Microtubule reorientation in shoots precedes bending during the gravitropic response of cut snapdragon spikes

The microtubule reorientation during the gravitropic bending of cut snapdragon (Antirrhinum majus L.) spikes was investigated. Using indirect immunofluorescence methods, we examined changes in microtubule orientation in the cortex, endodermis and pith tissues of the shoot bending zone, in response to gravistimulation. Our results show that dense microtubule arrays were visible throughout the cortical, endodermal and pith shoot tissues, and that the transverse orientation of the microtubules (perpendicular to the growth axis) was specifically associated with the shoot growing bending zone. Microtubules showed gravity-induced kinetics of changes in their orientation, which occurred only in the upper stem flank and preceded shoot bending. While this observation, that the gravity-induced microtubule orientation precedes bending, was previously reported only in special above-ground organs such as coleoptiles and hypocotyls, our present study is the first to show that such patterns of change occur in mature flowering shoots. These changes were exhibited first in the upper flank of the cortex and then in the upper flank of the endodermis. No changes in microtubule orientation were observed in the cortex or endodermis tissues of the lower flanks or in the pith, suggesting that these tissues continue to grow during shoot gravistimulation. Our results imply that microtubules may be involved in growth cessation of the upper shoot flank occurring during the gravitropic bending of snapdragon cut spikes.     

The microtubule cytoskeleton in hyphae ofUromyces phaseoli germlings: Its relationship to the region of nucleation and to the F-actin cytoskeleton

Summary The microtubule and F-actin cytoskeleton of nondifferentiated germlings ofUromyces phaseoli was studied using immunofluorescence methodologies. The microtubules were oriented mostly parallel to the longitudinal axis of the hypha. Microtubule depolymerizing agents, such as cold, demecolcine, griseofulvin and nocodazole, were effective in destroying the microtubule network, but not the F-actin system. Repolymerization of microtubules, following release from these agents, occurred first in the hyphal apices and not near the nuclei or spindle pole bodies. It was concluded that the microtubule nucleating region in such fungal cells is located in the apical regions. Enhanced microtubule arrays were visualized following incubation of the cells in taxol, an agent known to favor microtubule polymerization.     

Consistent handedness of microtubule helical arrays in maize and Arabidopsis primary roots

Summary The orientation of cortical microtubules in plant cells has been extensively studied, in part because of their influence on the expansion of most plant cell types. Cortical microtubules are often arranged in helical arrays, which are well known to occur with a specific pitch as a function of development or experimental treatment; however, it is not known if the handedness of helical arrays can also be specified. We have studied the handedness of helical arrays by using Vibratome sectioning of maize primary roots and confocal microscopy of Arabidopsis primary roots. In cortical cells of maize roots, the helical array was found to have the same handedness at a given position, not only for the cells of a single root, but also for the cells of more than one hundred roots examined. Quantification of angular distribution of apparent individual microtubules showed that defined regions of the root were composed of cells with highly uniform microtubule orientation. In the region between transverse and longitudinal microtubules (5–10.5 mm from the tip), the array formed a right-handed helix, and basal of cells with longitudinal microtubules (11.5–15 mm from the tip), the array formed a left-handed helix. Similarly, in epidermal cells of Arabidopsis roots right-handed helical arrays were found in the region between transverse and longitudinal microtubules. These results suggest that, in addition to the orientation of microtubules, the handedness of helical microtubule arrays is under cellular control.Abbreviations Cy3 indocarbocyanine - PBS phosphate-buffered saline - PIPES piperazine-N,N-bis-[2-ethanesulfonic acid]     

Characterization of the asymmetric growth of gravistimulated snapdragon spikes by stem and cell dimension analyses

Growth patterns of detached spikes of gravistimulated snapdragon (Antirrhinum majus L.) were analyzed in detail. The length increment of 5-mm marked subsections in the upper and lower flanks of the stem-bending zone was measured during gravistimulation using time-lapse photographs. At the onset of bending, a negative relative growth rate of the upper flank was detected, followed by increased relative growth rate in both lower and upper flanks. Consequently, a differential stem growth pattern was obtained during gravistimulation, which was significantly and specifically abolished by calcium antagonists reported previously to inhibit stem curvature of snapdragon. The differential growth patterns resulted from dynamic modifications of the cell dimensions in the epidermal and cortical stem layers. Bending started with both shrinking and widening of the epidermal cells and a parallel decrease in length and height of cortical cells at the upper stem flank. These changes were accompanied with a concomitant increase in length and height of the cortical cells on the lower stem flank, followed by a growth increase of epidermal cells. Our results suggest that both the epidermal and cortical cells play an important role in gravitropic shoot bending of snapdragon.     

