Root system architecture and gravitropism in the oil palm.

The oil palm (Elaeis guineensis Jacq.) has a root system consisting of primary (or order 1) roots, which are either orthogravitropic (R1 VD, with positive gravitropism) or diagravitropic (R1 H). Their statenchyma have very similar characteristics (mainly vacuolated, large cells). However, their statoliths sediment along the longitudinal wall in R1 H and along the distal wall in R1 VD (furthest cell wall from the apical meristem, opposite the proximal wall). Order 2 roots may have vertical upward (R2 VU) or downward growth (R2 VD) or even horizontal growth (R2 H). In all cases, the statoliths are located near the lower wall of the statocyte (distal in R2 VD, proximal in R2 VU and longitudinal in R2 H). Order 3 roots are usually agravitropic. When they grow upwards, R3 VU, their amyloplasts are located near the proximal wall. Likewise, the growth direction of R4 varies, but they have little or no statolith sedimentation. Roots with marked gravitropism (positive or negative) have amyloplasts that can sediment along different walls. But, irrespective of amyloplast position in the statocytes, the direction of root growth may be stable. The relation between the different reactions of roots and different sensitivity to auxin or to a curvature-halting signal is discussed.     

Gravity perception requires statoliths settled on specific plasma membrane areas in characean rhizoids and protonemata

Summary. The noninvasive infrared laser micromanipulation technique (optical tweezers, optical trapping) and centrifugation were used to study susception and perception, the early events in the gravitropic pathway of tip-growing characean rhizoids and protonemata. Reorientation of the growth direction in both cell types was only initiated when at least 2–3 statoliths settled on specific areas of the plasma membrane. This statolith-sensitive plasma membrane area is confined to the statolith region (10–35 μm behind the tip) in positively gravitropic rhizoids, whereas in negatively gravitropic protonemata, this area is limited to the apical plasma membrane (0–10 μm). Statolith sedimentation towards the sensitive plasma membrane areas is mediated by the concerted action of actin and gravity. The process of sedimentation, the pure physical movement, of statoliths is not sufficient to initiate graviresponses in both cell types. It is concluded that specific statolith-sensitive plasma membrane areas play a crucial role in the signal transduction pathway of gravitropism. These areas may represent the primary sites for gravity perception and may transform the information derived from the gravity-induced statolith sedimentation into physiological signals which trigger the molecular mechanisms of the opposite graviresponses in characean rhizoids and protonemata. Received September 10, 2001 Accepted November 16, 2001     

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.     

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     

Early development and gravitropic response of lateral roots in Arabidopsis thaliana

Root system architecture plays an important role in determining nutrient and water acquisition and is modulated by endogenous and environmental factors, resulting in considerable developmental plasticity. The orientation of primary root growth in response to gravity (gravitropism) has been studied extensively, but little is known about the behaviour of lateral roots in response to this signal. Here, we analysed the response of lateral roots to gravity and, consistently with previous observations, we showed that gravitropism was acquired slowly after emergence. Using a lateral root induction system, we studied the kinetics for the appearance of statoliths, phloem connections and auxin transporter gene expression patterns. We found that statoliths could not be detected until 1 day after emergence, whereas the gravitropic curvature of the lateral root started earlier. Auxin transporters modulate auxin distribution in primary root gravitropism. We found differences regarding PIN3 and AUX1 expression patterns between the lateral root and the primary root apices. Especially PIN3, which is involved in primary root gravitropism, was not expressed in the lateral root columella. Our work revealed new developmental transitions occurring in lateral roots after emergence, and auxin transporter expression patterns that might explain the specific response of lateral roots to gravity.     

Growth and gravitropism of corn roots in solution

Growth and early gravitropic responses of corn roots in solution have been studied using time-lapse photography. Aeration was required for both root growth and gravitropism. The optimum pH for gravitropism was in the range 5 to 6. The bending response seemed to be greater for roots in non-buffered solution than in buffered solution. Fastest growth and maximum curvature occurred with about 0.2 mol m⁻³ Ca²⁺. Under some conditions, the gravitropic response started with apparently negligible time delay after the start of the gravitropic stimulus. This may denote graviperception in or near the elongation zone itself. This mechanism for early but relatively weak gravitropism may help to explain a variety of gravitropic responses such as the ‘early wrong way’ curvature, and the behaviour of roots whose columella cells lack amyloplasts. More rapid bending appears to start at about 20 min, which is consistent with observations on roots in humid air and with the accepted statolith model of perception in the root cap.     

