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.     

Cell division in caffeine-induced binucleate protonemal cells ofAdiantum. II. Formation of the preprophase band and cell division in centrifuged and non-centrifuged cells

Organization of microtubules (MTs) in relation to the behavior of nuclei was examined in dividing binucleate cells ofAdiantum capillus-veneris L. To induce binucleate cells, caffeine, an inhibitor of formation of the cell plate, was applied at 4 mM to synchronously dividing protonemal cells during cytokinesis (Murata and Wada 1993). Formation of the preprophase band (PPB) during the next cell cycle was examined in non-centrifuged and centrifuged cells. The two nuclei were separated or associated with one another in both non-centrifuged and centrifuged cells, although the location of the nuclei in the cylindrical protonemal cells was different (Murata and Wada 1993). Irrespective of centrifugation, a single PPB was formed around the nuclei in cells with associated nuclei. Two PPBs were formed in cells with separated nuclei in centrifuged cells. Patterns of mitosis and cytokinesis varied, depending on the location of the PPB and the distribution of the nuclei. The role of the nucleus in formation of the PPB is discussed.     

Cell division of binuclear cells induced by caffeine: Spindle organization and determination of division plane

Synchronously dividing binuclear cells were induced in root tips ofTriticum turgidum by caffeine treatment. Spindle and other microtubular configurations of such cells were studied using tubulin immunofluorescence and electron microscopy. The binuclear cells developed one, two or three preprophase microtubule bands longitudinally, transversely or rarely in a cross configuration. During the mitotic entry binuclear cells formed prophase spindles separately around each nucleus. When the nuclei were located fairly apart, their spindle structures developed independently throughout all mitotic phases. But when the nuclei were located closely together their metaphase and anaphase spindles shared a common polar region. However, the two spindles in such cells retained their functional autonomy. They display structurally independent minipoles in the common polar region. After anaphase the neighbouring nonsister chromosome groups of nuclei divided by a common polar region come to lie close together and in telophase, become enclosed by a common nuclear envelope. During cytokinesis of binuclear cells cell plates were formed only between sister nuclei. These cell plates may develop normally or may curve or branch giving rise to aberrant daughter cell walls. The peculiar mode of spindle and spindle polar region organization of binuclear cells and determination of the division plane in them are discussed.     

Formation of a phragmosome-like structure in centrifuged protonemal cells of Adiantum capillus-veneris L.

A phragmosome (PS) is a transvacuolar aggregation of cytoplasm that develops in the plane of future cytokinesis and is found specifically in highly vacuolated cells. Although protonemal cells of Adiantum capillus-veneris L. usually do not form a PS, a PS-like structure developed at the site of a preprophase band (PPB) of microtubules (MTs) when the nucleus and endoplasm were displaced from the division site by centrifugation, leaving a PPB in the cortical cytoplasm. The PS-like structure contained endoplasmic MTs, F-actin, oil droplets and mitochondria. The structure did not develop when the cells were centrifuged before the formation of a PPB. Application of amiprophos-methyl (APM) before development of the PPB strongly inhibited the formation of the PS-like structure after centrifugation. The PS-like structure was dispersed after cytokinesis which occurred in the region of the displaced nucleus. Treatment with APM after the formation of the PS-like structure arrested the cell cycle at the M phase and inhibited the degradation of this structure. These results suggest that development of a PS-like structure is associated both with the formation of a PPB and with the stage of the cell cycle. Received: 9 July 1996 / Accepted: 12 September 1996     

Alteration of division polarity and preprophase band orientation in stomatogenesis by light

We have developed an experimental system in which the irradiation with a red light pulse induces stomatal disorientation in the hypocotyl epidermis ofCucumis sativus L. In this system, the orientation of the division plane in guard mother cells was not defined correctly. Preprophase bands formed in these cells but their orientation was abnormal.     

