Can a Ca2+ pump in the endoplasmic reticulum of the Lepidium root be the trigger for rapid changes in membrane potential after gravistimulation?

Since gravistimulation is followed by alterations in the external current symmetry (Behrens et al., 1982), the effect of gravistimulation on cellular membrane potential was investigated using conventional glass microelectrode techniques. The resting potential of statocytes in a vertically oriented root is approx. -118 mV. Upon gravistimulation, the membrane potential is temporarily depolarized (lag time = 2 s) to a potential of approx. -93 mV. This depolarization is only observed in statocytes located on the physically lower root flank while those on the corresponding upper flank become weakly hyperpolarized (approx. -13 mV). These results reflect altered ion fluxes across the plasma membrane. The perception of gravistimulus was suggested to result from a pressure of the amyloplasts on the distal endoplasmic reticulum (ER) of the statocytes (Sievers and Volkmann, 1972). A causal relationship between changes in ER-amyloplast interactions and the rapid alterations in plasma membrane potential described above is not known. A candidate for such an intracellular messenger is Ca2+. As a first step in establishing the validity of such an assumption, we have isolated ER membranes from roots. When incubated with micromolar concentrations of Ca2+, the vesicular membrane fraction accumulates Ca2+. The accumulation is ATP-dependent and -specific and is directly coupled to ATP hydrolysis since a protonophore shows no inhibitory effect. Thus, in analogy to the sarcoplasmic reticulum of muscle, regulation of an ER-localized Ca2+ compartment might be an important step in such complex processes as stimulus-transduction in gravitropism.     

Kinetics of amyloplast sedimentation in gravistimulated maize coleoptiles

Inner mesophyll cells from coleoptiles of Zea mays L. cv. Merit were fixed after varying periods of gravistimulation. A statistically significant amount (17–21%) of amyloplast sedimentation occurred in these cells after 30 s of gravistimulation. The presentation time is approx. 40 s or less. The accumulation of amyloplasts near the new lower wall shows a linear relationship to the logarithm of the gravistimulation time (r=0.92 or higher). The intercept of this line with the baseline value of amyloplasts in vertical coleoptiles indicates that the number of amyloplasts on the new lower wall begins increasing 11–15 s after the onset of gravistimulation. Direct observations of living cells confirm that many amyloplasts sediment within less than 15–30 s. These rapid kinetics are consistent with the classical statolith hypothesis of graviperception involving the sedimentation of amyloplasts to the vicinity of the new lower wall.     

Amyloplast sedimentation kinetics in gravistimulated maize roots

Amyloplast sedimentation in gravistimulated maize (Zea mays L.) roots was measured using the change in angle from the center of the cell to each amyloplast as an index of sedimentation. Using tissue fixed after gravistimulation, the relationship between mean amyloplast angle and the duration of gravistimulation was found to be linear when plotted on a logarithmic time scale. Extrapolated values for the onset of angular change are 5.9 s after the start of gravistimulation for the entire population of amyloplasts and 11.8 s for lead amyloplasts. By multiplying the instantaneous angular velocity (in radians) by the cell center to amyloplast radius, it is possible to calculate the initial sedimentation velocity to be 19.1 m min^-1 at 5.9 s. During sedimentation, the mean amyloplast angles surpass the calculated cell corner angle of 123° at 2.2 min for all amyloplasts and at 19 s for lead amyloplasts near the new lower wall. Thus, substantial sedimentation occurs within the presentation time, calculated to be 4.1 min. These kinetics are consistent with several hypotheses of graviperception.Symbol t_p presentation time     

Microtubules in statocytes from roots of cress (Lepidium sativum L.)

Summary Statocytes in root caps ofLepidium sativum L. were examined by means of ultrathin serial sections to evaluate the amount and distribution of cortical microtubules. The microtubules encircle the cell, oriented normal to the root length axis. In the distal cell edges, microtubules form a network, separating the distal complex of endoplasmic reticulum from the plasmalemma. Preprophase bands in meristem cells are observable rarely, structures which can be regarded as nucleating sites for microtubules are lacking. During ageing of the root cap cells, the number of microtubules increases in combination with a decrease of microtubule length. Development of the roots on a horizontal clinostat preserves a younger developmental stage of the microtubule system regarding amount and length of the individual microtubules. Evidence for an involvement of microtubules in graviperception is low, whereas their role in orienting cellulose microfibrils cannot be ruled out. Compression of the distal network of microtubules after centrifugation of the roots indicates that microtubules in statocytes ofLepidium sativum L. roots might function in stabilizing the distal complex of endoplasmic reticulum.     

