Subnucleolar location of fibrillarin and NopA64 in Lepidium sativum root meristematic cells is changed in altered gravity

Histone acetylation of 45S rDNA correlates with disrupted nucleolar organization during heat stress response in Zea mays L

The role of the nucleolus in plant response to heat stress remains largely obscure. Our current efforts focused on exploring the underlying mechanism by which nucleolar disorganization is regulated in heat stressed-maize lines. Here, two maize lines, a heat-sensitive line, ZD958, and a heat-tolerant line, ZDH, were submitted to heat stress for investigating their association with the nucleolar disruption. Immunofluorescence staining showed that nucleolar disruption increased with prolonged treatment time. After heat treatment, a significant change in nucleolus organization was observed in the ZD958 line, but the ZDH line showed mild alteration. Moreover, actinomycin D (ActD)-induced nucleolus fission led to inhibition of maize growth under the normal condition. The ZD958 line exhibited a significant increase in the level of H3K9ac and H4K5ac of the 45S rDNA accompanied by a higher transcription of the 5'-external transcribed spacer (ETS) region, while the line ZDH showed a slight increase in histone acetylation levels and the transcriptional initiation at this site after heat treatment. To our knowledge, this is the first report providing a comparative insight between heat stress, rDNA histone modifications, and nucleolus disintegration in a heat-tolerant ZDH compared with a heat-sensitive line ZD958. Our investigation might assist maize breeders in obtaining heat-tolerant lines by targeting nucleoli using epigenetics.

Cell cycle acceleration and changes in essential nuclear functions induced by simulated microgravity in a synchronized Arabidopsis cell culture

Zero gravity is an environmental challenge unknown to organisms throughout evolution on Earth. Nevertheless, plants are sensitive to altered gravity, as exemplified by changes in meristematic cell proliferation and growth. We found that synchronized Arabidopsis-cultured cells exposed to simulated microgravity showed a shortened cell cycle, caused by a shorter G2/M phase and a slightly longer G1 phase. The analysis of selected marker genes and proteins by quantitative polymerase chain reaction and flow cytometry in synchronic G1 and G2 subpopulations indicated changes in gene expression of core cell cycle regulators and chromatin-modifying factors, confirming that microgravity induced misregulation of G2/M and G1/S checkpoints and chromatin remodelling. Changes in chromatin-based regulation included higher DNA methylation and lower histone acetylation, increased chromatin condensation, and overall depletion of nuclear transcription. Estimation of ribosome biogenesis rate using nucleolar parameters and selected nucleolar genes and proteins indicated reduced nucleolar activity under simulated microgravity, especially at G2/M. These results expand our knowledge of how meristematic cells are affected by real and simulated microgravity. Counteracting this cellular stress is necessary for plant culture in space exploration.

Simulated microgravity, Mars gravity, and 2g hypergravity affect cell cycle regulation, ribosome biogenesis, and epigenetics in Arabidopsis cell cultures

Gravity is the only component of Earth environment that remained constant throughout the entire process of biological evolution. However, it is still unclear how gravity affects plant growth and development. In this study, an in vitro cell culture of Arabidopsis thaliana was exposed to different altered gravity conditions, namely simulated reduced gravity (simulated microgravity, simulated Mars gravity) and hypergravity (2g), to study changes in cell proliferation, cell growth, and epigenetics. The effects after 3, 14, and 24-hours of exposure were evaluated. The most relevant alterations were found in the 24-hour treatment, being more significant for simulated reduced gravity than hypergravity. Cell proliferation and growth were uncoupled under simulated reduced gravity, similarly, as found in meristematic cells from seedlings grown in real or simulated microgravity. The distribution of cell cycle phases was changed, as well as the levels and gene transcription of the tested cell cycle regulators. Ribosome biogenesis was decreased, according to levels and gene transcription of nucleolar proteins and the number of inactive nucleoli. Furthermore, we found alterations in the epigenetic modifications of chromatin. These results show that altered gravity effects include a serious disturbance of cell proliferation and growth, which are cellular functions essential for normal plant development.

