The KEULE gene is involved in cytokinesis in Arabidopsis

We present evidence to show that the KEULE gene of Arabidopsis is involved in cytokinesis. Mutant keule embryos have large multinucleate cells with gapped or incomplete cross walls, as well as cell wall stubs that are very similar to those observed upon caffeine inhibition of cytokinesis in plants. These defects are observed in all populations of dividing cells in the mutant, including calli, but less frequently in mature cells. Cell division appears to be slowed down, and the planes of cell division are often misoriented. In late embryos and seedlings, cross-wall formation usually appears complete, suggesting that the requirement for KEULE during cytokinesis is not absolute. Nonetheless, keule mutants die as seedlings with large polyploid cells. The bloated surface layer of keule seedlings does not uniformly behave like wild-type epidermis, and patches of this layer assume characteristics of the underlying ground tissue. The cytokinesis defect of keule mutants may influence aspects of cellular differentiation.     

Mutations in the Caenorhabditis elegans Gene vab-3 Reveal Distinct Roles in Fate Specification and Unequal Cytokinesis in an Asymmetric Cell Division

Asymmetric cell divisions in which a precursor cell distributes fate potential unequally between the two daughter cells represent one of the major mechanisms for fate specification during development. Such mechanisms suggest at least two distinct cellular activities: factors that act to establish asymmetry in the precursor cell and factors that are distributed or activated unequally and function to make the daughter cells different from each other. In Caenorhabditis elegans , cytokinesis of the first division of the male-specific postembryonic blast cell B is unequal, and the two daughters adopt different fates. Others have observed that the genes lin-17 and lin-44 are required, respectively, to establish and to orient this asymmetric division. Mutations in lin-17 and lin-44 coordinately disrupt cytokinesis and fate specification. We describe the function of the gene vab-3 in the B cell lineage. Mutations in vab-3 disrupt the fate of the anterior daughter of B, B.a. However, unlike lin-17 and lin-44 , mutations in vab-3 can disrupt fate without the corresponding disruption of unequal cytokinesis. Analysis of lin-17; vab-3 double mutants suggests that vab-3 acts after lin-17 for B.a fate specification. Double mutant analysis has also identified additional functions of lin-17 in the B lineage subsequent to this first division.     

Observations on induced amitosis in Acanthamoeba

Methods are described for the induction of amitotic cell division in Acanthamoeba rhysodes. Induced amitotic cell division in this organism is similar to normal cytokinesis in many respects, however, the nucleus is partitioned during its interphase state so that the daughter products of amitosis are not viable. It is proposed that the induction of amitotic cell division causes the amoeba to produce the normal cytoplasmic components responsible for cell division in the absence of nuclear mitosis. This is not a normal stage in the amoeba's life cycle and it appears to be a genetic defect unique to this strain of Acanthamoeba.Evidence is presented that induction of amitotic cell division requires protein synthesis but not ribonucleic acid synthesis. Further, induced amoebae require a period of adhesion to a foreign substrate before they are capable of amitosis. The pattern of amitotic cell division could be interpreted as a segregation of discrete cytoplasmic units, generated during induction.     

A change in sister chromatid behavior precedes nuclear division in Saccharomyces cerevisiae

We used a genetic assay to monitor the behavior of sister chromatids during the cell cycle. We show that the ability to induce sister chromatid exchanges (SCE) with ionizing radiation is maximal in budded cells with undivided nuclei and then decreases prior to nuclear division. SCE can be induced in cells arrested in G2 using either nocodazole or cdc mutants. These data show that sister chromatids have two different states prior to nuclear division. We suggest that the sister chromatids of cir. III, a circular derivative of chromosome III, separate (anaphase A) prior to spindle elongation (anaphase B). Other interpretations are also discussed. SCE can be induced in cdc mutants that arrest in G2 and in nocodazole-treated cells, suggesting that mitotic checkpoints arrest cells prior to sister chromatid separation. Received: 3 July 1996 / Accepted: 4 October 1996     

RNA levels in colchicine-resistant mutants of Chlamydomonas reinhardi

A colchicine-resistant mutant of Chlamydomonas reinhardi (col⁻_{10, 12}) which appears blocked in the final stages of the cell division cycle is shown to have an RNA: protein ratio over four times that which is observed in wild-type cultures. This does not appear to be simply a consequence of reduced overall growth rate, because a comparable reduction in the overall growth rate of wild type by caffeine inhibition did not produce such a large rise in RNA content. The RNA levels in six other colchicine-resistant mutants, which have various abnormalities in colchicine-binding activity, has also been investigated. Two of them have elevated RNA levels.     

