Cell shape and cell division

The correlation between cell shape elongation and the orientation of the division axis described by early cell biologists is still used as a paradigm in developmental studies. However, analysis of early embryo development and tissue morphogenesis has highlighted the role of the spatial distribution of cortical cues able to guide spindle orientation. In vitro studies of cell division have revealed similar mechanisms. Recent data support the possibility that the orientation of cell division in mammalian cells is dominated by cell adhesion and the associated traction forces developed in interphase. Cell shape is a manifestation of these adhesive and tensional patterns. These patterns control the spatial distribution of cortical signals and thereby guide spindle orientation and daughter cell positioning. From these data, cell division appears to be a continuous transformation ensuring the maintenance of tissue mechanical integrity.     

Manipulation of starch granule size distribution in potato tubers by modulation of plastid division

Starch granule size is an important parameter for starch applications in industry. Starch granules are formed in amyloplasts, which are, like chloroplasts, derived from proplastids. Division processes and associated machinery are likely to be similar for all plastids. Essential roles for FtsZ proteins in plastid division in land plants have been revealed. FtsZ forms the so-called Z ring which, together with inner and outer plastid division rings, brings about constriction of the plastid. It has been shown that modulation of the expression level of FtsZ may result in altered chloroplast size and number. To test whether FtsZ is also involved in amyloplast division and whether this, in turn, may affect the starch granule size in crop plants, FtsZ protein levels were either reduced or increased in potato. As shown previously in other plant species, decreased StFtsZ1 protein levels in leaves resulted in a decrease in the number of chloroplasts in guard cells. More interestingly, plants with increased StFtsZ1 protein levels in tubers resulted in less, but larger, starch granules. This suggests that the stoichiometry between StFtsZ1 and other components of the plastid division machinery is important for its function. Starch from these tubers also had altered pasting properties and phosphate content. The importance of our results for the starch industry is discussed.     

Surface and shape changes during cell division

Summary Rat kangaroo cells (PtK₂) were studied with scanning and transmission electron microscopy in order to correlate shape changes during the cell cycle with the presence or absence of microvilli and stress fibers. During interphase, bundles of actin are prominent in the cytoplasm, and microvilli are localized over and around the centrally positioned nucleus. As mitosis begins, the interphase bundles of actin and the microvilli disappear, but the mitotic cells maintain a flattened shape. At metaphase the cell is still so flat that both the chromosomes and spindle apparatus are visible through the intact cell membrane. Microvilli reappear in late anaphase above the chromosomes and poles. Before cleavage begins, microvilli increase in number until they cover the apical surface of the cell. At the same time, the cell increases in height so that the chromosomes and mitotic apparatus can no longer be detected through the cell membrane. During cleavage, microvilli continue to cover the cell in a uniform manner but become greatly diminished in number after cytokinesis is completed and the cells flatten and enter interphase. It is suggested that the microvilli organize a network of actin filaments which interact with cortical myosin to produce the cell rounding prior to cleavage.     

Organ shape: controlling oriented cell division

One mechanism by which organisms control the shape of growing organs is by regulating the orientation of cell divisions. This occurs in the Drosophila wing under the control of genes previously implicated in regulating cell polarity.     

SUN regulates vegetative and reproductive organ shape by changing cell division patterns

One of the major genes controlling the elongated fruit shape of tomato (Solanum lycopersicum) is SUN. In this study, we explored the roles of SUN in vegetative and reproductive development using near isogenic lines (NILs) that differ at the sun locus, and SUN overexpressors in both the wild species LA1589 (Solanum pimpinellifolium) and the cultivar Sun1642 background. Our results demonstrate that SUN controls tomato shape through redistribution of mass that is mediated by increased cell division in the longitudinal and decreased cell division in the transverse direction of the fruit. The expression of SUN is positively correlated with slender phenotypes in cotyledon, leaflet, and floral organs, an elongated ovary, and negatively correlated with seed weight. Overexpression of SUN leads to more extreme phenotypes than those shown in the NILs and include thinner leaf rachises and stems, twisted leaf rachises, increased serrations of the leaflets, and dramatically increased elongation at the proximal end of the ovary and fruit. In situ hybridizations of the NILs showed that SUN is expressed throughout the ovary and young fruit, particularly in the vascular tissues and placenta surface, and in the ovules and developing seed. The phenotypic effects resulting from high expression of SUN suggest that the gene is involved in several plant developmental processes.     

