Growth, ageing and death of a photoautotrophic plant cell culture

 Batch cultures of photoautotrophic cell suspensions of Chenopodiumrubrum L., growing in an inorganic medium on CO₂ under a daily balanced light–dark regime of 16 : 8 h could be maintained for approximately 100 d without subcultivation. The long-lived cultures showed an initial cell division phase of 4 weeks, followed by a stationary phase of another 4 weeks, after which ageing and progressive cell death reduced the number of living cells and the cultures usually expired after another 3–4 weeks. These developmental phases of the cell culture were characterised with respect to photosynthetic performance, dark respiration, content of phytohormones and capacity of cell division. Cell division of the majority of the cells finished in the G1- or G0-phase of the cell cycle, caused by a pronounced decline in the endogenous levels of auxin and cytokinins. Supply of these growth factors to resting cells resulted in resumption of cytokinesis, at least by some of the cells. However, responsiveness to the phytohomones declined during the stationary phase, and subcultivation was no longer possible beyond day 60 when the phases of ageing and death commenced. Ageing was characterised by a further decline in the photosynthetic capacity of the cells, by a climacteric enhancement of dark respiration, but also by a slight increase in the level of IAA and cytokinins concomitant with a decrease in ethylene. Similarities and differences between the development of batch-cultured photoautotrophic cells of C. rubrum and that of a leaf are discussed with respect to using the cell culture as a model for a leaf. Received: 30 April 1999 / Accepted: 21 August 1999     

Cytosolic heteroglycans in photoautotrophic and in heterotrophic plant cells

In plants several ‘starch-related’ enzymes exist as plastid- and cytosol-specific isoforms and in some cases the extraplastidial isoforms represent the majority of the enzyme activity. Due to the compartmentation of the plant cells, these extraplastidial isozymes have no access to the plastidial starch granules and, therefore, their in vivo function remained enigmatic. Recently, cytosolic heteroglycans have been identified that possess a complex pattern of the monomer composition and glycosidic bonds. The glycans act both as acceptors and donors for cytosolic glucosyl transferases. In autotrophic tissues the heteroglycans are essential for the nocturnal starch-sucrose conversion. In this review we summarize the current knowledge of these glycans, their interaction with glucosyl transferases and their possible cellular functions. We include data on the heteroglycans in heterotrophic plant tissues and discuss their role in intracellular carbon fluxes that originate from externally supplied carbohydrates.     

Cell Cycle Control in Arabidopsis

Although the basic mechanism of cell cycle control is conservedamong eukaryotes, its regulation differs in each type of organism.Plants have unique developmental features that distinguish themfrom other eukaryotes. These include the absence of cell migration,the formation of organs throughout the entire life-span fromspecialized regions called meristems, and the potency of non-dividingcells to re-enter the cell cycle. The study of plant cell cyclecontrol genes is expected to contribute to the understandingof these unique developmental phenomena. The principal regulatorsof the eukaryotic cell cycle, the cyclin-dependent kinases (CDKs)and cyclins, are conserved in plants. This review focuses oncell cycle regulation in the plant Arabidopsis thaliana . Whileexpression of one Arabidopsis CDK gene, Cdc2aAt, was positivelycorrelated with the competence of cells to divide, expressionof a mitotic-like cyclin, cyc1At, was almost exclusively confinedto dividing cells. The expression of the Arabidopsis -type cyclinsappears to be an early stage in the response of plant cellsto external and internal stimuli. Arabidopsis thaliana (L.) Heynh.; cell cycle; CDK; cyclin; plant development; plant hormone     

DNA损伤与细胞周期调控

DNA损伤和损伤后修复可引起细胞周期阻滞,这一事件由三个阶段组成：损伤的识别,损伤信号的传递以及细胞周期阻滞.在某些情况,这种细胞周期阻滞会失效.     

DNA repair mechanisms in dividing and non-dividing cells

DNA damage created by endogenous or exogenous genotoxic agents can exist in multiple forms, and if allowed to persist, can promote genome instability and directly lead to various human diseases, particularly cancer, neurological abnormalities, immunodeficiency and premature aging. To avoid such deleterious outcomes, cells have evolved an array of DNA repair pathways, which carry out what is typically a multiple-step process to resolve specific DNA lesions and maintain genome integrity. To fully appreciate the biological contributions of the different DNA repair systems, one must keep in mind the cellular context within which they operate. For example, the human body is composed of non-dividing and dividing cell types, including, in the brain, neurons and glial cells. We describe herein the molecular mechanisms of the different DNA repair pathways, and review their roles in non-dividing and dividing cells, with an eye toward how these pathways may regulate the development of neurological disease.     

Control of Cell Cycle and Cell Growth by Molecular Chaperones

Cells adapt their size to both intrinsic and extrinsic demands and, among them, those that stem from growth and proliferation rates are crucial for cell size homeostasis. Here we revisit mechanisms that regulate cell cycle and cell growth in budding yeast. Cyclin Cln3, the most upstream activator of Start, is retained at the endoplasmic reticulum in early G1 and released by specific chaperones in late G1 to initiate the cell cycle. On one hand, these chaperones are rate-limiting for release of Cln3 and cell cycle entry and, on the other hand, they are required for key biosynthetic processes. We propose a model whereby the competition for specialized chaperones between growth and cycle machineries could gauge biosynthetic rates and set a critical size threshold at Start.     