Microtubule distribution in gravitropic protonemata of the mossCeratodon

Summary Tip cells of dark-grown protonemata of the mossCeratodon purpureus are negatively gravitropic (grow upward). They possess a unique longitudinal zonation: (1) a tip group of amylochloroplasts in the apical dome, (2) a plastid-free zone, (3) a zone of significant plastid sedimentation, and (4) a zone of mostly non-sedimenting plastids. Immunofluorescence of vertical cells showed microtubules distributed throughout the cytoplasm in a mostly axial orientation extending through all zones. Optical sectioning revealed a close spatial association between microtubules and plastids. A majority (two thirds) of protonemata gravistimulated for >20 min had a higher density of microtubules near the lower flank compared to the upper flank in the plastid-free zone. This apparent enrichment of microtubules occurred just proximal to sedimented plastids and near the part of the tip that presumably elongates more to produce curvature. Fewer than 5% of gravistimulated protonemata had an enrichment in microtubules near the upper flank, whereas 14% of vertical protonemata were enriched near one of the side walls. Oryzalin and amiprophos-methyl (APM) disrupted microtubules, gravitropism, and normal tip growth and zonation, but did not prevent plastid sedimentation. We hypothesize that a microtubule redistribution plays a role in gravitropism in this protonema. This appears to be the first report of an effect of gravity on microtubule distribution in plants.Abbreviations APM amiprophos-methyl - DIC differential interference contrast - DMSO dimethyl sulfoxide - EGTA ethylene glycolbis-(-amino-ethylether) N,N,N',N'-tetraacetic acid - FITC fluorescein isothiocyanate - GS gravitropic stimulus - MT microtubule - PIPES piperazine-N,N'-bis-2-ethanesulfonic acid     

甘蔗茎尖细胞有丝分裂过程中微管骨架的变化

利用改进的冰冻切片法结合间接免疫荧光标记技术对甘蔗茎尖细胞有丝分裂过程中微管骨架的变化进行了研究。结果表明, 在甘蔗茎尖细胞有丝分裂过程中存在4种循序变化的典型微管列阵,即周质微管、早前期微管带、纺锤体微管及成膜体微管。同时, 还观察到在各种典型微管列阵相互转变过程中存在各种微管列阵的过渡状态。甘蔗茎尖正在伸长的幼叶部位细胞的周质微管主要为与细胞伸长轴相垂直的横向周质微管; 茎尖幼叶部位伸长缓慢细胞的微管主要为纵向及斜向排列的周质微管,在甘蔗茎尖幼叶基部初生增粗分生组织处, 横向、斜向、纵向及随机排列的周质微管列阵均有分布。在少数分裂前期的细胞中, 发现细胞具有2条早前期微管带, 其具体功能还不清楚。表明甘蔗茎尖细胞微管列阵的变化与许多双子叶植物及部分单子叶植物具有共同的变化规律, 进一步证明微管骨架的周期性变化在植物中具有普遍性。     

甘蔗茎尖细胞有丝分裂过程中微管骨架的变化

利用改进的冰冻切片法结合间接免疫荧光标记技术对甘蔗茎尖细胞有丝分裂过程中微管骨架的变化进行了研究。结果表明,在甘蔗茎尖细胞有丝分裂过程中存在4种循序变化的典型微管列阵,即周质微管、早前期微管带、纺锤体微管及成膜体微管。同时,还观察到在各种典型微管列阵相互转变过程中存在各种微管列阵的过渡状态。甘蔗茎尖正在伸长的幼叶部位细胞的周质微管主要为与细胞伸长轴相垂直的横向周质微管：茎尖幼叶部位伸长缓慢细胞的微管主要为纵向及斜向排列的周质微管,在甘蔗茎尖幼叶基部初生增粗分生组织处,横向、斜向、纵向及随机排列的周质微管列阵均有分布。在少数分裂前期的细胞中,发现细胞具有2条早前期微管带,其具体功能还不清楚。表明甘蔗茎尖细胞微管列阵的变化与许多双子叶植物及部分单子叶植物具有共同的变化规律,进一步证明微管骨架的周期性变化在植物中具有普遍性。     

Taxol maintains organized microtubule patterns in protoplasts which lead to the resynthesis of organized cell wall microfibrils

