Centrifugation causes adaptation of microfilaments Studies on the transport of statoliths in gravity sensingChara rhizoids

Summary The actin cytoskeleton is involved in the positioning of statoliths in tip growingChara rhizoids. The balance between the acropetally acting gravity force and the basipetally acting net out-come of cytoskeletal force results in the dynamically stable position of the statoliths 10–30 m above the cell tip. A change of the direction and/or the amount of one of these forces in a vertically growing rhizoid results in a dislocation of statoliths. Centrifugation was used as a tool to study the characteristics of the interaction between statoliths and microfilaments (MFs). Acropetal and basipetal accelerations up to 6.5 g were applied with the newly constructed slow-rotating-centrifuge-microscope (NIZEMI). Higher accelerations were applied by means of a conventional centrifuge, namely acropetally 10–200 g and basipetally 10–70 g. During acropetal accelerations (1.4–6 g), statoliths were displaced to a new stable position nearer to the cell vertex (12–6.5 m distance to the apical cell wall, respectively), but they did not sediment on the apical cell wall. The original position of the statoliths was reestablished within 30 s after centrifugation. Sedimentation of statoliths and reduction of the growth rates of the rhizoids were observed during acropetal accelerations higher than 50 g. When not only the amount but also the direction of the acceleration were changed in comparison to the natural condition, i.e., during basipetal accelerations (1.0–6.5 g), statoliths were displaced into the subapical zone (up to 90 m distance to the apical cell wall); after 15–20 min the retransport of statoliths to the apex against the direction of acceleration started. Finally, the natural position in the tip was reestablished against the direction of continuous centrifugation. Retransport was observed during accelerations up to 70 g. Under the 1 g condition that followed the retransported statoliths showed an up to 5-fold increase in sedimentation time onto the lateral cell wall when placed horizontally. During basipetal centrifugations 70 g all statoliths entered the basal vacuolar part of the rhizoid where they were cotransported in the streaming cytoplasm. It is concluded that the MF system is able to adapt to higher mass accelerations and that the MF system of the polarly growing rhizoid is polarly organized.Abbreviations g gravitational acceleration (9.81 m/s²) - MF microfilament - NIZEMI Niedergeschwindigkeits-Zentrifugen-Mikroskop (slow-rotating-centrifuge-microscope)     

Regulation of the position of statoliths inChara rhizoids

Summary The behavior of statoliths in rhizoids differently oriented with respect to the gravity vector indicates that there are cytoskeleton elements which exert forces on the statoliths, mostly in the longitudinal directions. Compared to the sum of the forces acting on a statolith, the gravitational force is a relatively small component,i.e., less than 1/5 of the cytoskeleton force. The balance is disturbed by displacing the rhizoid from the normal vertical orientation. It is also reversibly disturbed by cytochalasin B such that some statoliths move against the gravity force. Phalloidin stabilizes the position of the statoliths against cytochalasin B. We infer that microfilaments are involved in controlling the position of statoliths, and that there is a considerable tension on these microfilaments. The vibration frequency of the microfilaments corresponding to this tension is in the ultrasonic range.Visiting Professor on a grant from Deutsche Forschungsgemeinschaft.     

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     

Actomyosin-mediated statolith positioning in gravisensing plant cells studied in microgravity

The positioning and gravity-induced sedimentation of statoliths is crucial for gravisensing in most higher and lower plants. In positively gravitropic rhizoids and, for the first time, in negatively gravitropic protonemata of characean green algae, statolith positioning by actomyosin forces was investigated in microgravity (<10(-4) g) during parabolic flights of rockets (TEXUS/MAXUS) and during the Space-Shuttle flight STS 65. In both cell types, the natural position of statoliths is the result of actomyosin forces which compensate the statoliths' weight in this position. When this balance of forces was disturbed in microgravity or on the fast-rotating clinostat (FRC), a basipetal displacement of the statoliths was observed in rhizoids. After several hours in microgravity, the statoliths were loosely arranged over an area whose apical border was in the same range as in 1 g, whereas the basal border had increased its distance from the tip. In protonemata, the actomyosin forces act net-acropetally. Thus, statoliths were transported towards the tip when protonemata were exposed to microgravity or rotated on the FRC. In preinverted protonemata, statoliths were transported away from the tip to a dynamically stable resting position. Experiments in microgravity and on the FRC gave similar results and allowed us to distinguish between active and passive forces acting on statoliths. The results indicate that actomyosin forces act differently on statoliths in the different regions of both cell types in order to keep the statoliths in a position where they function as susceptors and initiate gravitropic reorientation, even in cells that had never experienced gravity during their growth and development.     

