Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

Cell elongation and division probability during the Escherichia coli growth cycle.

A new method is presented for determining the growth rate and the probability of cell division (separation) during the cell cycle, using size distributions of cell populations grown under steady-state conditions. The method utilizes the cell life-length distribution, i.e., the probability that a cell will have any specific size during its life history. This method was used to analyze cell length distributions of six cultures of Escherichia coli, for which doubling times varied from 19 to 125 min. The results for each culture are in good agreement with a single model of growth and division kinetics: exponential elongation of cells during growth phase of the cycle, and normal distributions of length at birth and at division. The average value of the coefficient of variation was 13.5% for all strains and growth rates. These results, based upon 5,955 observations, support and extend earlier proposals that growth and division patterns of E. coli are similar at all growth rates and, in addition, identify the general growth pattern of these cells to be exponential.     

THE OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN THE INTERNODES OF TROPICAL MONOCOTYLEDONS

The distribution of percent of dividing nuclei, parenchyma cell length, total cell number per internode, and total internode length were determined for successive internodes in the apex and growing vegetative internodes of 23 tropical species in 17 families of monocotyledons. Basal intercalary meristems (IM) were found in representatives of Commelinaceae, Cyperaceae, Flagellariaceae, Poaceae, Restionaceae, and Marantaceae. Uninterrupted meristems (UM) which are confined progressively to the upper region of the internode and are not isolated meristematic regions were found in the Costaceae, Dioscoreaceae, Philesiaceae, Smilacaceae, Agavaceae, Araceae, Arecaceae, Liliaceae, Pandanaceae, and Zingiberaceae. Both IM and UM were found in different species of Orchidaceae. The only morphological trait correlated with meristem type was presence of sheathing leaf bases in all species with IM. Both IM and UM are interpreted as extensions of the primary elongating meristem; the IM is disjunct, and the UM is continuous with it. The phytomer growth unit and the presence of internodal IM's cannot be applied generally to the monocotyledons.     

OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN EQUISETUM

Quantitative data on cell length and mitotic index in elongating internodes of seven species of Equisetum are presented as evidence for the occurrence of two patterns of internode development in rhizomes of different species. In rhizomes of three species of subgenus Equisetum (E. arvense, E. diffusum, E. telmateia) uninterrupted meristems are present, characterized by acropetal internode maturation. In rhizomes of four species of subgenus Hippochaetae (E. hyemale, E. variegatum, E. scripoides, and E. laevigatum) intercalary meristems are present. The number and locations of intercalary meristems are described for an aerial shoot of E. diffusum. The absence or diminution of intercalary meristems from rhizomes of a variety of vascular plants with intercalary meristems in aerial shoots is discussed from the standpoint of adaptive significance.     

SHOOT GROWTH AND HISTOGENESIS OF TREES POSSESSING DIVERSE PATTERNS OF SHOOT DEVELOPMENT

Shoot growth and histogenesis were followed in five unrelated tree taxa possessing inherently diverse patterns of shoot development. Following the resumption of growth in spring, each species differs quantitatively in the number of internodes elongating contemporaneously, in rates and duration of internodal elongation and seasonal periodicity of shoot growth. The basic pattern of internode elongation and histogenesis is qualitatively similar in each of the dicotyledonous species observed irrespective of growth habit or final form of the shoot produced. During the intial phase of internode development, growth is essentially uniform throughout young internodes, corresponding to an active period of cell division during which time pith cells increase in size to about one-third their final length. Subsequently, the pattern of cell division shifts progressively upward concomitant with increased elongation and maturation of pith cells in the basal portion of developing internodes. Thereafter, a wave of cell division accompanied by cell elongation continues to proceed acropetally until growth finally ceases in the distal portion of each internode. As long as internode elongation continues, frequently at distances 15–20 cm below the shoot apex, cell divisions still occur in the distal growing portion. As successive portions of each internode mature acropetally, final length of pith cells becomes relatively uniform throughout the internode. During the process of internode growth and development, cell lengths increase only two- to threefold, whereas cell numbers increase ten- to 30-fold, indicating the dominant role of cell division and increases in cell number to final internode length. Morphological patterns of shoot expression associated with differences in internode lengths along the axis of either preformed or neoformed shoots, as well as sylleptic branches, are due to differences in cell number rather than final cell length. Significant variations in final internode lengths along the axis of episodic shoots, caused by either endogenous or exogenous factors, are also attributed to differences in cell number.     