Amyloplasts are necessary for full gravitropic sensitivity in roots of Arabidopsis thaliana

The observation that a starchless mutant (TC7) of Arabidopsis thaliana (L.) Heynh. is gravitropic (T. Caspar and B.G. Pickard, 1989, Planta 177, 185–197) raises questions about the hypothesis that starch and amyloplasts play a role in gravity perception. We compared the kinetics of gravitropism in this starchless mutant and the wild-type (WT). Wild-type roots are more responsive to gravity than TC7 roots as judged by several parameters: (1) Vertically grown TC7 roots were not as oriented with respect to the gravity vector as WT roots. (2) In the time course of curvature after gravistimulation, curvature in TC7 roots was delayed and reduced compared to WT roots. (3) TC7 roots curved less than WT roots following a single, short (induction) period of gravistimulation, and WT, but not TC7, roots curved in response to a 1-min period of horizontal exposure. (4) Wild-type roots curved much more than TC7 roots in response to intermittent stimulation (repeated short periods of horizontal exposure); WT roots curved in response to 10 s of stimulation or less, but TC7 roots required 2 min of stimulation to produce a curvature. The growth rates were equal for both genotypes. We conclude that WT roots are more sensitive to gravity than TC7 roots. Starch is not required for gravity perception in TC7 roots, but is necessary for full sensitivity; thus it is likely that amyloplasts function as statoliths in WT Arabidopsis roots. Furthermore, since centrifugation studies using low gravitational forces indicated that starchless plastids are relatively dense and are the most movable component in TC7 columella cells, the starchless plastids may also function as statoliths.Abbreviations S2 story two - S3 story three - WT wild-type     

Gravitropic bending of cress roots without contact between amyloplasts and complexes of endoplasmic reticulum

The polar arrangement of cell organelles in Lepidium root statocytes is persistently converted to a physical stratification during lateral centrifugation (the centrifugal force acts perpendicular to the root long axis) or by apically directed centrifugation combined with cytochalasin-treatment. Lateral centrifugation (10 min, 60 min at 10\g or 50\g) causes displacement of amylplasts to the centrifugal anticlinal cell wall and shifting of the endoplasmic reticulum (ER) complex to the centripetal distal cell edge. After 60 min of lateral centrifugation at 10\g or 50\g all roots show a clear gravitropic curvature. The average angle of curvature is about 40° and corresponds to that of roots stimulated gravitropically in the horizontal position at 1\g in spite of the fact that the gravistimulus is 10-or 50-fold higher. Apically directed centrifugation combined with cytochalasin B (25 g\ml^-1) or cytochalasin D (2.5 g\ml^-1) incubation yields statocytes with the amyloplasts sedimented close to the centrifugal periclinal cell wall and ER cisternae accumulated at the proximal cell pole. Gravitropic stimulation for 30 min in the horizontal position at 1\g and additional 3 h rotation on a clinostat result in gravicurvature of cytochalasin B-treated centrifuged (1 h at 50\g) roots, but because of retarded root growth the angle of curvature is lower than in control roots. Cytochalasin D-treatment during centrifugation (20 min at 50\g) does not affect either root growth or gravicurvature during 3 h horizontal exposure to 1\g relative to untreated roots. As lateral centrifugation enables only short-term contact between the amyloplasts and the distal ER complex at the onset of centrifugation and apically directed centrifugation combined with cytochalasin-treatment even exclude any contact the integrity of the distal cell pole need not necessarily be a prerequisite for graviperception in Lepidium root statocytes.Abbreviations CB cytochalasin B - CD cytochalasin D - ER endoplasmic reticulum - g gravitational acceleration     

Light-enhanced perception of gravity in stems of intact pea seedlings

Dark-grown, 6-d-old pea seedlings (Pisum sativum L. cv. Alaska) do not respond gravitropically to brief (approx. 3 min) horizontal presentations, but seedlings given a pulse of red light (R) 16–24 h earlier respond to such stimuli by vigorous curvature of the epicotyl. With continuous horizontal stimulation (approx. 100 min), the kinetics and extent of the gravitropic response are almost identical in irradiated and dark-control plants. Prior R thus increases graviperception without altering the rate-limiting steps underlying the generation of curvature. This effect of R on graviperception develops slowly; seedlings studied only a few hours after R show differences in the kinetics of the gravitropic response, but not in presentation time. Neither the kinetics nor the extent of gravitropic curvature should be used as criteria for establishing changes in primary processes in gravitropism.     