How to activate a plant gravireceptor. Early mechanisms of gravity sensing studied in characean rhizoids during parabolic flights

Early processes underlying plant gravity sensing were investigated in rhizoids of Chara globularis under microgravity conditions provided by parabolic flights of the A300-Zero-G aircraft and of sounding rockets. By applying centrifugal forces during the microgravity phases of sounding rocket flights, lateral accelerations of 0.14 g, but not of 0.05 g, resulted in a displacement of statoliths. Settling of statoliths onto the subapical plasma membrane initiated the gravitropic response. Since actin controls the positioning of statoliths and restricts sedimentation of statoliths in these cells, it can be calculated that lateral actomyosin forces in a range of 2 x 10(-14) n act on statoliths to keep them in place. These forces represent the threshold value that has to be exceeded by any lateral acceleration stimulus for statolith sedimentation and gravisensing to occur. When rhizoids were gravistimulated during parabolic plane flights, the curvature angles of the flight samples, whose sedimented statoliths became weightless for 22 s during the 31 microgravity phases, were not different from those of in-flight 1g controls. However, in ground control experiments, curvature responses were drastically reduced when the contact of statoliths with the plasma membrane was intermittently interrupted by inverting gravistimulated cells for less than 10 s. Increasing the weight of sedimented statoliths by lateral centrifugation did not enhance the gravitropic response. These results provide evidence that graviperception in characean rhizoids requires contact of statoliths with membrane-bound receptor molecules rather than pressure or tension exerted by the weight of statoliths.     

轮藻假根中的平衡石在回转器水平回转时的运动

利用回转器重现了在ＴＥＸＵＳ火箭抛物线飞行的微重力实验中轮藻假根内平衡石和假根基部方向的运动。在快速回转器上回转１５ｍｉｎ时,假根中的平衡石复合体中心离假根顶端的距离比在原来沿重力方向生长的假根中的距离增加了６０％。细胞松弛素Ｄ的实验证实平衡石的这种运动是和肌动蛋白丝相关,而且在重力场中作用于平衡石的向基力也是肌动蛋白丝产生的。因此回转器和细胞松弛素Ｄ的实验证实了在地球上,平衡石的位置取决于作用方     

轮藻假根中的平衡石在回转器水平回转时的运动(英文)

利用回转器重现了在TEXUS火箭抛物线飞行的微重力实验中轮藻假根内平衡石向假根基部方向的运动。在快速回转器上回转15 min时,假根中的平衡石复合体中心离假根顶端的距离比在原来沿重力方向生长的假根中的距离增加了60%。细胞松弛素D的实验证实平衡石的这种运动是和肌动蛋白丝相关,而且在重力场中作用于平衡石的向基力也是肌动蛋白丝产生的。因此回转器和细胞松弛素D的实验证实了在地球上,平衡石的位置取决于作用方向相反的重力和肌动蛋白丝作用力的动态平衡的假说。然后在快速回转器上,平衡石中心在一个新的位置上维持了30 min左右的稳定,也就是出现了一个新的动态平衡状态。这一新的状态是在原先的向着假根顶端的重力和向着假根基部的肌动蛋白丝作用力的平衡在回转器上被打破后再经约有15 min时达到的。更进一步的快速回转器实验还展示了可能因平衡石位置的这一变化而启动的肌动蛋白丝的再组织和由此产生的平衡石向假根顶端方向再转运的过程。快速和慢速回转器实验在这里的结果有差异,推测是和回转器上颗粒的振幅随回转器转速的增加而减小有关。加之,轮藻假根的单细胞性质,因此在假根处于回转轴上时,快速回转器是更适合这项模拟失重的研究。总之,在失重条件下平衡石和肌动蛋白丝的关系是可以利用回转器来研究的。     

Actomyosin-mediated statolith positioning and sedimentation in gravisensing plant cells studied in microgravity

Graviresponding and tip-growing characean rhizoids and protonemata possess a highly efficient actin-based system to control and correct the position of their statoliths, a prerequisite for gravisensing. Acropetally and basipetally acting actomyosin forces and gravity are the components of the statolith positioning system that also directs sedimenting statoliths to cell-type specific ,oraviperception sites at the plasma membrane where the graviresponse is initiated. These results encourage to propose that similar cytoskeleton-mediated mechanisms for gravity sensing may exist in higher plant statocytes.     

