Ectopic divisions in vascular and ground tissues of Arabidopsis thaliana result in distinct leaf venation defects

Leaf venation patterns vary considerably between species and between leaves within a species. A mechanism based on canalization of auxin transport has been suggested as the means by which plastic yet organized venation patterns are generated. This study assessed the plasticity of Arabidopsis thaliana leaf venation in response to ectopic ground or procambial cell divisions and auxin transport inhibition (ATI). Ectopic ground cell divisions resulted in vascular fragments between major veins, whereas ectopic procambial cell divisions resulted in additional, abnormal vessels along major veins, with more severely perturbed lines forming incomplete secondary and higher-order venation. These responses imply limited vascular plasticity in response to unscheduled cell divisions. Surprisingly, a combination of ectopic ground cell divisions and ATI resulted in massive vascular overgrowth. It is hypothesized that the vascular overproduction in auxin transport-inhibited wild-type leaves is limited by simultaneous differentiation of ground cells into mesophyll cells. Ectopic ground cell divisions may negate this effect by providing undifferentiated ground cells that respond to accumulated auxin by differentiation into vascular cells.     

Key divisions in the early Arabidopsis embryo require POL and PLL1 phosphatases to establish the root stem cell organizer and vascular axis

Arabidopsis development proceeds from three stem cell populations located at the shoot, flower, and root meristems. The relationship between the highly related shoot and flower stem cells and the very divergent root stem cells has been unclear. We show that the related phosphatases POL and PLL1 are required for all three stem cell populations. pol pll1 mutant embryos lack key asymmetric divisions that give rise to the root stem cell organizer and the central vascular axis. Instead, these cells divide in a superficially symmetric fashion in pol pll1 embryos, leading to a loss of embryonic and postembryonic root stem cells and vascular specification. We present data that show that POL/PLL1 drive root stem cell specification by promoting expression of the WUS homolog WOX5. We propose that POL and PLL1 are required for the proper divisions of shoot, flower, and root stem cell organizers, WUS/WOX5 gene expression, and stem cell maintenance.     

Dynamic analysis of epidermal cell divisions identifies specific roles for COP10 in Arabidopsis stomatal lineage development

Stomatal development in Arabidopsis thaliana has been linked to photoreceptor-perceived light through several components of the photomorphogenic switch, whose lack of function is often seedling-lethal. CONSTITUTIVE PHOTOMORPHOGENIC 10 (COP10) is an important component of this switch, its loss of function producing stomatal clusters. Exploiting the reduced lethality of the cop10-1 mutant we characterized the developmental basis of its stomatal phenotype. Constitutive, light-independent stomata overproduction accounts for half of cop10-1 stomatal abundance and appears very early in development. Clusters are responsible for the remaining stomata excess and build-up progressively at later stages. Serial impressions of living cotyledon epidermis allowed a dynamic, quantitative analysis of stomatal lineage types by reconstructing their division histories. We found that COP10 adjusts the initiation frequency and extension of stomatal lineages (entry and amplifying asymmetric divisions) and represses stomatal fate in lineage cells; COP10 also supervises the orientation of spacing divisions in satellite lineages, preventing the appearance of stomata in contact. Aberrant accumulation of the proliferating stomatal lineage cell marker TMMpro::TMM-GFP showed that the abundant cop10-1 stomatal lineages maintained extended and ectopic competence for stomatal fate. Expression of stomatal development master genes suggests that the mutant does not bypass major molecular actors in this process. cop10-1 first leaf produces trichomes and apparently normal pavement cells, but functionally and morphologically aberrant stomata; COP10 operates genetically in parallel to the stomatal repressor SDD1 and does not generally affect epidermal cell differentiation, but seems to operate on stomatal lineages where it controls specific cell-lineage and cell-signaling developmental mechanisms.     

Hypothesis: cell volume limits cell divisions

Mammalian somatic cells and also cells of the yeast Saccharomyces cerevisiae are capable of undergoing a limited number of divisions. Reaching the division limit is referred to, apparently not very fortunately, as replicative aging. A common feature of S. cerevisiae cells and fibroblasts approaching the limit of cell divisions in vitro is attaining giant volumes. In yeast cells this phenomenon is an inevitable consequence of budding so it is not causally related to aging. Therefore, reaching a critically large cell volume may underlie the limit of cell divisions. A similar phenomenon may limit the number of cell divisions of cultured mammalian cells. The term replicative (generative) aging may be therefore illegitimate.     