Novel, Moon and Mars, partial gravity simulation paradigms and their effects on the balance between cell growth and cell proliferation during early plant development

Clinostats and Random Positioning Machine (RPM) are used to simulate microgravity, but, for space exploration, we need to know the response of living systems to fractional levels of gravity (partial gravity) as they exist on Moon and Mars. We have developed and compared two different paradigms to simulate partial gravity using the RPM, one by implementing a centrifuge on the RPM (RPM^HW), the other by applying specific software protocols to driving the RPM motors (RPM^SW). The effects of the simulated partial gravity were tested in plant root meristematic cells, a system with known response to real and simulated microgravity. Seeds of Arabidopsis thaliana were germinated under simulated Moon (0.17 g) and Mars (0.38 g) gravity. In parallel, seeds germinated under simulated microgravity (RPM), or at 1 g control conditions. Fixed root meristematic cells from 4-day grown seedlings were analyzed for cell proliferation rate and rate of ribosome biogenesis using morphometrical methods and molecular markers of the regulation of cell cycle and nucleolar activity. Cell proliferation appeared increased and cell growth was depleted under Moon gravity, compared with the 1 g control. The effects were even higher at the Moon level than at simulated microgravity, indicating that meristematic competence (balance between cell growth and proliferation) is also affected at this gravity level. However, the results at the simulated Mars level were close to the 1 g static control. This suggests that the threshold for sensing and responding to gravity alteration in the root would be at a level intermediate between Moon and Mars gravity. Both partial g simulation strategies seem valid and show similar results at Moon g-levels, but further research is needed, in spaceflight and simulation facilities, especially around and beyond Mars g levels to better understand more precisely the differences and constrains in the use of these facilities for the space biology community.

Novel simulators of partial gravity show that the threshold for plants to sense and respond to gravity in their roots lies somewhere between gravity levels of the Moon and Mars. An international team led by Jack van Loon from the VU University Medical Center in Amsterdam, the Netherlands, reconfigured the hardware (include centrifugation) and software (new algorithm) on a microgravity-simulating instrument known as a Random Positioning Machine to produce fractional levels of gravity similar to those found on Mars (0.38 g) and the Moon (0.17 g). They germinated seeds of thale cress (Arabidopsis thaliana) in these devices, and showed in 4-day old seedlings that the balance between cell proliferation and cell growth in the roots was out of whack in both simulated microgravity and Moon gravity levels, but not in a simulated Mars scenario.

Inside a plant nucleus: discovering the proteins

Nuclear proteins are a vital component of eukaryotic cell nuclei and have a profound effect on the way in which genetic information is stored, expressed, replicated, repaired, and transmitted to daughter cells and progeny. Because of the plethora of functions, nuclear proteins represent the most abundant components of cell nuclei in all eukaryotes. However, while the plant genome is well understood at the DNA level, information on plant nuclear proteins remains scarce, perhaps with the exception of histones and a few other proteins. This lack of knowledge hampers efforts to understand how the plant genome is organized in the nucleus and how it functions. This review focuses on the current state of the art of the analysis of the plant nuclear proteome. Previous proteome studies have generally been designed to search for proteins involved in plant response to various forms of stress or to identify rather a modest number of proteins. Thus, there is a need for more comprehensive and systematic studies of proteins in the nuclei obtained at individual phases of the cell cycle, or isolated from various tissue types and stages of cell and tissue differentiation. All this in combination with protein structure, predicted function, and physical localization in 3D nuclear space could provide much needed progress in our understanding of the plant nuclear proteome and its role in plant genome organization and function.

