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 microm 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 microm 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 (Fc) was about 86% less (Fc = 0.016 pN) than the gravity force (Fg = 0.11 pN). Treatment with CD reduced Fc 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.

Immunolocalization of actin in root statocytes of Lens culinaris L.

Lentil root statocytes show a strict structural polarity of their organelles with respect to the g vector. These cells are involved in the perception of gravity and are responsible for the orientation of the root. Actin filaments take part in the positioning of their organelles and could also be involved in the transduction of the gravitropic signal. A pre-embedding immunogold silver technique was carried out with a monoclonal antibody in order to study the distribution of actin cytoskeleton in the statocytes at the electron microscopic level. Some areas were never labelled (cell wall, vacuole, nucleoplasm, mitochondria, starch grains of the amyloplasts) or very slightly labelled (stroma of the amyloplasts). The labelling was scattered in the cytoplasm always close to, or on the nuclear and amyloplast envelopes and the tonoplast. Associations of 2 to 6 dots in file were observed, but these short files were not oriented in one preferential direction. They corresponded to a maximum distance of 0.9 micron. This work demonstrated that each statocyte organelle was enmeshed in an actin web of short filaments arranged in different ways. The images obtained by rhodaminephalloidin staining were in accordance with those of immunogold labelling. The diffuse fluorescence of the cytoplasm could be explained by the fact that the meshes of the web should be narrow. The vicinity of actin and of the amyloplasts envelope could account for the movement of these organelles that was observed in spatial microgravity.

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.

Statoliths motions in gravity-perceiving plant cells: does actomyosin counteract gravity?

Statocytes from plant root caps are characterized by a polar arrangement of cell organelles and sedimented statoliths. Cortical microtubules and actin microfilaments contribute to development and maintenance of this polarity, whereas the lack of endoplasmic microtubules and prominent bundles of actin microfilaments probably facilitates sedimentation of statoliths. High-resolution video microscopy shows permanent motion of statoliths even when sedimented. After immunofluorescence microscopy using antibodies against actin and myosin II the most prominent labeling was observed at and around sedimented statoliths. Experiments under microgravity have demonstrated that the positioning of statoliths depends on the external gravitational force and on internal forces, probably exerted by the actomyosin complex, and that transformation of the gravistimulus evidently occurs in close vicinity to the statoliths. These results suggest that graviperception occurs dynamically within the cytoplasm via small-distance sedimentation rather than statically at the lowermost site of sedimentation. It is hypothesized that root cap cells are comparing randomized motions with oriented motions of statoliths and thereby perceiving gravity.

Effect of horizontal clinorotation on the root system development and on lipid breakdown in rapeseed (Brassica napus) seedlings.

Seedlings of Brassica napus were cultivated on a slowly rotating clinostat (1 rpm) or in the vertical control for 5 d. The root growth, the cotyledonary reserves and the transport of 14C-labeled sucrose from cotyledons to root system were studied in both cultural conditions. The biomass (fresh weight) of the root system was 35% higher in the horizontally clinorotated seedlings than in the controls. This increase was correlated with a greater degradation of reserve lipids and faster accumulation of sucrose in the cotyledons. The activity of isocitrate lyase, one of the two enzymes necessary to conversion of lipids into glucids, was also greater in the cotyledons of clinorotated seedlings. The labeling distribution of 14C in the cotyledons, the hypocotyl and the root system after 30, 60 and 120 min of application of 14C-labeled sucrose on the cotyledons showed higher translocation of the cotyledonary sucrose to the root system of clinorotated seedlings. In addition, we studied the effects of clinorotation on the biomass of the excised root system (of 10 d old seedlings) cultivated in a medium containing 1% sucrose. The horizontally clinorotated root system grew more than that of the controls. These results showed that the horizontal clinorotation acted on the root system growth and provoked a higher sucrose translocation from source to sink, i.e. from cotyledons to root system.

Changes in hormonal balance and meristematic activity in primary root tips on the slowly rotating clinostat and their effect on the development of the rapeseed root system.