Effects of prolonged omnilateral gravistimulation on the ultrastructure of statocytes and on the graviresponse of roots

Statocytes of vertically growing roots of Lepidium sativum L. exhibit a strict polarity: The nucleus is positioned near the proximal periclinal cell wall, amyloplasts are sedimented on a complex of rough endoplasmic reticulum (ER) consisting of parallel cisternae near the distal periclinal cell wall.When 24 h old, vertically grown roots are rotated for an additional 20 h on a horizontal clinostat, this polarity is destroyed. Furthermore, the prolonged omnilateral stimulation leads to a damage of the statocytes, which in some cases ends in the self-destruction of the sensitive cells. The different components of the ultrastructural respones of the statocytes are: Displacement of the nucleus; changes in amount and distribution of the ER; loss of amyloplast starch; confluence of lipid droplets to large aggregates: a considerable increase of the lytic compartment. In addition, even anticlinal cell walls may be lysed up to small stumps. As all these effects are clearly restricted to the statocytes, only these cells are able to respond to the continuously changing direction of the gravity vector, thus perceiving gravity as such.After being exposed horizontally, the graviresponse of rotated roots is delayed as compared to the controls. About 20% of the rotated roots do not respond (curve) at all, but grow perpendicular in relation to the gravity vector. Perception of gravity is inevitably correlated with the polarity and the integrity of the statocytes.Abbreviation ER endoplasmic reticulum A preliminary report was presented at the Fall Meeting of the German Society for Cell Biology in Salzburg, Austria, September 1979 (Hensel and Sievers 1979)This paper represents part of a dissertation (D 5) of W. H.     

Gravity induced changes in intracellular potentials in statocytes of cress roots

Two glass microelectrodes were inserted from opposite sides of the root cap into statocytes of Lepidium sativum L. immersed in medium with or without cytochalasin D (CD). Intracellular potentials (Eis) of statocytes were measured with reference to an earthed electrode in the bathing solution. In the absence of CD, Ei values were -160 +/- 2 mV (n = 52) in vertical roots. During the recording of Eis, the roots were tilted from the vertical by 45 degrees so that in a tilted root one electrode was on the upper side and the other on the lower side; after 5 min the roots were returned to the vertical. At approximately 64 s after tilting (lasting 5-15 s) there was a transient lowering of Ei (more negative) by an average of 4.7 mV on both the upper and lower sides (n = 52). In some cases, this decrease in Ei was preceded by a transitory increase. Returning the roots to the vertical resulted in a response similar to that obtained by tilting. In roots treated with CD at a concentration of 3 (microM for 1 h, the initial Ei was -145 +/- 2 mV (n = 43), and the lowering of Ei on position change (tilting or returning) was smaller (2.0 mV) in some statocytes (n = 50) and higher (8.1 mV) in others (n = 14) compared to control roots (without and with DMSO). A higher concentration (10 microM) of CD and longer treatment (2 h) further reduced the decrease in Ei (1.1 mV) on position change (n = 26). The observed effects of CD support the hypothesis that statoliths in statocytes are anchored by actin filaments to the plasma membrane and/or to the cortical endoplasmic reticulum. Movement of statoliths during the first step of graviperception may lead to stress changes in actin filaments, affecting the transmembrane potential and also the Ei.     

The polarity of statocytes and the gravisensitivity of roots are dependent on the concentration of calcium in statocytes

In order to examine a possible role of calcium in graviperception, the calcium ionophore A23187 was used to elevate the concentration of free cytoplasmic calcium in statocytes of the roots of Lepidium sativum L. After a brief incubation (30 min) in a medium that contained 10 micromoles A23187 and 5.5 micromoles CaCl2, 50% of the roots bent gravitropically during a subsequent 2 h of horizontal exposure, with an angle of curvature that varied from 5 degrees to 70 degrees. The corresponding statocytes exhibited a polar arrangement of cell organelles as did the controls. However, in statocytes from 50% of the roots which were not curved after gravistimulation a portion of the distal endoplasmic reticulum (ER) complex was displaced in the direction of gravity within 30 min of horizontal exposure. After washing of the briefly treated roots for 24 h with 1% dimethylsulfoxide the percentage of gravitropically bending roots increased to approximately 80%, but the angle of curvature amounted to only 5 degrees-10 degrees. Longer treatment (2 h) with A23187 caused a complete loss of graviresponsiveness which was accompanied by disintegration of statocyte polarity. We concluded from these results that i) calcium is involved in graviperception and ii) gravisensitivity depends on the integrity of statocytes.     

