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

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.

Nodal endoplasmic reticulum, a specialized form of endoplasmic reticulum found in gravity-sensing root tip columella cells

The endoplasmic reticulum (ER) of columella root cap cells has been postulated to play a role in gravity sensing. We have re-examined the ultrastructure of columella cells in tobacco (Nicotiana tabacum) root tips preserved by high-pressure freezing/freeze-substitution techniques to gain more precise information about the organization of the ER in such cells. The most notable findings are: the identification of a specialized form of ER, termed "nodal ER," which is found exclusively in columella cells; the demonstration that the bulk of the ER is organized in the form of a tubular network that is confined to a peripheral layer under the plasma membrane; and the discovery that this ER-rich peripheral region excludes Golgi stacks, vacuoles, and amyloplasts but not mitochondria. Nodal ER domains consist of an approximately 100-nm-diameter central rod composed of oblong subunits to which usually seven sheets of rough ER are attached along their margins. These domains form patches at the interface between the peripheral ER network and the ER-free central region of the cells, and they occupy defined positions within central and flanking columella cells. Over one-half of the nodal ER domains are located along the outer tangential walls of the flanking cells. Cytochalasin D and latrunculin A cause an increase in size and a decrease in numbers of nodal ER domains. We postulate that the nodal ER membranes locally modulate the gravisensing signals produced by the sedimenting amyloplasts, and that the confinement of all ER membranes to the cell periphery serves to enhance the sedimentability of the amyloplasts in the central region of columella cells.

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.

CYTOSKELETAL PERSPECTIVES ON ROOT GROWTH AND MORPHOGENESIS

Growth and development of all plant cells and organs relies on a fully functional cytoskeleton comprised principally of microtubules and microfilaments. These two polymeric macromolecules, because of their location within the cell, confer structure upon, and convey information to, the peripheral regions of the cytoplasm where much of cellular growth is controlled and the formation of cellular identity takes place. Other ancillary molecules, such as motor proteins, are also important in assisting the cytoskeleton to participate in this front-line work of cellular development. Roots provide not only a ready source of cells for fundamental analyses of the cytoskeleton, but the formative zone at their apices also provides a locale whereby experimental studies can be made of how the cytoskeleton permits cells to communicate between themselves and to cooperate with growth-regulating information supplied from the apoplasm.

Central root cap cells are depleted of endoplasmic microtubules and actin microfilament bundles: implications for their role as gravity-sensing statocytes

Indirect immunofluorescence, using monoclonal antibodies to actin and tubulin, applied to sections of root tips of Lepidium, Lycopersicon, Phleum, and Zea, revealed features of the cytoskeleton that were unique to the statocytes of their root caps. Although the cortical microtubules (CMTs) lay in dense arrays against the periphery of the statocytes, these same cells showed depleted complements of endoplasmic microtubules (EMTs) and of actin microfilament (AMF) bundles, both of which are characteristic of the cytoskeleton of other post-mitotic cells in the proximal portion of the root apex. The scarcity of the usual cytosketetal components within the statocytes is considered responsible for the exclusion of the larger organelles (e.g., nucleus, plastids, ER elements) from the interior of the cell and for the absence of cytoplasmic streaming. Furthermore, the depletion of dense EMT networks and AMF bundles in statocyte cytoplasm is suggested as being closely related to the elevated cytoplasmic calcium content of these cells which, in turn, may also favour the formation of the large sedimentable amyloplasts by not permitting plastid divisions. These latter organelles are proposed to act as statoliths due to their dynamic interactions with very fine and highly unstable AMFs which enmesh the statoliths and merge into peripheral AMFs-CMTs-ER-plasma membrane complexes. Rather indirect evidence for these interactions was provided by showing enhanced rates of statolith sedimentation after chemically-induced disintegration of CMTs. All these unique properties of the root cap statocytes are supposed to effectively enhance the gravity-perceptive function of these highly specialized cells.

Biology of plant cells in microgravity and under clinostating

Experimental data on plant cell reproduction, growth, and differentiation in spaceflight and under clinostating that partially reproduce the biological effects of microgravity are elucidated. The rearrangements of organelle structural and functional organization in unicellular plant organisms as well as in meristematic, differentiating, and differentiated cells of multicellular organisms in these conditions are considered. The focus is on the changes in the interrelations of prokaryotic and eukaryotic organisms under altered gravity. Ideas on the acceleration of differentiation and aging of cells in microgravity and clinostating and the organism's adaptive possibilities for carrying out its own functions are discussed.

