Electron tomographic analysis of somatic cell plate formation in meristematic cells of Arabidopsis preserved by high-pressure freezing

We have investigated the process of somatic-type cytokinesis in Arabidopsis (Arabidopsis thaliana) meristem cells with a three-dimensional resolution of approximately 7 nm by electron tomography of high-pressure frozen/freeze-substituted samples. Our data demonstrate that this process can be divided into four phases: phragmoplast initials, solid phragmoplast, transitional phragmoplast, and ring-shaped phragmoplast. Phragmoplast initials arise from clusters of polar microtubules (MTs) during late anaphase. At their equatorial planes, cell plate assembly sites are formed, consisting of a filamentous ribosome-excluding cell plate assembly matrix (CPAM) and Golgi-derived vesicles. The CPAM, which is found only around growing cell plate regions, is suggested to be responsible for regulating cell plate growth. Virtually all phragmoplast MTs terminate inside the CPAM. This association directs vesicles to the CPAM and thereby to the growing cell plate. Cell plate formation within the CPAM appears to be initiated by the tethering of vesicles by exocyst-like complexes. After vesicle fusion, hourglass-shaped vesicle intermediates are stretched to dumbbells by a mechanism that appears to involve the expansion of dynamin-like springs. This stretching process reduces vesicle volume by approximately 50%. At the same time, the lateral expansion of the phragmoplast initials and their CPAMs gives rise to the solid phragmoplast. Later arriving vesicles begin to fuse to the bulbous ends of the dumbbells, giving rise to the tubulo-vesicular membrane network (TVN). During the transitional phragmoplast stage, the CPAM and MTs disassemble and then reform in a peripheral ring phragmoplast configuration. This creates the centrifugally expanding peripheral cell plate growth zone, which leads to cell plate fusion with the cell wall. Simultaneously, the central TVN begins to mature into a tubular network, and ultimately into a planar fenestrated sheet (PFS), through the removal of membrane via clathrin-coated vesicles and by callose synthesis. Small secondary CPAMs with attached MTs arise de novo over remaining large fenestrae to focus local growth to these regions. When all of the fenestrae are closed, the new cell wall is complete. Few endoplasmic reticulum (ER) membranes are seen associated with the phragmoplast initials and with the TVN cell plate that is formed within the solid phragmoplast. ER progressively accumulates thereafter, reaching a maximum during the late PFS stage, when most cell plate growth is completed.     

Roles for kinesin and myosin during cytokinesis

Cytokinesis in higher plants involves the phragmoplast, a complex cytoplasmic structure that consists of microtubules (MTs), microfilaments (MFs) and membrane elements. Both MTs and MFs are essential for cell plate formation, although it is not clear which motor proteins are involved. Some candidate processes for motor proteins include transport of Golgi vesicles to the plane of the cell plate and the spatiotemporal organization of the cytoskeletal elements in order to achieve proper deposition and alignment of the cell plate. We have focused on the kinesin-like calmodulin binding protein (KCBP) and, more broadly, on myosins. Using an antibody that constitutively activates KCBP, we find that this MT motor, which is minus-end directed, contributes to the organization of the spindle and phragmoplast MTs. It does not participate in vesicle transport; rather, because of the orientation of the phragmoplast MTs, it is supposed that plus-end kinesins fill this role. Myosins, on the other hand, based on their inhibition with 2,3-butanedione monoxime and 1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine (ML-7), are associated with the process of post-mitotic spindle/phragmoplast alignment and with late lateral expansion of the cell plate. They are also not the principal motors involved in vesicle transport.     