The morphometry of the root system, the meristematic activity and the level of indole-3-acetic acid (IAA), abscisic acid (ABA) and zeatin in the primary root tips of rapeseed seedlings were analyzed as functions of time on a slowly rotating clinostat (1 rpm) or in the vertical controls (1 rpm). The fresh weight of the root system was 30% higher throughout the growth period (25 days) in clinorotated seedlings. Morphometric analysis showed that the increase in biomass on the clinostat was due to greater primary root growth, earlier initiation and greater elongation of the secondary roots, which could be observed even in 5-day-old seedlings. However, after 15 days, the growth of the primary root slowed on the clinostat, whereas secondary roots still grew faster in clinorotated plants than in the controls. At this time, the secondary roots began to be initiated closer to the root tip on the clinostat than in the control. Analysis of the meristematic activity and determination of the levels in IAA, ABA and zeatin in the primary root tips demonstrated that after 5 days on the clinostat, the increased length of the primary root could be the consequence of higher meristematic activity and coincided with an increase in both IAA and ABA concentrations. After 15 days on the clinostat, a marked increase in IAA, ABA and zeatin, which probably reached supraoptimal levels, seems to cause a progressive disturbance of the meristematic cells, during a decrease of primary root growth between 15 and 25 days. These modifications in the hormonal balance and the perturbation of the meristematic activity on the clinostat were followed by a loss of apical dominance, which was responsible for the early initiation of secondary roots, the greater elongation of the root system and the emergence of the lateral roots near the tip of the primary root.

Effect of microgravity on the cell cycle in the lentil root.

Characteristics of the cell cycle in cortical regions (0-0.6 mm from the root-cap junction) of the primary root of lentil (Lens culinaris L.) during germination in the vertical position on earth were determined by iododeoxyuridine labelling and image analysis. All cells were in the G1 phase at the beginning of germination and the duration of the first cell cycle was about 25 h. At 29 h, around 14% of the cortical nuclei were still in the G2 or M phases of the first cell cycle, whereas 53 and 33% of the nuclei were respectively in the G1 or S phase of the second cell cycle. In parallel, the cell cycle was analysed in root tips of lentil seedlings grown in space during the IML 2 mission (1994), (1) on the 1-g centrifuge for 29 h, (2) on the l-g centrifuge for 25 h and placed in microgravity for 4 h, (3) in microgravity for 29 h, (4) in microgravity for 25 h and placed on the 1-g centrifuge for 4 h. The densitometric analysis of nuclear DNA content showed that in microgravity there were less cells in DNA synthesis and more cells in G1 than in the controls on the 1-g centrifuge (flight and ground). The comparison of the sample grown continuously on the 1-g centrifuge in space and of the sample grown first in l-g and then in microgravity indicated that 4 h of microgravity modified cell cycle, increasing the percentage of cells in the G1 phase. On the contrary, the transfer from microgravity to the 1-g centrifuge (for 4 h) did not provoke any significant change in the distribution of the nuclear DNA content. Thus the effect of microgravity could not be reversed by a 4 h centrifugation. As the duration of the first cell cycle in the lentil root meristem is about 25 h, the results obtained are in agreement with the hypothesis that the first cell cycle and/or the second G1 phase was lengthened in absence of gravity. The difference observed in the distribution of the nuclear DNA content in the two controls could he due to the fact that the 1g control on board was subjected to a period of 15 min of microgravity for photography 25 h after the hydration of the seeds, which indicated an effect of short exposure to weightlessness. The mitotic index of cortical cells was greater on the 1-g centrifuge in space than in any other sample (flight and ground) which could show an effect of the centrifugation on the mitosis.

Gravisensing in roots.

The mode of gravisensing in higher plants is not yet elucidated. Although, it is generally accepted that the amyloplasts (statoliths) in the root cap cells (statocytes) are responsible for susception of gravity. However, the hypothesis that the whole protoplast acts as gravisusceptor cannot be dismissed. The nature of the sensor that is able to transduce and amplify the mechanical energy into a biochemical factor is even more controversial. Several cell structures could potentially serve as gravireceptors: the endoplasmic reticulum, the actin network, the plasma membrane, or the cytoskeleton associated with this membrane. The nature of the gravisusceptors and gravisensors is discussed by taking into account the characteristics of the gravitropic reaction with respect to the presentation time, the threshold acceleration, the reciprocity rule, the deviation from the sine rule, the movement of the amyloplasts, the pre-inversion effect, the response of starch free and intermediate mutants and the effects of cytochalasin treatment. From this analysis, it can be concluded that both the amyloplasts and the protoplast could be the gravisusceptors, the former being more efficient than the latter since they can focus pressure on limited areas. The receptor should be located in the plasma membrane and could be a stretch-activated ion channel.