Functional ultrastructure of the plant nucleolus

Nucleoli are nuclear domains present in almost all eukaryotic cells. They not only specialize in the production of ribosomal subunits but also play roles in many fundamental cellular activities. Concerning ribosome biosynthesis, particular stages of this process, i.e., ribosomal DNA transcription, primary RNA transcript processing, and ribosome assembly proceed in precisely defined nucleolar subdomains. Although eukaryotic nucleoli are conservative in respect of their main function, clear morphological differences between these structures can be noticed between individual kingdoms. In most cases, a plant nucleolus shows well-ordered structure in which four main ultrastructural components can be distinguished: fibrillar centers, dense fibrillar component, granular component, and nucleolar vacuoles. Nucleolar chromatin is an additional crucial structural component of this organelle. Nucleolonema, although it is not always an unequivocally distinguished nucleolar domain, has often been described as a well-grounded morphological element, especially of plant nucleoli. The ratios and morphology of particular subcompartments of a nucleolus can change depending on its metabolic activity which in turn is correlated with the physiological state of a cell, cell type, cell cycle phase, as well as with environmental influence. Precise attribution of functions to particular nucleolar subregions in the process of ribosome biosynthesis is now possible using various approaches. The presented description of plant nucleolar morphology summarizes previous knowledge regarding the function of nucleoli as well as of their particular subdomains not only in the course of ribosome biosynthesis.

Light and gravity signals synergize in modulating plant development

Tropisms are growth-mediated plant movements that help plants to respond to changes in environmental stimuli. The availability of water and light, as well as the presence of a constant gravity vector, are all environmental stimuli that plants sense and respond to via directed growth movements (tropisms). The plant response to gravity (gravitropism) and the response to unidirectional light (phototropism) have long been shown to be interconnected growth phenomena. Here, we discuss the similarities in these two processes, as well as the known molecular mechanisms behind the tropistic responses. We also highlight research done in a microgravity environment in order to decouple two tropisms through experiments carried out in the absence of a significant unilateral gravity vector. In addition, alteration of gravity, especially the microgravity environment, and light irradiation produce important effects on meristematic cells, the undifferentiated, highly proliferating, totipotent cells which sustain plant development. Microgravity produces the disruption of meristematic competence, i.e., the decoupling of cell proliferation and cell growth, affecting the regulation of the cell cycle and ribosome biogenesis. Light irradiation, especially red light, mediated by phytochromes, has an activating effect on these processes. Phytohormones, particularly auxin, also are key mediators in these alterations. Upcoming experiments on the International Space Station will clarify some of the mechanisms and molecular players of the plant responses to these environmental signals involved in tropisms and the cell cycle.

Three major nucleolar proteins migrate from nucleolus to nucleoplasm and cytoplasm in root tip cells of Vicia faba L. exposed to aluminum

Results from our previous investigation indicated that Al could affect the nucleolus and induce extrusion of silver-staining nucleolar particles containing argyrophilic proteins from the nucleolus into the cytoplasm in root tip cells of Vicia faba L. So far, the nucleolar proteins involved have not been identified. It is well known that nucleophosmin (B23), nucleolin (C23), and fibrillarin are three major and multifunctional nucleolar proteins. Therefore, effects of Al on B23, C23, and fibrillarin in root tip cells of V. faba exposed to 100 μM Al for 48 h were observed and analyzed using indirect immunofluorescence microscopy and Western blotting. The results from this work demonstrated that after 100 μM of Al treatment for 48 h, B23 and C23 migrated from the nucleolus to the cytoplasm and fibrillarin from the nucleolus to the nucleoplasm. In some cells, fibrillarin was present only in the cytoplasm. Western blotting data revealed higher expression of the three major nucleolar proteins in Al-treated roots compared with the control and that the B23 content increased markedly. These findings confirmed our previous observations.