Summary We have investigated the effects of microtubule stabilizing conditions upon microtubule patterns in protoplasts and developed a new method for producing protoplasts which have non-random cortical microtubule arrays. Segments of elongating pea epicotyl tissue were treated with the microtubule stabilizing drug taxol for 1 h before enzymatic digestion of the cell walls in the presence of the drug. Anti-tubulin immunofluorescence showed that 40 M taxol preserved regions of ordered microtubules. The microtubules in these regions were arranged in parallel arrays, although the arrays did not always show the transverse orientation seen in the intact tissue. Protoplasts prepared without taxol had microtubules which were random in distribution. Addition of taxol to protoplasts with random microtubule arrangements did not result in organized microtubule arrays. Taxol-treated protoplasts were used to determine whether or not organized microtubule arrays would affect the organization of cell wall microfibrils as new walls were regenerated. We found that protoplasts from taxol-treated tissue which were allowed to regenerate cell walls produced organized arrays of microfibrils whose patterns matched those of the underlying microtubules. Protoplasts from untreated tissue synthesized microfibrils which were disordered. The synthesis of organized microfibrils by protoplasts with ordered microtubules arrays shows that microtubule arrangements in protoplasts influence the arrangement of newly synthesized microfibrils.Abbreviations DIC differential interference contrast - DMSO dimethyl sulfoxide - FITC fluorescein isothiocyanate - IgG immunoglobulin G - PIPES piperazine-N,N-bis[2-ethane-sulfonic acid] - PBS phosphate buffered saline     

Heat stress affects the organization of microtubules and cell division in Nicotiana tabacum cells

To investigate the effects of heat stress on the plant cytoskeleton, the structure of microtubule arrays in N. tabacum suspension cells incubated at 38 or 42°C was analysed. Whilst incubation at 42 °C resulted in the disruption of the majority of cellular microtubules after 30 min, in cells exposed to 38 °C all the microtubule arrays were preserved even after 12 h of incubation, although their organization was altered. The most susceptible were the microtubules of the mitotic spindle and the phragmoplast. Several abnormalities were observed: (i) splitting of the spindle into several parts; (ii) elongation of the spindles; (iii) formation of microtubule asters in mitotic cells, and (iv) elongation of phragmoplast microtubules. Exposure of cells to 38 °C caused a decrease in the mitotic index but an accumulation of telophase cells. The recovery of normal microtubule organization occurred after 12 h. Treatment of the cells subjected to heat stress conditions with an inhibitor of protein synthesis, cycloheximide, did not prevent either the alterations of microtubule organization or accumulation of cells containing phragmoplasts. Therefore, heat shock proteins do not seem to be directly responsible for the microtubule disorganization induced by heat stress.     

The effect of brefeldin A on the redistribution of osmiophilic particles and the gravitropic response of rye coleoptiles

Summary The occurrence of IAA-inducible osmiophilic particles (OPs) in the periplasmic space of epidermal cells in the upper and lower flank (UF, LF) of gravistimulated rye coleoptile segments was analyzed employing brefeldin A (BFA) as an inhibitor of secretion at the plasma membrane. A 2 h horizontal gravistimulation of untreated samples caused a duplication of OPs in the periplasmic space of epidermal cells at the growth-inhibited UF as compared to the LF of upward bending coleoptile segments. In contrast to this, the number of OPs within the cytoplasm close to the plasma membrane of epidermal cells was similar at both flanks. BFA caused an inhibition of graviresponsive growth and prevented the occurrence of OPs in the periplasmic space of the epidermal cells of the UF and the LF. Likewise, growth of vertically oriented coleoptile segments was inhibited by BFA. Growth inhibition of both gravistimulated and control segments was accompanied by a twofold increase of the occurrence of cytoplasmic OPs. The results illustrate that the occurrence of OPs within the periplasmic space of the epidermal cells depends on secretion processes. Furthermore they provide evidence that their increased occurrence in the growth-inhibited UF during gravistimulation is due to their inhibited infiltration into the cell walls. We suggest that thereby wall loosening is temporarily prevented.Abbreviations BFA brefeldin A - LF lower flank - OP osmiophilic particle - UF upper flank Dedicated to Professor Dr. Dieter Klämbt on the occasion of his 65th birthday     

Morphology and Microtubule Organization in Arabidopsis Roots Exposed to Oryzalin or Taxol