Plant cells on earth and in space

Two quite different types of plant cells are analysed with regard to transduction of the gravity stimulus: (i) Unicellular rhizoids and protonemata of characean green algae; these are tube-like, tip-growing cells which respond to the direction of gravity. (ii) Columella cells located in the center of the root cap of higher plants; these cells (statocytes) perceive gravity. The two cell types contain heavy particles or organelles (statoliths) which sediment in the field of gravity, thereby inducing the graviresponse. Both cell types were studied under microgravity conditions (10(-4) g) in sounding rockets or spacelabs. From video microscopy of living Chara cells and different experiments with both cell types it was concluded that the position of statoliths depends on the balance of two forces, i.e. the gravitational force and the counteracting force mediated by actin microfilaments. The actomyosin system may be the missing link between the gravity-dependent movement of statoliths and the gravity receptor(s); it may also function as an amplifier.     

Micromanipulation of statoliths in gravity-sensing Chara rhizoids by optical tweezers

Infrared laser traps (optical tweezers) were used to micromanipulate statoliths in gravity-sensing rhizoids of the green alga Chara vulgaris Vail. We were able to hold and move statoliths with high accuracy and to observe directly the effects of statolith position on cell growth in horizontally positioned rhizoids. The first step in gravitropism, namely the physical action of gravity on statoliths, can be simulated by optical tweezers. The direct laser microirradiation of the rhizoid apex did not cause any visible damage to the cells. Through lateral positioning of statoliths a differential growth of the opposite flank of the cell wall could be induced, corresponding to bending growth in gravitropism. The acropetal displacement of the statolith complex into the extreme apex of the rhizoid caused a temporary decrease in cell growth rate. The rhizoids regained normal growth after remigration of the statoliths to their initial position 10–30 m basal to the rhizoid apex. During basipetal displacement of statoliths, cell growth continued and the statoliths remigrated towards the rhizoid tip after release from the optical trap. The resistance to statolith displacement increased towards the nucleus. The basipetal displacement of the whole complex of statoliths for a long distance (>100 m) caused an increase in cell diameter and a subsequent regaining of normal growth after the statoliths reappeared in the rhizoid apex. We conclude that the statolith displacement interferes with the mechanism of tip growth, i.e. with the transport of Golgi vesicles, either directly by mechanically blocking their flow and/or, indirectly, by disturbing the actomyosin system. In the presence of the actin inhibitor cytochalasin B the optical forces required for acropetal and basipetal displacement of statoliths were significantly reduced to a similar level. The lateral displacement of statoliths was not changed by cytochalasin B. The results indicate: (i) the viscous resistance to optical displacement of statoliths depends mainly on actin, (ii) the lateral displacement of statoliths is not impeded by actin filaments, (iii) the axially directed actin-mediated forces against optical displacement of statoliths (for a distance of 10 m) are stronger in the basipetal than in the acropetal direction, (iv) the forces acting on single statoliths by axially oriented actin filaments are estimated to be in the range of 11–110 pN for acropetal and of 18–180 pN for basipetal statolith displacements.Abbreviation CB cytochalasin B This work was supported by the Bundesminister für Forschung und Technologie, and by Fonds der Chemischen Industrie. We thank Professor Dr. A. Sievers (Botanisches Institut, Universität Bonn, Germany) for helpful discussions.     

Cytoplasmic streaming inChara rhizoids

Summary In-vivo videomicroscopy ofChara rhizoids under 10^–4g demonstrated that gravity affected the velocities of cytoplasmic streaming. Both, the acropetal and basipetal streaming velocities increased on the change to microgravity. The endogenous difference in the velocities of the oppositely directed cytoplasmic streams was maintained under microgravity, yet the difference was diminished as the basipetal streaming velocity increased more than the acropetal streaming velocity. Direction and structure of microfilaments labeled by rhodamine-phalloidin had not changed after 6 min of microgravity.Abbreviations g gravitational acceleration - Nizemi slow rotating centrifuge microscope - Texus technological experiments under reduced gravity     

The endogenous difference in the rates of acropetal and basipetal cytoplasmic streaming inChara rhizoids is enhanced by gravity

