The Role of the Ameiotic1 Gene in the Initiation of Meiosis and in Subsequent Meiotic Events in Maize

Understanding the initiation of meiosis and the relationship of this event with other key cytogenetic processes are major goals in studying the genetic control of meiosis in higher plants. Our genetic and structural analysis of two mutant alleles of the ameiotic1 gene (am1 and am1-praI) suggest that this locus plays an essential role in the initiation of meiosis in maize. The product of the ameiotic1 gene affects an earlier stage in the meiotic sequence than any other known gene in maize and is important for the irreversible commitment of cells to meiosis and for crucial events marking the passage from premeiotic interphase into prophase I including chromosome synapsis. It appears that the period of ameiotic1 gene function in meiosis at a minimum covers the interval from some point during premeiotic interphase until the early zygotene stage of meiosis. To study the interaction of genes in the progression of meiosis, several double meiotic mutants were constructed. In these double mutants (i) the ameiotic1 mutant allele was brought together with the meiotic mutation (afd1) responsible for the fixation of centromeres in meiosis; and with the mutant alleles of the three meiotic genes that control homologous chromosome segregation (dv1, ms43 and ms28), which impair microtubule organizing center organization, the orientation of the spindle fiber apparatus, and the depolymerization of spindle filaments after the first meiotic division, respectively; (ii) the afd1 mutation was combined with two mutations (dsy1 and as1) affecting homologous pairing; (iii) the ms43 mutation was combined with the as1, the ms28 and the dv1 mutations; and (iv) the ms28 mutation was combined with the dv1 mutation and the ms4 (polymitotic1) mutations. An analysis of gene interaction in the double mutants led us to conclude that the ameiotic1 gene is epistatic over the afd1, the dv1, the ms43 and the ms28 genes but the significance of this relationship requires further analysis. The afd gene appears to function from premeiotic interphase throughout the first meiotic division, but it is likely that its function begins after the start of the ameiotic1 gene expression. The afd1 gene is epistatic over the two synaptic mutations dsy1 and as1 and also over the dv1 mutation. The new ameiotic*-485 and leptotene arrest*-487 mutations isolated from an active ROBERTSON's Mutator stocks take part in the control of the initiation of meiosis.     

Minispindles and cytoplasmic domains in microsporogenesis of orchids

Summary Changes in the microtubular cytoskeleton during meiosis and cytokinesis in hybrid moth orchids were studied by indirect immunofluorescence. Lagging chromosomes not incorporated into telophase nuclei after first meiotic division behave as small extra nuclei. Events in the microtubular cycle associated with these micronuclei are similar to and synchronous with those of the principal nuclei. During second meiotic division the micronuclei trigger formation of minispindles which are variously oriented with respect to the two principal spindles. After meiosis, radial systems of microtubules measure cytoplasmic domains around each nucleus in the coenocyte. Cleavage planes are established in regions where opposing radial arrays interact and the cytoplasm cleaved around micronuclei is proportionately smaller than that around the four principal nuclei. These observations clearly demonstrate that nuclei in plant cells are of fundamental importance in microtubule organization and provide strong evidence in support of our recently advanced hypothesis that division planes in simultaneous cytokinesis following meiosis are determined by establishment of cytoplasmic domains via radial systems of nuclear-based microtubules rather than by division sites established before nuclear division.Abbreviations DMSO dimethylsulfoxide - FITC fluorescein isothiocyanate - MTOC microtubule organizing center - PBS phosphate buffered saline - PPB preprophase band of microtubules     