The effect of oxidizing agents on cell division and shape

Certain oxidizing agents such as vitaminK(VK) and lipid peroxides were found to suppress an increase in cytoplasmic Ca2+ concentration by growth factors, and inhibit on cell proliferation. These oxidizing agents induced a marked change in cell shape. In a detailed analysis of each phase in the cell cycle, the inhibition of an increase in cytoplasmic Ca2+ and cell division occurred only when the agents were added at G0/G1 phase. The addition to S or M phase cells did not influence in cytoplasmic Ca2+ and cell division. These experimental results suggest that these oxidizing agents may inhibit the transfer of stimulation signals from growth factors by acting on cell membrane sites and suppress subsequent DNA replication and mitotic division.     

Orientation of cortical microtubules correlates with cell shape and division direction

Summary Cortical microtubules in the epidermis of regeneratingGraptopetalum plants were examined by in situ immunofluorescence. Paradermal slices of tissue were prepared by a method that preserves microtubule arrays and also maintains cell junctions. To test the hypothesis that cortical microtubule arrays align perpendicular to the direction of organ growth, arrays were visualized and their orientation quantified. A majority of microtubules are in transverse orientation with respect to the organ axis early in shoot development when the growth habit is uniform. Later in development, when growth habit is non-uniform and the tissue is contoured, cortical microtubules are increasingly longitudinal and oblique in orientation. Microtubules show only a minor change in orientation at the site of greatest curvature, the transition zone of a developing leaf. To assess the role of the division plane on orientation of arrays, the pattern of microtubules was examined in individual cells of common shape. Cells derived from transverse divisions have predominately transverse cortical arrays, whereas cells derived from oblique and longitudinal divisions have non-transverse arrays. The results show that, regardless of the stage of development, microtubules orient with respect to cell shape and plane of division. The results suggest that cytoskeletal function is best considered in small domains of growth within an organ.Abbrevations DMSO dimethylsulfoxide - EGTA ethylene glycol-bis-(ß-aminoethyl ether)-N, N, N, N-tetra acetic acid - FITC fluorescein isothiocyanate - MTSB microtubule stabilizing buffer - PBS phosphate buffered saline     

Development of leaf shape

Variation among vascular plants in the initiation and patterning of leaves results in a diverse array of leaf shape, including the strap-like leaf of many grasses and the broad lamina of most eudicots. Recent findings highlight the importance of interactions between the shoot apical meristem (SAM) and developing leaf primordia in axis specification and the establishment of leaf shape. Global regulators of epigenetic states have been implicated in these interactions and may play a role in distinguishing founder cells and stem cells within the SAM.     

Programmed cell death remodels lace plant leaf shape during development

Programmed cell death (PCD) functions in the developmental remodeling of leaf shape in higher plants, a process analogous to digit formation in the vertebrate limb. In this study, we provide a cytological characterization of the time course of events as PCD remodels young expanding leaves of the lace plant. Tonoplast rupture is the first PCD event in this system, indicated by alterations in cytoplasmic streaming, loss of anthocyanin color, and ultrastructural appearance. Nuclei become terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling positive soon afterward but do not become morphologically altered until late stages of PCD. Genomic DNA is fragmented, but not into internucleosomal units. Other cytoplasmic changes, such as shrinkage and degradation of organelles, occur later. This form of PCD resembles tracheary element differentiation in cytological execution but requires unique developmental regulation so that discrete panels of tissue located equidistantly between veins undergo PCD while surrounding cells do not.     

Leaf expansion and cell division are affected by reducing absorbed light before but not after the decline in cell division rate in the sunflower leaf

The effect of absorbed photosynthetic photon flux density (PPFD) on leaf expansion is a key issue for analysing the phenotypic variability between plants and for modelling feedback loops. Expansion and epidermal cell division in leaf 8 of sunflower were analysed in a series of five experiments where absorbed photosynthetic photon flux density (PPFD) was reduced either by shading or by covering part of the leaf area. These treatments were imposed at different times during leaf development. Expansion and cell division were affected by a reduction in absorbed PPFD only in the first part of leaf development, while the leaf area was less than 2% of its final value and while absolute expansion rate was slow. In contrast, it was not affected if imposed later when the leaf was visible and absolute expansion rate was at maximum. A reduction in absorbed PPFD caused the same reduction in expansion and in cell division whether it was due to a reduction in incident PPFD or to a reduction in photosynthetic leaf area, suggesting that carbon metabolism was involved. Relative expansion rate recovered to control levels when relative division rate began to decline, in all experiments and in all zones of a leaf. This was probably linked to the source–sink transition, after which the leaf had such a high priority in carbon allocation that it was largely insensitive to changes in absorbed PPFD. The final leaf area was therefore closely related to the cumulated PPFD absorbed by the plant from leaf initiation to the end of exponential cell division.     

