Acid-induced wall loosening is confined to the accelerating region of the root growing zone

The aims of this study were to quantify developmental differences in acid growth along the root axis and to determine whether these differences were due to alterations in cell turgor or cell wall properties. The apoplast pH of maize roots growing in hydroponics was altered from pH 7.0 to pH 3.4 using 2 mol m^-3 citrate-phosphate buffer or unbuffered solutions. Whole root elongation rate rapidly increased and measurement of the local growth profile indicated that this increase in growth occurred in young cells in the accelerating zone (apical 0-4 mm) while more proximal growing cells were unaffected. Unbuffered solutions of identical pH produced qualitatively similar results. Single cell turgor pressures were unchanged between pH treatments both longitudinally and radially in the root tip. This suggests that the rapid acid-induced changes in growth rate were due to an increase in cell wall loosening. Single cell osmotic pressure and water potential were not significantly different between pH treatments. Acid pH caused net solute import at the root tip to increase 3- to 4-fold, which, coupled with the maintenance of turgor and osmotic pressure, indicated that solute import was not limiting expansion. Thus, acidic solutions cause an increase in growth in accelerating but not decelerating regions. It has been shown for the first time that acid growth in intact, growing roots is not due to differences in turgor, assigning these changes to cell wall properties. Possible cell wall biochemical alterations are discussed.     

Cdk1-dependent control of membrane-trafficking dynamics

Cyclin-dependent kinase 1 (Cdk1) is required for initiation and maintenance of polarized cell growth in budding yeast. Cdk1 activates Rho-family GTPases, which polarize the actin cytoskeleton for delivery of membrane to growth sites via the secretory pathway. Here we investigate whether Cdk1 plays additional roles in the initiation and maintenance of polarized cell growth. We find that inhibition of Cdk1 causes a cell surface growth defect that is as severe as that caused by actin depolymerization. However, unlike actin depolymerization, Cdk1 inhibition does not result in a massive accumulation of intracellular secretory vesicles or their cargoes. Analysis of post-Golgi vesicle dynamics after Cdk1 inhibition demonstrates that exocytic vesicles are rapidly mistargeted away from the growing bud, possibly to the endomembrane/vacuolar system. Inhibition of Cdk1 also causes defects in the organization of endocytic and exocytic zones at the site of growth. Cdk1 thus modulates membrane-trafficking dynamics, which is likely to play an important role in coordinating cell surface growth with cell cycle progression.     

Maintenance requirements in Bacillus sphaericus 1593M under dual substrate limitations estimated at zero growth rate in a total cell retention culture

The maintenance requirement(s) exhibited by Bacillus sphaericus1593M was clearly established from the data on the steady state continuous cultures wherein the growth yield was found to be increasing with increase in dilution rate (4). In the present study we have attempted to estimate the maintenance coefficient of B. sphaericus 1593M by achieving zero growth rate in a total cell retention culture (TCRC). Zero growth rate was achieved at a cell density of 5.57?g?l^?1 which remained constant reflecting a steady state in the TCRC. Based on the substrate flux calculations, maintenance coefficient was found to be 0.087?h^?1. Further, using this data of maintenance coefficient, steady state cell density and substrate profiles in continuous cultures of B. sphaericus 1593M were predicted by simulation, which showed a close relationship to that of the experimental data.     

Effect of external pH on yield,morphology and cell components ofCandida utilis growing in continuous culture

The physiological state ofCandida utilis growing in continuous culture was controlled by external pH and dilution rate. The pH of the medium influences the physiological mechanisms coupled with the maintenance of living functions. The consumption of nitrogen source for biomass formation is a linear function of growth rate, independent of external pH. The statistical distribution of cell volumes was identified as log-normal where the dispersion is indirectly proportional to the maintenance coefficient and the mean of the logarithm of cell volume is a linear combination of specific growth rate and maintenance coefficient. The content of RNA, protein, glucan and mannan in dry biomass may also be expressed as a simple function of specific growth rate and maintenance coefficient.     