MONOPLASTIDIC CELL DIVISION IN LOWER LAND PLANTS

In many bryophytes and vascular cryptogams mitosis and/or meiosis takes place in cells containing a single plastid. In monoplastidic cell division plastid polarity assures that nuclear and plastid division are infallibly coordinated. The two major components of plastid polarity are morphogenetic plastid migration and microtubule organization at the plastids. Before nuclear division the plastid migrates to a position intersecting the future division plane. This morphogenetic migration is a reliable marker of division polarity in cells with and without a preprophase band of microtubules (PPB). The PPB, which predicts the future division plane before mitosis, is a characteristic feature of land plants and its insertion into the cytokinetic apparatus marks the evolution of a cortical microtubule system and a commitment to meristematic growth. Microtubule systems associated with plastid division, the axial microtubule system (AMS) in mitosis and the quadripolar microtubule system (QMS) in meiosis, contribute to predictive positioning of plastids and participate directly in spindle ontogeny. Division polarity in monoplastidic sporocytes is remarkable in that division sites are selected prior to the two successive nuclear divisions of meiosis. Plastid arrangement prior to meiosis determines the future spore domains in monoplastidic sporocytes, whereas in polyplastidic sporocytes the spore nuclei play a major role in claiming cytoplasmic domains. It is hypothesized that predivision microtubule systems associated with monoplastidic cell division are early forming components of the mitotic apparatus that serve to orient the spindle and insure equal apportionment of nucleus and plastids. “Can it be supposed that cytoplasm would be intrusted with so important a task as the preparation of a chloroplast for each of the four nuclei that are later to preside over the spores before there is any indication that such nuclear division is to take place?” Bradley Moore Davis, 1899     

The DUET gene is necessary for chromosome organization and progression during male meiosis in Arabidopsis and encodes a PHD finger protein

Progression through the meiotic cell cycle is an essential part of the developmental program of sporogenesis in plants. The duet mutant of Arabidopsis was identified as a male sterile mutant that lacked pollen and underwent an aberrant male meiosis. Male meiocyte division resulted in the formation of two cells instead of a normal tetrad. In wild type, male meiosis extends across two successive bud positions in an inflorescence whereas in duet, meiotic stages covered three to five bud positions indicating defective progression. Normal microspores were absent in the mutant and the products of the aberrant meiosis were uni- to tri-nucleate cells that later degenerated, resulting in anthers containing largely empty locules. Defects in male meiotic chromosome organization were observed starting from diplotene and extending to subsequent stages of meiosis. There was an accumulation of meiotic structures at metaphase 1, suggesting an arrest in cell cycle progression. Double mutant analysis revealed interaction with dyad, a mutation causing chromosome cohesion during female meiosis. Cloning and molecular analysis of DUET indicated that it potentially encodes a PHD-finger protein and shows specific expression in male meiocytes. Taken together these data suggest that DUET is required for male meiotic chromosome organization and progression.     

Microtubules become more dynamic but not shorter during preprophase band formation: a possible "search-and-capture" mechanism for microtubule translocation

The dynamic behavior of the microtubule cytoskeleton plays a crucial role in cellular organization, but the physical mechanisms underlying microtubule (re)organization in plant cells are poorly understood. We investigated microtubule dynamics in tobacco BY-2 suspension cells during interphase and during the formation of the preprophase band (PPB), the cytoskeletal structure that defines the site of cytokinesis. Here we show that after 2 h of microtubule accumulation in the PPB and concurrent disappearance elsewhere in the cortex, the PPB is completed and starts to breakdown exponentially already 20 min before the onset of prometaphase. During formation of the PPB, the dynamic instability, i.e., the stochastic alternating between growing and shrinking phases, of the cortical microtubules outside the PPB increases significantly, but the microtubules do not become shorter. Based on this, as well as on the cross-linking of microtubules in the PPB and the lack of evidence for motor involvement, we propose a "search-and-capture" mechanism for PPB formation, in which the regulation of dynamic instability causes the cortical microtubules to become more dynamic and possibly longer, while the microtubule cross-linking activity of the developing PPB preferentially stabilizes these "searching" microtubules. Thus, microtubules gradually disappear from the cortex outside the PPB and aggregate to the forming PPB.     

Microtubule reorganization accompanying preprophase band formation in guard mother cells ofAvena sativa L.