Movement of Cell Organelles and the Geotropic Curvature in Roots of Norway Spruce (Picea abies)

The geotropic development in roots of Norway spruce [(Picea abies (L.)] H. Karst, has been followed by light and electron microscopy and compared with the movement of cell organelles (statoliths) in the root cap cells. The geotropic curvature develops in two phases: (a) an initial curvature in the root cap region, which results in an asymmetry in the extreme root tip and which appears after about 3 h stimulation in the horizontal position; and (b) the geotropic curvature in the basal parts of the root tip, which after 8 h is distributed over the entire elongation zone. A graphic extrapolation, based on measurements of the root curvatures after various stimulation periods, indicates a presentation time in the range of 8 to 10 min. The root anatomy and ultrastructure have been examined in detail in order to obtain information as to which organelles may act as gravity receptors. The root cap consists of a central core (columella) distinct from the peripheral part. The core contains three to four rows of parenchymatic cells each consisting of 15 to 18 storeys of statocyte cells with possibly mobile cell organelles. Amyloplasts and nuclei have been found to be mobile in the root cap cells, and the movement of both types of organelles has been followed after inversion of the seedlings and stimulation in the horizontal position for various periods of time at 4°C and 21°C. Three-dimensional reconstructions of spruce root cap cells based on serial sectioning and electron microscopy have been performed. These demonstrate that the endoplasmic reticulum (ER)-system and the vacuoles occupy a considerable part of the statocyte cell. For this reason the space available for free movement of single statolith particles is highly restricted.     

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.     

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 effect of the external medium on the gravitropic curvature of rice (Oryza sativa, Poaceae) roots

The roots of rice seedlings, growing in artificial pond water, exhibit robust gravitropic curvature when placed perpendicular to the vector of gravity. To determine whether the statolith theory (in which intracellular sedimenting particles are responsible for gravity sensing) or the gravitational pressure theory (in which the entire protoplast acts as the gravity sensor) best accounts for gravity sensing in rice roots, we changed the physical properties of the external medium with impermeant solutes and examined the effect on gravitropism. As the density of the external medium is increased, the rate of gravitropic curvature decreases. The decrease in the rate of gravicurvature cannot be attributed to an inhibition of growth, since rice roots grown in 100 Osm/m3 (0.248 MPa) solutions of different densities all support the same root growth rate but inhibit gravicurvature increasingly with increasing density. By contrast, the sedimentation rate of amyloplasts in the columella cells is unaffected by the external density. These results are consistent with the gravitational pressure theory of gravity sensing, but cannot be explained by the statolith theory.     

The Starch Statolith Hypothesis and the Interaction of Amyloplasts and Endoplasmic Reticulum in Root Geotropism

Roots of cress (Lepidium sativum L, ) seedlings continuouslystimulated at an angle of 135°—root tips pointingobliquely upwards—develop a larger final geotropic curvaturethan roots stimulated at 45° or 90°. This well-knownbehaviour has previously been interpreted as support for thestarch statolith hypothesis. In the present experiments two groups of cress and lettuce (Lactucasativa L.) seedlings were used: (a) the control group in whichthe roots were allowed to curve without adjustment of the stimulationangle, and (b) the test group in which the roots were readjustedat different time intervals to the original stimulation angle.They were stimulated continuously at 45°, 90°, or 135°and the development of root curvatures was followed over a periodof 5–8 h. Initially (1–2 h) the rate of curvature was approximatelythe same for 135° and 90° control and tested cress andlettuce roots. Thereafter the test roots stimulated at 135°followed a linear curvature pattern. Seedlings stimulated at45° and 90° did not show the same linearity in curvaturedevelopment in the test group. The rates of curvature in thetest group were generally higher than in the control group atangles less than 135°. Cress seedlings were examined by light and electron microscopyin order to follow the movement of the cell organelles in thestatocytes. In the statocytes of roots of test seedlings thestarch statoliths were located in the position attained beforethe first readjustment of the stimulation angle. In the statocytesof control roots the starch statoliths followed the curvatureof the root tip sliding along the cell walls and attaining therest position as in normally orientated roots. The behaviour of control and readjusted roots is interpretedas a result of interaction between starch statoliths and endoplasmicreticulum membranes.     

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.     

Cytochalasin D does not inhibit gravitropism in roots

It is generally thought that sedimenting plastids are responsible for gravity sensing in higher plants. We directly tested the model generated by the current statolith hypothesis that the gravity sensing that leads to gravitropism results from an interaction between the plastids and actin microfilaments. We find that the primary roots of rice, corn, and cress undergo normal gravitropism and growth even when exposed to cytochalasin D, a disruptor of actin microfilaments. These results indicate that an interaction between amyloplasts and the actin cytoskeleton is not critical for gravity sensing in higher plants and weaken the current statolith hypothesis.     