Synchronous cell divisions in Anacystis nidulans Richter

Summary The blue-green alga, Anacystis nidulans was successfully synchronized in continuous light (15,000 lux) by a 14 hr cycle, consisting of 8 hrs at 26°C, and 6 hrs at 32°C, coupled with periodic dilution of the algal suspension to a constant cell number at the end of each cycle. The alga continues to grow at the lower temperature, and a division burst begins 2 hrs after transfer to the higher temperature.

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Zusammenfassung Die Cyanophycee Anacystis nidulans wurde in Dauerlicht von 15 000 Lux vollständig synchronisiert. Die Cyclen bestehen aus 8 Std bei 26° C und 6 Std bei 32° C, an ihrem Ende wurde jeweils mit frischer Nährlösung auf die Ausgangszellzahl von 3·10⁷ Zellen/ml verdünnt. Bei beiden Temperaturen kann sich die Alge vermehren, unter den synchronisierenden Bedingungen des Temperaturwechsels beginnt der Teilungsschub (Verdopplung der Zellzahl) bei 32° C nach 2 Std.

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Pringsheim (1968) renamed this alga as Lauterbornia nidulans.     

Real-time lineage analysis reveals oriented cell divisions associated with morphogenesis at the shoot apex of Arabidopsis thaliana

Precise knowledge of spatial and temporal patterns of cell division, including number and orientation of divisions, and knowledge of cell expansion, is central to understanding morphogenesis. Our current knowledge of cell division patterns during plant and animal morphogenesis is largely deduced from analysis of clonal shapes and sizes. But such an analysis can reveal only the number, not the orientation or exact rate, of cell divisions. In this study, we have analyzed growth in real time by monitoring individual cell divisions in the shoot apical meristems (SAMs) of Arabidopsis thaliana. The live imaging technique has led to the development of a spatial and temporal map of cell division patterns. We have integrated cell behavior over time to visualize growth. Our analysis reveals temporal variation in mitotic activity and the cell division is coordinated across clonally distinct layers of cells. Temporal variation in mitotic activity is not correlated to the estimated plastochron length and diurnal rhythms. Cell division rates vary across the SAM surface. Cells in the peripheral zone (PZ) divide at a faster rate than in the central zone (CZ). Cell division rates in the CZ are relatively heterogeneous when compared with PZ cells. We have analyzed the cell behavior associated with flower primordium development starting from a stage at which the future flower comprises four cells in the L1 epidermal layer. Primordium development is a sequential process linked to distinct cellular behavior. Oriented cell divisions, in primordial progenitors and in cells located proximal to them, are associated with initial primordial outgrowth. The oriented cell divisions are followed by a rapid burst of cell expansion and cell division, which transforms a flower primordium into a three-dimensional flower bud. Distinct lack of cell expansion is seen in a narrow band of cells, which forms the boundary region between developing flower bud and the SAM. We discuss these results in the context of SAM morphogenesis.     

The ndhF chloroplast gene detected in all vascular plant divisions

A 335-bp segment of the NADH dehydrogenase F (ndhF) gene from a representative of each nonflowering vascular plant division (Coniferophyta, Filicophyta, Ginkgophyta, Gnetophyta, Lycophyta, Psilophyta, Sphenophyta) has been sequenced and aligned with those of rice, tobacco, an orchid and a liverwort. Because ndhF is apparently absent in the genus Pinus L. (Coniferophyta), it has been speculated that this gene may be absent in the gymnosperms. However, this study suggests that the absence of the ndhF gene in Pinus may be unique and is not a general characteristic of the gymnosperms.Abbreviations ndhF NADH dehydrogenase F - PCR polymerase chain reactionThis research was supported by grants from NSF LASER/EPS-CoR 92-96-ADP-02, LEGSF RD-A-13 (1991–1994), and by the Department of Plant Biology, Louisiana State University.     