Summary Measurements of cytoplasmic streaming inChara rhizoids were made by a streak-photography method combined with dark-field illumination. The velocity of cytoplasmic streaming in the acropetal direction was faster than in the basipetal direction. The difference in the streaming velocities in both morphological directions was apparently due to endogenous forces. In addition to this, a small difference attributable to gravity was superimposed if the rhizoid was oriented parallel to the gravity vector. The difference in the endogenous forces underlying the oppositely directed streams may be as high as about 12-fold the force imposed by gravity but, on average, it is about 5-fold the gravity force. In the normal vertical position of the rhizoid, this endogenously generated difference is enhanced by the gravity effect. In contrast to the variability of streaming rate due to endogenous forces, the effect of the gravity force is relatively uniform. The difference between acropetal and basipetal streaming velocities is perpetuated over the whole range of lowered velocities after treatment with cytochalasin B. After prolonged incubation in cytochalasin B, the basipetal streaming stops earlier than the acropetal streaming. A difference in the length of filaments on both sides of the streaming machinery in rhizoids is proposed as the structural basis for the difference in velocities.     

Contributions of space experiments to the study of gravitropism

The study of gravitropism in space has permitted the discovery that statoliths are not completely free to sediment in the gravisensing cells of roots. These organelles are attached to actin filaments via motor proteins (myosin) which are responsible for their displacement from the distal pole of the cell toward the proximal pole when the seedlings are transferred from a 1g centrifuge in space to microgravity. On the ground, the existence of the link between the statoliths and the actin network could not be established because the gravity force is much greater than the force exerted by the motor proteins. This finding led to a new hypothesis on gravisensing. It has been proposed that statoliths can exert tensions in the actin network which become asymmetrical when the root is stimulated in the horizontal position on the ground. The space experiments have confirmed to some extent the results obtained on gravisensitivity with clinostats, although these devices do not simulate microgravity correctly. Reexamination of the means of estimating gravisensitivity has led to the conclusion that the perception and the transduction phases could be very short (that is, within a second). This data is consistent with the fact that the statoliths are attached to the actin filament and do not have to move a long distance to exert forces on the actin network. It has also been demonstrated that gravity regulates the gravitropic bending in order to avoid the overshooting of the vertical direction on the ground. The roots, which are stimulated and placed in microgravity, are not subjected to this regulation and curve more than roots stimulated continuously. However, the curvature of roots or of coleoptiles that takes place in microgravity can be greatly reduced by straightening the extremity of the organs.     

Lentil root statoliths reach a stable state in microgravity

 The kinetics of the movement of statoliths in gravity-perceiving root cap cells of Lens culinaris L. and the force responsible for it have been analysed under 1 g and under microgravity conditions (S/MM-03 mission of Spacehab 1996). At the beginning of the experiment in space, the amyloplasts were grouped at the distal pole of the statocytes by a root-tip-directed 1-g centrifugal acceleration. The seedlings were then placed in microgravity for increasing periods of time (13, 29, 46 or 122 min) and chemically fixed. During the first 29 min of microgravity there were local displacements (mean velocity: 0.154 μm min⁻¹) of some amyloplasts (first at the front of the group and then at the rear). Nevertheless, the group of amyloplasts tended to reconstitute. After 122 min in microgravity the bulk of amyloplasts had almost reached the proximal pole where further movement was blocked by the nucleus. After a longer period in microgravity (4 h; experiment carried out 1994 during the IML 2 mission) the statoliths reached a stable position due to the fact that they were stopped by the nucleus. The position was similar to that observed in roots grown continuously in microgravity. Treatment with cytochalasin D (CD) did not stop the movement of the amyloplasts but slowed down the velocity of their displacement (0.019 μm min⁻¹). Initial movement patterns were the same as in control roots in water. Comparisons of mean velocities of amyloplast movements in roots in space and in inverted roots on earth showed that the force responsible for the movement in microgravity (F_c) was about 86% less (F_c = 0.016 pN) than the gravity force (F_g = 0.11 pN). Treatment with CD reduced F_c by two-thirds. The apparent viscosity of the statocyte cytoplasm was found to be 1 Pa s or 3.3 Pa s for control roots or CD treated roots, respectively. Brownian motion or elastic forces due to endoplasmic reticulum membranes do not cause the movement of the amyloplasts in microgravity. It is concluded that the force transporting the statoliths is caused by the actomyosin system. Received: 22 March 1999 / Accepted: 18 December 1999     

The response to gravity is correlated with the number of statoliths in Chara rhizoids.