The Arabidopsis HINKEL gene encodes a kinesin-related protein involved in cytokinesis and is expressed in a cell cycle-dependent manner

Plant cytokinesis starts in the center of the division plane, with vesicle fusion generating a new membrane compartment, the cell plate, that subsequently expands laterally by continuous fusion of newly arriving vesicles to its margin. Targeted delivery of vesicles is assisted by the dynamic reorganization of a plant-specific cytoskeletal array, the phragmoplast, from a solid cylinder into an expanding ring-shaped structure. This lateral translocation is brought about by depolymerization of microtubules in the center, giving way to the expanding cell plate, and polymerization of microtubules along the edge. Whereas several components are known to mediate cytokinetic vesicle fusion [8-10], no gene function involved in phragmoplast dynamics has been identified by mutation. Mutations in the Arabidopsis HINKEL gene cause cytokinesis defects, such as enlarged cells with incomplete cell walls and multiple nuclei. Proper targeting of the cytokinesis-specific syntaxin KNOLLE [8] and lateral expansion of the phragmoplast are not affected. However, the phragmoplast microtubules appear to persist in the center, where vesicle fusion should result in cell plate formation. Molecular analysis reveals that the HINKEL gene encodes a plant-specific kinesin-related protein with a putative N-terminal motor domain and is expressed in a cell cycle-dependent manner similar to the KNOLLE gene. Our results suggest that HINKEL plays a role in the reorganization of phragmoplast microtubules during cell plate formation.     

Fine structure studies on phragmoplast and cell plate formation

Formation and development of phragmoplast and cell plate were studied in endosperm of Haemanthus katherinae Bak. The same cells were studied with the light and electron microscope. Several cells were studied with time-lapse microcinematography before fixation. This permitted comparison of structures during their development on both the light and electron microscope level. Movements of fibrillar components of the phragmoplast and spindle were analyzed and their transport properties were correlated with formation of the cell plate. Change of arrangement of microtubules, transport of vesicles which form the cell plate, and formation of vesicles and microtubules has also been discussed.     

Accumulation of F-actin during cytokinesis inAllium. Correlation with microtubule distribution and the effects of drugs

Summary The distribution of F-actin in the phragmoplast/cell plate complex of formaldehyde-fixedAllium root cells was visualized with rhodaminephalloidin (RP). Increased RP fluorescence appears in late anaphase in a broad zone between separating chromosomes. The fluorescence is mostly amorphous in appearance and does not resemble the distinct actin fibers seen in interphase cells. The actin becomes more concentrated near the midplane by telophase and takes the form of a relatively bright layer of fluorescence adjacent to the forming cell plate. This distribution differs markedly from that of phragmoplast microtubules (MTs) which extend back from the plate toward the daughter nuclei. F-actin continues to accumulate in new parts of the expanding phragmoplast, while RP fluorescence gradually decreases near older portions of the plate. It disappears completely near the new wall in most interphase cells. Treatment of root tips with cytochalasin B or D before fixation markedly reduces RP fluorescence, but phragmoplast MTs remain. Colchicine or oryzalin treatment leads to the disappearance of both phragmoplast actin and MTs. The possible function of actin in the phragmoplast/cell plate complex is discussed.Abbreviations CB cytochalasin B - CD cytochalasin D - CIPC isopropyl N-(3-chlorophenyl-)carbamate - DIC differential interference contrast - MT microtubule - PBS phosphate buffered saline - PM plasmalemma - RP rhodamine-phalloidin     

Microtubule organization during successive microsporogenesis in Allium cepa and simultaneous cytokinesis in Nicotiana tabacum