Geminin Coordinates Cell Cycle and Developmental Control

Growth and differentiation are two major themes in embryonic development. Numerous cell divisions have to be regulated on the path from a unicellular embryo, the zygote, to the multicellular structures of a mature being. Numerous functions, specializations and cellular identities have to be generated, in order to form a complex and mature animal. Numerous mechanisms have to control the correct assignment and acquisition of cellular fates, as well as the right timing and allocation of cells. Therefore, a strict coordination has to occur between embryonic patterning and the cell cycle. From this point of view, dual roles or mutual interactions of typical proliferation and developmental control genes are likely. Recently, new light was shed on these issues by identifying the nuclear protein Geminin as a molecular coordinator between the cell cycle and axial patterning. We summarize the role of Geminin in cell cycle, in the embryonic patterning controlled by Hox genes, providing insights into cell cycle regulators in embryonic development, and, conversely, typical developmental control genes in cell cycle regulation.     

Hormonal Control of Gene Expression During Reactivation of the Cell Cycle in Tobacco Mesophyll Protoplasts

The roles of auxin and cytokinin in cell cycle reactivation were studied during the first 48 h of culture of mesophyll protoplasts of Nicotiana tabacum. Using hormone delay and withdrawal studies we found that auxin was required by 0–4 h of culture, whereas cytokinin was not required until hour 10–12, which is 6–10 h before S phase. Cycloheximide blocks division, indicating that protein synthesis is required. In an effort to detect a molecular response to either hormone, we examined the expression of the cell cycle marker, cdc2. Cdc2 expression was detected by 12 h of culture, coincident with the timing of the cytokinin requirement and well before the entry into S. However, cdc2 was partially induced by either auxin or cytokinin alone, suggesting that cdc2 expression is not the primary target of either hormone. Our hormone delay experiments suggest that there are separate signal transduction pathways leading from auxin and from cytokinin to reactivation of the cell cycle and that these pathways converge before S. The underlying mechanisms for these distinct pathways remain to be elucidated. Received November 4, 1997; accepted October 7, 1998     

Development and application of photoautotrophic micropropagation plant system

Research has revealed that most chlorophyllous explants/plants in vitro have the ability to grow photoautotrophically (without sugar in the culture medium), and that the low or negative net photosynthetic rate of plants in vitro is not due to poor photosynthetic ability, but to the low CO₂ concentration in the air-tight culture vessel during the photoperiod. Moreover, numerous studies have been conducted on improving the in vitro environment and investigating its effects on growth and development of cultures/plantlets on nearly 50 species since the concept of photoautotrophic micropropagation was developed more than two decades ago. These studies indicate that the photoautotrophic growth in vitro of many plant species can be significantly promoted by increasing the CO₂ concentration and light intensity in the vessel, by decreasing the relative humidity in the vessel, and by using a fibrous or porous supporting material with high air porosity instead of gelling agents such as agar. This paper reviews the development and characteristics of photoautotrophic micropropagation systems and the effects of environmental conditions on the growth and development of the plantlets. The commercial applications and the perspective of photoautotrophic micropropagation systems are discussed.     

Expression of Cell Cycle Genes in Shoot Apical Meristems

This article reviews cell proliferation in the shoot apical meristem. The morphology and function of the meristem depends on the positional control of cell growth and division. The review describes the historical framework of research in this area and then discusses the regulatory pathways that might link developmental controls to the core cell cycle machinery.     

HMGB1对小鼠系膜细胞细胞周期及细胞周期相关蛋白表达的影响

该实验以小鼠系膜细胞MMC为研究对象,以重组HMGB1为刺激物,通过检测细胞周期的变化及细胞PCNA、CyclinD1、CDK4和p16的表达水平,初步探讨HMGB1对系膜细胞的细胞周期及其相关调控因子的影响。选取小鼠系膜细胞MMC为研究对象,随机分为对照组及0.05mg/LHMGB1刺激组,经流式细胞术检测发现HMGB1能够上调小鼠系膜细胞中S期细胞所占比例;免疫细胞化学检测显示,PCNA蛋白在小鼠系膜细胞中的表达上调;通过RT-PCR技术及Western blot技术检测到小鼠系膜细胞中CyclinD1 mRNA和蛋白以及CDK4蛋白的高表达情况,而p16蛋白的表达呈时间依赖性降低。由此可见,HMGB1可能是通过上调CyclinD1/CDK4的表达,并下调p16的表达,促进细胞从G_0/G_1期进入S期,介导了小鼠系膜细胞的异常增殖,可能是HMGB1参与狼疮性肾炎发病的可能机制之一。     

Cell Cycle Control: A Complex Issue

Do p27Kip1 and p21Cip1 function as activators or inhibitors of D cyclin-cdk4 activity? Attempts to answer this question and thus to understand how cdk4—a key cell cycle regulator—becomes active have produced conflicting data. In this perspective, we summarize the results of studies addressing the effects of p27Kip1 and p21Cip1 on the assembly and activation of D cyclin-cdk4 complexes. Emphasis is placed on our experimental findings, which support a model of cell cycle control in which p27Kip1 and p21Cip1 stabilize D cyclin-cdk4 complexes but inhibit D cyclin-cdk4 activity.