Growth of cell populations with arbitrarily distributed cycle durations. I. basic model

A discrete model is proposed describing the growth of cell populations with arbitrary frequency distributions of cycle durations. The model assumes that each cell divides into two cells at the end of its cycle, and that each new cell is assigned an individual cycle duration according to a probability distribution that can be arbitrarily defined. The increase in the cell number is calculated, either from the numbers of cells at earlier time points or from the initial conditions of the population, by a recurrence formula; it is also approximated by the optimal exponential function, whose parameters are determined by the initial conditions. The appropriate average cycle duration is shown not to be the arithmetic or geometric mean, but rather the solution to a more complex equation. Age distributions are calculated and compared with those found in the literature. The results of the model calculations are compared with computer simulations and with observed data on populations of the ciliate Tetrahymena geleii.     

An anti-HLA class I monoclonal antibody alters the progression in the cell cycle of phytohemagglutinin-activated human T lymphocytes

The monomorphic anti-HLA Class I monoclonal antibody 01.65 inhibits the incorporation of tritiated thymidine ([3H]TdR) in Phytohemagglutinin (PHA)-activated human T lymphocytes. Our data indicate that 01.65 affects the average duration of the cell cycle by increasing the length of the early S subphase. As a consequence of the increase in the doubling time of the cell population, the absolute number of cells at harvesting time was reduced in 01.65-treated cultures compared to that of untreated cultures. The lengthening of the S-phase and the decrease in the cell number can together quantitatively account for the reduction of [3H]TdR incorporation observed in 01.65-treated cultures.     

Meristem differentiation in Riella helicophylla (Bory et Mont.) Mont. under the influence of auxin or antiauxin

During the development of the unistratose gemmae of Riella helicophylla, the single intercalary meristem of the very young gemmae is subdivided into two lateral meristems. The duration of the cell reproduction cycle increases from the margin to the median part of the gemmae. This polarization within the meristem disappears after addition of the antiauxin PCIB to the culture medium. PCIB leads to a retardation or blockage of the cell cycle during the light period of the culture. Under the influence of PCIB the amount of starch in the chloroplasts is strikingly increased, probably because of a reduction of starch degradation. Addition of sugars compensates the effect of PCIB on the cell cycle. The effects of PCIB are counteracted by auxin. The results are taken as evidence that auxin plays a role in directing the transport of substances needed for the continuation of the cell reproduction cycle between adjacent cells of the meristem.Abbreviations IAA indole-3-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - PCIB p-chlorophenoxyisobutyric acid - G-l-P glucose-l-phosphat - EDTA ethylenediaminetetraacetic acid     

Control of cell length in Bacillus subtilis.

During inhibition of deoxyribonucleic acid synthesis in Bacillus subtilis 168 Thy-minus Tryp-minus, the rate of length extension is constant. A nutritional shift-up during thymine starvation causes an acceleration in the linear rate of length extension. During a nutritional shift-up in the presence of thymine, the rate of length extension gradually increases, reaching a new steady state at about 50 min before the new steady-state rate of cell division is reached. The steady-state rates of nuclear division and length extension are reached at approximately the same time. The ratio of average cell length to numbers of nuclei per cell in exponential cultures is constant over a fourfold range of growth rates. These observations are consistent with: (i) surface growth zones which operate at a constant rate of length extension under any one growth condition, but which operate at an absolute rate proportional to the growth rate of the culture, (ii) a doubling in number of growth zones at nuclear segregation, and (iii) a requirement for deoxyribonucleic acid replication for the doubling in a number of sites.     

Differences in Cell Cycle Duration of Sister Cells in Secondary Root Meristems of Cocos nucifera L.

Cell lengths of Gl/Gl pairs of sister cells and in mitotic packetsof cells were measured in secondary roots of Cocos nuciferaL. Due to symplastic growth cell length ratios of sister cells,over a cell cycle, remained constant for the dwarf, hybrid andtall varieties of C. nucifera; the mean cell length ratio, betweenthe larger and smaller cell, was between 116:1 and 1 –20:1.From the cell length ratios, mean differences in cell cycleduration for sister cells were calculated to lie between 0–22and 0–27 of the average cell cycle duration. Cumulative distributions of the cell length ratios of sistercells, when converted to differences in cell cycle durationand plotted on semi-log graph paper deviated from an exponentialdistribution. In 86 per cent of the sister cells studied, thelarger cell of a sister pair divided first, however in the remaining14 per cent, half had the smaller cell of a sister pair dividingfirst while in the other half of the sister pairs, the cellswere of equal length but one had entered mitosis ahead of itssister. These results support neither cell cycle models basedon a cell sizer control per se nor on a random transition frominactive to active parts of the cell cycle.     

