Mechanical and electrical anisotropy in Mimosa pudica pulvini

Thigmonastic or seismonastic movements in Mimosa pudica, such as the response to touch, appear to be regulated by electrical, hydrodynamical and chemical signal transduction. The pulvinus of Mimosa pudica shows elastic properties, and we found that electrically or mechanically induced movements of the petiole were accompanied by a change of the pulvinus shape. As the petiole falls, the volume of the lower part of the pulvinus decreases and the volume of the upper part increases due to the redistribution of water between the upper and lower parts of the pulvinus. This hydroelastic process is reversible. During the relaxation of the petiole, the volume of the lower part of the pulvinus increases and the volume of the upper part decreases. Redistribution of ions between the upper and lower parts of a pulvinus causes fast transport of water through aquaporins and causes a fast change in the volume of the motor cells. Here, the biologically closed electrochemical circuits in electrically and mechanically anisotropic pulvini of Mimosa pudica are analyzed using the charged capacitor method for electrostimulation at different voltages. Changing the polarity of electrodes leads to a strong rectification effect in a pulvinus and to different kinetics of a capacitor discharge if the applied initial voltage is 0.5 V or higher. The electrical properties of Mimosa pudica''s pulvini were investigated and the equivalent electrical circuit within the pulvinus was proposed to explain the experimental data. The detailed mechanism of seismonastic movements in Mimosa pudica is discussed.Key words: electrophysiology, plant electrostimulation, pulvinus, Mimosa pudica, charged capacitor method, electrical circuits, ion channels     

The plasma membrane aquaporin NtAQP1 is a key component of the leaf unfolding mechanism in tobacco

Epinastic leaf movement of tobacco is based on differential growth of the upper and lower leaf surface and is distinct from the motor organ-driven mechanism of nyctinastic leaf movement of, for example, mimosa species. The epinastic leaf movement of tobacco is observed not only under diurnal light regimes but also in continuous light, indicating a control by light and the circadian clock. As the transport of water across membranes by aquaporins is an important component of rapid plant cell elongation, the role of the tobacco aquaporin Nt aquaporin (AQP)1 in the epinastic response was studied in detail. In planta NtAQP1-luciferase (LUC) activity studies, Northern and Western blot analyses demonstrated a diurnal and circadian oscillation in the expression of this plasma membrane intrinsic protein (PIP)1-type aquaporin in leaf petioles, exhibiting peaks of expression coinciding with leaf unfolding. Cellular water permeability of protoplasts isolated from leaf petioles was found to be high in the morning, i.e. during the unfolding reaction, and low in the evening. Moreover, diurnal epinastic leaf movement was shown to be reduced in transgenic tobacco lines with an impaired expression of NtAQP1. It is concluded that the cyclic expression of PIP1-aquaporin represents an important component of the leaf movement mechanism.     

The pulvinus endodermal cells and their relation to leaf movement in legumes of the Brazilian cerrado

Legume pulvini have a clearly delimited endodermis, whose variable content has been associated with the velocity and type of leaf movement: pulvini in leaves with fast nastic movement contain starch grains; pulvini in leaves with slow nastic movements have calcium oxalate crystals as well as starch grains in the endodermis. However, the studies carried out to date have involved few legume species. This study therefore purported to examine the consistency of this hypothesis in other legumes. Thus, the structure and content of the pulvinus endodermal cells of nine legumes of the Brazilian cerrado, with different types and velocities of leaf movement, were investigated: slow nyctinastic and heliotropic movements ( BAUHINIA RUFA, COPAIFERA LANGSDORFFII, SENNA RUGOSA - Caesalpinioideae; ANDIRA HUMILIS and DALBERGIA MISCOLOBIUM - Faboideae; STRYPHNODENDRON POLYPHYLLUM - Mimosoideae), slow heliotropic movement ( ZORNIA DIPHYLLA - Faboideae), and fast seismonastic and slow nyctinastic and heliotropic movements ( MIMOSA RIXOSA and MIMOSA FLEXUOSA - Mimosoideae). Samples were prepared following standard plant anatomy and ultrastructure techniques. The endodermis of all the species contains starch grains. In the species displaying only slow movements, calcium oxalate prismatic crystals were observed in addition to starch grains, except in ZORNIA DIPHYLLA. In conclusion, oxalate crystals occur only in endodermal cells of pulvini that display slow movements, while starch grains are always present in pulvinus endodermal cells of plants with any kind of movement.     