Microtubule cytoskeleton organization during microspore mother cell (MMC) meiosis in Allium cepa L. and microsporogenesis in Nicotiana tabacum L. was examined. The MMC microtubules (MTs) were short and well dispersed in the cytoplasm of both taxa. As the MMCs of both species entered metaphase of meiosis I, the MTs constructed a spindle that facilitated the chromosomes to orient in the meridian plane. At anaphase of meiosis I, the spindle MTs differentiated into two types: one MT type became short, pulled the chromosomes toward the two poles, and was designated as centromere MTs; the second type of MT connected the two poles, and was designated as pole MTs. In A. cepa, where successive cytokinesis was observed, pole MTs assumed a tubbish shape. Some new short MTs aggregated in the meridian plane and constricted to form a phragmoplast, which developed into a cell plate, divided the cytoplasm into two parts and produced a dyad. However, in tobacco, a phragmoplast was not generated in anaphase of meiosis I and II and cytokinesis did not occur. The spindle MTs depolymerized and reorganized the radial arrangement of MTs from the nucleate surface to the periplasm during anaphase. Following telophase of meiosis II, the cytoplasm produced centripetal furrows, which met in the center of the cell and divided it into four parts, serving as a form of cytokinesis. In this process, MTs appeared to bear no relationship to cytokinesis.     

Quantitative analysis of changes in actin microfilament contribution to cell plate development in plant cytokinesis

Background  

Plant cells divide by the formation of new cross walls, known as cell plates, from the center to periphery of each dividing cell. Formation of the cell plate occurs in the phragmoplast, a complex structure composed of membranes, microtubules (MTs) and actin microfilaments (MFs). Disruption of phragmoplast MTs was previously found to completely inhibit cell plate formation and expansion, indicative of their crucial role in the transport of cell plate membranes and materials. In contrast, disruption of MFs only delays cell plate expansion but does not completely inhibit cell plate formation. Despite such findings, the significance and molecular mechanisms of MTs and MFs remain largely unknown.     

Caffeine inhibition of cytokinesis: effect on the phragmoplast cytoskeleton in livingTradescantia stamen hair cells

Summary The distribution of microtubules and actin microfilaments during caffeine-induced inhibition of cell plate formation has been studied in livingTradescantia stamen hair cells. Previous studies have shown that caffeine allows cell plate initiation but prevents its completion, resulting in binucleate cells. In the present study, confocal microscopy of cells microinjected with fluorescent brain tubulin or phalloidin, and cultured in the presence 5 mM caffeine, revealed that the initiation and early lateral expansion phase of the phragmoplast occur normally. However, caffeine completely inhibits the formation of the cytoskeletal torus which occurs in untreated cells during the late stages of cell plate and phragmoplast expansion. Caffeine further causes the disintegration of the incomplete cell plate. The results allow us to distinguish two phases in cell plate and phragmoplast growth: the initiation and early expansion phase, which is not affected by caffeine, and the late lateral expansion phase, which is completely inhibited in the presence of caffeine. Also in this study, the use of a high phalloidin concentration has revealed structural detail about the actin microfilaments involved in cell plate formation: microfilaments are observed that link the expanding edge of the phragmoplast with the cortical division site. In addition, cortical actin patches are observed within the actin depleted zone that might play a role in guidance of phragmoplast and cell plate expansion.     

Cytoskeletal dynamics in living plant cells

Microtubules and microfilaments have been imaged in living plant cells and their dynamic changes recorded during division, growth and development. Carboxyfluorescein labeled brain tubulin has been injected into cells that are maintained in an active state in a culture chamber on the microscope stage. Subsequent imaging with the confocal microscope reveals microtubules in the preprophase band, the mitotic apparatus, the phragmoplast, and the cortical array. The structural changes of these microtubules have been observed during transitional stages. In addition, their dynamic features are demonstrated by depolymerization in elevated calcium, low temperature, and in the drug oryzalin, and by repolymerization when returned to normal conditions. Examination of living Tradescantia stamen hair cells, which have been injected with fluorescent phalloidin to label the actin microfilaments, reveals hitherto undisclosed aspects of the preparation of the division site and dynamics of the phragmoplast cytoskeleton. During prophase microfilaments occur throughout the cell cortex, with those in the region of the preprophase band becoming transversely aligned. At nuclear envelope breakdown, these specifically disassemble, leaving a circumferential zone from which microfilaments remain absent throughout division. During cytokinesis microfilaments arise within the phragmoplast, oriented parallel to the microtubules, but excluded from the zone where the MTs overlap and where cell plate vesicles aggregate. The phragmoplast microfilaments, in a manner similar to microtubules, shorten in length, expand in girth, and eventually disassemble when the cell plate is complete.     