Morphological analysis of nuclear separation and cell division during the life cycle of Escherichia coli.

Quantitative electron microscope observations were performed on Escherichia coli B/r after balanced growth with doubling times (tau) of 32 and 60 min. The experimental approach allowed the timing of morphological events during the cell cycle by classifying serially sectioned cells according to length. Visible separation of the nucleoplasm was found to coincide with the time of termination of chromosome replication as predicted by the Cooper-Helmstetter model. The duration of the process of constrictive cell division (10 min) appeared to be independent of the growth rate for tau equals 60 min or less but to increase with increase doubling time in more slowly growing cells. Physiological division, i.e., compartmentalization prior to physical separation of the cells, was only observed to occur in the last minute of the cell cycle. The morphological results indicate that cell elongation continues during the division process in cells with tau equals 32 min, but fails to continue in cells with tau equals 60 min.     

Influence of the existence of a resting state on the decay of synchronization in cell culture

General relationships between the distribution of cell doubling times and the growth pattern of an initially synchronized cell population are applied to the model proposed by Smith and Martin (1973) in which the mitotic cycle or "B" phase is preceded by a random-exit resting "A" state. Results show that culture synchronization decays so rapidly as to be virtually unobservable unless the time spent by a cell in the B phase is at least equal to that spent in the A state. If synchronization persists over several mitotic cycles, the growth pattern is determined to a much greater extent by variation in the duration of the B phase than by the probability of exit from the A state. Accordingly the growth pattern of a cell population, like the doubling time distribution which governs the pattern, is of limited usefulness in detecting the existence of a resting state.     

The effect of growth-promoting agents on replication and cell cycle withdrawal in cultures of epidermal keratinocytes

Summary Epidermal keratinocytes grow in culture to form a stratified squamous epithelium. These cultures contain a replicating as well as a terminally differentiating population and undergo surface desquamation. Epidermal growth factor (EGF) and cholera toxin are usually employed as growth-promoting agents because they reduce the population doubling time; that is, the period required to increase the total cell number twofold. There are three ways in which this reduction in population doubling time could be achieved: (a) the time for one cell cycle or the cell cycle length may be shortened; (b) the number of cells that withdraw from the cell cycle and terminally differentiate may be reduced; or (c) the number of cells that desquamate into the medium over a set period of time may be reduced. We have explored these possibilities in growing cultures of epidermal keratinocytes using a newly developed double-label assay. This assay gives a measure of both cell length and cell cycle withdrawal. Results show that the growth enhancement induced by EGF and cholera toxin can be attributed primarily to a reduction in cell cycle withdrawal and, to a lesser degree, to a reduction in cell cycle length. EGF and cholera toxin have no significant effect on the rate of desquamation. A linear correlation was noted between cell cycle lengths and withdrawal, suggesting an interconnection between the rate of cell renewal and the likelihood of undergoing terminal differentiation. This research was supported by grant DE04511 from the National Institute of Dental Research, Bethesda, MD, and gifts from the University Hospital Auxilliary, Health Sciences Center, SUNY Stony Brook, and the Suffolk County Volunteer Firefighter Fund.     

Estimation of cell cycle parameters from double labeling experiments

Double labeling of cell populations with radioactive thymidine yields two types of differently labeled nuclei. Their numbers and the number of unlabeled nuclei can be used to estimate doubling times, T, and S-phase lengths, S. As of yet, such estimations have been performed either for stationary populations in which proliferation and losses are in balance, or for exponentially growing populations in which all cells have the same cycle duration. We calculate S and T for the more general type of cell population with arbitrarily distributed frequencies of cycle durations. The calculations do not require more mathematical or computational effort. We obtain three main results: (i) The estimation of T and S does not require explicit knowledge of the frequency distribution of cycle durations; (ii) in particular, equivalent estimates for T and S are obtained for both types of growing cell populations without losses, one with arbitrarily distributed cycle durations and one with the same cycle duration for all cells; and (iii) for small labeling indices, the estimate for S from the general model approaches the S-phase length of a stationary population and the estimate for T from the general model approaches the generation time of a stationary population, multiplied by the constant factor 1n(2). These relationships are valuable tools for reinterpreting results derived under the assumption of stationarity, which are considerably easier to obtain.     