Effects of Light and Growth Substances on the Diurnal Movements of the Leaflets of Mimosa pudica

The leaflets of Mimosa pudica show diurnal opening and closingmovements for some days when they are removed in pairs fromthe parent plant along with the pulvini and a small sectionof the rachis, and floated in water. The movements can be measuredfrom changes in the angle of the blades. When the leaflets arekept in darkness they show a diurnal movement of opening inthe morning at the same time as opening occurs in light. Light,however, is necessary to prevent the leaflets from closing duringthe day after an initial opening. Blue light was most effectivein maintaining the open condition throughout the day. Low concentrationsof indole-3-acetic acid (IAA) and gibberellic acid (GA) promotedopening of the leaflets while higher concentrations preventedtheir closure at night. IAA and GA acted synergistically inreducing dark-closure. Growth substances also prevented daylightclosure of the leaflets in the dark. The results arre discussedin relation to the changes in the osmotic relations of pulvinarcells regulating leaf movements.     

Les rythmes endogènes des mouvements foliaires chez Mimosa pudica L.

Abstract

Endogenous Rhythms in Mimosa pudica L. Leaf Movements.

The rhythmic movements performed by the leaves of the “Sensitive plant”, Mimosa pudica L., observed by time lapse photography, result of periodical turgor variations taking place in the parenchymatous cells of specialized motor organs. These turgor variations are associated with membrane permeability changes and ionic movements. These leaf movements allow to specify the temporal organization of this plant. Statistical analysis of observed periodicities in leaf movement shows that, in alternating conditions of light and dark (L/D:14/10) three distinct rhythms exist: a circadian rhythm synchronized by the photoperiodic cycle (τ = 24 hrs), and two ultradian rhythms with mean period values 3.8 hrs and 0.5 hrs respectively. In constant conditions from germination (L/L), the leaf behavior is strongly modified, but the three period values are found again (mean period values of 25.1 hrs, 3.5 hrs and 0.6 hrs respectively). The occurence of many rhythms with various periods taking place in the same organ is discussed in reference to observations effected on other biological subjects. Then, it appears that the period value within 2 and 4 hrs may be considered as a characteristic one in plants.     

Circadian rhythmicity in excised samanea pulvini: I. Sucrose-white light interactions

The rhythmic movement of excised Samanea saman pulvini incubated in H(2)O or 50 mm sucrose was monitored during extended periods of white light (cool white fluorescent, 2,000 ft-c), darkness, or alternating white light (16 hr) and darkness (8 hr). In continuous white light, the rhythm damps at an intermediate angle after only one cycle, whether pulvini are incubated in sucrose or H(2)O. The rhythm also damps after the first cycle when darkened pulvini are incubated in H(2)O, but it persists for several cycles if sucrose is available. Sucrose depresses the mesor (average angle) during extended dark periods in Samanea, as in Albizzia julibrissin, but it increases the mesor if supplied during white light-dark cycles. With the latter irradiation schedule, oscillations persist for several cycles whether pulvini are supplied with H(2)O or sucrose, but closure is incomplete when pulvini are incubated in sucrose.     