Narrowing of the preprophase microtubule band is not required for cell division plane determination in cultured plant cells

Summary. In most higher-plant cells, cortical microtubules form a tightly focused preprophase band (PPB) that disappears with the onset of prometaphase, but whose location defines the future location of the cell plate at the end of cytokinesis. It is unclear whether the PPB microtubules themselves designate the precise area where the cell plate will insert, or rather if these microtubules are responding to a hierarchical signal(s). Here we show that narrowing of the microtubules within the PPB zone is not necessary for proper division plane determination. In cultured tobacco BY-2 cells in which PPB microtubules are depolymerized, the phragmoplast can still accurately locate and insert at the proper site. The data do not support a role for PPB microtubule narrowing in focusing the signal that is used later by the phragmoplast to position the cell plate; rather, proper phragmoplast positioning is more likely a consequence of a non-microtubule positional element. Although the PPB microtubules do not directly mark the division site, we show that they are required for accurate spindle positioning, an activity that presets the future growth trajectory of the phragmoplast and is necessary for insuring high-fidelity cell plate positioning. Correspondence and reprints: Department of Biology, Pennsylvania State University, University Park, PA 16802, U.S.A. Present address: Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia, U.S.A.     

Oblique cell plate formation in tobacco BY-2 cells originates in double preprophase bands

Under specific experimental conditions, the formation of oblique cell plates was observed in tobacco BY-2 cells. Examination of this process, using an inverted microscope and immunofluorescence microscopy, revealed that the oblique cell plates were formed in cells that had double preprophase bands (PPBs). The formation of the oblique cell plates is discussed with a relationship to PPBs.     

Reprogramming of cells following wounding in pea (Pisum sativum L.) roots. I. Cell division and differentiation of new vascular elements

Summary Normally quiescent cortical tissue of pea roots can be induced, by severing the adjacent vascular cylinder of the root, to undergo redifferentiation to form new files of tracheary and sieve elements which bridge the wound. The development of vascular transfer cells is also induced. Redifferentiation is normally accompanied by division of the original cortical cells. The planes of cell division, especially those preceding sieve element formation, are aligned very precisely in adjacent cells, to produce smooth files of cells. In roots wounded 3–4 mm from the apex, bands of microtubules in the periphery of the cells (pre-prophase bands) form at sites which correspond to the expected planes of cell division.     

Divide and conquer: cytokinesis in plant cells

Plant cells divide in two by constructing a new cell wall (cell plate) between daughter nuclei after mitosis. Golgi-derived vesicles are transported to the equator of a cytoskeletal structure called a phragmoplast, where they fuse together to form the cell plate. Orientation of new cell walls involves actindependent guidance of phragmoplasts and associated cell plates to cortical sites established prior to mitosis. Recent work has provided new insights into how actin filaments and other proteins in the phragmoplast and cell plate contribute to cytokinesis. Newly discovered mutations have identified a variety of genes required for cytokinesis or its spatial regulation.     

Mitotic cyclin distribution during maize cell division: Implications for the sequence diversity and function of cyclins in plants

Summary Cyclin proteins are components of the regulatory system that controls the orderly progression of the events of cell division. Their sub-cellular location, as well as their fluctuating abundance and their affinities for the cyclin-dependent kinases (CDKs) to which they bind, determine their successive roles during the cell cycle. Here we employ species-specific antibodies to monitor changes in quantity and location of four maize cyclins and maize Cdc2-kinase in dividing maize root tip cells. Maize cyclin Ia occurs in the nuclear matrix and is released when the nuclear envelope breaks down. In contrast, cyclin Ib is cytoplasmic until prophase; it associates transiently with the nuclear envelope and preprophase band (PPB) just before these structures break down and then associates with the condensed chromosomes and spindle region before declining at anaphase. Cyclin II and Cdc2 also occur in the PPB. Occurrence of cyclin Ib and Cdc2 at the PPB concurrent with initiation of breakdown is consistent with previous studies in which microinjection of cyclin-dependent protein kinase indicated that removal of the PPB at the time of nuclear-envelope breakdown is catalysed by a CDK. While cyclins Ia and III are predominantly nuclear prior to mitosis, cyclins Ib and II are predominantly cytoplasmic until prophase then become nuclear. The initial cytoplasmic retention of cyclins Ib and II correlates with their possession of a sequence similar to the cytoplasmic-retention signal of animal cyclin B1. Cyclin II binds to all microtubule arrays during the cell cycle, becoming markedly concentrated in the phragmoplast, and cyclin III associates with the spindle and then the phragmoplast. Cdc2 also occurs in the phragmoplast. Persistence of mitotic cyclins and CDK after mitosis into the cytokinetic stage, as seen in maize, is not paralleled in animal cells, where the cytokinetic mid-body is not so labelled, presumably reflecting the key role of the phragmoplast apparatus in plant cell division.Abbreviations CDK cyclin-dependent kinase - CRS cytoplasmicretention signal - NE nuclear envelope - NEB nuclear-envelope breakdown - NLS nuclear-location signal - PPB preprophase band - FITC fluorescein isothiocyanate - TRITC tetramethylrhodamine isothiocyanate     