Cytoplasmic free Ca2+ in Arabidopsis roots changes in response to touch but not gravity.

Changes in cytoplasmic Ca2+ concentration ([Ca2+]i) have been proposed to be involved in signal transduction pathways in response to a number of stimuli, including gravity and touch. The current hypothesis proposes that the development of gravitropic bending is correlated with a redistribution of [Ca2+]i in gravistimulated roots. However, no study has demonstrated clearly the development of an asymmetry of this ion during root curvature. We tested this hypothesis by quantifying the temporal and spatial changes in [Ca2+]i in roots of living Arabidopsis seedlings using ultraviolet-confocal Ca(2+)-ratio imaging and vertical stage fluorescence microscopy to visualize root [Ca2+]i. We observed no changes in [Ca2+]i associated with the graviresponse whether monitored at the whole organ level or in individual cells in different regions of the root for up to 12 h after gravistimulation. However, touch stimulation led to transient increases in [Ca2+]i in all cell types monitored. The increases induced in the cap cells were larger and longer-lived than in cells in the meristematic or elongation zone. One millimolar La3+ and 100 microM verapamil did not prevent these responses, whereas 5 mM EGTA or 50 microM ruthenium red inhibited the transients, indicating an intracellular origin of the Ca2+ increase. These results suggest that although touch responses of roots may be mediated through a Ca(2+)-dependent pathway, the gravitropic response is not associated with detectable changes in [Ca2+]i.

Statocyte polarity and gravisensitivity in seedling roots grown in microgravity.

Space experiments have offered a unique opportunity to analyse the mechanism of gravisensing in plant roots. It has been shown that the strict structural polarity of statocytes observed on the ground is perturbed in microgravity: the amyloplasts move towards the proximal half of the cell and, at least in some cases, the nucleus becomes located further away from the (proximal) plasma membrane. It has thus been demonstrated that the amyloplasts do not move freely in the cytoplasm. Experiments using cytochalasin B (or D) have indicated that these organelles are attached to the actin network, probably by motor proteins. These findings have led to a new hypothesis on gravisensing the basis of which is that the tension in the actin filaments resulting from interaction with the statoliths would be transmitted to stretch-activated ion channels located in the plasma membrane (Sievers et al., 1991, In: Lloyd (ed) The cytoskeletal basis of plant growth and form, Academic Press, London New York, pp 169-182). Recently, it has been shown that the sensitivity of roots grown under 1 g conditions in orbit is less than that of roots grown in microgravity or under simulated weightlessness on clinostats. Since the location of the amyloplasts in microgravity is different from that in 1 g, the greater sensitivity observed could be due to different tensions in the actin network.

The response to auxin of rapeseed (Brassica napus L.) roots displaying reduced gravitropism due to transformation by Agrobacterium rhizogenes.

It has recently been documented that, compared to untransformed controls, the roots of oilseed rape (Brassica napus L. CV CrGC5) seedlings transformed by Agrobacterium rhizogenes A4 show a reduced gravitropic reaction (Legue et al. 1994, Physiol Plant 91: 559-566). After stimulation at 90 degrees C or 135 degrees, the transformed root tips curve. but never reach a vertical orientation. In the present study, we investigated the causes of reduced gravitropic bending observed in stimulated transformed root tips. First, we localized the gravitropic curvature in normal and in transformed roots after 1.5 h of stimulation. The cells involved in root curvature (target cells) corresponded at the cellular level to the apical part of the zone of increasing cell length. In transformed roots grown in the vertical position, these cells showed a reduction in cell length compared to controls. Because auxin is considered to be the gravitropic mediator, the response of normal and transformed roots to exogenous auxin was studied. Indole-3-acetic acid (IAA) was applied along the first 3 mm using resin beads loaded with the hormone. In comparison to normal roots, transformed roots showed reduced bending toward the bead at all points of bead application. Moreover, the cells which responded to IAA corresponded to the target cells involved in the gravitropic reaction. The level of endogenous IAA was lower in transformed roots. Thus, it was concluded that the modified behavior of transformed roots during gravitropic stimulation could be due to differences either in IAA levels or in reactivity of the target cells to the message from the cap.