Transport processes in stimulated and non-stimulated leaves of Mimosa pudica

Summary Using energy-dispersive X-ray microanalysis, the concentrations of ions, especially potassium and chlorine, were determined in different tissues of primary and tertiary pulvini of Mimosa pudica. It was shown that stimulating the leaf was followed by ion displacements which were most striking in the outer extensor cells, resulting in turgor loss. Since Ca concentration remains relatively constant in cell walls of collapsed cells, the changes of K concentration are best described by the K:Ca ratio. After stimulation the K:Ca ratio dropped in the outer extensor of the primary pulvinus from 775.3 to 2.37 in the cytoplasm, and from 542.2 to 9.25 in the cell wall. Changes in chlorine content were less striking in the primary pulvinus. The KCl ratios in some cases were lower than 1.0, which indicates that Cl content can increase, while K content is diminished. In the non-stimulated tertiary pulvini the outer extensor cells show high concentrations of Cl, but much lower Cl concentrations were found after stimulation. In contrast to the primary pulvinus the K content of the tertiary pulvini is very low. In the vascular tissues of both primary and tertiary pulvini stimulation is followed by a release of K and Cl out of the sieve element cytoplasm into the apoplast. K then appears accumulated in the cell walls of the collenchymatous tissue. These displacements lead to the assumption that the collenchymatous apoplast temporarily functions as a reservoir for K and to a lesser extent for Cl. With regard to the mechanism of leaf movement after stimulation, the accumulation of ions in the apoplast seems to be initiated by the decrease of water potential triggered by an apoplastic accumulation of unloaded sucrose (Fromm and Eschrich 1988a). The resulting turgor release in the outer extensor is accompanied by an efflux of ions.Supported by the Deutsche Forschungsgemeinschaft     

Basic properties of the circadian leaf movements of Oxalis regnellii, and period change due to lithium ions

The circadian leaf movement of Oxalis regnellii Mig, has been investigated. The three leaflets of a stalk were normally synchronized, and under the experimental conditions chosen they showed a period of 26.2 ± 0.1 h. Cutting off one or two leaflets led to a successive decrease of the period length (25.7 ± 0.1 and 25.1 ± 0.3 h resp.). It was possible to phase shift the leaf movements by mechanical means (advance of 1.6 ± 0.3 h).
Lithium ions, added permanently to the transpiration stream, increased the period length of the leaf movements by more than one hour (with 10 m M Li⁺). A 24 h pulse of 20 m M LiCl caused a permanent 2–3 h phase delay of the circadian rhythm. Four-h pulses, on the other hand, provoked only transient phase delays, the magnitude being dependent on the phase of application. Lithium concentrations were determined for different regions of leaves and pulvini. It was shown that leaf segments had considerably lower concentrations than pulvini. No significant difference in the lithium concentration was observed between the upper and lower part of pulvini.
In the light leaf position was strongly correlated with water uptake and the consequences for applications of substances to the circadian system via the transpiration stream is discussed. A simple model of the oscillatory system and reactions connected to it is discussed.     

Elemental analysis of freeze-dried thin sections of Samanea motor organs: barriers to ion diffusion through the apoplast

Leaflet movements in the legume Samanea saman are dependent upon massive redistribution of potassium (K), chloride (Cl), and other solutes between opposing (extensor and flexor) halves of the motor organ (pulvinus). Solutes are known to diffuse through the apoplast during redistribution. To test the possibility that solute diffusion might be restricted by apoplastic barriers, we analyzed elements in the apoplast in freeze-dried cryosections of pulvini using scanning electron microscopy/x-ray microanalysis. Large discontinuities in apoplastic K and Cl at the extensor-flexor interface provide evidence for a barrier to solute diffusion. The barrier extends from the epidermis on upper and lower sides of the pulvinus to cambial cells in the central vascular core. It is completed by hydrophobic regions between phloem and cambium, and between xylem rays and surrounding vascular tissue, as deduced by discontinuities in apoplastic solutes and by staining of fresh sections with lipid-soluble Sudan dyes. Thus, symplastic pathways are necessary for ion redistribution in the Samanea pulvinus during leaflet movement. In pulvini from leaflets in the closed state, all cells on the flexor side of the barrier have high internal as well as external K and Cl, whereas cells on the extensor side have barely detectable internal or external K or Cl. Approximately 60% of these ions are known to migrate to the extensor during opening; all return to the flexor during subsequent closure. We propose that solutes lost from shrinking cells in the outer cortex diffuse through the apoplast to plasmodesmata-rich cells of the inner cortex, collenchyma, and phloem; and that solutes cross the barrier by moving through plasmodesmata.     

Aquaporins and plant transpiration

Although transpiration and aquaporins have long been identified as two key components influencing plant water status, it is only recently that their relations have been investigated in detail. The present review first examines the various facets of aquaporin function in stomatal guard cells and shows that it involves transport of water but also of other molecules such as carbon dioxide and hydrogen peroxide. At the whole plant level, changes in tissue hydraulics mediated by root and shoot aquaporins can indirectly impact plant transpiration. Recent studies also point to a feedback effect of transpiration on aquaporin function. These mechanisms may contribute to the difference between isohydric and anisohydric stomatal regulation of leaf water status. The contribution of aquaporins to transpiration control goes far beyond the issue of water transport during stomatal movements and involves emerging cellular and long‐distance signalling mechanisms which ultimately act on plant growth.     