Systems Analysis of Shoot Apical Meristem Growth and Development: Integrating Hormonal and Mechanical Signaling

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem maintenance and in controlling meristematic properties, such as cell proliferation. Self-organizing transport processes, which are still only partially understood, lead to the patterned accumulation of auxin at particular positions, where organs will grow out. A major downstream target of auxin-mediated growth regulation is the cell wall, which is a determinant for both growth rates and growth distribution, but feedbacks with metabolism and the synthetic capacity of the cytoplasm are crucial as well. Recent work has also pointed at a potential role of mechanical signals in growth coordination, but the precise mechanisms at work remain to be elucidated.     

Emerging aspects of ER organization in root hair tip growth: Lessons from RHD3 and atlastin

Cell polarity is a fundamental aspect of eukaryotic cells. A central question for cell biologists is how the polarity of a cell is established and maintained. Root hairs are exceptionally polarized structures formed from specific root epidermal cells. The morphogenesis of root hairs is characterized by the localized cell growth in a small dome at the tip of the hair, a process called tip growth. Root hairs are thus an attractive model system to study the establishment and maintenance of cell polarity in eukaryotes. Research on Arabidopsis root hairs has identified a plethora of molecular and cellular components that are important for root hair tip growth. Recently, studies on RHD3 and Atlastin have revealed a surprising similarity with respect to the role of the tubular ER network in tip growth of root hairs in plants and the axonal outgrowth of corticospinal neurons in neurological disorders known as hereditary spastic paraplegia (HSP). In this mini-review, we highlight recent progress in understanding of the function and regulation of RHD3 in the generation of the tubular ER network and discussed ways in which RHD3 could be involved in the establishment and maintenance of root hair tip growth.     

The effect of temperature on bacterial degradation of EDTA in pH-auxostat

The effect of temperature on the maximum specific growth rate and the cell yield was studied during cultivation of two bacterial strains (LPM-4 and Pseudomonas sp. LPM-410) on EDTA under unlimited cell growth conditions in a pH-auxostat. Both strains displayed linear dependence of reciprocal biomass yield against reciprocal specific growth rate, from which the values of rate of substrate expenditure for cell maintenance and the “maximum” yield (i.e., hypothetical yield without cell maintenance processes) were estimated. Analysis of the maximum yield values based on mass–energy balance theory suggested that oxidation of the carboxylic acid side chains of EDTA by a monooxygenase had zero or low energetic efficiency. An Arrhenius equation with different values of Arrhenius parameters within different temperature ranges gave a good fit with the temperature dependence of both growth rate and biomass yield. Specific growth rates of both strains showed a more pronounced temperature dependence than did the cell yields. A possible kinetic mechanism was suggested which might be responsible for the modes of the temperature dependences of specific growth rate and yield that were found. The mechanism is based on a hypothetical key substance governing the metabolic flows, which is formed in a zero-order reaction and destroyed in a first-order reaction, both rate constants depending on temperature according to the Arrhenius law.     

Minichromosome Maintenance Proteins as Markers for Proliferation Zones During Embryogenesis

Regulation of cell proliferation is of fundamental importance for growth andprevention of cancer. An obligatory step in the regulation of cell proliferation is thecontrol of the initiation of DNA synthesis. The minichromosome maintenance (MCM)proteins are essential DNA replication factors crucial for initiating DNA synthesis onceevery cell cycle. Recent studies show that the level of MCM proteins is stringentlyregulated to correlate with cell proliferation and carcinogenesis. Here we discuss ourrecent data, which highlights the usefulness of minichromosome maintenance (MCM)gene expression for detecting proliferating cells as well as zones containingproliferative/stem cells during embryogenesis in a whole organism.     