Early wrong-way response occurs in orthogravitropism of maize roots treated with lithium

Millet, B. and Pickard, B. G. 1988. Early wrong-way response occurs in orthogravitropism of maize roots treated with lithium. - Physiol. Plant. 72: 555–559.
Application of lithium ions to tips of roots of Zea mays L. cv. Silver Queen shifts the direction of initial orthogravitropic curvature from downward to upward. The production of this putatively incidental perturbation of orthogravitropic bending kinetics by a pharmacological agent might provide insight into both ortho- and plagiogravitro-pism. Additionally, the protocol of the experiments bears on recent claims that mucilage external to the root cap plays an essential role in gravitropism. External mucilage was removed before roots were stimulated, yet they reached about 50 degrees gravitropic curvature in an hour.     

A polarized cell: the root statocyte

In the gravity-perceiving cells (statocytes), located in the centre of the root cap, polarity is expressed in the arrangement of the organelles since, in most genera, the nucleus and the endoplasmic reticulum are maintained at the opposite ends of each cell by actin. Polarity is also evident in the distribution of plasmodesmata, which are more numerous in the transverse walls than in the longitudinal walls. The centre of each statocyte is depleted of microtubules (they are only located at the periphery) but is occupied by numerous amyloplasts (statoliths), denser than the cytoplasm. The amyloplasts do not contribute to the inherent structural polarity since their position is dependent upon the gravity vector. This article focuses on new microscopic analyses and on data obtained from experiments performed in microgravity, which have contributed to our better understanding of the architecture of the actin web implicated in the perception of gravity. Depending upon the plant, the actin network seems to be formed of single filaments arranged in various ways, or, of thin bundles of actin filaments. The amyloplasts are enmeshed in this web of actin and their envelopes are associated with it, but they can have autonomous movement via myosin in the absence of gravity. From calculations of the value of the force necessary to move one amyloplast in the lentil root, and from videomicroscopy performed with living statocytes of maize roots, it is hypothesized that actin microfilaments could be orientated in an overall diagonal direction in the statocyte. These observations could help in understanding how slight amyloplast movements may trigger and transmit the gravitropic signal.     

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)     

Restitution of polarity in statocytes from centrifuged roots*

Abstract The structural polarity of statocytes from cress roots is changed by centrifugation. Upon low- dose centrifugation (3000 g min), the extent of stratification depends on statocyte position, i.e., central statocytes are affected more than lateral ones. Upon higher doses of centrifugation (60,000 and 360,000 g min), a uniform density gradient is established in all statocytes. If, after centrifugation, the roots are exposed to gravity again, the endoplasmic reticulum (ER) cisternae are relocated parallel to the periclinal cell walls within a few minutes; this relocation is independent of the direction of gravity in relation to the root axis, and independent of the previously applied centrifugation dose. This supports the notion that polarity is determined genetically. Cytochalasin B treatment, before and during centrifugation, totally inhibits the relocation of ER. After removing the drug by rinsing the roots, the statocytes restore cell polarity and relocate ER. These results indicate that relocation of ER cisternae may be mediated by microfilaments. When centrifuged roots are exposed to 1 g in the horizontal position, the latent period of gravitropism increases by 8–10 min relative to controls, regardless of the previously applied centrifugation doses. The kinetics of curvature are virtually identical. Since the increase in the latent period coincides with the time needed for most statocytes to restore the distal cell pole, it is evident that perception of gravity is correlated to the integrity of the distal cell pole.     