Control of phloem unloading by action potentials in Mimosa

In the sensitive plant, Mimosa pudica , action potentials arise when the leaves are touched and they trigger a sudden decrease in turgor of the pulvinar motor cells, which causes the leaf to close. These potentials may travel through the phloem and they appear to influence pulvinar phloem unloading after stimulation. Mature leaves were exposed to ¹⁴CO₂ and phloem translocation was observed by autoradiography. In unstimulated pulvini, labeled photoassimilates were restricted to the phloem. However, after stimulation, the ¹⁴C-label appeared to be concentrated in the extensor region of the motor cortex. Since stimulation elicits an action potential, it is suggested that it also triggers phloem unloading of sucrose in the pulvini.     

Ultrastructural Features of Cortical Parenchyma Cells ('Motor Cells') in Stamen Filaments of Berberis canadensis Mill. and Tertiary Pulvini of Mimosa pudica L.

The comparative study of Berberis stamen filaments and Mimosatertiary pulvini shows many common features in their corticalparenchyma ultrastructure. In both mature organs, cells haveunusual sinuous walls with numerous plasmodesmata, tannins inthe vicinity of the nucleus, microfilaments and microfibrilsin tannin vacuoles. Nevertheless, in Mimosa pulvini, the abaxialcells have thicker walls than the adaxial cells while in stamenfilaments the reverse is true; therefore the dissymmetric organizationof the organ walls is the opposite. We will discuss: (1) the problems of chemical fixation of excitableplant cells; (2) relations between the direction of bendingand the thickness of cell walls; (3) the location of tanninsand (4) the existence of microfilaments and microfibrils. Key words: Berberis canadensis Mill., berberry, Mimosa pudica L., sensitive plant, Stamen, Pulvinus, Ultrastructure, Seismonastic movement     

On the role of tannin vacuoles in several nastic leaf responses

Summary Experiments were done to relate the presence of tannin vacuoles to plant movements. When surveyed, 4 out of 10 species exhibited rapid thigmonastic or nyctinastic movements, and only those 4 species had tannin vacuoles in their motor cells. Interestingly, plants ofAlbizzia julibrissin whose seed was obtained in New Haven, Connecticut had rapid nyctinasty and tannin vacuoles in the tertiary pulvini, whereasAlbizzia collected in Winston-Salem, North Carolina had neither. Nyctinasty ofMimosa pudica andAlbizzia NH, both of which had tannin vacuoles, was at the rate of 4.2 °/min, whereasAlbizzia WS andSamanea saman, neither of which had tannin vacuoles, exhibited leaflet closure at the rate of 2.0–2. 1 °/min. Pretreatment with the Ca²⁺ channel agonist Bay K-8644 increased, and with the antagonist verapamil decreased, the rate of leaflet closure only inMimosa andAlbizzia NH. Pretreatment with the Ca²⁺ inhibitor Erythrosin B substantially blocked reopening only inMimosa andAlbizzia NH. We conclude that tannin vacuoles assist nastic movements to be rapid by acting as a store of Ca²⁺ and releasing it as a second messenger upon mechanical perturbation or darkness.Abbreviations FR far-red irradiation - MP mechanical perturbation - TV tannin vacuoles     

Light-Stimulated Inositol Phospholipid Turnover in Samanea saman Pulvini : Increased Levels of Diacylglycerol

Leaflet movement in Samanea saman is driven by an endogenous circadian clock and by light. We are investigating whether the effects of light on leaflet movement are mediated by increased inositol phospholipid turnover. We demonstrated previously that irradiation of excised pulvini with 15 to 30 seconds of white light decreases the levels of phosphatidylinositol monophosphate and phosphatidylinositol bisphosphate and increases the levels of inositol phosphates. We now report that the diacylglycerol level increases after 30 seconds of white light but returns to below the control level after 10 minutes of white light.     