Tissue slices from living root caps as a model system in which to study cytodifferentiation of polar cells

Tissue slices of living root caps of cress (Lepidium sativum L.), two to three cell layers in thickness, were prepared by a microsurgical procedure. The viability, cellular structures and cytoplasmic movement of the cells were examined in the light microscope. Nuclei, amyloplasts, vacuoles and endoplasmic reticulum were identified and their positions confirmed after fixation and observation of the same cells in the electron microscope. The distribution of microtubules was shown by immunocytochemistry. During germination, microtubules appear first at the distal edges of the statocytes, while in mature statocytes a distal domain of criss-crossed microtubules could be distinguished from a proximal domain with transversally oriented microtubules. Microfilaments in young statocytes form a nuclear enclosure; in mature statocytes bundles of microfilaments fan out into the cell cortex. The transition from statocytes to secretion cells is accompanied by a more pronounced cortical network of microfilaments, while the nucleus-associated microfilaments remain visible. It is suggested that these microfilaments play a role in the positioning of the nucleus and the translocation of endoplasmic reticulum.

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

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

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

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

Cytodifferentiation of polar plant cells: formation and turnover of endoplasmic reticulum in root statocytes

By application of cytochalasin D (10 micrograms/ml) the distribution and turnover of endoplasmic reticulum (ER) in root statocytes of cress (Lepidium sativum L.) was studied. After 7 min of incubation, the distal ER complex, in 20-h-old control statocytes consisting of stacked cisternae, was disintegrated and redistributed. The amyloplasts sedimented into the most distal part of the cell. When the incubation time was increased up to 4 h, ER was formed near the nucleus and accumulated at the proximal cell pole. Thus microfilaments are suggested to be involved in (i) stabilization of the distal ER complex and (ii) the ER translocation from the forming site into the distal cell pole. By morphometric measurements the volume of story II (story III) statocytes was calculated to be 2400 microns3 (3000 microns3), containing an ER area of 1824 microns2 (2400 microns2). From the net ER formation rate of 5.2 microns2/min (story II) and 4.6 microns2/min (story III) and the net decrease rate of 23 microns2/min (story II) and 39.2 microns2/min (story III), a total ER formation rate of about 27 microns2/min (story II) and 43 microns2/min (story III) was estimated.

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.

Cytodifferentiation of polar plant cells: use of anti-microtubular agents during the differentiation of statocytes from cress roots (Lepidium sativum L.)

The development of the structural polarity of statocytes from cress roots (Lepidium sativum L.) was studied in a time- and stage-dependent manner. outgrowing radicles had statocytes with abundant lipid droplets, sparsely developed endoplasmic reticulum (ER) and nuclei located at the proximal cell poles. During differentiation, coincidentally the lipid droplets disappeared while rough ER increased in length. The ER was translocated into the distal cell pole to establish a complex of stacked ER. Microtubules occurred first at the distal cell edges. As a second step, ER was produced in the vicinity of the nucleus and was also translocated distally. By application of the antimicrotubular agents heavy water (90%), colchicine (10(-4) mol l-1) and triethyl lead chloride (20 micromoles l-1), the involvement of microtubules in these events was studied. Triethyl lead chloride led to a complete cessation of differentiation; root-cap cells remained at a stage without polar arrangement of the ER. Colchicine affected the development of structural polarity slightly, as shown by a higher density of cortical ER cisternae. Heavy water inhibited the translocation of ER almost completely and yielded ER located also in the cell center. All anti-microtubular agents inhibited cell division and the differentiation of the distal cell layer of the dermatocalyptrogen into statocytes. It is hypothesized that microtubules serve as anchoring sites for microfilaments, which actually mediate the translocation of the ER. Hence, an intact system of microtubules and microfilaments is necessary for the expression of structural polarity.

Polarity of root statocytes--relevance for graviperception

During outgrowth of the radicle of cress (Lepidium sativum L.) the statocytes of the root cap develop a structural polarity with the nucleus at the proximal cell pole and a complex of endoplasmic reticulum (ER) at the distal cell pole. Amyloplasts sediment upon this complex of ER. During all stages of development the cytoskeleton (microtubules, microfilaments) is involved in positioning of the ER. The structural polarity of the statocytes develops independently of gravity, as indicated by corresponding results from fast and slow rotating clinostats and roots grown under microgravity in orbit. Disturbance of the structural polarity is possible by application of drugs, influencing microtubules and microfilaments. If, by rotation of roots on slow rotating clinostats or centrifugation, the structural polarity of the statocytes is changed, the ability of the roots to perceive gravity is affected also.

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

The effect of cytochalasin B (CB; 25 micrograms ml-1 in 1% dimethylsulfoxide, DMSO) upon the structural polarity of statocytes in cress roots is demonstrated. If normal, vertically grown roots are incubated in CB, the structural polarity of the statocytes is altered according to the developmental stage of the root. Statocytes from young roots (13 or 17 hours, additionally 7 hours CB) are characterized by proximal ER cisternae and a sparsely developed distal ER-complex. Statocytes from older roots (24 hours, additionally 7 hours CB) still accumulate distal ER, as in control roots, but at the proximal cell pole in the vicinity of the nucleus additional ER is found. These effects are reversed by washing out the drug in DMSO. Growth of the roots under a continuous supply of CB yields statocytes with sedimented nuclei, proximal ER and almost no distal ER. Together with quantitative data from morphometric studies, a dynamic model of the expression of inherent cell polarity in structural polarity is proposed.