Cell-cycle parameters of soybean (Glycine max L.) cells growing in suspension culture: Suitability of the system for genetic studies

Analysis of cell cycle activity and population dynamics in heterogeneous plant cell suspensions using flow cytometry

Flow cytometry was used to measure cell cycle parameters in Solanum aviculare plant cell suspensions. Methods for bromodeoxyuridine (BrdU) labeling of plant nuclei were developed so that cell cycle times and the proportion of cells participating in growth could be determined as a function of culture time and conditions. The percentage of cells active in the cell cycle at 25 degrees C decreased from 52% to 19% within 7.6 d of culture; presence of a relatively large proportion of non-active cells was reflected in the results for culture growth. While the maximum specific growth rate of the suspensions at 25 degrees C was 0.34 d-1 (doubling time: 2.0 d), the specific growth rate of active cells was significantly greater at 0.67 d-1, corresponding to a cell cycle time of 1.0 d. A simple model of culture growth based on exponential and linear growth kinetics and the assumption of constant cell cycle time was found to predict with reasonable accuracy the proportion of active cells in the population as a function of time. Reducing the temperature to 17 degrees C lowered the culture growth rate but prolonged the exponential growth phase compared with 25 degrees C; the percentage of cells participating in the cell cycle was also higher. Exposure of plant cells to different agitation intensities in shake flasks had a pronounced effect on the distribution of cells within the cell cycle. The proportion of cells in S phase was 1.8 times higher at a shaker speed of 160 rpm than at 100 rpm, while the frequency of G0 + G1 cells decreased by up to 27%. Because of the significant levels of intraculture heterogeneity in suspended plant cell systems, flow cytometry is of particular value in characterizing culture properties and behavior.

Individual selection, culture and manipulation of higher plant cells

Due to the heterogeneity in morphology, physiological and morphogenetical capabilities of higher plant cells in mass culture, the development of methods for individually culturing defined cells seemed to be useful and necessary. Individual cell culture represents a powerful tool for studies on the physiology of different cell types, the analysis of differentiation programs, the genetic manipulation of plant cells and cell-cell interactions. An improved microculture system based on a computer-controlled set-up for the efficient selection, transfer and individual culture of defined higher plant cells until regeneration of whole plants is described. Related experimental approaches for individually manipulating higher plant cells under controlled conditions, such as electrofusion of defined pairs of protoplasts and subprotoplasts, cell reconstruction and intranuclear microinjection of protoplasts and karyoplasts - mainly performed with cells of the crop plant Brassica napus L. - are presented.

Organellar DNA synthesis in permeabilized soybean cells

Cultured cells of Glycine max (L.) Merr. v. Corsoy were permeabilized by treatment with L-α-lysophosphatidylcholine (LPC). The permeabilized cells were capable of uptake and incorporation of deoxynucleoside triphosphates into DNA. Incorporation of exogenous nucleotides into DNA was linear for at least 90 minutes and the initial rate of incorporation approached 50% of the theoretical in vivo rate of DNA synthesis. However, DNA synthesis in the permeabilized cells was unaffected by the potent DNA polymerase α inhibitor, aphidicolin. Analysis of newly synthesized DNA by molecular hybridization revealed that only organellar DNA was synthesized by the permeabilized cells. The LPC treated cells were also permeable to a protein as large as DNase I. The permeabilized cells were capable of RNA and protein synthesis as indicated by incorporation of radiolabeled UTP and leucine, respectively, into acid-precipitable material.

Isolation and characterization of a gene encoding meso-diaminopimelate dehydrogenase fromGlycine max

A detailed characterization of the lysine biosynthetic pathway in plants is yet to be completed. It is, however, assumed that the diaminopimelic acid pathway exists in the plant kingdom, as commonly described forEscherichia coli.Modification and refinement of lytic complementation, a technique previously utilized in bacterial systems, facilitated the isolation of a functional Diaminopimelate Dehydrogenase gene from aGlycine max nuclear gene library. The isolated gene codes for the enzyme meso-diaminopimelate dehydrogenase. The coding capacity for the enzyme was originally contained on a 6.6kb fragment in a Charon 4a soybean gene bank. Subcloning of the 6.6kb fragment resulted in the recombinant plasmid pMW75. Subsequent subcloning resulted in a 4.05 kb fragment contained in pLW14. One region of homology was observed upon hybridization to EcoR1 digested soybean DNA. Homologous sequences were also observed in Triticum DNA.Meso-diaminopimelate dehydrogenase activity was demonstrated inGlycine max embryos. Maximum enzymatic activity of the cloned enzyme was observed at a pH of 8.0. The enzyme encoded by the soybean gene has an apparent molecular weight of 67 000.

Isopropyl-N(3-chlorophenyl) carbamate (CIPC) induced chromosomal loss in soybean: a new tool for plant somatic cell genetics

The properties and uses of tissue culture partial haploid soybean cell lines are explored. Partial haploid lines were prepared by CIPC treatment of a genetic heterozygote, and compared to this heterozygote and to homozygous cell lines corresponding to the parental genotypes from which the heterozygote was derived. Cell lines which lack chromosomes were characterized physiologically and with respect to a variety of isozyme markers. Often the loss of chromosomes revealed a phenotype corresponding to a recessive parental genotype. In some cases, however, new phenotypes were observed indicating a complex genotype and suggests the interaction of several genes. The implications of this for plant breeding are discussed.CIPC also was used as a tool to dissect a complex phenotype which arose as the result of mutagenesis. A mutant cell line which required asparagine for growth but also had acquired the ability to grow on allantoin as a sole source of nitrogen was treated with CIPC to remove chromosomes. The requirement for asparagine could be separated from the ability to use allantoin, demonstrating that these phenotypes were the result of separate mutations.