Summary In order to study developmental changes in microtubule organization attending the formation of a longitudinally oriented preprophase band, the guard mother cells ofAvena were examined using a new procedure for anti-tubulin immunocytochemistry on large epidermal segments. We found that the interphase band (IMB) of transverse cortical microtubules present in these cells following asymmetric division is replaced after subsidiary cell formation by mesh-like to radial microtubules that extend throughout the cytoplasm. Many of the Mts are also grouped in bundles. Gradually, this intermediate array is succeeded by longitudinal elements of the PPB. Thus, preprophase band formation is accompanied by a 90° shift in Mt orientation, with a radial arrangement serving as an intermediate stage. The micrographs are most consistent with the rearrangement of intact Mts, although changes in Mt assembly are possible as well. The role of the IMB in guard mother cells is also discussed.Abbreviations GMC guard mother cell - IMB interphase microtubule band - Mt microtubule - PPB preprophase band     

Rearrangements of Microtubules Involved in Establishing Cell Division Planes Start Immediately after DNA Synthesis and Are Completed just before Mitosis

This work concerns an aspect of spatial regulation of cell division, the development of the preprophase band (PPB) of microtubules. The PPB is significant in plant development because its position in the dividing cell indicates where the new cell wall will be inserted[mdash]an important site for control of histogenesis. We have categorized and determined the durations of stages in the development of PPBs, and have established their timing relative to the S-, G2-, and mitotic phases of the cell cycle. Roots of wheat seedlings were supplied with bromodeoxyuridine in continuous and pulse-chase treatments. Cells that were in the S-phase were identified and changes in their microtubule arrays were monitored by double immunolabeling. PPB initiation was detectable as early as the end of the S-phase as a narrowing of the preceding interphase array of microtubules. Development continued throughout G2 to a mature, narrow PPB, which existed only briefly and then eroded during the transition to the prophase mitotic spindle. The microtubule rearrangements of PPB development showed that preparation of the future site and plane of division in higher plant cells begins just after DNA replication and is completed just before mitosis.     

Demarcation of the cortical division zone in dividing plant cells

Somatic cytokinesis in higher plants involves, besides the actual construction of a new cell wall, also the determination of a division zone. Several proteins have been shown to play a part in the mechanism that somatic plant cells use to control the positioning of the new cell wall. Plant cells determine the division zone at an early stage of cell division and use a transient microtubular structure, the preprophase band (PPB), during this process. The PPB is formed at the division zone, leaving behind a mark that during cytokinesis is utilized by the phragmoplast to guide the expanding cell plate toward the correct cortical insertion site. This review discusses old and new observations with regard to mechanisms implicated in the orientation of cell division and determination of a cortical division zone.     

Microtubule-associated AIR9 recognizes the cortical division site at preprophase and cell-plate insertion

In plants, the preprophase band (PPB) of microtubules marks the cortical site where the cross-wall will fuse with the parental wall during cytokinesis . This band disappears before metaphase, and it is not known how the division plane is "memorized". One idea is that the PPB leaves behind molecules involved in the maturation of the cell plate . Here, we report on the proteomic isolation of a novel 187 kDa microtubule-associated protein, AIR9, conserved in land plants and trypanosomatid parasites. AIR9 decorates cortical microtubules and the PPB but is downregulated during mitosis. AIR9 reappears at the former PPB site precisely when the cortex is contacted by the outwardly growing cytokinetic apparatus. AIR9 then moves inward on the new cross-wall and thus forms a torus. Truncation studies show that formation of the torus requires a repeated domain separate from AIR9's microtubule binding site. Cell plates induced to insert outside the predicted division site do not elicit an AIR9 torus, suggesting that AIR9 recognizes a component of the former PPB. Such misplaced walls remain immature, based on their prolonged staining for the cell-plate polymer callose. We propose that AIR9 may be part of the mechanism ensuring the maturation of those cell plates successfully contacting the "programmed" cortical division site.     

The Cell Cycle Timing of Centromeric Chromatin Assembly in Drosophila Meiosis Is Distinct from Mitosis Yet Requires CAL1 and CENP-C