Gravitropism and starch statoliths in an Arabidopsis mutant

The mutant TC 7 of Arabidopsis thaliana (L.) Heynh. has been reported to be starch-free and still exhibit root gravitropism (T. Caspar and B. G. Pickard 1989, Planta 177, 185–197). This is not consistent with the hypothesis that plastid starch has a statolith function in gravity perception. In the present study, initial light microscopy using the same mutant showed apparently starch-free statocytes. However, ultrastructural examination detected residues of amyloplast starch grains in addition to the starch-depleted amyloplasts. Applying a point-counting morphometric method, the starch grains in the individual amyloplasts in the mutant were generally found to occupy more than 20% and in a few cases up to 60% of the amyloplast area. In the wild type (WT) the starch occupied on average 98 % of the amyloplast area and appeared as densely packed grains. The amyloplasts occupied 13.9% of the area of the statocyte in the mutant and 23.3% of the statocyte area in the WT. Sedimentation of starch-depleted amyloplasts in the mutant was not detected after 40 min of inversion while in the WT the amyloplasts sedimented at a speed of 6 m · h^-1. The gravitropic reactivity and the curvature pattern were also examined in the WT and the mutant. The time-courses of root curvature in the WT and the mutant showed that when cultivated under standard conditions for 60 h in darkness, the curvatures were 83° and 44°, respectively, after 25 h of continuous stimulation in the horizontal position. The WT roots curved significantly more rapidly and with a more normal gravitropic pattern than those of the mutant. These results are discussed in relation to the results previously obtained with the mutant and with respect to the starch-statolith hypothesis.Abbreviation WT wild type This work was supported by grants from Norwegian Research Council for Science and the Humanities (NAVF) which we gratefully acknowledge. We would also like to thank Dr. Timothy Caspar, Michigan State University, East Lansing, USA, for providing us with the seeds of TC 75.     

An Arabidopsis E3 ligase, SHOOT GRAVITROPISM9, modulates the interaction between statoliths and F-actin in gravity sensing

Higher plants use the sedimentation of amyloplasts in statocytes as statolith to sense the direction of gravity during gravitropism. In Arabidopsis thaliana inflorescence stem statocyte, amyloplasts are in complex movement; some show jumping-like saltatory movement and some tend to sediment toward the gravity direction. Here, we report that a RING-type E3 ligase SHOOT GRAVITROPISM9 (SGR9) localized to amyloplasts modulates amyloplast dynamics. In the sgr9 mutant, which exhibits reduced gravitropism, amyloplasts did not sediment but exhibited increased saltatory movement. Amyloplasts sometimes formed a cluster that is abnormally entangled with actin filaments (AFs) in sgr9. By contrast, in the fiz1 mutant, an ACT8 semidominant mutant that induces fragmentation of AFs, amyloplasts, lost saltatory movement and sedimented with nearly statically. Both treatment with Latrunculin B, an inhibitor of AF polymerization, and the fiz1 mutation rescued the gravitropic defect of sgr9. In addition, fiz1 decreased saltatory movement and induced amyloplast sedimentation even in sgr9. Our results suggest that amyloplasts are in equilibrium between sedimentation and saltatory movement in wild-type endodermal cells. Furthermore, this equilibrium is the result of the interaction between amyloplasts and AFs modulated by the SGR9. SGR9 may promote detachment of amyloplasts from AFs, allowing the amyloplasts to sediment in the AFs-dependent equilibrium of amyloplast dynamics.     

Mechanotransduction in root gravity sensing cells

The analysis of the dose-response curve of the gravitropic reaction of lentil seedling roots has shown that these organs are more sensitive when they have been grown in microgravity than when they have been grown on a 1 g centrifuge in space before gravistimulation. This difference of gravisensitivity is not due to the volume or the density of starch grains of statoliths, which are about the same in both conditions (1 g or microgravity). However, the distribution of statoliths within the statocyte may be responsible for this differential sensitivity, since the dispersion of these organelles is greater in microgravity than in 1 g. When lentil roots grown in microgravity or in 1 g are stimulated at 0.93 g for 22 min, the amyloplasts sediment following two different trajectories. They move from the proximal half of the statocytes toward the lower longitudinal wall in the microgravity grown sample and from the distal half toward the longitudinal wall in the 1 g grown sample. At the end of the stimulation, they reach a similar position within the statocytes. If the roots of both samples are left in microgravity for 3 h, the amyloplasts move toward the cell centre in a direction that makes an average angle of 40 degrees with respect to the lower longitudinal wall. The actin filaments, which are responsible for this movement, may have an overall orientation of 40 degrees with respect to this wall. Thus, when roots grown in microgravity are stimulated on the minicentrifuge the amyloplasts slide on the actin filaments, whereas they move perpendicular to them in 1 g grown roots. Our results suggest that greater sensitivity of seedling roots grown in microgravity should be due to greater dispersion of statoliths, to better contacts between statoliths and the actin network and to greater number of activated mechanoreceptors. One can hypothesize that stretch activated ion channels (SACs) located in the plasma membrane are responsible for the transduction of gravistimulus. These SACs may be connected together by elements of the cytoskeleton lining the plasma membrane and to the actin filaments. They could be stimulated by the action of statoliths on the actin network and/or on these elements of the cytoskeleton which link the mechanoreceptors (SACs).