A Novel Plant Kinesin-Related Protein Specifically Associates with the Phragmoplast Organelles

In higher plants, the formation of the cell plate during cytokinesis requires coordinated microtubule (MT) reorganization and vesicle transport in the phragmoplast. MT-based kinesin motors are important players in both processes. To understand the mechanisms underlying plant cytokinesis, we have identified AtPAKRP2 (for Arabidopsis thaliana phragmoplast-associated kinesin-related protein 2). AtPAKRP2 is an ungrouped N-terminal motor kinesin. It first appeared in a punctate pattern among interzonal MTs during late anaphase. When the phragmoplast MT array appeared in a mirror pair, AtPAKRP2 became more concentrated near the division site, and additional signal could be detected elsewhere in the phragmoplast. In contrast, the previously identified AtPAKRP1 protein is associated specifically with bundles of MTs in the phragmoplast at or near their plus ends. Localization of the tobacco homolog(s) of AtPAKRP2 was altered by treatment of brefeldin A in BY-2 cells. We discuss the possibility that AtPAKRP1 plays a role in establishing and/or maintaining the phragmoplast MT array, and AtPAKRP2 may contribute to the transport of Golgi-derived vesicles in the phragmoplast.     

Monoclonal antibodies to a fern spermatozoid detect novel components of the mitotic and cytokinetic apparatus in higher plant cells

Summary The aim of this study was to search for uncharacterized components of the plant cytoskeleton using monoclonal antibodies raised against spermatozoids of the fernPteridium (Marc et al. 1988). The cellular distribution of crossreacting immunoreactive material during the division cycle in wheat root tip cells was determined by immunofluorescence microscopy and compared to the fluorescence pattern obtained with antitubulin. Five antibodies are of special interest. Pas1D3 and Pas5F4 detect a diffuse cytoplasmic material, which, during mitosis, follows the distribution of microtubules (MTs) at the nuclear surface and in the preprophase band (PPB), spindle and phragmoplast. The immunoreactive material codistributes specifically with MT arrays of the mitotic apparatus and does not associate with interphase cortical MTs. Pas5D8 is relevant to the PPB and spatial control of cytokinesis. It binds in a thin layer at the cytoplasmic surface throughout the cell cycle, except when its coverage is transiently interrupted by an exclusion zone at the PPB site and later at the same site when the phragmoplast fuses with the parental cell wall.Pas2G6 reacts with a component of basal bodies and the flagellar band in thePteridium spermatozoid and recognizes irregularly shaped cytoplasmic vesicles in wheat cells. During interphase these particles form a cortical network.Pas6D7 binds to dictyosomes and dictyosome vesicles. At anaphase the vesicles accumulate at the equator and subsequently condense into the cell plate.Abbreviations MT microtubule - PPB preprophase band     

Cell division in leaves ofNicotiana

Summary Young leaves ofNicotiana tabacum were fixed in glutaraldehyde-formaldehyde followed by osmium tetroxide. The fine structure of dividing cells was studied. Before prophase a band of microtubules was observed between the nucleus and the cell wall at a position judged as the future plane of division. The microtubules in the band are 4–6 units deep and relatively closely packed, giving sections of the band a characteristic appearance. Micro-tubules of the mitotic spindle, the phragmoplast, and the preprophase band are morphologically similar. Some of the microtubules of the mitotic spindle and the phragmoplast have an undulate appearance. It is suggested that the undulate microtubules may have been fixed at a time when microwaves were traveling along them. The cell plate is formed by a fusion of small smooth surfaced vesicles and small coated vesicles. Fusion of small vesicles results first in larger vesicles and then in a meshwork of new cell-wall material surrounded by new regions of plasma membrane. Most of the vesicles are derived from dictyosomes and may be produced before and during prophase as well as during later stages of division. The ER may also contribute some vesicles to the cell plate.     