CENP-A (CID in flies) is the histone H3 variant essential for centromere specification, kinetochore formation, and chromosome segregation during cell division. Recent studies have elucidated major cell cycle mechanisms and factors critical for CENP-A incorporation in mitosis, predominantly in cultured cells. However, we do not understand the roles, regulation, and cell cycle timing of CENP-A assembly in somatic tissues in multicellular organisms and in meiosis, the specialized cell division cycle that gives rise to haploid gametes. Here we investigate the timing and requirements for CID assembly in mitotic tissues and male and female meiosis in Drosophila melanogaster, using fixed and live imaging combined with genetic approaches. We find that CID assembly initiates at late telophase and continues during G1 phase in somatic tissues in the organism, later than the metaphase assembly observed in cultured cells. Furthermore, CID assembly occurs at two distinct cell cycle phases during male meiosis: prophase of meiosis I and after exit from meiosis II, in spermatids. CID assembly in prophase I is also conserved in female meiosis. Interestingly, we observe a novel decrease in CID levels after the end of meiosis I and before meiosis II, which correlates temporally with changes in kinetochore organization and orientation. We also demonstrate that CID is retained on mature sperm despite the gross chromatin remodeling that occurs during protamine exchange. Finally, we show that the centromere proteins CAL1 and CENP-C are both required for CID assembly in meiosis and normal progression through spermatogenesis. We conclude that the cell cycle timing of CID assembly in meiosis is different from mitosis and that the efficient propagation of CID through meiotic divisions and on sperm is likely to be important for centromere specification in the developing zygote.     

Microtubule distribution in dv, a maize meiotic mutant defective in the prophase to metaphase transition

Microsporogenesis in Zea mays, the meiotic reduction of diploid sporocytes to haploid microspores, proceeds through a well-defined developmental sequence. The ability to generate mutants that affect the process makes this an ideal system for elucidating the role of the cytoskeleton during plant development. We have used immunofluorescence microscopy to compare microtubule distribution in wild-type and mutant microsporocytes. During normal meiosis the distribution of microtubules follows a specific temporal and spatial pattern that reflects the polar nature of microspore formation. Perinuclear microtubule staining increases and the nucleus elongates in the future spindle axis during late prophase I. Metaphase I spindles with highly focused poles align along the long axis of the anther locule. Cytokinesis occurs perpendicular to the spindle axis. The second division axis shifts 90 degrees with respect to the first division plane, thereby yielding an isobilateral tetrad of microspores. Microtubule distribution patterns during meiosis suggest that a nuclear envelope-associated microtubule organizing center (MTOC) controls the organization of cytoplasmic microtubules and contributes to spindle formation. The meiotic mutant dv is defective in the transition from a prophase microtubule array to a metaphase spindle. Instead of converging to form focused poles, the metaphase spindle poles remain diffuse as in prometaphase. This defect correlates with several abnormalities in subsequent developmental events including the formation of multinucleate daughter cells, multiple microspindles during meiosis II, multiple phragmoplasts, polyads of microspores, and cytoplasmic microtubule foci. These results suggest that dv is a mutation that affects MTOC organization.     

Arabidopsis TANGLED identifies the division plane throughout mitosis and cytokinesis

BACKGROUND: In premitotic plant cells, the future division plane is predicted by a cortical ring of microtubules and F-actin called the preprophase band (PPB). The PPB persists throughout prophase, but is disassembled upon nuclear-envelope breakdown as the mitotic spindle forms. Following nuclear division, a cytokinetic phragmoplast forms between the daughter nuclei and expands laterally to attach the new cell wall at the former PPB site. A variety of observations suggest that expanding phragmoplasts are actively guided to the former PPB site, but little is known about how plant cells "remember" this site after PPB disassembly. RESULTS: In premitotic plant cells, Arabidopsis TANGLED fused to YFP (AtTAN::YFP) colocalizes at the future division plane with PPBs. Strikingly, cortical AtTAN::YFP rings persist after PPB disassembly, marking the division plane throughout mitosis and cytokinesis. The AtTAN::YFP ring is relatively broad during preprophase/prophase and mitosis; narrows to become a sharper, more punctate ring during cytokinesis; and then rapidly disassembles upon completion of cytokinesis. The initial recruitment of AtTAN::YFP to the division plane requires microtubules and the kinesins POK1 and POK2, but subsequent maintenance of AtTAN::YFP rings appears to be microtubule independent. Consistent with the localization data, analysis of Arabidopsis tan mutants shows that AtTAN plays a role in guidance of expanding phragmoplasts to the former PPB site. CONCLUSIONS: AtTAN is implicated as a component of a cortical guidance cue that remains behind when the PPB is disassembled and directs the expanding phragmoplast to the former PPB site during cytokinesis.     