Influence of Temperature on Glucose Utilization by Pseudomonas fluorescens

The influence of temperature on the conversion of glucose into cell material and into energy for maintenance was determined for Pseudomonas fluorescens by a steady-state turbidity method and by a substrate utilization method. Conversion of glucose into cell material was measured as yield; conversion of glucose into energy for maintenance was measured as specific maintenance, the minimum dilution rate in continuous culture below which a steady state is not possible. The values obtained by the two methods were nearly identical; with both, the yield and specific maintenance decreased with decreasing temperature. The specific maintenance consumption rate (milligrams of glucose taken up per milligram of cell dry weight per hour at zero growth) was also calculated by the substrate utilization method and found to decrease with decreasing temperature. However, the amount of glucose consumed per generation for maintenance increased with decreasing temperature. This increased glucose consumption for maintenance may provide a partial explanation for the decrease in yield at low temperatures. Small amounts of glucose were also converted into pigment at all temperatures tested, with the greatest amount formed at 20 C.     

Cultivation of Saccharomyces cerevisiae in continuous culture

Summary Glucose limited growth of a respiratory deficient mutant of Saccharomyces cerevisiae was studied in continuous culture under steady state conditions. The maximal growth rate, the Michaelis constant, the cell yield, the maintenance coefficient and the ethanol yield of the growing cell population were determined. The steady state concentrations of cells, glucose and ethanol were measured as functions of the dilution rate and compared with theoretical predictions. A far-reaching agreement between theory and experiment was observed. The decrease of the cell yield in the range of low dilution rates is well explained by introducing the concept of maintenance energy in the general theory of continuous cultures. A deviation of the cell yield from the predicted values, which has been found in the range of high dilution rates, is discussed.     

Repair of wounded monolayers of cultured bovine aortic endothelial cells is inhibited by calcium spirulan,a novel sulfated polysaccharide isolated from Spirulina platensis

Calcium spirulan (Ca-SP) is a novel sulfated polysaccharide isolated from a blue-green alga Spirulina platensis. Ca-SP inhibits thrombin by activation of heparin cofactor II. Therefore, it could serve as an origin of anti-atherogenic medicines. Since maintenance of vascular endothelial cell monolayers is important for prevention of vascular lesions such as atherosclerosis, the effect of Ca-SP at 20 microg/ml or less on the repair of wounded bovine aortic endothelial cell monolayers in culture was investigated in the present study. When the monolayers were wounded and cultured in the presence of Ca-SP, the polysaccharide inhibited the appearance of the cells in the wounded area. The inhibition was also observed even when the repair was promoted by excess basic fibroblast growth factor, which is one of the autocrine growth factors that are involved in the endothelial cell monolayer maintenance. On the other hand, Ca-SP inhibited the cell growth and the incorporation of [3H]thymidine into the acid-insoluble fraction of proliferating endothelial cells, suggesting that Ca-SP inhibits endothelial cell proliferation. From these results, it is concluded that Ca-SP may retard the repair process of damaged vascular endothelium through inhibition of vascular endothelial cell proliferation by induction of a lower ability to respond to stimulation by endogenous basic fibroblast growth factor.     

Negative control of cell proliferation in eukaryotes

Abstract. Control over cell growth in eukaryotes is predominantly achieved by regular transition of cells from proliferation to rest and vice versa as a result of a co-ordinated inter-relationship between intracellular growth inhibitors and extracellular growth stimulators (mitogens). The ability to cease and resume growth on demand implies the existence of a refined intracellular regulatory network including both positive and negative control elements. We review here evidence that resting cells are able to produce molecules with antiproliferative activity, some of which behave as short-lived repressor proteins. A number of genes coding for growth inhibitory molecules have been identified. However, it is not yet certain whether the same molecules ensure the maintenance of a resting state. It has become apparent that immediate growth arrest or growth resumption require not only a rapid production of inhibitors and stimulators but also their biochemical transformation (e.g. phosphorylation or dephosphorylation) and/ or translocation within the cell, whereby one and the same molecule can be a growth inhibitor or fulfil some other function in the cell cycle, according to circumstances or context. At present, three levels of negative cell growth control can be tentatively outlined: 1 rapid appearance of growth inhibitory molecules to bring about temporary arrest at critical checkpoints of the proliferative cycle; 2 transition of a cell to proliferative rest with continuous production of growth inhibitor(s); 3 long-lasting maintenance of the resting state provided for by complex intracellular changes not connected with production of growth inhibitor(s).     