Zellteilung und Bildung von Innenschalen beiHantzschia amphioxys undAchnanthes coarctata

Before cytokinesis, the identically constructed chromatophores ofHantzschia amphioxys andAchnanthes coarctata are transformed into less effigurated bodies. In normal cytokinesis, the course of mitosis, chromatophore division, and cleavage furrowing are exactly synchronized. The division of the chromatophore appears as a passive process, i.e. intersection by the cleavage furrow. In inequal cell divisions before the formation of inner valves cytokinesis can take place without chromatophore division. Once chromatophore division without mitosis and cytokinesis was observed. InHantzschia there are three types of inner valve formation, inAchnanthes coarctata only two. The inner valves develop under unfavorable growth conditions, the cells possessing them, however, are not resting spores as in some other diatoms. InHantzschia, auxospore formation is suppressed under the cultural conditions used, the cells multiply intensely without diminution.

     

A cytokinesis-defective mutant of Arabidopsis ( cyt1 ) characterized by embryonic lethality, incomplete cell walls, and excessive callose accumulation

The genetic control of cell division in eukaryotes has been addressed in part through the analysis of cytokinesis-defective mutants. Two allelic mutants of Arabidopsis ( cyt1–1 and cyt1–2 ) altered in cytokinesis and cell-wall architecture during embryogenesis are described in this report. Mutant embryos appear slightly abnormal at the heart stage and then expand to form a somewhat disorganized mass of enlarged cells with occasional incomplete walls. In contrast to the keule and knolle mutants of Arabidopsis and the cyd mutant of pea, which also exhibit defects in cytokinesis during embryogenesis, cyt1 embryos cannot be rescued in culture, are desiccation-intolerant at maturity, and produce cell walls with excessive callose as revealed through staining with the aniline blue fluorochrome, Sirofluor. Some cyt1 defects can be partially phenocopied by treatment with the herbicide dichlobenil, which is thought to interfere with cellulose biosynthesis. The distribution of unesterified pectins in cyt1 cell walls is also disrupted as revealed through immunocytochemical localization of JIM 5 antibodies. These features indicate that CYT1 plays an essential and unique role in plant growth and development and the establishment of normal cell-wall architecture.     

Isolation of novel pre-mRNA splicing mutants of Schizosaccharomyces pombe

 New prp (pre-mRNA processing) mutants of the fission yeast Schizosaccharomyces pombe were isolated from a bank of 700 mutants that were either temperature sensitive (ts^-) or cold sensitive (cs^-) for growth. The bank was screened by Northern blot analysis with probes complementary to S. pombe U6 small nuclear RNA (sn RNA), the gene for which has a splicesomal (mRNA-type) intron. We identified 12 prp mutants that accumulated the U6 snRNA precursor at the nonpermissive temperature. All such mutants were also found to have defects in an early step of TFIID pre-mRNA splicing at the nonpermissive temperature. Complementation analyses showed that seven of the mutants belong to six new complementation groups designated as prp8 and prp10-prp14, whereas the five other mutants were classified into the known complementation groups prp1, prp2 and prp3. Interestingly, some of the isolated prp mutants produced elongated cells at the nonpermissive temperature, which is a phenotype typical of cell division cycle (cdc) mutants. Based on these findings, we propose that some of the wild-type products from these prp ⁺ genes play important roles in the cellular processes of pre-mRNA splicing and cell cycle progression. Received: 15 April 1996/Accepted: 9 July 1996     

Inducible sfi dependent division inhibition in Escherichia coli

Summary Brief exposure of an Escherichia coli tif lon strain to 40° results in subsequent prolonged inhibition of cell division (part of the SOS response), which is completely and specifically suppressed by sfiA and sfiB mutations. This sfi dependent division inhibition requires protein synthesis during the 40° incubation period, implying the existence of a tif-inducible protein which results in cell division arrest. sfi dependent division inhibition is also induced early during thymine starvation in tif ⁺ cells; at later times a sfi independent mechanism of division arrest is invoked as well.In lon mutants, known to lack a protease, the sfi dependent division inhibition is amplified, perhaps due to stabilization of the inducible protein involved in division arrest. In these strains the P1 lysogenization defect and the filamentation observed after a nutritional shift-up are sfi dependent, suggesting that P1 infection and nutritional shift-up may also induce the protein involved in division arrest. Bacteria are known to increase in size following a shift-up. Thus the latter observation suggests that the SOS response may be not only a last resort in time of distress but also a means permitting better adaptation of the cells to their environment.After five years of heroic struggle against cancer, Jacqueline George passed away 14 August 1979. Despite weakened health and debilitating therapy she continued to stimulate and participate in the work of the microbial genetics group which she had created     

cAMP uncouples DNA synthesis and cell division in Tetrahymena

Tetrahymena cells were synchronized by a differential density labeling method. Millimolar concentrations of db-cAMP will cause a delay in the division of the synchronized cells only if added before late S period. The effective concentrations will also cause a precocious initiation of DNA synthesis during G2 just prior to cell division, suggesting that the cyclic nucleotide causes an uncoupling of the program of DNA synthesis and that of karyo- and cytokinesis.     