In contrast to higher plants, Chara rhizoids have single membrane-bound compartments that appear to function as statoliths. Rhizoids were generated by germinating zygotes of Chara in either soil water (SW) medium or artificial pond water (APW) medium. Differential-interference-contrast microscopy demonstrated that rhizoids form SW-grown plants typically contain 50 to 60 statoliths per cell, whereas rhizoids from APW-grown plants contain 5 to 10 statoliths per cell. Rhizoids from SW are more responsive to gravity than rhizoids from APW because (a) SW rhizoids were oriented to gravity during vertical growth, whereas APW rhizoids were relatively disoriented, and (b) curvature of SW rhizoids was 3 to 4 times greater throughout the time course of curvature. The growth rate of APW rhizoids was significantly greater than that of SW-grown rhizoids. This latter result suggests that APW rhizoids are not limited in their ability for gravitropic curvature by growth and that these rhizoids are impaired in the early stages of gravitropism (i.e. gravity perception). Plants grown in APW appeared to be healthy because of their growth rate and the vigorous cytoplasmic streaming observed in the rhizoids. This study is comparable to earlier studies of gravitropism in starch-deficient mutants of higher plants and provides support for the role of statoliths in gravity perception.     

Tip-localized actin polymerization and remodeling,reflected by the localization of ADF,profilin and villin,are fundamental for gravity-sensing and polar growth in characean rhizoids

Polar organization and gravity-oriented, polarized growth of characean rhizoids are dependent on the actin cytoskeleton. In this report, we demonstrate that the prominent center of the Spitzenkörper serves as the apical actin polymerization site in the extending tip. After cytochalasin D-induced disruption of the actin cytoskeleton, the regeneration of actin microfilaments (MFs) starts with the reappearance of a flat, brightly fluorescing actin array in the outermost tip. The actin array rounds up, produces actin MFs that radiate in all directions and is then relocated into its original central position in the center of the Spitzenkörper. The emerging actin MFs rearrange and cross-link to form the delicate, subapical meshwork, which then controls the statolith positioning, re-establishes the tip-high calcium gradient and mediates the reorganization of the Spitzenkörper with its central ER aggregate and the accumulation of secretory vesicles. Tip growth and gravitropic sensing, which includes control of statolith positioning and gravity-induced sedimentation, are not resumed until the original polar actin organization is completely restored. Immunolocalization of the actin-binding proteins, actin-depolymerizing factor (ADF) and profilin, which both accumulate in the center of the Spitzenkörper, indicates high actin turnover and gives additional support for the actin-polymerizing function of this central, apical area. Association of villin immunofluorescence with two populations of thick undulating actin cables with uniform polarity underlying rotational cytoplasmic streaming in the basal region suggests that villin is the major actin-bundling protein in rhizoids. Our results provide evidence that the precise coordination of apical actin polymerization and dynamic remodeling of actin MFs by actin-binding proteins play a fundamental role in cell polarization, gravity sensing and gravity-oriented polarized growth of characean rhizoids.Abbreviations ADF Actin-depolymerizing factor - CD Cytochalasin D - MF Microfilament     

Rhizoids and protonemata of characean algae: model cells for research on polarized growth and plant gravity sensing

Gravitropically tip-growing rhizoids and protonemata of characean algae are well-established unicellular plant model systems for research on gravitropism. In recent years, considerable progress has been made in the understanding of the cellular and molecular mechanisms underlying gravity sensing and gravity-oriented growth. While in higher-plant statocytes the role of cytoskeletal elements, especially the actin cytoskeleton, in the mechanisms of gravity sensing is still enigmatic, there is clear evidence that in the characean cells actin is intimately involved in polarized growth, gravity sensing, and the gravitropic response mechanisms. The multiple functions of actin are orchestrated by a variety of actin-binding proteins which control actin polymerisation, regulate the dynamic remodelling of the actin filament architecture, and mediate the transport of vesicles and organelles. Actin and a steep gradient of cytoplasmic free calcium are crucial components of a feedback mechanism that controls polarized growth. Experiments performed in microgravity provided evidence that actomyosin is a key player for gravity sensing: it coordinates the position of statoliths and, upon a change in the cell's orientation, directs sedimenting statoliths to specific areas of the plasma membrane, where contact with membrane-bound gravisensor molecules elicits short gravitropic pathways. In rhizoids, gravitropic signalling leads to a local reduction of cytoplasmic free calcium and results in differential growth of the opposite subapical cell flanks. The negative gravitropic response of protonemata involves actin-dependent relocation of the calcium gradient and displacement of the centre of maximal growth towards the upper flank. On the basis of the results obtained from the gravitropic model cells, a similar fine-tuning function of the actomyosin system is discussed for the early steps of gravity sensing in higher-plant statocytes.     