Effects of glycine on dark-and ligh-induced pulvinar movements and modifications of proton fluxes in the pulvinus of Mimosa pudica during glycine uptake

Glycine (1–50 mM) increases the rate of the dark-induced (scotonastic) movements and decreases the amplitude and the rate of the light-induced (photonastic) movements of the secondary pulvini of Mimosa pudica leaves. The uptake of glycine is accompanied by a long-lasting dose-dependent increase in the alkalinity of the bathing medium of the excised pulvini. The data are in agreement with a H⁺-glycine co-transport mechanism within the pulvinar cells. Fusicoccin (50 M), known to promote H⁺–K⁺ exchange, antagonizes the effects of glycine on the movements and the alkalization of the bathing medium of the excised pulvini. The present results argue for the hypothesis that proton fluxes mediate the scotonastic and photonastic pulvinar movements.Abbreviations Gly glycine - FC fusicoccin - P₁ primary pulvinus - P₂ secondary pulvinus     

Electrical evidence for rhythmic changes in the cotransport of sucrose and hydrogen ions in Samanea pulvini

Measurements with microelectrodes implanted into Samanea saman (Jacq.) Merrill leaf pulvini showed that membrane potentials were rhythmically sensitive to the application of sucrose. The magnitude of the electrical depolarizations induced by sucrose were dependent on the concentration of H⁺ in the medium, yet changes in [H⁺] alone did not greatly affect the potential. During sucrose-induced electrical depolarization, there was a slight increase in the pH of the bathing medium; both effects were abolished by high levels of K⁺, Na⁺ or Ca²⁺ in the medium. These observations indicate that H⁺ enter the cells by some cooperative action with sucrose. A model of H⁺-substrate cotransport is proposed in which a sugar carrier in the membrane is made more permeable by the attachment of a proton. The rhythmic nature of this proposed cotransport may be related to circadian leaf-movements in this plant.     

Ultradian Rhythms in Desmodium

The leaves of Desmodium gyrans (L.F.) DC show circadian movements in the terminal and ultradian movements of the lateral leaflets. The movements are due to swelling and shrinking of motor cells in special organs. The anatomy of these pulvini is described for the lateral leaflets. Data from electrophysiological recordings using microelectrodes inserted into the lateral pulvini, together with treatments that affect the proton pumps and ion channels, have been used to develop a physiological model of the ultradian leaflet movement. It explains the oscillations in the motor cells as being due to a change between a pump state and depolarization. During the pump state, ions are taken up, causing water influx and swelling of the motor cells. Depolarization causes loss of ions and water efflux (the motor cells shrink). The roles of calcium and the phosphatidyl inositol signal chain are discussed on the basis of experiments using chemical agents that affect these processes. Since calcium oscillations are known to occur in organisms in both time and space, an attempt has been made to simulate the situation in Desmodium pulvini by a model of specially coupled oscillators. Effects of different other treatments of the lateral pulvini are discussed. Oscillations in the minute range seem to be more common and some might be related to turgor regulation and ion uptake comparable to the situation in Desmodium. The ultradian control of the lateral pulvini and the circadian control of the terminal pulvini are apparently based on different mechanisms.     

Distribution of Potassium, Chloride and Calcium and Capacity of Hydrogen Ion Excretion in Various Parts of the Mimosa Pudica Plant

The contents of K, Cl and Ca were determined in various partsof Mimosa pudica plant, namely in young internode (YI), agedinternode (AI), upper half (UP1), and lower half (LP1) of theprimary pulvinus, secondary pulvinus (P2), petiole (Pt) andleaflet blade (B1). Potassium is found in the following distributionYI = LP1 > P2 > UP1 > Pt > AI > B1 and is thuslocalized in parts showing a great metabolic activity. Chloridefollows the distribution of K except for the petiole and toa lesser extent for the young internode. Calcium content islow in the parts showing a high plasticity (pulvini and younginternode). A relationship is thus suggested between the amountof Ca and the rate of the movements which can be performed.The capacity for spontaneous and fusicoccin-induced H⁺ excretionis greater in the plant parts exhibiting the higher K content.This suggests that larger H⁺ fluxes may be required in tissuesshowing a high metabolic activity. Mimosa pudica, ion content, H⁺ excretion