In roots of Arabidopsis thaliana, we examined the effects oflow concentrations of microtubule inhibitors on the polarityof growth and on the organization of microtubule arrays. Intact6 d old seedlings were transplanted onto plates containing inhibitors,and sampled 12 h, 24 h and 48 h later. Oryzalin, a compoundthat causes microtubule depolymerization, stimulates the radialexpansion of roots. The amount of radial swelling is linearlyproportional to the logarithm of the oryzalin concentration,from the response threshold, 170 nM, to 1 µM. Cells inthe zone of division were slightly more sensitive to oryzalinthan were cells in the zone of pure elongation. Radial swellingis also stimulated by taxol, a compound that causes microtubulepolymerization. Taxol at 1 µM causes little swelling,but at 10µM causes extensive radial swelling of cellsin the elongation zone, and does not affect cells in the divisionzone. To examine the microtubules in these roots, we used methacrylatesections with immunofluorescence microscopy. At all concentrationsof oryzalin, cortical arrays are disorganized and depleted ofmicrotubules, and the microtubules themselves often appear fragmented.These effects increase in severity with concentration, but areunmistakable at 170 nM. In taxol, cortical arrays appear tobe more intensely stained than those of controls. At 10 µM,many cells in growing regions of the stele have longitudinalmicrotubules, whereas many cells in the cortex appear to havetransversely aligned microtubules. Taxol affects microtubulesin cells of division and elongation zones to the same extent,despite the observed difference in growth. We conclude thatthe precise, spatial pattern of cortical microtubules may notbe primarily responsible for controlling growth anisotropy;and that control over growth anisotropy may differ between dividingand non-dividing cells. (Received December 6, 1993; Accepted June 7, 1994)     

Spline and flagellar microtubules are resistant to mitotic disrupter herbicides

Summary Microtubule arrays in developing spermatogenous cells of pteridophytes have unique microtubule organizing centers and post-translation modifications of tubulin. Sensitivity of these arrays to the microtubule-destabilizing effects of the mitotic disrupter herbicides was examined by immunofluorescence, transmission and immunogold electron microscopy. Acetylated, stabilized arrays, such as the spline, and microtubules of the basal bodies and flagella are formed after the final mitotic division and are resistant to these herbicides. Non-acetylated, dynamic arrays that exist prior to the final mitosis, such as interphase and mitotic arrays, are eliminated by all of these herbicides, with symptomology (arrested prometaphase, lobed nuclei, irregular cell plate formation) similar to that observed in other land plants. The only exception to the instability of these mitotic microtubule arrays are the few microtubules that are collected by kinetochores into short tufts. The presence of structurally-distinguishable MTOCs, such as the blepharoplast, did not confer resistance, despite the anchoring of the minus ends of the microtubules. Simultaneous treatment with herbicide and 5-bromodeoxyuridine (BrdU), with subsequent detection with anti-BrdU of cells that had gone through S-phase during the BrdU incubation, reveals that only acetylated arrays formed prior to herbicide treatment are resistant. These data indicate that only actively polymerizing, dynamic microtubule arrays are sensitive to the destabilizing effects of the mitotic disrupter herbicides.Abbreviations MTOC microtubule organizing center - BrdU 5-bromodeoxyuridine     

冰冻切片法在植物微管骨架研究中的应用

介绍了冰冻切片法研究植物微管骨架的一般程序和技术上的一些改进,结果证明,改进的冰冻切片技术,可以对植物不同类型的细胞进行很好的标记。实验结果表明,甘蔗正在迅速伸长的幼叶分布的微管类型主要是与细胞伸长轴方向垂直的周质微管,幼叶基部尤其是第三幼叶基部分布的主要是与细胞伸长轴方向平行的周质微管。表明冰冻切片法在植物微管骨架的研究中具有广阔的应用前景。     

Ultrastructural effects of the herbicide chlorpropham (CIPC) in root tip cells of wheat