Aphidicolin as synchronizing agent in root tip meristems of Haplopappus gracilis

Aphidicolin, a reversible inhibitor of nuclear DNA replication, was tested as syncrhonizing agent in root tip meristems of Haplopappus gracilis. Embryos (i.e. decoated seeds) or 3-day-old seedlings were used to this purpose. After a 24 h treatment with the drug, a high level of synchrony was observed in both experimental materials for two cell cycles as assessed by determining the accumulation of cells in the S and M phases of the cycle. Highest synchronization was obtained with germinating embryos, possibly owing to a low degree of synchrony already existing in this system.

Uptake of isolated plant chromosomes by plant protoplasts

For mass isolation of plant metaphase chromosomes, cultured cells of wheat (Triticum monococcum) and parsley (Petroselinum hortense) were synchronized by hydroxyurea and colchicine treatment. This synchronization procedure resulted in high mitotic synchrony, especially in suspension cultures of parsley in which 80% of the cells were found to be at the metaphase stage. Mitotic protoplasts isolated from these synchronized cell cultures served as a source for isolation of chromosomes. The described isolation and purification method yielded relatively pure chromosome suspension. The uptake of the isolated plant chromosomes into recipient wheat, parsley, and maize protoplasts was induced by polyethylene-glycol treatment. Cytological studies provided evidences for uptake of plant chromosomes into plant protoplasts.

An efficient plating system for rapid isolation of mutants from plant cell suspensions

A plating system for cell suspensions of soybean, SB-1, (Glycine max L. cv. 'Mandarin') and Datura innoxia D.I. (Mill) was developed using feeder cells. The characteristics of the system are: a) the efficiency of plating (EOP) is high (0.5-0.6), b) over a range of 10-300 plated clumps the EOP is constant, c) the growth rate of plated cells resembles that of suspension cultures (generation time 24 hr.). Clumps with few or with many cells have similar plating efficiencies.Employing the plating system, a mutant resistant to 8 azaguanine (8AG) was isolated from SB-1 in 7 days and purified and tested within an additional 3 weeks. Feeder plates were used to selectively re-isolate 8 AG resistant and maltose utilizing mutants from a 1000-fold excess of wild type cells.The plating technique also can be utilized to isolate auxotrophic mutants since free amino acids are not produced by the feeder suspension. Other applications of this plating technique are discussed.

DNA replication in soybean protoplasts and suspension-cultured cells: Comparison of exponential and fluorodeoxyuridine synchronized cultures

Cell-suspension cultures of soybean (Glycine max (L.) Merr., line SB-1) have been used to study DNA replication. Cells or protoplasts incorporate either radioactive thymidine or 5-bromodeoxyuridine (BUdR) into DNA. The DNA has been extracted as large molecules which can be visualized by autoradiography. Nuclei were isolated and lysed on slides thus avoiding degradation of DNA by a cytoplasmic endonuclease. The autoradiograms demonstrated that DNA synthesis occurs at several sites tandemly arranged on single DNA molecules separated by center to center distances ranging from 10 to 30 μm. Velocity sedimentations through alkaline gradients confirm the lengths of the replicated regions seen in autoradiograms. By using velocity sedimentation it also has been possible to demonstrate that replication proceeds by the synthesis of very small (4-6S) DNA intermediates which join to form the larger, replicon-size pieces seen in autoradiograms. Both small (4-6S) and large (20-30S) intermediates are observed in synchronized and exponential cultures. However, after synchronization with fluorodeoxyuridine (FUdR) the rate of DNA synthesis is reduced. Since the size of intermediates is not reduced by FUdR treatment, it is concluded that the slower rate of replication results from a reduction in the number of tandem replication units but not in the rate at which they are elongated. After FUdR treatment, the density analogue of thymidine, BUdR, can be substituted for almost all of the thymidine residue in DNA, resulting in a buoyant density increase (in CsCl) from 1.694 to 1.747 g/cm(3). Using this density analogue it is possible to estimate the amount of template DNA attached to new replication sites. When this is done, it can be shown that synchronized cells initiate replication at about 5,000 different sites at the beginning of S. (Each such site will replicate to an average length of 20 μm.) Use of BUdR also substantiates that at early stages of replication, very small replicated regions (<8S) exist which are separated by unreplicated segments of DNA which replicate at a later time. Most of these conclusions agree with the pattern of DNA replication established for animal cells. However, a major difference appears to be that after prolonged inhibition of soybean cell replication with FUdR, very small, as well as replicon-size intermediates accumulate when replication is restored. This indicates that regulation of replication in these cells may be different from animal cells.

Temperature response of the cell cycle of Haplopappus gracilis in suspension culture and its significance to the G1 transition probability model

The effects of temperature on the cell cycle of Haplopappus gracilis suspension cultures were analysed by the fraction of labelled mitoses method. Sphase in these cultures shows a different temperature optimum as compared to optima derived for G2 and mitosis. G1 phase has a much lower Q10 than the other cell cycle phases and shows no temperature optimum between 22 and 34° C. These results are discussed in relation to a transition probability model of the cell cycle proposed by Smith and Martin (Proc. Natl. Acad. Sci. USA 70, 1263-1267, 1973), in which each cell has a time independent probability of initiating the transition into another round of DNA replication and division. The implications of such a model for cell cycle analysis are discussed and a tentative model for a probabilistic transition trigger is advanced.