Cell cycle length, cell size, and proliferation rate in hydra stem cells

We have analyzed the cell cycle parameters of interstitial cells in Hydra oligactis. Three subpopulations of cells with short, medium, and long cell cycles were identified. Short-cycle cells are stem cells; medium-cycle cells are precursors to nematocyte differentiation; long-cycle cells are precursors to gamete differentiation. We have also determined the effect of different cell densities on the population doubling time, cell cycle length, and cell size of interstitial cells. Our results indicate that decreasing the interstitial cell density from 0.35 to 0.1 interstitial cells/epithelial cell (1) shortens the population doubling time from 4 to 1.8 days, (2) increases the [3H]thymidine labeling index from 0.5 to 0.75 and shifts the nuclear DNA distribution from G2 to S phase cells, and (3) decreases the length of G2 in stem cells from 6 to 3 hr. The shortened cell cycle is correlated with a significant decrease in the size of interstitial stem cells. Coincident with the shortened cell cycle and increased growth rate there is an increase in stem cell self-renewal and a decrease in stem cell differentiation.     

The AGCVIII kinase Dw2 modulates cell proliferation,endomembrane trafficking,and MLG/xylan cell wall localization in elongating stem internodes of Sorghum bicolor

Stems of bioenergy sorghum (Sorghum bicolor L. Moench.), a drought-tolerant C4 grass, contain up to 50 nodes and internodes of varying length that span 4–5 m and account for approximately 84% of harvested biomass. Stem internode growth impacts plant height and biomass accumulation and is regulated by brassinosteroid signaling, auxin transport, and gibberellin biosynthesis. In addition, an AGCVIII kinase (Dw2) regulates sorghum stem internode growth, but the underlying mechanism and signaling network are unknown. Here we provide evidence that mutation of Dw2 reduces cell proliferation in internode intercalary meristems, inhibits endocytosis, and alters the distribution of heteroxylan and mixed linkage glucan in cell walls. Phosphoproteomic analysis showed that Dw2 signaling influences the phosphorylation of proteins involved in lipid signaling (PLDδ), endomembrane trafficking, hormone, light, and receptor signaling, and photosynthesis. Together, our results show that Dw2 modulates endomembrane function and cell division during sorghum internode growth, providing insight into the regulation of monocot stem development.     

Kinetics of the nuclear division cycle of Aspergillus nidulans.

We have analyzed the cell cycle kinetics of Aspergillus nidulans by using the DNA synthesis inhibitor hydroxyurea (HU) and a temperature-sensitive cell cycle mutant nimT that blocks in G2. HU rapidly inhibits DNA synthesis (S), and as a consequence progression beyond S to mitosis (M) is blocked. Upon removal of HU the inhibition is rapidly reversible. Conidia (asexual spores) of nimT were germinated at restrictive temperature to synchronize germlings in G2 and then downshifted to permissive temperature in the presence of HU. This procedure synchronizes the germlings at the beginning of S in the second cell cycle after spore germination. We have measured the total duration of S, G2, and M as the time required for these cells to recover from the HU block and undergo the next nuclear division. The duration of S was defined by the time course of sensitivity to reintroduction of HU during recovery from the initial HU block. The cell cycle time was measured as the nuclear doubling time, and the duration of mitosis was determined from the mitotic index. The duration of G1 was calculated by subtracting the combined durations of S, G2, and M from the nuclear doubling time, and the length of G2 was calculated by subtracting S and M from the aggregate length of S, G2, and M. We have also determined the duration of the phases of the cell cycle during the first cycle after spore germination. In these experiments spores were germinated directly in HU without first being blocked in G2. Because the durations of G1, S, G2, and M for the first cell cycle after spore germination were identical with those previously determined for spores presynchronized at the beginning of S in the second cell cycle, we conclude that dormant conidia of A. nidulans are arrested at, or before, the start of S.     

Investigations on the cell cycle of haploid and diploid tissue cultures of Datura innoxia Mill. and its synchronization

Using a ¹⁴C/³H double-labelling technique, the influence of kinetic on the length of the cell cycle of meristematic cells in haploid and diploid callus cultures of Datura innoxia was determined. The total length of the cell cycle of haploid cells as compared to that of diploid cells was reduced by 2.3 h (-kinetin) or 1.4 h (+kinetin). Furthermore, the addition of kinetin to the nutrient solution also reduces cell cycle duration at both ploidy levels. For synchronization of the cell cycle, a fluorodesoxyuridine/thymidine system was successfully employed. Apparently, the reduction of total cell cycle duration of cycling cells due to treatment with kinetin occurred at the expense of the G₁phase. Nevertheless, kinetin seems to exert an influence on the transition of cells from the G₂ into the M phase as well.Abbreviations FUdR fluorodeoxyuridine - HU hydroxyurea - IAA nidole acetic acid