The present ultrastructural investigation on the effects of 50 M chlorpropham (previously called CIPC) on growing roots of wheat (Triticum aestivum (L.) Thell cv. Vergina) was undertaken to clarify the mechanism of a carbamate herbicide action in plant cells, since the wide range of responses of plant cells to carbamate herbicides is based mainly on immunofluorescence studies. Cells of control roots contained abundant microtubules both in interphase and mitotic arrays. In chlorpropham-treated roots, however, no microtubules could be detected at all, neither in dividing nor in differentiating cells. Cycling cells became binucleate, polyploid or contained incomplete cell walls, the result of inhibition of cytokinesis. In long-term drug treatments (24 h or more) the affected cells entered a new cycle, which, however, did not progress beyond mid-metaphase. The nuclei of binucleate cells initiated prophase synchronously. Small vacuoles and Golgi vesicles were trapped within the nucleoplasm of the multilobed nuclei. In roots recovering from 8 h chlorpropham treatment, cells continued to exhibit polyploid nuclei, intranuclear vacuoles and incomplete walls. Microtubules reappeared but they were sparse and lacked a definite orientation. Preprophase cells did not form normal preprophase bands of microtubules, while mitotic cells occasionally contained microtubules bound to chromosomes and converged to minipoles. It is concluded that chlorpropham disorganized directly microtubules in addition to irreversibly affecting microtubule organizing centres, which failed to further support microtubule arrays.     

Asymmetric CLASP-dependent nucleation of noncentrosomal microtubules at the trans-Golgi network

Proper organization of microtubule arrays is essential for intracellular trafficking and cell motility. It is generally assumed that most if not all microtubules in vertebrate somatic cells are formed by the centrosome. Here we demonstrate that a large number of microtubules in untreated human cells originate from the Golgi apparatus in a centrosome-independent manner. Both centrosomal and Golgi-emanating microtubules need gamma-tubulin for nucleation. Additionally, formation of microtubules at the Golgi requires CLASPs, microtubule-binding proteins that selectively coat noncentrosomal microtubule seeds. We show that CLASPs are recruited to the trans-Golgi network (TGN) at the Golgi periphery by the TGN protein GCC185. In sharp contrast to radial centrosomal arrays, microtubules nucleated at the peripheral Golgi compartment are preferentially oriented toward the leading edge in motile cells. We propose that Golgi-emanating microtubules contribute to the asymmetric microtubule networks in polarized cells and support diverse processes including post-Golgi transport to the cell front.     

The effect of low temperature on the microtubules in root meristem cells of spring and winter cultivars of wheat Triticum aestivum L

We have studied the response of the interphase and mitotis microtubule arrays in root meristem cells of spring and winter cultivars of wheat Triticum aestivum L. (Moskovskaya 35 and Moskovskaya 39) during cold stress (1 h at 0 degrees C) and acclimation to cold (3-48 h at 0 degrees C). Our data show that interphase microtubules are more resistant to cold than mitotic arrays in both cultivars. During cold stress the density of endoplasmic microtubules increases in interphase cells of winter plants, yet no changes are detected in cells of spring plants. In mitotic cells of both wheat cultivars the density of microtubules within the kinetochore fibers decreases, yet this effect is more evident in the cells of spring plants. During acclimation to cold of both cultivars, we have observed the disorganization of the interphase cortical arrays and the enhanced growth of endoplasmic microtubule arrays, composed of microtubule converging centers. However, the reaction of mitotic microtubule arrays differs in the cells of winter and spring plants. In winter plants, during prophase diffuse tubulin "halo" accumulates first at perinuclear area, followed by the appearance of the microtubule converging centers. In spring plants, we have observed the formation of the prophase spindle, yet later the prophase spindle is not detected. Metaphase cells of both cultivars show similar aberrations of the mitotic spindle, accumulation of abnormal metaphases and the excessive formation of microtubule converging centers. In telophase cells of both cultivars, acclimation induces similar reaction, resulting in the disorganization of the phragmoplast and the formation of multiple microtubule converging centers. The latter are detected in the perinuclear areas of the daughter cells in winter plants and in the cortical cytoplasm of cells in spring plants. Our data point to the common pathways of microtubule response to cold treatment (0 degrees C). The excessive formation of the microtubule converging centers indicates the activation of microtubule assembly during prolonged cold treatment.     

Basipetal propagation of gravity-induced surface pH changes along primary roots of <Emphasis Type="Italic">Lepidium sativum</Emphasis> L.