Functional modulation of cardiac form through regionally confined cell shape changes

Developing organs acquire a specific three-dimensional form that ensures their normal function. Cardiac function, for example, depends upon properly shaped chambers that emerge from a primitive heart tube. The cellular mechanisms that control chamber shape are not yet understood. Here, we demonstrate that chamber morphology develops via changes in cell morphology, and we determine key regulatory influences on this process. Focusing on the development of the ventricular chamber in zebrafish, we show that cardiomyocyte cell shape changes underlie the formation of characteristic chamber curvatures. In particular, cardiomyocyte elongation occurs within a confined area that forms the ventricular outer curvature. Because cardiac contractility and blood flow begin before chambers emerge, cardiac function has the potential to influence chamber curvature formation. Employing zebrafish mutants with functional deficiencies, we find that blood flow and contractility independently regulate cell shape changes in the emerging ventricle. Reduction of circulation limits the extent of cardiomyocyte elongation; in contrast, disruption of sarcomere formation releases limitations on cardiomyocyte dimensions. Thus, the acquisition of normal cardiomyocyte morphology requires a balance between extrinsic and intrinsic physical forces. Together, these data establish regionally confined cell shape change as a cellular mechanism for chamber emergence and as a link in the relationship between form and function during organ morphogenesis.     

The OKRA leaf shape mutation in cotton is active in all cell layers of the leaf

Okra (L2O) is a semidominant mutation of cotton (Gossypium barbadense) that alters leaf shape by increasing the length of lobes and decreasing lamina expansion. Chimeras containing L2O and wild-type tissue were generated using Semigamy (Se), a mutation that blocks syngamy during fertilization and produces haploid maternal/paternal chimeral progeny at low frequency. In sectorial chimeras, changes in leaf morphology coincide with the boundary between mutant and wild-type tissues, suggesting that L2O does not regulate a laterally diffusible factor within the leaf. However, in mericlinal or periclinal chimeras, the presence of L2O in tissue derived from any of the three histogenic layers (L1, L2, or L3) of the shoot apical meristem produced leaves with a partial mutant phenotype. The presence of L2O in the epidermis (an L1 derivative), or in the subepidermal mesophyll of the leaf (L2 derivatives) reduced the growth of the lamina and thus increased the depth of leaf lobes. The presence of L2O in the middle mesophyll of the lamina and the vasculature of major lateral veins (L3 derivatives) had no local effect on the expansion of the lamina, but significantly increased lobe length. These results demonstrate that L2O is active in every tissue layer of the leaf.     

Cell shape and division in Escherichia coli: experiments with shape and division mutants.

Double mutants which carry mutations in genes (rodA, pbpA) required for cell elongation (i.e., maintenance of rod shape) in combination with mutations in genes (ftsA, ftsI, ftsQ, or ftsZ) required for septation were constructed. Such mutants were able to grow for about two mass doublings at a normal rate at the restrictive temperature (42 degrees C). The morphology of the cells formed under these conditions was interpreted by assuming the existence of a generalized system for peptidoglycan growth together with two additional systems which modify the shape of the growing peptidoglycan layer. The results also showed that different fts genes probably control different stages in septation. ftsZ (sulB or sfiB) appears to be required for the earliest step in septation, ftsQ and ftsI (pbpB or sep) are required for a later step or steps, and ftsA is required only for the latest stages in septation.     