Mutations in the pilz group genes disrupt the microtubule cytoskeleton and uncouple cell cycle progression from cell division in Arabidopsis embryo and endosperm

Organised cell division and expansion play important roles in plant embryogenesis. To address their cellular basis, we have analysed Arabidopsis abnormal-embryo mutants which were isolated for their characteristic phenotype: mutant embryos are small, mushroom-shaped ("pilz") and consist of only one or few large cells each containing one or more variably enlarged nuclei and often cell wall stubs. These 23 mutants represent four genes, PFIFFERLING, HALLIMASCH, CHAMPIGNON, and PORCINO, which map to different chromosomes. All four genes have very similar mutant phenotypes although porcino embryos often consisted of only one large cell. The endosperm did not cellularise and contained a variably reduced number of highly enlarged nuclei. By contrast, genetic evidence suggests that these genes are not required for gametophyte development. Expression of cell cycle genes, Cdc2a, CyclinA2 and CyclinB1, and the cytokinesis-specific KNOLLE gene was not altered in mutant embryos. However, KNOLLE syntaxin accumulated in patches but no KNOLLE-positive structure resembling a forming cell plate occurred in mitotic cells. A general defect in microtubule assembly was observed in all mutants. Interphase cells lacked cortical microtubules, and spindles were absent from mitotic nuclei although in rare cases, short stubs of microtubules were attached to partially condensed chromosomes. Our results suggest that the cellular components affected by the pilz group mutations are necessary for continuous microtubule organisation, mitotic division and cytokinesis but do not mediate cell cycle progression.     

Drc1p/Cps1p, a 1,3-beta-glucan synthase subunit, is essential for division septum assembly in Schizosaccharomyces pombe.

Schizosaccharomyces pombe divides by medial fission through the use of an actomyosin-based contractile ring. A division septum is formed centripetally, concomitant with ring constriction. Although several genes essential for cytokinesis have been described previously, enzymes that participate in the assembly of the division septum have not been identified. Here we describe a temperature-sensitive mutation, drc1-191, that prevents division septum assembly and causes mutant cells to arrest with a stable actomyosin ring. Unlike the previously characterized cytokinesis mutants, which undergo multiple mitotic cycles, drc1-191 is the first cytokinesis mutant that arrests with two interphase nuclei. Interestingly, unlike drc1-191, drc1-null mutants proceed through multiple mitotic cycles, leading to the formation of large cells with many nuclei. drc1 is allelic to cps1, which encodes a 1,3-beta-glucan synthase subunit. We conclude that Drc1p/Cps1p is not required for cell elongation and cell growth, but plays an essential role in assembly of the division septum. Furthermore, it appears that constriction of the actomyosin ring might depend on assembly of the division septum. We discuss possible mechanisms that account for the differences in the phenotypes of the drc1-191 and the drc1-null mutants and also reflect the potential links between Drc1p and other cytokinesis regulators.     

A pupal lethal mutation with a paternally influenced maternal effect on embryonic development in Drosophila melanogaster

The maternal effect and zygotic phenotype of l(1)pole hole (l(1)ph) is described. l(1)ph is a zygotic lethal mutation which affects cell division of adult precursor cells in Drosophila larvae. The locus is located in 2F6 on the salivary gland chromosome map and four alleles have been characterized. Germ-line clonal analysis of amorphic alleles indicates that l(1)ph has a maternal effect lethal phenotype. Two lethal phenotypes are observed among embryos derived from female germ-line clones homozygous for amorphic alleles dependent upon the zygotic activity of l(1)ph⁺ introduced via the sperm. Class 1: If no wild-type dose of the gene is introduced, embryos form abnormal blastoderms in which nuclear migration and cell formation is disrupted leading to an ill-defined cuticular pattern. Class 2: If a wild-type copy of the gene is introduced, blastoderm cells do not form beneath the pole cells (the pole hole phenotype); subsequently such embryos are missing cuticular structures posterior to the seventh abdominal segment (the torso phenotype). When the zygotic activity l(1)ph⁺ is modulated using position effect variegation a new phenotype is observed among class 2 embryos in which torso embryos are twisted along their longitudinal axis.     