A role of microtubules in the polarity of statocytes from roots of Lepidium sativum L.

When roots of Lepidium sativum L. are immersed in a colchicine solution (10^-4 mol l^-1), the cortical microtubules of statocytes are affected such that the dense network ofmicrotubules at the distal cell edges, between the endoplasmic reticulum and the plasma membrane, disappears almost completely, whereas the microtubules, lining the anticlinal cell walls are reduced only to a limited extent. Upon inversion of colchicine-pretreated roots, the distal complex of endoplasmic reticulum sinks into the interior of the statocyte. Germination of seeds in the cold (3–4°C) leads to a retardation of statocyte development; the elaborated system of endoplasmic reticulum is lacking, and only a few microtubules are observable, lining the plasma membrane along the anticlinal cell walls. During an additional 4 h at 24°C, groups of microtubules develop near the plasma membrane in the distal one-third of the statocytes, coaligning with newly synthesized cisternae of the endoplasmic reticulum. It is proposed that, particularly at the distal statocyte pole, microtubules in coordination with cross-bridging structures, act in stabilizing the polar arrangement of the distal endoplasmic reticulum and, in turn, facilitate an integrated function of amyloplasts, endoplasmic reticulum and plasma membrane in graviperception.Abbreviations ER endoplasmic reticulum - MT microtubule     

Cytochalasin B affects the structural polarity of statocytes from cress roots (Lepidium sativum L.)

Summary The effect of cytochalasin B (CB; 25 ·ml^–1 in 1% dimethylsulfoxide, DMSO) upon the structural polarity of statocytes in cress roots is demonstrated. If normal, vertically grown roots are incubated in CB, the structural polarity of the statocytes is altered according to the developmental stage of the root. Statocytes from young roots (13 or 17 hours, additionally 7 hours CB) are characterized by proximal ER cisternae and a sparsely developed distal ER-complex. Statocytes from older roots (24 hours, additionally 7 hours CB) still accumulate distal ER, as in control roots, but at the proximal cell pole in the vicinity of the nucleus additional ER is found. These effects are reversed by washing out the drug in DMSO. Growth of the roots under a continuous supply of CB yields statocytes with sedimented nuclei, proximal ER and almost no distal ER. Together with quantitative data from morphometric studies, a dynamic model of the expression of inherent cell polarity in structural polarity is proposed.Abbreviations CB cytochalasin B - DMSO dimethylsulfoxide - ER endoplasmic reticulum Preliminary results were presented at the joint Annual Meeting of the Belgian and German Society for Cell Biology, Bonn, 18–22 March 1985; Eur. J. Cell Biol. 36 (Suppl. 7), 1985, 25.Dedicated to Professor Dr. A.Betz on the occasion of his 65th birthday.     

Electrical properties of the vertically growing root tip of Lepidium sativum L.

Electrical transmembrane potential differences and resistances in different tissues of intact root tips of Lepidium sativum L. were investigated in a humid atmosphere by conventional glass-microelectrode techniques with the reference electrode at the surface (apoplast) of the root. The resting potential (inside negative) in cells of the root cap rose from-80 mV in external cell layers (secretion cells) to approx.-140 mV in central cells (statocytes). Measurements of the electric input resistance within the apoplast of the root tip (calyptra, meristem and elongation zone) yielded a preference for longitudinal contact (resistance per length of tissue approx. 3.4 GOhm m^-1) compared with transversal contact (approx. 14 GOhm m^-1). Similarly, the symplastic coupling expressed as the characteristic length (L) where a signal is reduced to 1/c compared with the origin yielded L _y =390 m in the longitudinal (y) direction and L _x =140 m in the transversal (x) direction. Cable analytical treatment of the symplastic input resistances (approx. 10 MOhm) resulted in low membrane resistances in the y-direction at the ends of cells compared with the membrane resistances in the x-direction (approx. 0.2 Ohm m²) of the lateral membranes in the approximately cylindrical cells. This anisotropy is discussed in terms of model calculations. The resistivity of the symplast was calculated to be about 2.5 Ohm m. The input current-voltage relationship displayed a slight curvature with increasing slope for the more negative membrane potential typical of membranes with electrogenic pumps. Even after massive electrical stimulation in the range from-50 to-150mV carried out to trace current-voltage curves, electrical excitations (action potentials) were not detected in the cells investigated.Abbreviations el voltage recording electrodes - R resistance - V _r resting potential     

Effects of submergence on development and gravitropism in the coleoptile of Oryza sativa L.