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.     

Apical meristem exhaustion during determinate primary root growth in the moots koom 1 mutant of Arabidopsis thaliana

An indeterminate developmental program allows plant organs to grow continuously by maintaining functional meristems over time. The molecular mechanisms involved in the maintenance of the root apical meristem are not completely understood. We have identified a new Arabidopsis thaliana mutant named moots koom 1 (mko1) that showed complete root apical meristem exhaustion of the primary root by 9?days post-germination. MKO1 is essential for maintenance of root cell proliferation. In the mutant, cell division is uncoupled from cell growth in the region corresponding to the root apical meristem. We established the sequence of cellular events that lead to meristem exhaustion in this mutant. Interestingly, the SCR and WOX5 promoters were active in the mko1 quiescent center at all developmental stages. However, during meristem exhaustion, the mutant root tip showed defects in starch accumulation in the columella and changes in auxin response pattern. Therefore, contrary to many described mutants, the determinate growth in mko1 seedlings does not appear to be a consequence of incorrect establishment or affected maintenance of the quiescent center but rather of cell proliferation defects both in stem cell niche and in the rest of the apical meristem. Our results support a model whereby the MKO1 gene plays an important role in the maintenance of the root apical meristem proliferative capacity and indeterminate root growth, which apparently acts independently of the SCR/SHR and WOX5 regulatory pathways.     

Stoichiometric and kinetic characterisation of Nitrobacter in mixed culture by decoupling the growth and energy generation processes

The growth, maintenance and lysis processes of Nitrobacter were characterised. A Nitrobacter culture was enriched in a sequencing batch reactor (SBR). Fluorescent in situ hybridisation showed that Nitrobacter constituted 73% of the bacterial population. Batch tests were carried out to measure the oxygen uptake rate and/or nitrite consumption rate when both nitrite and CO2 were in excess, and in the absence of either of these two substrates. The results obtained, along with the SBR performance data, allowed the determination of the maintenance coefficient and in situ cell lysis rate of Nitrobacter. Nitrobacter spends a significant amount of energy for maintenance, which varies considerably with the specific growth rate. At maximum growth, Nitrobacter consume nitrite at a rate of 0.042 mgN/mgCOD(biomass) . h for maintenance purposes, which increases more than threefold to 0.143 mgN/mgCOD(biomass) . h in the absence of growth. In the SBR, where Nitrobacter grew at 40% of its maximum growth rate, a maintenance coefficient of 0.113 mgN/mgCOD . h was found, resulting in 42% of the total amount of nitrite being consumed for maintenance. The above three maintenance coefficient values obtained at different growth rates appear to support the maintenance model proposed in Pirt (1982). The in situ lysis rate of Nitrobacter was determined to be 0.07/day under aerobic conditions at 22 degrees C and pH 7.3. Further, the maximum specific growth rate of Nitrobacter was estimated to be 0.02/h (0.48/day). The affinity constant of Nitrobacter with respect to nitrite was determined to be 1.50 mgNO2(-)-N/L, independent of the presence or absence of CO2.     