AtMAC stabilizes the phragmoplast by crosslinking microtubules and actin filaments during cytokinesis

The phragmoplast, a structure crucial for the completion of cytokinesis in plant cells, is composed of antiparallel microtubules (MTs) and actin filaments (AFs). However, how the parallel structure of phragmoplast MTs and AFs is maintained, especially during centrifugal phragmoplast expansion, remains elusive. Here, we analyzed a new Arabidopsis thaliana MT and AF crosslinking protein (AtMAC). When AtMAC was deleted, the phragmoplast showed disintegrity during centrifugal expansion, and the resulting phragmoplast fragmentation led to incomplete cell plates. Overexpression of AtMAC increased the resistance of phragmoplasts to depolymerization and caused the formation of additional phragmoplasts during cytokinesis. Biochemical experiments showed that AtMAC crosslinked MTs and AFs in vitro, and the truncated AtMAC protein, N-CC1, was the key domain controlling the ability of AtMAC. Further analysis showed that N-CC1(51–154) is the key domain for binding MTs, and N-CC1(51–125) for binding AFs. In conclusion, AtMAC is the novel MT and AF crosslinking protein found to be involved in regulation of phragmoplast organization during centrifugal phragmoplast expansion, which is required for complete cytokinesis.     

Dynamics of phragmoplastin in living cells during cell plate formation and uncoupling of cell elongation from the plane of cell division.

The cell plate is formed by the fusion of Golgi apparatus-derived vesicles in the center of the phragmoplast during cytokinesis in plant cells. A dynamin-like protein, phragmoplastin, has been isolated and shown to be associated with cell plate formation in soybean by using immunocytochemistry. In this article, we demonstrate that similar to dynamin, phragmoplastin polymerizes to form oligomers. We fused soybean phragmoplastin with the green fluorescence protein (GFP) and introduced it into tobacco BY-2 cells to monitor the dynamics of early events in cell plate formation. We demonstrate that the chimeric protein is functional and targeted to the cell plate during cytokinesis in transgenic cells. GFP-phragmoplastin was found to appear first in the center of the forming cell plate, and as the cell plate grew outward, it redistributed to the growing margins of the cell plate. The redistribution of phragmoplastin may require microtubule reorganization because the microtubule-stabilizing drug taxol inhibited phragmoplastin redistribution. Our data show that throughout the entire process of cytokinesis, phragmoplastin is concentrated in the area in which membrane fusion is active, suggesting that phragmoplastin participates in an early membrane fusion event during cell plate formation. Based on the dynamics of GFP-phragmoplastin, it appears that the process of cell plate formation is completed in two phases. The first phase is confined to the cylinder of the phragmoplast proper and is followed by a second phase that deposits phragmoplast vesicles in a concentric fashion, resulting in a ring of fluorescence, with the concentration of vesicles being higher at the periphery. In addition, overexpression of GFP-phragmoplastin appears to act as a dominant negative, slowing down the completion of cell plate formation, and often results in an oblique cell plate. The latter appears to uncouple cell elongation from the plane of cell division, forming twisted and elongated cells with longitudinal cell divisions.     

Action of taxol on mitosis: modification of microtubule arrangements and function of the mitotic spindle in Haemanthus endosperm