Effects of gravitropic stress on the development of the primary root of lentil seedlings grown in space.

Root growth and cell differentiation were analysed in lentil seedlings grown (1) in microgravity (F microg), (2) on the 1 x g centrifuge (F1 x g), (3) in microgravity and placed on the 1 x g centrifuge for 4 h [F(microg + 1 x g)], (4) on the 1 x g centrifuge and placed in microgravity for 4 h [F(1 x g + microg)]. In microgravity, there were strong oscillations of the root tip, even when the seedlings were grown first on the 1 x g centrifuge [F(1 x g + microg)]. In the [F(microg + 1 x g)] sample, the roots grown in microgravity were oblique with respect to the 1 x g acceleration when the seedlings were placed on the centrifuge. They were therefore gravistimulated. However, root length was similar in the 4 samples after 29 h of growth and growth rate of the root was the same between 25 h and 29 h although it appeared to be slightly greater in the [F(microg + 1 x g)] sample. Cell elongation was analysed as a function of the distance from the root cap junction. Cell length was similar in the seedlings grown in microgravity or on the 1 x g centrifuge. The transfer from the 1 x g centrifuge to microgravity [F(1 x g + microg)] did not modify cell elongation in the roots. Cell length in the roots which were grown in microgravity and gravistimulated [F(microg + 1 x g)] was different from that observed in microgravity but this was only due to gravistimulation. Thus, gravity does not have an effect on cell elongation when the roots are strictly oriented in the vertical position but it does as soon as the root tip deviates from this orientation.

Sensitivity to gravistimulus of lentil seedling roots grown in space during the IML 1 Mission of Spacelab.

The gravitropic curvature of seedlings of lentil (Lens culunaris L. cv. Verte du Puy) grown in microgravity and stimulated on the 1 g centrifuge for 5 to 60 min was followed by time lapse photography in near weightlessness in the frame of the IML 1 Mission of Spacelab. In microgravity, the root tip could overshoot the direction of the 1 g acceleration after bending whereas roots stimulated on the ground did not reach the direction of the gravity vector. On earth, there is, therefore, a regulation (inhibition of root curvature), which is gravity dependent. In space, the initial rate of curvature as well as the amplitude of curvature varied as a function of the quantity of stimulation (Q, in gmin). For a given quantity of stimulation, the rate of curvature remained constant for 80 min. The bending has thus a certain inertia, which is linked to the mechanism of differential growth. The presentation time (Tp) of the lentil root was calculated by extrapolation to zero curvature of the regression line representing either the initial rate of curvature or the amplitude of curvature at 2 h after the end of the stimulation. Tp was estimated to 27 and 26 s, respectively. These results confirm the values of Tp obtained by clinostats, and they also lead to a reconsideration of the causes of the kinetics of root curvature.

Densitometric analysis of nuclear DNA content in lentil roots grown in space.

Lentil seedlings were grown for 28 h in space, on board Spacelab (IML 1 Mission) and growth of the primary root was analysed. The length of cortical cells was less in near weightlessness than on the 1 g centrifuge (flight control) and mitotic index was lower but there was no apparent perturbation in the mitosis. To further investigate which phase of cell cycle was modified, densitometric analysis of nuclear DNA content in cortical cells was carried out by the mean of an image processing system (SAMBA). In microgravity there was a decrease in DNA synthesis and a promotion of the arrest in the G2 phase of cell cycle. These results, and other ones obtained elsewhere on a slowly rotating clinostat, led us to think that in microgravity the perturbation of the gravisensing cells and/or the absence of convection could account for the modification of cell growth registered in the primary root.