Microbial Maintenance: A Critical Review on Its Quantification

Microbial maintenance is an important concept in microbiology. Its quantification, however, is a subject of continuous debate, which seems to be caused by (1) its definition, which includes nongrowth components other than maintenance; (2) the existence of partly overlapping concepts; (3) the evolution of variables as constants; and (4) the neglect of cell death in microbial dynamics. The two historically most important parameters describing maintenance, the specific maintenance rate and the maintenance coefficient, are based on partly different nongrowth components. There is thus no constant relation between these parameters and previous equations on this subject are wrong. In addition, the partial overlap between these parameters does not allow the use of a simple combination of these parameters. This also applies for combinations of a threshold concentration with one of the other estimates of maintenance. Maintenance estimates should ideally explicitly describe each nongrowth component. A conceptual model is introduced that describes their relative importance and reconciles the various concepts and definitions. The sensitivity of maintenance on underlying components was analyzed and indicated that overall maintenance depends nonlinearly on relative death rates, relative growth rates, growth yield, and endogenous metabolism. This quantitative sensitivity analysis explains the felt need to develop growth-dependent adaptations of existing maintenance parameters, and indicates the importance of distinguishing the various nongrowth components. Future experiments should verify the sensitivity of maintenance components under cellular and environmental conditions.     

Yeast src homology region 3 domain-binding proteins involved in bud formation

The yeast protein Bem1p, which bears two src homology region 3 (SH3) domains, is involved in cell polarization. A Rho-type GTPase, Rho3p, is involved in the maintenance of cell polarity for bud formation, and the rho3 defect is suppressed by a high dose of BEM1. Mutational analysis revealed that the second SH3 domain from the NH2 terminus (SH3-2) of Bem1p is important for the functions of Bem1p in bud formation and in the suppression of the rho3 defect. Boi2p, which bound to SH3-2 Bem1p, was identified using the two-hybrid system. Boi2p has a proline-rich sequence that is critical for displaying the Boi2p-Bem1p two-hybrid interaction, an SH3 domain in its NH2-terminal half, and a pleckstrin homology domain in its COOH-terminal half. A BOI2 homologue, BOI1, was identified as a gene whose overexpression inhibited cell growth. Cells overexpressing either BOI1 or BOI2 were arrested as large, round, and unbudded cells, indicating that the Boi proteins affect cell polarization. Genetic analysis revealed that BOI1 and BOI2 are functionally redundant and important for cell growth. delta boi1 delta boi2 cells became large round cells or lysed with buds, displaying defects in bud formation and in the maintenance of cell polarity. Analysis using several truncated versions of BOI2 revealed that the COOH-terminal half, which contains the pleckstrin homology domain is essential for the function of Boi2p in cell growth, while the NH2- terminal half is not, and the NH2-terminal half might be required for modulating the function of Bem1p. Overproduction of either Rho3p or the Rho3p-related GTPase Rho4p suppressed the boi defect. These results demonstrate that Rho3p GTPases and Boi proteins function in the maintenance of cell polarity for bud formation.     

Microbial growth dynamics on the basis of individual budgets

The popular theories for microbial dynamics by Monod, Pirt and Droop are shown to be special cases of a model for individual budgets, in which growth and maintenance are on the expense of reserve materials. The dynamics of reserve materials is a first order process with a relaxation time proportional to cell length; maintenance is proportional to cell volume, and uptake, which depends hyperbolically on substrate density, is proportional to cell volume as well. Because of the latter, population dynamics depends on the behaviour of the individuals in a simple way, such that the cell volume distribution has no quantitative effect.When uptake is proportional to the surface area of the cell, which is realistic from a physical point of view, the relation between the individual level and the population one becomes more complicated and the cell size and shape distribution affects population dynamics. It is shown how the changing shape of rods modifies uptake and, consequently, growth.The concept of energy conductance, defined as the ratio, of the maximum surface area specific uptake and the volume specific energy reserve has been introduced in the analysis of microbial dynamics. The first tentative results indicate that the value for E. coli is close to the mean value for a wide variety of animals.Properties of the model for cell suspension at constant substrate densities are analyzed and tested against a variety of experimental data from the literature on both the individual and the population level.