Effects of lead on the morphology and structure of the nucleolus in the root tip meristematic cells of Allium cepa L

To study the toxic mechanisms of lead (Pb) in plants, the effects of Pb on the morphology and structure of the nucleolus in root tip meristematic cells of Allium cepa var. agrogarum L. were investigated. Fluorescence labeling, silver-stained indirect immunofluorescent microscopy and western blotting were used. Fluorescence labeling showed that Pb ions were localized in the meristematic cells and the uptake and accumulation of Pb increased with treatment time. At low concentrations of Pb (1-10 μM) there were persistent nucleoli in some cells during mitosis, and at high concentration (100 μM) many of the nucleolar organizing regions were localized on sticky chromosomes in metaphase and anaphase cells. Pb induced the release of particles containing argyrophilic proteins to be released from the nucleus into the cytoplasm. These proteins contained nucleophosmin and nucleolin. Pb also caused the extrusion of fibrillarin from the nucleus into the cytoplasm. Western blotting demonstrated the increased expression of these three major nucleolar proteins under Pb stress.

Cell proliferation and plant development under novel altered gravity environments

Gravity is a key factor for life on Earth. It is the only environmental factor that has remained constant throughout evolution, and plants use it to modulate important physiological activities; gravity removal or alteration produces substantial changes in essential functions. For root gravitropism, gravity is sensed in specialised cells, which are capable of detecting magnitudes of the g vector lower than 10(-3) . Then, the mechanosignal is transduced to upper zones of the root, resulting in changes in the lateral distribution of auxin and in the rate of auxin polar transport. Gravity alteration has consequences for cell growth and proliferation rates in root meristems, which are the basis of the developmental programme of a plant, in which regulation via auxin is involved. The effect is disruption of meristematic competence, i.e. the strict coordination between cell proliferation and growth, which characterises meristematic cells. This effect can be related to changes in the transport and distribution of auxin throughout the root. However, similar effects of gravity alteration have been found in plant cell cultures in vitro, in which neither specialised structures for gravity sensing and signal transduction, nor apparent gravitropism have been described. We postulate that gravity resistance, a general mechanism of cellular origin for developing rigid structures in plants capable of resisting the gravity force, could also be responsible for the changes in cell growth and proliferation parameters detected in non-specialised cells. The mechanisms of gravitropism and graviresistance are complementary, the first being mostly sensitive to the direction of the gravity vector, and the second to its magnitude. At a global molecular level, the consequence of gravity alteration is that the genome should be finely tuned to counteract a type of stress that plants have never encountered before throughout evolution. Multigene families and redundant genes present an advantage in that they can experience changes without the risk of being deleterious and, for this reason, they should play a key role in the response to gravitational stress.

Plant cell gravisensitivity and adaptation to microgravity

A short overview on the effects of real and simulated microgravity on certain cell components and processes, including new information obtained recently, is presented. Attention is focused on the influence of real and simulated microgravity on plant cells that are not specialised to gravity perception and on seed formation. The paper considers the possibility of full adaptation of plants to microgravity, and suggests some questions for future plant research in order to make decisions on fundamental and applied problems of plant space biology.

Aluminum can induce alterations in the cellular localization and expression of three major nucleolar proteins in root tip cells of Allium cepa var. agrogarum L

A 50 μM aluminum (Al) could induce nucleolar materials containing the argyrophilic proteins scattered in the nuclei and extruded from the nuclei into the cytoplasm in the root tip cells of Allium cepa. Unfortunately, what kinds of nucleolar proteins are affected has not been reported till now. In order to go deeper into the understanding of the cytological effects of Al on nucleolus and nucleolar proteins, alterations in the cellular localization and expression of three major nucleolar proteins: nucleophosmin, nucleolin, and fibrillarin were further examined under the treatment with Al in the root tip cells of A. cepa in the present study. Cytological effects of Al on nucleolus were observed by silver-staining method and three major nucleolar proteins: nucleophosmin, nucleolin, and fibrillarin were examined by western blotting. The results indicated that in the presence of 50 μM Al for 48 h the nucleolar proteins were translocated from nucleolus to nucleoplasm and cytoplasm. Western blotting data demonstrated the relatively higher expression of the three major nucleolar proteins when compared with control. Evidence from the present investigation indicated that Al had toxic effects on Ag-NOR proteins, nucleophosmin and nucleolin, and other kinds of nucleolar proteins, fibrillarin.