中枢神经系统发育的不对称细胞分裂机制

无论在无脊椎动物还是脊椎动物中,组成中枢神经系统（CNS）的大多数细胞都是由极性神经祖细胞不对称分裂而来。通过简要综述果蝇（Drosophila melanogaste）成神经母细胞（NB）不对称分裂机制,并与近年来在脊椎动物不对称细胞分裂上取得的研究成果相比较,尝试找出两个系统的相似性和相异性。     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Oriented cell divisions and muscle satellite cell heterogeneity

Satellite cells are crucial for maintaining muscle homeostasis and for regeneration following injury. In this issue, Kuang et al. (2007) reveal that muscle satellite cells are a heterogeneous mixture of stem cells and committed myogenic progenitors. They show that asymmetric division of stem cells in the satellite cell niche is a mechanism for generating these two populations.     

不对称分裂中命运决定子的调节机制

不对称细胞分裂是果蝇等无脊椎动物以及脊椎动物神经发生过程中神经干细胞分化的基本机制.命运决定子的极性定位及其选择性分配,作为不对称细胞分裂中的重要环节,在子细胞命运决定方面发挥至关重要的作用.本文综述了在中枢及外周神经系统发育期间,不对称分裂中调节Numb等命运决定子靶向定位的影响因素及命运决定子的效应机制,并简要探讨命运决定子调节机制的进化保守性.     

Oriented cell divisions in the extending germband of Drosophila

Tissue elongation is a general feature of morphogenesis. One example is the extension of the germband, which occurs during early embryogenesis in Drosophila. In the anterior part of the embryo, elongation follows from a process of cell intercalation. In this study, we follow cell behaviour at the posterior of the extending germband. We find that, in this region, cell divisions are mostly oriented longitudinally during the fast phase of elongation. Inhibiting cell divisions prevents longitudinal deformation of the posterior region and leads to an overall reduction in the rate and extent of elongation. Thus, as in zebrafish embryos, cell intercalation and oriented cell division together contribute to tissue elongation. We also show that the proportion of longitudinal divisions is reduced when segmental patterning is compromised, as, for example, in even skipped (eve) mutants. Because polarised cell intercalation at the anterior germband also requires segmental patterning, a common polarising cue might be used for both processes. Even though, in fish embryos, both mechanisms require the classical planar cell polarity (PCP) pathway, germband extension and oriented cell divisions proceed normally in embryos lacking dishevelled (dsh), a key component of the PCP pathway. An alternative means of planar polarisation must therefore be at work in the embryonic epidermis.     

A nuclear protein associated with cell divisions in plants

A nuclear protein, present in carrot meristems and rapidly proliferating cultured cells of carrot (Daucus carota L.) has been identified by the use of a monoclonal antibody (MAb 21D7). By combining the techniques of two-dimensional polyacrylamide gel analysis and blotting separated proteins onto nitrocellulose sheets, it was shown that the antibody detected a single polypeptide of apparent molecular mass (M _r) of 45000 and an isoelectric focusing point (pI) of 6.7. This protein was found by subcellular fractionation and immunofluorescence to be highly concentrated in the nucleoli of somatic and zygotic embryos of a wide range of plants. It was not detectable in logarthmically growing cells ofEscherichia coli, yeast, embryos ofDrosophila melanogaster or cultured C3H mouse cells. These data indicate that this protein is a highly conserved non-histone protein associated with nuclei of rapidly dividing plant cells.Abbreviations M _r apparent molecular mass - Da dalton - Ig immunoglobulins - MAb monoclonal antibody - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - 2-D gel two-dimensional gel electrophoresis - 2,4-D 2,4-dichlorophenoxyacetic acid     

Supernumerary cell divisions following meiosis in the spider plant

A cultivar ofChlorophytum elatum var.variegatum (spider plant, 2n=28) from Agra, India, is characterized by one, two or three supernumerary divisions of the cells of the microspore tetrad. During these divisions the chromosomes neither synapse nor split, nor are any spindles formed. The distribution of chromosomes is random, with the result that the microspores vary considerably in chromosome number. Only a small percentage of the pollen produced has the full haploid set. The genetics of supernumerary divisions is still under investigation.