Transduction of the gravity stimulus in the root statocyte.

The amyloplasts of root statocytes are considered to be the perceptors of gravity. However, their displacement and the starch they contain are not required for gravisensing. The mechanism of the transduction of gravistimulus remains therefore controversial. It is well known that the amplitude of the stimulus is dependent upon the intensity of the acceleration and the inclination of the root with respect to gravity. This strongly supports the hypothesis that the stimulus results in a mechanical effect (pressure or tension) on a cellular structure. Three cellular components are proposed as possible candidates for the role of transducer: the actin filaments, the endoplasmic reticulum and the plasma membrane with its ion channels. Recent results obtained in the frame of the IML 1 Mission of Spacelab show that the endoplasmic reticulum should rather be responsible for the termination of the stimulus. The contacts of amyloplasts with the distal ER could therefore be involved in the regulation of root growth.

[Microgravity and root gravitropism]. / Microgravite et gravitropisme racinaire.

Space experiments permit to understand better some phases of the gravitropic reaction which occurs when the orientation of the root changes in the gravitational field. In gravisensing cells (statocytes in the root cap), the nucleus is attached to the cell periphery, close to the plasma membrane, by actin filaments. The location of the amyloplasts (statoliths) depends also greatly on these elements of the cytoskeleton. A short period in microgravity (5 min.) modifies the location of the nucleus and of the amyloplasts in the statocytes. The tensions exerted by these very dense organelles on the actin network disappear and this network undergoes a relaxation. The kinetics of gravitropic curvature is also better understood. In fact, gravitropic reaction is regulated by a mechanism depending on gravity. In roots grown in space, then stimulated for 1 h on a 1 g centrifuge, and replaced in microgravity, the regulation limiting the curvature does not occur. It is hypothesized that the sedimentation of the amyloplasts on the endoplasmic reticulum placed at the basal pole of the statocytes could be responsible for this regulation. The contacts between these two organelles should have also a role in root growth. This hypothesis will be tested in our next space experiment (July 94). The experiments in near weightlessness also permit to determine the presentation time which is the duration of stimulation necessary to evoke a slight but significant curvature. Presentation time is 27 s. This short period allows a slight movement of the amyloplasts only (around 0.45 micrometer). The sequence of events leading to the curvature of the root is now well established: the first signal is the separation of the endoplasmic reticulum and the amyloplasts, when the root is subjected to a change in orientation. It is followed by the pressure of these organelles on the actin network which transmits this mechanical effect to the plasma membrane. The transduction of the effect occurs then by the activation of the ions channels (Ca++) and the carrier of a growth inhibitor (auxin), both located in the plasma membrane. This growth inhibitor provokes an asymmetrical growth in the distal part of the meristem and the proximal part of the cell elongation zone. At last, when the root tip reaches the direction of gravity, the amyoloplasts sediment on the endoplasmic reticulum and induce a signal of termination of the curavature.

Actin filaments responsible for the location of the nucleus in the lentil statocyte are sensitive to gravity.

The location of the nucleus in statocytes or lentil roots grown: 1), at 1 g on the ground, 2), on a 1 g centrifuge in space, 3), in simulated microgravity on a slowly rotating clinostat (0.9 rmp) 4), in microgravity in space was investigated and statistically evaluated. In cells differentiated at 1 g on the ground, the nuclear membrane was almost in contact with the plasmalemma lining the proximal cell wall, whereas in statocytes of roots crown on the clinostat there was a distance of 0.47 micrometers (horizontal clinorotation) and or 0.76 micrometers (vertical clinorotation) between these membranes. However, in microgravity the nucleus was the most displaced, 0.87 micrometers from the proximal cell wall. Centrifugation of vertically grown roots in the root-tip direction showed that the threshold of centrifugal force to detach all nuclei from the proximal cell wall was about 40 g. In statocytes developed in the presence of cytochalasin B at 1 g the nuclei were sedimented on the amyloplasts at the distal cell pole, demonstrating that the location of the nucleus depends on actin filaments. The results obtained are in agreement with the hypothesis that gravity causes a tension of actin filaments and that this part of the cytoskeleton undergoes a relaxation in microgravity.