Caryopses of rice (Oryza sativa L. cv. Sasanishiki) were germinated in air or under water. In submerged seedlings a twofold increase in coleoptile growth rate and an inhibition of root growth was observed. The amount of starch in the amyloplasts of submerged coleoptiles was substantially reduced compared to the air-grown control plants and plastids had a proplastidic character. During the rapid elongation of coleoptiles under water, the osmotic concentration of the press sap remained constant, whereas in air-grown coleoptiles a decrease was measured. Determination of curvature of gravistimulated air-grown and submerged shoots was carried out by placing the coleoptiles horizontally in air of 98% relative humidity. Air-grown coleoptiles reached a vertical orientation within 5 h after onset of gravistimulation. In coleoptiles germinated under water the first signs of consistent negative gravitropic bending occurred after 4–5 h and curvature was complete after 24 h. During the first 5 h of gravistimulation the water-grown coleoptiles grew at an average rate of 0.39 mm·h^–1, whereas in air-grown coleoptiles a rate of 0.27 mm·h^–1 was measured. Concomitant with the delayed onset of gravitropic bending of the water-grown coleoptiles, a change in plastid ultrastructure and an increase in starch content was observed. We conclude that the gravitropic responsiveness of the rice coleoptile depends on the presence of starch-filled amyloplasts.We wish to thank H.-J. Ensikat for technical assistance with the scanning electron microscopy. Supported by the Bundesminister für Forschung und Technologie and the Deutsche Forschungsgemeinschaft.     

Cytoplasmic streaming affects gravity-induced amyloplast sedimentation in maize coleoptiles

Living maize (Zea mays L.) coleoptile cells were observed using a horizontal microscope to determine the interaction between cytoplasmic streaming and gravity-induced amyloplast sedimentation. Sedimentation is heavily influenced by streaming which may (1) hasten or slow the velocity of amyloplast movement and (2) displace the plastid laterally or even upwards before or after sedimentation. Amyloplasts may move through transvacuolar strands or through the peripheral cytoplasm which may be divided into fine cytoplasmic strands of much smaller diameter than the plastids. The results indicate that streaming may contribute to the dynamics of graviperception by influencing amyloplast movement.     

Oriented movement of statoliths studied in a reduced gravitational field during parabolic flights of rockets

During five rocket flights (TEXUS 18, 19, 21, 23 and 25), experiments were performed to investigate the behaviour of statoliths in rhizoids of the green alga Chara globularia Thuill. and in statocytes of cress (Lepidium sativum L.) roots, when the gravitational field changed to approx. 10^–4 · g (i.e. microgravity) during the parabolic flight (lasting for 301–390 s) of the rockets. The position of statoliths was only slightly influenced by the conditions during launch, e.g. vibration, acceleration and rotation of the rocket. Within approx. 6 min of microgravity conditions the shape of the statolith complex in the rhizoids changed from a transversely oriented lens into a longitudinally oriented spindle. The center of the statolith complex moved approx. 14 m and 3.6 m in rhizoids and root statocytes, respectively, in the opposite direction to the originally acting gravity vector. The kinetics of statolith displacement in rhizoids demonstrate that the velocity was nearly constant under microgravity whereas it decreased remarkably after inversion of rhizoids on Earth. It can be concluded that on Earth the position of statoliths in both rhizoids and root statocytes depends on the balance of two forces, i.e. the gravitational force and the counteracting force mediated by microfilaments.Abbreviations ER endoplasmic reticulum - g 9.806 m · s^–2 - MF microfilament - TEXUS Technologische Experimente unter Schwerelosigkeit (technological experiments under reduced gravity) Dedicated to Professor Wolfgang Haupt on the occasion of his 70th birthday     

Short-and long-term effects of cadmium on transmembrane electric potential (E m) in maize roots