Nuclear localization of endogenous RGK proteins and modulation of cell shape remodeling by regulated nuclear transport

The members of the RGK small GTP-binding protein family, Kir/Gem, Rad, Rem and Rem2, are multifunctional proteins that regulate voltage-gated calcium channel activity and cell shape remodeling. Calmodulin (CaM) or CaM 14-3-3 are regulators of RGK functions and their association defines the subcellular localization of RGK proteins. Abolition of CaM association results in the accumulation of RGK proteins in the nucleus, whereas 14-3-3 binding maintains them in the cytoplasm. Kir/Gem possesses nuclear localization signals (NLS) that mediate nuclear accumulation through an importin alpha5-dependent pathway (see Mahalakshmi RN, Nagashima K, Ng MY, Inagaki N, Hunziker W, Béguin P. Nuclear transport of Kir/Gem requires specific signals and importin alpha5 and is regulated by Calmodulin and predicted service phosphorylations. Traffic 2007; doi: 10.1111/j.1600-0854.2007.00598.x). Because the extent of nuclear localization depends on the RGK protein and the cell type, the mechanism and regulation of nuclear transport may differ. Here, we extend our analysis to the other RGK members and show that Rem also binds importin alpha5, whereas Rad associates with importins alpha3, alpha5 and beta through three conserved NLS. Predicted phosphorylation of a serine residue within the bipartite NLS affects, as observed for Kir/Gem, nuclear accumulation of Rem, but not that of Rad or Rem2. We also identify an additional regulatory phosphorylation for all RGK proteins that prevents binding of 14-3-3 and thereby interferes with their cytosolic relocalization by 14-3-3. Functionally, nuclear localization of RGK proteins contributes to the suppression of RGK-mediated cell shape remodeling. Importantly, we show that endogenous RGK proteins are localized predominantly in the nucleus of individual cells of the brain cortex 'in situ' as well as in primary hippocampal cells, indicating that transport between the nucleus and their site of action in the cytoplasm (i.e., cytoskeleton, endoplasmic reticulum or plasma membrane) is of physiological relevance for the regulation of RGK protein function.     

Effects of nitrogen on mesophyll cell division and epidermal cell elongation in tall fescue leaf blades

Leaf elongation rate (LER) in grasses is dependent on epidermal cell supply (number) and on rate and duration of epidermal cell elongation. Nitrogen (N) fertilization increases LER. Longitudinal sections from two genotypes of tall fescue (Festuca arundinacea Schreb.), which differ by 50% in LER, were used to quantify the effects of N on the components of epidermal cell elongation and on mesophyll cell division. Rate and duration of epidermal cell elongation were determined by using a relationship between cell length and displacement velocity derived from the continuity equation. Rate of epidermal cell elongation was exponential. Relative rates of epidermal cell elongation increased by 9% with high N, even though high N increased LER by 89%. Duration of cell elongation was approximately 20 h longer in the high- than in the low-LER genotype regardless of N treatment. The percentage of mesophyll cells in division was greater in the high- than in the low-LER genotype. This increased with high N in both genotypes, indicating that LER increased with cell supply. Division of mesophyll cells adjacent to abaxial epidermal cells continued after epidermal cell division stopped, until epidermal cells had elongated to a mean length of 40 micrometers in the high-LER and a mean length of 50 micrometers in the low-LER genotype. The cell cycle length for mesophyll cells was calculated to be 12 to 13 hours. Nitrogen increased mesophyll cell number more than epidermal cell number: in both genotypes, the final number of mesophyll cells adjacent to each abaxial epidermal cell was 10 with low N and 14 with high N. A spatial model is used to describe three cell development processes relevant to leaf growth. It illustrates the overlap of mesophyll cell division and epidermal cell elongation, and the transition from epidermal cell elongation to secondary cell wall deposition.     

形态变化对叶片表面温度的影响

干旱区植物叶片形态可塑性是植物适应高温干旱环境的重要生存策略, 但目前仍缺乏直观的数据予以证明。该研究应用热成像技术和图像分析技术, 同步测定真实叶片与模拟叶片的叶温、形态及风速、辐射和温度等环境参数。研究结果显示: 在干旱、高温环境下, 除了蒸腾, 叶片形态变化也是调控叶温的重要因子。干旱区植物叶片变小, 有利于加速叶片与环境的物质及热量交换, 从而达到降低叶温的目的。样地数据显示, 在高温、低风速环境下, 叶片宽度每减少1 cm, 叶片表面温度降低约2.1 ℃, 而模拟叶片叶宽度每减少1 cm, 叶片表面温度降低0.60-0.86 ℃。该研究对深入理解植物生存策略与环境适能力具有重要意义。