Dual localized kinesin‐12 POK2 plays multiple roles during cell division and interacts with MAP65‐3

Kinesins are versatile nano‐machines that utilize variable non‐motor domains to tune specific motor microtubule encounters. During plant cytokinesis, the kinesin‐12 orthologs, PHRAGMOPLAST ORIENTING KINESIN (POK)1 and POK2, are essential for rapid centrifugal expansion of the cytokinetic apparatus, the phragmoplast, toward a pre‐selected cell plate fusion site at the cell cortex. Here, we report on the spatio‐temporal localization pattern of POK2, mediated by distinct protein domains. Functional dissection of POK2 domains revealed the association of POK2 with the site of the future cell division plane and with the phragmoplast during cytokinesis. Accumulation of POK2 at the phragmoplast midzone depends on its functional POK2 motor domain and is fine‐tuned by its carboxy‐terminal region that also directs POK2 to the division site. Furthermore, POK2 likely stabilizes the phragmoplast midzone via interaction with the conserved microtubule‐associated protein MAP65‐3/PLEIADE, a well‐established microtubule cross‐linker. Collectively, our results suggest that dual localized POK2 plays multiple roles during plant cell division.     

The role of ethylene in the control of cell division in cultured pea root tips: a mechanism to explain the excision effect

Summary A transitory cell division block, or excision effect, occurs in the meristem of roots after excision and transfer to culture medium. This block can be induced, in intact seedling roots, by exogenous treatment with ethylene gas. With continuous treatment, the block is longer and the recovery less than after a 4 hour pulse. In excised roots the excision effect can be eliminated by treatment with an inhibitor of ethylene synthesis (aminoethoxyvinylglycine) or action (silver thiosulfate). These experiments provide evidence to support the hypothesis that ethylene from the wounded end of an excised root is involved in a process resulting in a transitory block in cell cycle progression in the meristem. The implications of this hypothesis are discussed.     

Themes and variations in prokaryotic cell division

Perhaps the biggest single task facing a bacterial cell is to divide into daughter cells that contain the normal complement of chromosomes. Recent technical and conceptual breakthroughs in bacterial cell biology, combined with the flood of genome sequence information and the excellent genetic tools in several model systems, have shed new light on the mechanism of prokaryotic cell division. There is good evidence that in most species, a molecular machine, organized by the tubulin-like FtsZ protein, assembles at the site of division and orchestrates the splitting of the cell. The determinants that target the machine to the right place at the right time are beginning to be understood in the model systems, but it is still a mystery how the machine actually generates the constrictive force necessary for cytokinesis. Moreover, although some cell division determinants such as FtsZ are present in a broad spectrum of prokaryotic species, the lack of FtsZ in some species and different profiles of cell division proteins in different families suggests that there are diverse mechanisms for regulating cell division.     

Roles of actin-depleted zone and preprophase band in determining the division site of higher-plant cells, a tobacco BY-2 cell line expressing GFP-tubulin

Summary. The mode of cytokinesis, especially in determining the site of cell division, is not well understood in higher-plant cells. The division site appears to be predicted by the preprophase band of microtubules that develop with the phragmosome, an intracellular structure of the cytoplasm suspending the nucleus and the mitotic apparatus in the center. As the preprophase band disappears during mitosis, it is thought to leave some form of memory on the plasma membrane to guide the growth of the new cell plate at cytokinesis. However, the intrinsic nature of this memory remains to be clarified. In addition to microtubules, microfilaments also dynamically change forms during cell cycle transition from the late G2 to the early G1 phase. We have studied the relationships between microtubules and microfilaments in tobacco BY-2 cells and transgenic BY-2 cells expressing a fusion protein of green-fluorescent protein and tubulin. At the late G2 phase, microfilaments colocalize with the preprophase band of microtubules. However, an actin-depleted zone which appears at late prometaphase is observed around the chromosomes, especially at metaphase, but also throughout anaphase. To study the functions of the actin-depleted zone, we disrupted the microfilament structures with bistheonellide A, a novel macrolide that depolymerizes microfilaments very rapidly even at low concentrations. The division planes became disorganized when the drug was added to synchronized BY-2 cells before the appearance of the actin-depleted zone. In contrast, the division planes appeared smooth, as in control cells, when the drug was added after the appearance of the actin-depleted zone. These results suggest that the actin-depleted zone may participate in the demarcation of the division site at the final stage of cell division in higher plants.Correspondence and reprints: Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba Prefecture 277-8562, Japan.     