Nucleolin level in plant root meristematic cells under chilling stress and recovery

Nucleolin and its homologues are multifunctional proteins which reside mainly in nucleoli of yeast, animal and plant cells. Hence, they are generally implicated in many stages of ribosome biosynthesis. In this study nucleolin was identified in root meristematic cell nucleoli of soybean plants subjected to chilling stress, recovered after chilling and under control conditions with the use of the immunogold electron microscopy technique. Soybean nucleoli exhibited various metabolic activities under these conditions (Stępiński, 2004). Current studies showed that the level of nucleolin, expressed as a number of gold grains per μm(2), varied in particular subnucleolar regions in the soybean root meristematic cell nucleoli. Labeling density changed in these regions when plants were subjected to the given treatment. Most abundantly this protein was present in dense fibrillar component (DFC) around fibrillar centers (FCs) in the nucleoli of recovered plants, while in the nucleoli of stressed plants this region contained the lowest level of nucleolin. It can be supposed that nucleolin participates in ribosome biogenesis and its level is correlated with metabolic activity of soybean nucleoli - the more active nucleoli, the higher level of nucleolin and vice versa.

Plant cell proliferation and growth are altered by microgravity conditions in spaceflight

Seeds of Arabidopsis thaliana were sent to space and germinated in orbit. Seedlings grew for 4d and were then fixed in-flight with paraformaldehyde. The experiment was replicated on the ground in a Random Positioning Machine, an effective simulator of microgravity. In addition, samples from a different space experiment, processed in a similar way but fixed in glutaraldehyde, including a control flight experiment in a 1g centrifuge, were also used. In all cases, comparisons were performed with ground controls at 1g. Seedlings grown in microgravity were significantly longer than the ground 1g controls. The cortical root meristematic cells were analyzed to investigate the alterations in cell proliferation and cell growth. Proliferation rate was quantified by counting the number of cells per millimeter in the specific cell files, and was found to be higher in microgravity-grown samples than in the control 1g. Cell growth was appraised through the rate of ribosome biogenesis, assessed by morphological and morphometrical parameters of the nucleolus and by the levels of the nucleolar protein nucleolin. All these parameters showed a depletion of the rate of ribosome production in microgravity-grown samples versus samples grown at 1g. The results show that growth in microgravity induces alterations in essential cellular functions. Cell growth and proliferation, which are strictly associated functions under normal ground conditions, appeared divergent after gravity modification; proliferation was enhanced, whereas growth was depleted. We suggest that the cause of these changes could be an alteration in the cell cycle regulation, at the levels of checkpoints regulating cell cycle progression, leading to a shortened G2 period.

Immunodetection of nucleolar proteins and ultrastructure of nucleoli of soybean root meristematic cells treated with chilling stress and after recovery

The nucleolar proteins, fibrillarin and nucleophosmin, have been identified immunofluorescently in the root meristematic cells of soybean seedlings under varying experimental conditions: at 25 degrees C (control), chilling at 10 degrees C for 3 h and 4 days and recovery from the chilling stress at 25 degrees C. In each experimental variant, the immunofluorescence signals were present solely at the nucleolar territories. Fluorescent staining for both proteins was mainly in the shape of circular domains that are assumed to correspond to the dense fibrillar component of the nucleoli. The fewest fluorescent domains were observed in the nucleoli of chilled plants, and the highest number was observed in the plants recovered after chilling. This difference in the number of circular domains in the nucleoli of each variant may indicate various levels of these proteins in each variant. Both the number of circular domains and the level of these nucleolar proteins changed with changes in the transcriptional activity of the nucleoli, with the more metabolically active cell having higher numbers of active areas in the nucleolus and higher levels of nucleolar proteins, and conversely. Electron microscopic studies revealed differences in the ultrastructure of the nucleoli in all experimental variants and confirmed that the number of fibrillar centres surrounded by dense fibrillar component was the lowest in the nucleoli of chilled plants, and the highest in the nucleoli of recovered seedlings.