Polarity of statocytes in lentil seedling roots grown in space (Spacelab D1 Mission).

A morphometric analysis of root statocytes was performed on seedlings of lentil (Lens culinaris L., cv. Verte du Puy) in order to determine the effects of microgravity on the polarity of these cells. Seedlings were grown: (1) on the ground, (2) in microgravity, (3) on a 1 g centrifuge in space, (4) first in microgravity and then placed on a 1 g centrifuge for 3 h. Dry seeds were hydrated in space (except for the ground control) for 25 h in darkness at 22 degrees C in the Biorack facility developed by the European Space Agency. At the end of the experiment, the seedlings were photographed and fixed in glutaraldehyde in the Biorack glove box. The average shape of the statocytes and the location of endoplasmic reticulum, amyloplasts and nucleus in the cells were analysed in the four samples. By considering the cell shape, it appears that the morphology of the statocytes on the ground was different from that observed in the space samples. Cell polarity was similar in microgravity and in the centrifuged samples except for the distribution of the amyloplasts. These organelles were not distributed at random in near zero gravity, and they were more numerous in the proximal than in the distal half. Moreover, the statoliths were more voluminous in microgravity than in the centrifuged samples. The nucleus was closer to the cell center in the statocytes of roots grown in microgravity than in statocytes of roots grown in microgravity and then placed on the 1 g centrifuge for 3 h. It is hypothesized that the nucleus is attached to the cell periphery and that its location is dependent upon gravity.

Graviperception of lentil seedling roots grown in space (Spacelab D1 Mission).

The growth and graviresponsiveness of roots were investigated in lentil seedlings (Lens culinaris L. cv. Verte du Puy) grown (1) in microgravity, (2) on a 1 g centrifuge in space, (3) in microgravity and then placed on the 1 g centrifuge for 3 h, (4) on the ground. Dry seeds were hydrated in space (except for the ground control) and incubated for 25 h at 22 degrees C in darkness. At the end of the experiment, the seedlings were photographed and fixed in glutaraldehyde in a Biorack glove box. Root length was similar for seedlings grown in space and for the ground and the 1 g centrifuge controls. The direction of root growth in the microgravity sample deviated strongly from the initial orientation of the roots of the dry seeds. This deviation could be due to spontaneous curvatures similar to those observed on clinostats. When lentil seedlings were first grown in microgravity for 25 h and then placed on the 1 g centrifuge for 3 h, their roots bent strongly under the effect of the centrifugal acceleration. The amplitude of root curvature on the centrifuge was not significantly different from that observed on ground controls growing in the vertical position and placed in the horizontal position for 3 h. The gravisensitivity of statocytes differentiated in microgravity was similar to that of statocytes differentiated on earth. There were no qualitative differences in the ultrastructural features of the gravisensing cells in microgravity and in the 1 g centrifuge and ground controls. However, the distribution of statoliths in the gravisensing cells was different in microgravity: most of them were observed in the proximal part of these cells. Thus, these organelles were not distributed at random, which is in contradiction with results obtained with clinostats. The distal complex of endoplasmic reticulum in the statocytes was not in contact with the amyloplasts. Contact and pressure of amyloplasts on the tubules were not prerequisites for gravisensing. The results obtained are not in agreement with the hypothesis that the distal endoplasmic reticulum would be the transducer of the action of the statoliths.

[Cellular differentiation and proliferation in corn roots grown in microgravity (Biocosmos 1985)]. / Differenciation et proliferation cellulaires dans des racines de mais cultive en microgravite (Biocosmos 1985).

A cytological study was performed on Maize roots (Zea mays) which were grown in space (flight Biocosmos 1985) and on control roots on earth. Two criteria were selected: cell elongation in the cortical zone in the four mm of the extremity of the root and mitotic activity of the meristem. The results show that in microgravity the length of the meristem is reduced of 1/3 and that its mitotic activity increases of about twofold comparatively to the synchronous control. In parallel the cell differentiation begin closer to the root cap junction. These results are discussed relative to the influence of gravistimulation on cell proliferation and cell differentiation in roots.