While there is ample evidence for a role of auxin in root gravitropism, the seeming rapidity of gravi-induced changes in electrical parameters has so far been an argument against auxin being a primary signal in gravitropic signal transmission. To address this problem, we re-investigated the effect of gravistimulation on membrane voltages of Lepidium sativum L. and Vigna mungo L. root cells. In our hands, gravistimulation did not induce changes in membrane voltage in cells of the root cap statenchyma, root meristem or apical elongation zone that can be correlated with the orientation of the cells relative to the gravity vector. While these results challenge a model of rapid electrically based signal transmission, there is evidence for a slower signal propagation along gravistimulated L. sativum roots. Using multiple proton-selective microelectrodes to simultaneously measure surface pH on opposite root flanks at different distances from the root tip, we observed gravi-induced asymmetric pH changes at the surface of all investigated root zones. Upon gravistimulation, the surface pH decreased on the physically upper root flank and increased on the lower flank. The pH asymmetry appeared first [2.1+/-0.4 min (mean +/- SD) after tilting] at the root cap and then - with incrementing lag times - at the meristem (after 2.5+/-0.3 min at 300 micro m from root tip; after 3.7+/-0.4 min at 700 micro m) and apical elongation zone (4.8+/-0.5 min at 1,000 micro m), suggesting a basipetal progression of differential surface acidification at a rate of 250-350 micro m min(-1), consistent with reported auxin transport rates.     

The contributions of microtubule stability and dynamic instability to adenovirus nuclear localization efficiency

Adenoviruses (Ads) utilize host cell microtubules to traverse the intracellular space and reach the nucleus in a highly efficient manner. Previous studies have shown that Ad infection promotes the formation of stable, posttranslationally modified microtubules by a RhoA-dependent mechanism. Ad infection also shifts key parameters of microtubule dynamic instability by a Rac1-dependent mechanism, resulting in microtubules with lower catastrophe frequencies, persistent growth phases, and a bias toward net growth compared to microtubules in uninfected cells. Until now it was unclear whether changes in RhoGTPase activity or microtubule dynamics had a direct impact on the efficiency of Ad microtubule-dependent nuclear localization. Here we have performed synchronous Ad infections and utilized confocal microscopy to analyze the individual contributions of RhoA activation, Rac1 activation, microtubule stability, dynamic behavior, and posttranslational modifications on Ad nuclear localization efficiency (NLE). We found that drug-induced suppression of microtubule dynamics impaired Ad NLE by disrupting the radial organization of the microtubule array. When the microtubule array was maintained, the suppression or enhancement of microtubule turnover did not significantly affect Ad NLE. Furthermore, RhoA activation or the formation of acetylated microtubules did not enhance Ad NLE. In contrast, active Rac1 was required for efficient Ad nuclear localization. Because Rac1 mediates persistent growth of microtubules to the lamellar regions of cells, we propose that Ad-induced activation of Rac1 enhances the ability of microtubules to "search and capture" incoming virus particles.     

Differential proton secretion in the apical elongation zone caused by gravistimulation is induced by a signal from the root cap

The extracellular proton activity along primary roots of Phleum pratense L. was measured using proton-selective microelectrodes. Removal of the root cap caused a reduction of the proton influx in the transitional region between the meristem and the apical elongation zone of the vertical root and inhibited the development of pH differences between the physically upper and lower flanks of the gravistimulated root. Disruption of the actin filament system of the root with 5 mmol m-3 cytochalasin D did not result in an altered proton flux and pH pattern compared with untreated vertical control roots, but inhibited the gravity-induced development of pH differences between the physically upper and lower root flanks as well as gravitropic curvature. These results provide evidence that pH changes following gravistimulation are induced by a signal transmitted from the root cap and that the actin filament system is involved in the gravity perception/transduction mechanism.     

Microtubule reorganization in tobacco BY-2 cells stably expressing GFP-MBD

 Microtubule organization plays an important role in plant morphogenesis; however, little is known about how microtubule arrays transit from one organized state to another. The use of a genetically incorporated fluorescent marker would allow long-term observation of microtubule behavior in living cells. Here, we have characterized a Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cell line that had been stably transformed with a gfp-mbd construct previously demonstrated to label microtubules (J. Marc et al., 1998, Plant Cell 10: 1927–1939). Fluorescence levels were low, but interphase and mitotic microtubule arrays, as well as the transitions between these arrays, could be observed in individual gfp-mbd-transformed cells. By comparing several attributes of transformed and untransformed cells it was concluded that the transgenic cells are not adversely affected by low-level expression of the transgene and that these cells will serve as a useful and accurate model system for observing microtubule reorganization in vivo. Indeed, some initial observations were made that are consistent with the involvement of motor proteins in the transition between the spindle and phragmoplast arrays. Our observations also support the role of the perinuclear region in nucleating microtubules at the end of cell division with a progressive shift of these microtubules and/or nucleating activity to the cortex to form the interphase cortical array. Received: 2 June 1999 / Accepted: 13 August 1999