We have studied the effect of taxol on mitosis in Haemanthus endosperm. Immuno-Gold Stain (IGS), a new immunocytochemical method (17), was used to visualize microtubules (MTs) in the light microscope. Observations on MT arrangements were correlated with studies in vivo. Chromosome movements are affected in all stages of mitosis which progresses over at least 10(4) range of taxol concentrations. The three most characteristic effects on MTs are: (a) enhancement of the lateral associations between MTs, seen especially during the reorganization of the polar region of the spindle, (b) promotion of MT assembly, leading to the formation of additional MTs in the spindle and MT arrays in the cytoplasm, and (c) an increase in MT stability, demonstrated in their increased cold resistance. In this report, the emphasis is on the primary, immediate effects, occurring in the first 30 min of taxol action. Effects are detected after a few mins, are reversible, and are concentration/time dependent. The spindle and phragmoplast are remarkably modified due to the enhancement of lateral associations of MTs and the formation of abundant nonkinetochore and polar, asterlike MTs. The equatorial region of the interzone in anaphase may be entirely depleted of MTs, and the spindle may break perpendicular to the spindle axis. Mitosis is completed in these conditions, providing evidence for the motile autonomy of each half-spindle. Trailing chromosome arms in anaphase are often stretched and broken. Chromosome fragments are transported away from the polar regions, i.e., in the direction opposite to that expected (5, 6). This supplies the first direct evidence of pushing by elongating MTs in an anastral higher plant spindle. These observations draw attention to the relation between the lateral association of MT ends to assembly/disassembly and to the role of such an interaction in spindle function and organization.     

Synthetic lipid (DOPG) vesicles accumulate in the cell plate region but do not fuse

The cell plate is the new cell wall, with bordering plasma membrane, that is formed between two daughter cells in plants, and it is formed by fusion of vesicles (approximately 60 nm). To start to determine physical properties of cell plate forming vesicles for their transport through the phragmoplast, and fusion with each other, we microinjected fluorescent synthetic lipid vesicles that were made of 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DOPG) into Tradescantia virginiana stamen hair cells. During interphase, the 60-nm wide DOPG vesicles moved inside the cytoplasm comparably to organelles. During cytokinesis, they were transported through the phragmoplast and accumulated in the cell plate region together with the endogenous vesicles, even inside the central cell plate region. Because at this stage microtubules are virtually absent from that region, while actin filaments are present, actin filaments may have a role in the transport of vesicles toward the cell plate. Unlike the endogenous vesicles, the synthetic DOPG vesicles did not fuse with the developing cell plate. Instead, they redistributed into the cytoplasm of the daughter cells upon completion of cytokinesis. Because the redistribution of the vesicles occurs when actin filaments disappear from the phragmoplast, actin filaments may be involved in keeping the vesicles inside the developing cell plate region.     

Phragmoplasts in the Absence of Nuclear Division

All land plants (embryophytes) use a phragmoplast for cytokinesis. Phragmoplasts are distinctive cytoskeletal structures that are instrumental in the deposition of new walls in both vegetative and reproductive phases of the life cycle. In meristems, the phragmoplast is initiated among remaining non-kinetochore spindle fibers between sister nuclei and expands to join parental walls at the site previously marked by the preprophase band of microtubules (PPB). The microtubule cycle and cell cycle are closely coordinated: the hoop-like cortical microtubules of interphase are replaced by the PPB just prior to prophase, the PPB disappears as the spindle forms, and the phragmoplast mediates cell plate deposition after nuclear division. In the reproductive phase, however, cortical microtubules and PPBs are absent and cytokinesis may be uncoupled from the cell cycle resulting in multinucleate cells (syncytia). Minisyncytia of 4 nuclei occur in microsporocytes and several (typically 8) nuclei occur in the developing megagametophyte. Macrosyncytia with thousands of nuclei may occur in the nuclear type endosperm. Cellularization of syncytia involves formation of adventitious phragmoplasts at boundaries of nuclear-cytoplasmic domains (NCDs) defined by radial microtubule systems (RMSs) emanating from non-sister nuclei. Once initiated in the region of microtubule overlap at interfaces of opposing RMSs, the adventitious phragmoplasts appear structurally identical to interzonal phragmoplasts. Phragmoplasts are constructed of multiple opposing arrays similar to what have been termed microtubule converging centers. The individual phragmoplast units are distinctive fusiform bundles of anti-parallel microtubules bisected by a dark mid-zone where vesicles accumulate and fuse into a cell plate.     

