgor/membrane potential probe (T-EP probe) monitored cell turgor and membrane potential simultaneously in single giant cells. The new probe consisted of a silicone oil-filled micropipette (oil-microelectrode), which conducted electric current. Measurements of turgor and hydraulic conductivity were performed as with the conventional cell pressure probe besides the membrane potential. In internodal cells of Chara corallina, steady state turgor (0.5-0.7 MPa) and resting potentials (-200 to ?220 mV) in APW, and hydraulic conductivity (0.07 to 0.21 × 10～⁵ m s^?1 MPa^?1) were measured with the new probe, and cells exhibited healthy cytoplasmic streaming for at least 24 h during measurements. When internodal cells of Chara corallina were treated with 30, 20, 10, and 5 mM KCI, turgor responded immediately to all concentrations, and the osmotic changes in the medium were measured. Action potentials, which brought the membrane potential to a steady depolarization that measured the concentration difference of K⁺ in the medium, were induced in a concentration — dependent delay and occurred only 30, 20, and 10 mM of KCl. When the solution was changed back to APW, the repolarization of membrane potential consisted of a quick and a following slow phase. During the quick phase, which took place immediately and lasted 1 to 3 min, the plasma membrane remained activated. The membrane was gradually deactivated in the slow phase, and entirely deactivated when the membrane potential recovered to the resting potential in APW. Although the activated plasma membrane was permeable to K⁺, no major ion channels were activated on the tonoplast, and therefore, internodal cells of Chara corallina did not regulate turgor when osmotic potential changed in the surrounding medium.