Elongation and cell division in Bdellovibrio bacteriovorus

Elongation and division of Bdellovibrio bacteriovorus were studied in axenic synchronous cultures. The cells elongate unidirectionally from one end attaining a length of several unit cells, and then divide into the corresponding number of cells. The length of the filament and, consequently, the progeny number, vary within the range of two to several dozen cells, according to the conditions used. A protein and a low molecular weight component are required for normal division.     

Asymmetric cell division of a triangular halophilic archaebacterium

Abstract The cell division of the halophilic archaebacterium, Haloarcula japonicus , which has a characteristic triangular shape in high salt concentration media, was analysed by time lapse microscopic cinematography. Cell division on an agar medium occured on average every 3.7 h. Cell plates were laid down asymmetrically, generating triangular or rhomboid shape daughter cells which then separated. Cell plate formation was clearly observed because the cells are flat and thin enough to see through using a conventional light microscope.     

A mutation of the CRUMPLED LEAF gene that encodes a protein localized in the outer envelope membrane of plastids affects the pattern of cell division, cell differentiation, and plastid division in Arabidopsis

We identified a novel mutation of a nuclear-encoded gene, designated as CRUMPLED LEAF (CRL), of Arabidopsis thaliana that affects the morphogenesis of all plant organs and division of plastids. Histological analysis revealed that planes of cell division were distorted in shoot apical meristems (SAMs), root tips, and embryos in plants that possess the crl mutation. Furthermore, we observed that differentiation patterns of cortex and endodermis cells in inflorescence stems and root endodermis cells were disturbed in the crl mutant. These results suggest that morphological abnormalities observed in the crl mutant were because of aberrant cell division and differentiation. In addition, cells of the crl mutant contained a reduced number of enlarged plastids, indicating that the division of plastids was inhibited in the crl. The CRL gene encodes a novel protein with a molecular mass of 30 kDa that is localized in the plastid envelope. The CRL protein is conserved in various plant species, including a fern, and in cyanobacteria, but not in other organisms. These data suggest that the CRL protein is required for plastid division, and it also plays an important role in cell differentiation and the regulation of the cell division plane in plants. A possible function of the CRL protein is discussed.     

Regulation of cell division and malignant transformation

The problem of regulation of cell division is essentially a problem of understanding regulation of transition from the resting state of a cell to the dividing state and vice versa. In malignancy the ability to revert back to a normal resting state is impaired. A model is presented which attempts to explain the control of the above transitions through control of uptake of essential nutrients by a transport-inhibitory protein. Experimental evidence in favour of the model is given.     

A new gene controlling the frequency of cell division per round of DNA replication in Escherichia coli

Summary A novel mutant of Escherichia coli, named cfcA1, was isolated from a temperature-sensitive dnaB42 strain, and found to have the following characteristics. Division arrest and lethality induced by inhibition of DNA replication was reduced and delayed in the cfcA1 dnaB42 strain, as compared with the parental dnaB42 strain. Two types of inhibition of division induced by the addition of nalidixic acid or hydroxyurea were suppressed by the cfcA1 mutation. Under permissive conditions for DNA replication, the colony forming ability of cfcA1 cells was significantly reduced as compared with that of cfc ⁺ cells; conversely the division rate of cfcA1 cells was higher than that of cfc ⁺ cells. The cfcA1 mutation partially restored division arrest induced in the thermosensitive ftsZ84 mutant at the restrictive temperature and suppresed the UV sensitivity of the lon mutation. The mutation was mapped at 79.2 min on the E. coli chromosome. Taking these properties into account, it is hypothesized that the cfcA gene is involved in determining the frequency of cell division per round of DNA replication by interacting with the FtsZ protein which is essential for cell division.