Centrin homologues in higher plants are prominently associated with the developing cell plate

Summary Centrin and calmodulin are members of the EF-hand calcium-binding superfamily of proteins. In this study we compared localisation and immunoblotting of centrin with calmodulin in several monocot (onion and wheat) and dicot (mung bean andArabidopsis) plants. We confirmed that an anti-calmodulin antibody recognised a 17 kDa protein in all species tested and localises to the cytoplasm, mitotic matrix and with microtubules of the preprophase band and phragmoplast. In contrast, immunoblotting using anti-centrin antibodies shows that plant centrins vary from 17 to 20 kDa. Immunofluorescence microscopy with anti-centrin antibodies revealed only weak centrin immunoreactivity in the cytoplasm, nucleus, nuclear envelope, phragmoplast and mitotic matrix in meristematic cells. There was a slightly more intense perinuclear labelling in large differentiating onion cells and between separating anaphase chromosomes. While centrin is known to localise to the mitotic spindle poles in animal and algal cells, there was no appreciable immunoreactivity at the spindle poles in higher plants. In contrast, there was an intense immunofluorescence signal with anti-centrin antibodies in the developing cell plate. Further characterisation of the cell plate labelling by immunogold electron microscopy shows centrin immunoreactivity was closely associated with vesicles in the cell plate. Our observations suggest that centrin may play a role in cell plate formation.Abbreviations BSA bovine serum albumin - MTs microtubules - MTOCs microtubule organising centres - PBS phosphate buffered saline - PBST phosphate buffered saline with Tween-20     

Meiotic cytokinetic apparatus in the formation of the linear spore tetrads of Conocephalum japonicum (Bryophyta)

Most bryophytes produce tetrahedral spore tetrads. However, linear spore tetrads have been reported to occur in Conocephalum japonicum (Thunb.) Grolle. In this study, the distribution of microtubules (MTs) during meiosis in C. japonicum was examined to determine the division pattern resulting in a linear tetrad. Spore mother cells in the pre-meiotic stage were cylindrical with randomly distributed cytoplasmic MTs. In the prophase-metaphase transition, spindle MTs replaced cytoplasmic MTs and a barrel-shaped spindle with two flattened poles developed. Cortical MT arrays were not detectable throughout meiosis. Although a phragmoplast appeared between sister nuclei in telophase-I, it disappeared without expanding to the parental cell wall. Metaphase-II spindles oriented parallel to the long axis of the cell and in tandem to each other resulted in a linear arrangement of telophase nuclei. Radial arrays of MTs developed from the nuclear surfaces and three phragmoplasts appeared among the four nuclei to produce four spores. Two phragmoplasts separating the paired sister nuclei appeared prior to the appearance of a phragmoplast between non-sister nuclei. The MT cycle is basically the same as that reported in meiosis of C. conicum, which produces non-linear tetrads. A morphometric study indicated that the difference in the division pattern between C. conicum and C. japonicum is due to a difference in the shape of spore mother cells. The cylindrical shape of sporocytes of C. japonicum restricts the orientation of spindles and phragmoplasts so that the four resultant spores are arranged linearly. Received: 22 April 1998 / Accepted: 15 May 1998     

Formation and function of phragmoplast during successive cytokinesis stages in higher plant meiosis

Cytoskeletal rearrangements were studied during meiotic telophase in a number of monocotyledonous plant species. Wild type and abnormal meiosis (in wide cereal hybrids, meiotic mutants and allolines) was analyzed. It was found that central spindle fibers that move centrifugally, along with newly-formed MTs, are the basis of phragmoplast formation and function in PMCs of monocotyledonous plant species with successive cytokinesis stages. A model for centrifugal movement of the meiotic phragmoplast is proposed; this model is a modification of the corresponding process during B-anaphase.