Statoliths and microfilaments in plant cells

Microfilaments have been demonstrated in rhizoids of Chara fragilis Desvaux by labelling of actin with rhodamine-conjugated phalloidin. Each rhizoid contains thick microfilament-bundles arranged longitudinally in the basal region. In the subapical and apical regions, much thinner bundles exist which contact the statoliths and encircle them in the form of a dense envelope. In root statocytes from Lepidium sativum L. the presence of an actin network is indicated by the fact that application of cytochalasin B (25 g·ml^-1 for 4 h) results in an approximately threefold increase in the rate of statolith (amyloplast) sedimentation relative to controls. It is concluded that in gravity-perceiving plant cells statoliths may trigger the transduction mechanism via actin filaments.Abbreviation CB cytochalasin B - ER endoplasmic reticulum - MF microfilament     

The Polar Organization of the Growing Chara Rhizoid and the Transport of Statoliths are Actin-Dependent*

Horizontally positioned Chara rhizoids continue growth without gravitropic bending when the statoliths are removed from the apex by basipetal centrifugation. The transport of statoliths in centrifuged rhizoids is bidirectional: 50–60 % of the statoliths are re-transported on a straight course to the apex at velocities from 1 to 14 μm . min^?1 increasing towards the rhizoid tip. The centrifuged statoliths which are located closest to the nucleus are basipetally transported and caught up in the cytoplasmic streaming of the cell. Those statoliths which are located near the apical side of the nucleus are transported either apically or basally. A de-novo-formation of statoliths was not observed. After retransport to the apex some statoliths transiently sediment, a process which can induce a local inhibition of cell wall growth. The rhizoid bends again gravitropically only if a few statoliths finally sediment in the apex; the more statoliths that sediment in the apex the shorter the radius of bending becomes. The transport of statoliths is mediated by actin filaments which form a network of thin filaments in the apical and subapical zone of the rhizoid, and thicker parallel bundles in the basal zone where cytoplasmic streaming occurs. Both subpopulations of actin filaments overlap in the nucleus zone.     

Amyloplast sedimentation dynamics in maize columella cells support a new model for the gravity-sensing apparatus of roots

Quantitative analysis of statolith sedimentation behavior was accomplished using videomicroscopy of living columella cells of corn (Zea mays) roots, which displayed no systematic cytoplasmic streaming. Following 90 degrees rotation of the root, the statoliths moved downward along the distal wall and then spread out along the bottom with an average velocity of 1.7 microm min(-1). When statolith trajectories traversed the complete width or length of the cell, they initially moved horizontally toward channel-initiation sites and then moved vertically through the channels to the lower side of the reoriented cell where they again dispersed. These statoliths exhibited a significantly lower average velocity than those sedimenting on distal-to-side trajectories. In addition, although statoliths undergoing distal-to-side sedimentation began at their highest velocity and slowed monotonically as they approached the lower cell membrane, statoliths crossing the cell's central region remained slow initially and accelerated to maximum speed once they reached a channel. The statoliths accelerated sooner, and the channeling effect was less pronounced in roots treated with cytochalasin D. Parallel ultrastructural studies of high-pressure frozen-freeze-substituted columella cells suggest that the low-resistance statolith pathway in the cell periphery corresponds to the sharp interface between the endoplasmic reticulum (ER)-rich cortical and the ER-devoid central region of these cells. The central region is also shown to contain an actin-based cytoskeletal network in which the individual, straight, actin-like filaments are randomly distributed. To explain these findings as well as the results of physical simulation experiments, we have formulated a new, tensegrity-based model of gravity sensing in columella cells. This model envisages the cytoplasm as pervaded by an actin-based cytoskeletal network that is denser in the ER-devoid central region than in the ER-rich cell cortex and is linked to stretch receptors in the plasma membrane. Sedimenting statoliths are postulated to produce a directional signal by locally disrupting the network and thereby altering the balance of forces acting on the receptors in different plasma membrane regions.