Meiotic weakness: the first division

Cell division is probably the most dramatic event in the life of a cell : the entire genetic material has to be equally distributed into the two daughter cells. Segregation errors have severe consequences and lead to either cell death or the generation of aneuploid cells and may cause the formation of tumors or tumor promoting mutations in somatic cells. In meiosis, they provoke the generation of aneuploid embryos and/or spontaneous abortions. Trisomies in humans, such as trisomy 21, are due to the missegregation of one chromosome in the first meiotic division in the oocyte. This review deals with the molecular mechanisms regulating the two meiotic divisions required for the generation of female haploid germ cells. Here we focus mainly on spindle assembly, and cell cycle regulation especially during the first meiotic division in mouse oocytes (excellent reviews have been written on the peculiar aspects of cell cycle regulation in meiosis II, such as the CSF arrest).     

Centriolin,a centriole-appendage protein,regulates peripheral spindle migration and asymmetric division in mouse meiotic oocytes

Unlike somatic cells mitosis, germ cell meiosis consists of 2 consecutive rounds of division that segregate homologous chromosomes and sister chromatids, respectively. The meiotic oocyte is characterized by an absence of centrioles and asymmetric division. Centriolin is a relatively novel centriolar protein that functions in mitotic cell cycle progression and cytokinesis. Here, we explored the function of centriolin in meiosis and showed that it is localized to meiotic spindles and concentrated at the spindle poles and midbody during oocyte meiotic maturation. Unexpectedly, knockdown of centriolin in oocytes with either siRNA or Morpholino micro-injection, did not affect meiotic spindle organization, cell cycle progression, or cytokinesis (as indicated by polar body emission), but led to a failure of peripheral meiotic spindle migration, large polar body emission, and 2-cell like oocytes. These data suggest that, unlike in mitotic cells, the centriolar protein centriolin does not regulate cytokinesis, but plays an important role in regulating asymmetric division of meiotic oocytes.     

Nuclear and microtubule dynamics of G2/M somatic nuclei during haploidization in germinal vesicle-stage mouse oocytes

During the haploidization process, it is expected that diploid chromosomes of somatic cells will be reduced to haploid for the generation of artificial gametes. In the present study, we aimed to use enucleated mouse oocytes at the germinal vesicle-stage (G2/M) as recipients for somatic cells that are also synchronized to the G2/M stage for haploidization. The reconstructed oocytes were then induced to undergo meiosis in vitro and observed for their nuclear morphology and microtubule network formation at various expected stages of the meiotic division. Following in vitro maturation, more than half (62/119, 52.1%) of the reconstructed oocytes completed the first round of meiosis-like division, as evidenced by the extrusion of pseudopolar bodies (PBs). However, accelerated PB extrusion, approximately 3-4 h earlier than that by control oocytes occurred. Furthermore, abnormally large pseudo-PBs, as large as four times the normal PB sizes, were observed. During the process of in vitro maturation at both the expected stages of metaphase I (MI) and metaphase II (MII), condensed chromosomes were observed in 38.7% and 55.2% of oocytes, respectively. However, two other types of nuclear configurations were also observed: 1) uneven distribution of chromatin and 2) an interphase-like nucleus, indicating deficiencies in chromosome condensation. Following oocyte activation, more than half (21/33, 63.6%) of the reconstructed oocytes with pseudo-PBs formed separated pseudopronuclei (PN), suggesting formation of functional spindles. The formation of bipolar spindle-like microtubule network at both the expected MI and MII stages during in vitro maturation was confirmed by immunohistochemistry. In summary, this study demonstrated that a high proportion of G2/M somatic nuclei appear to undergo meiosis-like division, in two successive steps, forming a pseudo-PB and two separate pseudo-PN upon in vitro maturation and activation treatment. Moreover, the enucleated geminal vesicle cytoplast retained its capacity for meiotic division following the introduction of a somatic G2/M nucleus.     