In this study, the effects of Cd on root growth, respiration, and transmembrane electric potential (E _m) of the outer cortical cells in maize roots treated with various Cd concentrations (from 1 μM to 1 mM) for several hours to one week were studied. The E _m values of root cells ranged between −120 and −140 mV and after addition of Cd they were depolarized immediately. The depolarization was concentration-dependent reaching the value of diffusion potential (E _D) when the Cd concentration exceeded 100 μM. The values of E _D ranged between −65 to −68 mV (−66 ± 1.42 mV). The maximum depolarization of E _m was registered approx. 2.5 h after addition of Cd to the perfusion solution and in some cases, partial (Cd > 100 μM) or complete repolarization (Cd < 100 μM) was observed within 8–10 h of Cd treatment. In the time-dependent experiments (0 to 168 h) shortly after the maximum repolarization of E _m a continuous concentration-dependent decrease of E _m followed at all Cd concentrations. Depolarization of E _m was accompanied by both increased electrolyte leakage and inhibition of respiration, especially in the range of 50 μM to 1 mM Cd, with the exception of root cells treated with 1 and 10 μM Cd for 24 and 48 h. Time course analysis of Cd impact on root respiration revealed that at higher Cd concentrations (> 50 μM) the respiration gradually declined (∼ 6 h) and then remained at this lowest level for up to 24 h. All the Cd concentrations used in this experiment induced significant inhibition of root elongation and concentrations higher than 100 μM stopped the root growth within the first day of Cd treatment. Our results suggest that Cd does not cause irreversible changes in the electrogenic plasma membrane H⁺ ATPase because fusicoccin, an H⁺ ATPase activator diminished the depolarizing effect of Cd on the E _m. The depolarization of E _m in the outer cortical cells of maize roots was the result of a cumulative effect of Cd on ATP supply, plasmalemma permeability, and activity of H⁺ ATPase.     

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.Abbreviations 9-HFCA 9-hydroxyfluorenecarboxylic acid - NPA naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid - IAA indole-3-acetic acid     

The action of gravity in agravitropic Zea primary roots: Effect of gravistimulation on the extracellular free-Ca2+ content in the 1-mm apical root tip in the dark

The action of gravity stimulation in darkness was examined in agravitropic primary roots of Zea mays L. (cv. Golden Cross Bantam 70). Contents of diffusible and nitric-acid-extractable Ca²⁺ in 1-mm apical tips of roots gravistimulated in the dark were measured by flowinjection analysis as free Ca²⁺ and bound Ca²⁺, respectively. The free-Ca²⁺ content increased transiently, reaching a maximum 0.5 h after gravistimulation. This transient increase was also observed when gravistimulation was applied by changing the orientation of the roots back from horizontal to vertical again. On the other hand, the bound-Ca²⁺ content decreased transiently following gravistimulation. Furthermore, when the root caps were treated with 10 mM 2-(N-morpholino) ethanesulfonic acid buffer, the elevation of free Ca²⁺ following gravistimulation was prevented. These results indicate that gravity perception and the initial transduction steps proceed in the dark, and moreover that the elevation of free Ca²⁺ brought about by the interaction of Ca²⁺/H ⁺ in the apoplast of root tips may be involved in transmission of the gravity signal.Abbreviations FIA flow-injection analysis - Mes 2-(N-morpholino) ethanesulfonic acid - Pipes 1,4-piperazinediethanesulfonic acid - Quin 2 2-[(2-bis-[carboxymethyl]amino-5-methylphenoxy)methyl]-6-methoxy-8-bis(carboxymethyl) aminoquinoline     

Root growth and statocyte polarity in lentil seedling roots grown in microgravity or on a slowly rotating clinostat

The ability of clinostats to simulate microgravity was evaluated by comparing lentil ( Lens culinnrias L. cv. Verte du Puy) seedlings grown in space (Spacelab D1 Mission) with seedlings grown on a slowly rotating elinostat. Seeds were germinated and incubated for 25.5 h at 22°C (1) in microgravity, (2) on a 1g-centrifuge in space. (3) on a slowly rotating elinostat and (4) on the ground. Morphological (root length and orientation) and ultrastructural (distribution of amyloplasts, location of the nucleus in statocytes) parameters were studied. For clinostat experiments, two different configurations were employed: the longitudinal axis of the root was parallel (horizontal elinorotation) or perpendicular (vertical elinorotation) to the axis of rotation. the same configurations were used for the lg-controls. Root length and orientation were similar for roots grown on the clinostat and in microgravity. The amyloplasts were identically distributed in statocytes of horizontally clinorolated roots and in statocytes differentiated in microgravity. However, the location of the nucleus was similar in vertically rotated roots and microgravity samples. Since the involvement of the nucleus in graviperception is not known, it can be concluded that horizontal clinorotation simulates microgravity better than vertical elinorotation.