开口箭大小孢子发生及雌雄配子体发育

利用石蜡切片技术,对百合科植物开口箭(Tupistra chinensis Baker)大小孢子发生及雌雄配子体发育进程进行胚胎学观察分析,以明确开口箭胚胎发育的特征,为百合科植物的研究提供生殖生物学依据。结果表明:(1)开口箭花药具有4个药室,花药壁的发育方式为基本型,由表皮、药室内壁、中层及绒毡层组成;绒毡层发育类型为分泌型,到四分体花药阶段绒毡层细胞开始解体退化,花药成熟时完全消失。(2)花粉母细胞减数分裂为连续型,依次形成二分体、四分体,四分体为左右对称形;成熟花粉为2-细胞花粉,具单萌发沟。(3)子房3室,倒生型胚珠6枚,双珠被,薄珠心;在花部的分化早期,由珠心顶端表皮下方分化出雌性孢原细胞,孢原细胞经过一次平周分裂形成周缘细胞和造孢细胞,造孢细胞发育为大孢子母细胞;大孢子母细胞第一次减数分裂后形成二分体,珠孔端的二分体孢子退化,合点端的二分体孢子继续第二次分裂,形成两个子细胞依次发育为二核胚囊、四核胚囊和八核胚囊;开口箭的胚囊发育类型为葱型。     

The Arabidopsis APC4 subunit of the anaphase-promoting complex/cyclosome (APC/C) is critical for both female gametogenesis and embryogenesis

The anaphase‐promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that is involved in regulating cell‐cycle progression. It has been widely studied in yeast and animal cells, but the function and regulation of the APC/C in plant cells are largely unknown. The Arabidopsis APC/C comprises at least 11 subunits, only a few of which have been studied in detail. APC4 is proposed to be a connector in the APC/C in yeast and animals. Here, we report the functional characterization of the Arabidopsis APC4 protein. We examined three heterozygous plant lines carrying apc4 alleles. These plants showed pleiotropic developmental defects in reproductive processes, including abnormal nuclear behavior in the developing embryo sac and aberrant cell division in embryos; these phenotypes differ from those reported for mutants of other subunits. Some ovules and embryos of apc4/+ plants also accumulated cyclin B protein, a known substrate of APC/C, suggesting a compromised function of APC/C. Arabidopsis APC4 was expressed in meristematic cells of seedlings, ovules in pistils and embryos in siliques, and was mainly localized in the nucleus. Additionally, the distribution of auxin was distorted in some embryos of apc4/+ plants. Our results indicate that Arabidopsis APC4 plays critical roles in female gametogenesis and embryogenesis, possibly as a connector in APC/C, and that regulation of auxin distribution may be involved in these processes.     

Cytokinesis-Based Constraints on Polarized Cell Growth in Fission Yeast

The rod-shaped fission yeast Schizosaccharomyces pombe, which undergoes cycles of monopolar-to-bipolar tip growth, is an attractive organism for studying cell-cycle regulation of polarity establishment. While previous research has described factors mediating this process from interphase cell tips, we found that division site signaling also impacts the re-establishment of bipolar cell growth in the ensuing cell cycle. Complete loss or targeted disruption of the non-essential cytokinesis protein Fic1 at the division site, but not at interphase cell tips, resulted in many cells failing to grow at new ends created by cell division. This appeared due to faulty disassembly and abnormal persistence of the cell division machinery at new ends of fic1Δ cells. Moreover, additional mutants defective in the final stages of cytokinesis exhibited analogous growth polarity defects, supporting that robust completion of cell division contributes to new end-growth competency. To test this model, we genetically manipulated S. pombe cells to undergo new end take-off immediately after cell division. Intriguingly, such cells elongated constitutively at new ends unless cytokinesis was perturbed. Thus, cell division imposes constraints that partially override positive controls on growth. We posit that such constraints facilitate invasive fungal growth, as cytokinesis mutants displaying bipolar growth defects formed numerous pseudohyphae. Collectively, these data highlight a role for previous cell cycles in defining a cell''s capacity to polarize at specific sites, and they additionally provide insight into how a unicellular yeast can transition into a quasi-multicellular state.     

Hormonal and cell division analyses in Watsonia lepida seedlings

The regeneration ability, cell division activity, auxin and cytokinin content of seedling regions and hypocotyl subsections of Watsonia lepida were studied. A total of 21 different cytokinins or conjugates were found in seedlings, with the highest cytokinin content in meristematic regions (root and shoot apical meristems). The greatest contribution to the cytokinin pool came from the biologically inactive cZRMP, suggesting that significant de novo synthesis was occurring. Five different auxins or conjugates were detected, being concentrated largely in the shoot apical meristem and leaves, IAA being the most abundant. Analysis of hypocotyl subsections (C1–C4) revealed that cell division was highest in subsection C2, although regeneration in vitro was significantly lower than in subsection C1. Anatomically, subsection C1 contains the apical meristem, and hence has meristematic cells that are developmentally plastic. In contrast, subsection C2 has cells that have recently exited the meristem and are differentiating. Despite high rates of cell division, cells in subsection C2 appear no longer able to respond to cues that promote proliferation in vitro. Auxin and cytokinin analyses of these subsections were conducted. Possibly, a lower overall cytokinin content, and in particular the free-base cytokinins, could account for this observed difference.