The cytoskeleton and spatial control of cytokinesis in the plant life cycle

Summary One of the intriguing aspects of development in plants is the precise control of division plane and subsequent placement of walls resulting in the specific architecture of tissues and organs. The placement of walls can be directed by either of two microtubule cycles. The better known microtubule cycle is associated with control of the future division plane in meristematic growth where new cells become part of tissues. The future daughter domains are determined before the nucleus enters prophase and the future site of cytokinesis is marked by a preprophase band (PPB) of cortical microtubules. The spindle axis is then organized in accordance with the PPB and, following chromosome movement, a phragmoplast is initiated in the interzone and expands to join with parental walls at the site previously occupied by the PPB. The alternative microtubule cycle lacks both the hooplike cortical microtubules of interphase and the PPB. Wall placement is determined by a radial microtubule system that defines a domain of cytoplasm either containing a nucleus or destined to contain a nucleus (the nuclear cytoplasmic domain) and controls wall placement at its perimeter. This more flexible system allows for cytoplasmic polarization and migration of nuclei in coenocytes prior to cellularization. The uncoupling of cytokinesis from karyokinesis is a regular feature of the reproductive phase in plants and results in specific, often unusual, patterns of cells which reflect the position of nuclei at the time of cellularization (e.g., the arrangement of spores in a tetrad, cells of the male and female gametophytes of angiosperms, and the distinctive cellularization of endosperm). Thus, both microtubule cycles are required for completion of plant life cycles from bryophytes to angiosperms. In angiosperm seed development, the two methods of determining the boundaries of domains where walls will be deposited are operative side by side. Whereas the PPB cycle drives embryo development, the radial-microtubule-system cycle drives the common nuclear type of endosperm development from the syncytial stage through cellularization. However, a switch to the PPB cycle can occur in endosperm, as it does in barley, when peripheral cells divide to produce a multilayered aleurone. The triggers for the switch between microtubule cycles, which are currently unknown, are key to understanding plant development.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Preprophasic microtubule systems and development of the mitotic spindle in hornworts (Bryophyta)

Summary Studies of monoplastidic mitosis in hornworts (Bryophyta) using transmission electron microscopy and indirect immunofluorescence staining of microtubules have revealed that two mutually perpendicular microtubule systems predict division polarity in preprophase. Events of cytoplasmic reorganization in preparation for division occur in the following order: migration of the single plastid to a position perpendicular to the division site, constriction of the plastid where its midpoint intersects the division site, development of an axial system of microtubules parallel to the elongating plastid isthmus, and appearance of an atypical preprophase band of microtubules (PPB). The PPB is asymmetrical with a tight band of microtubules on the side over the plastid isthmus and a broad band of widely spaced microtubules over the nucleus. The axial system contributes directly to development of the spindle. In prometaphase, the axial system separates at the equator and additional microtubule bundles project from polar regions, creating two opposing halfspindles. The PPB is still present during asymmetrical organization of the spindle and microtubules extending from the broad portion of the PPB to poles appear to be incorporated into the developing spindle. Dynamic changes in the microtubular cytoskeleton demonstrate (1) intimate relationship of plastid and nuclear division, (2) contribution of preprophase/prophase microtubule systems to spindle development in monoplastidic cells, and (3) dynamic reorientation of microtubules from one system to another.     

Development of the preprophase band from random cytoplasmic microtubules in guard mother cells of Allium cepa L.

The organization of microtubule (MT) arrays in the guard mother cells (GMCs) of A. cepa was examined, focussing on the stage at which a longitudinal preprophase band (PPB) is established perpendicular to all other division planes in the epidermis. In the majority of young GMCs, including those seen just after asymmetric division, MTs are distributed randomly throughout the cortex and inner regions of the cytoplasm. Few MTs are associated with the nuclear surface. As the GMCs continue to develop, MTs cluster around the nucleus and a PPB appears as a wide longitudinal band. Microtubules also become prominent between the nucleus and the periclinal and transverse walls, while they decrease in number along the radial longitudinal walls. The PPB progressively narrows by early prophase, and a transversely oriented spindle gradually ensheaths the nucleus. These observations indicate that the initial, broad PPB is organized by a rearrangement of the random cytoplasmic array of MTs. Additional reorganization is responsible for MTs linking the nucleus and the cortex in the future plane of the cell plate, and for narrowing of the PPB.Abbreviations GMC guard mother cell - MT microtubule - PPB preprophase band     

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