Organization of the nucleoli of soybean root meristematic cells at different states of their activity

A Promising Ash Supplementation Strategy in the Cultivation of Spirodela polyrrhiza Plants

An innovative approach to the management of waste in the form of ash obtained during biomass combustion is justified due to its specific properties, including the presence of macro- and microelements. The aim of the current study was to determine the concentration of ash obtained from Sorghum combustion regarding its fertilizer value and its effect on the cytological structures, physiological parameters, growth and development of Lemnaceae plants, thereby demonstrating the possibility of using this waste to supplement culture media. The analyses showed that the use of ash in the in vitro cultivation of Lemnaceae aquatic plants had a dose-dependent effect. The addition of 2% ash favorably affected the condition of plant roots, i.e., meristem elongation and an increase in nucleoli sizes as well as improving the chlorophyll content index, gas exchange parameters, chemical oxygen demand (COD) and plant vigor via PSII, which was confirmed by a chlorophyll fluorescence measurement. On the other hand, too high of a concentration, i.e., 10% ash, adversely affected the plant development and parameters studied. Concluding, the use of ash at a low concentration favorably affected the yielding of Spirodela polyrrhiza, whose biomass can be used for energy purposes in the production of bioethanol, plant biogas or the phytoremediation of industrial waters and leachate.

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.

Ultrastructural and Molecular Analyses Reveal Enhanced Nucleolar Activity in Medicago truncatula Cells Overexpressing the MtTdp2α Gene

The role of tyrosyl-DNA phosphodiesterase 2 (Tdp2) involved in the repair of 5'-end-blocking DNA lesions is still poorly explored in plants. To gain novel insights, Medicago truncatula suspension cultures overexpressing the MtTdp2α gene (Tdp2α-13C and Tdp2α-28 lines, respectively) and a control (CTRL) line carrying the empty vector were investigated. Transmission electron microscopy (TEM) revealed enlarged nucleoli (up to 44% expansion of the area, compared to CTRL), the presence of nucleolar vacuoles, increased frequency of multinucleolate cells (up to 4.3-fold compared to CTRL) and reduced number of ring-shaped nucleoli in Tdp2α-13C and Tdp2α-28 lines. Ultrastructural data suggesting for enhanced nucleolar activity in MtTdp2α-overexpressing lines were integrated with results from bromouridine incorporation. The latter revealed an increase of labeled transcripts in both Tdp2α-13C and Tdp2α-28 cells, within the nucleolus and in the extra-nucleolar region. MtTdp2α-overexpressing cells showed tolerance to etoposide, a selective inhibitor of DNA topoisomerase II, as evidenced by DNA diffusion assay. TEM analysis revealed etoposide-induced rearrangements within the nucleolus, resembling the nucleolar caps observed in animal cells under transcription impairment. Based on these findings it is evident that MtTdp2α-overexpression enhances nucleolar activity in plant cells.

Nucleolus-derived mediators in oncogenic stress response and activation of p53-dependent pathways

Rapid growth and division of cells, including tumor ones, is correlated with intensive protein biosynthesis. The output of nucleoli, organelles where translational machineries are formed, depends on a rate of particular stages of ribosome production and on accessibility of elements crucial for their effective functioning, including substrates, enzymes as well as energy resources. Different factors that induce cellular stress also often lead to nucleolar dysfunction which results in ribosome biogenesis impairment. Such nucleolar disorders, called nucleolar or ribosomal stress, usually affect cellular functioning which in fact is a result of p53-dependent pathway activation, elicited as a response to stress. These pathways direct cells to new destinations such as cell cycle arrest, damage repair, differentiation, autophagy, programmed cell death or aging. In the case of impaired nucleolar functioning, nucleolar and ribosomal proteins mediate activation of the p53 pathways. They are also triggered as a response to oncogenic factor overexpression to protect tissues and organs against extensive proliferation of abnormal cells. Intentional impairment of any step of ribosome biosynthesis which would direct the cells to these destinations could be a strategy used in anticancer therapy. This review presents current knowledge on a nucleolus, mainly in relation to cancer biology, which is an important and extremely sensitive element of the mechanism participating in cellular stress reaction mediating activation of the p53 pathways in order to counteract stress effects, especially cancer development.

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.

Nucleolar chromatin organization at different activities of soybean root meristematic cell nucleoli

Nucleolar chromatin, including nucleolus-associated chromatin as well as active and inactive condensed ribosomal DNA (rDNA) chromatin, derives mostly from secondary constrictions known as nucleolus organizer regions containing rDNA genes on nucleolus-forming chromosomes. This chromatin may occupy different nucleolar positions being in various condensation states which may imply different rDNA transcriptional competence. Sections of nucleoli originating from root meristematic cells of soybean seedlings grown at 25 °C (the control), then subjected to chilling stress (10 °C), and next transferred again to 25 °C (the recovery) were used to measure profile areas occupied by nucleolar condensed chromatin disclosed with sodium hydroxide methylation-acetylation plus uranyl acetate technique. The biggest total area of condensed chromatin was found in the nucleoli of chilled plants, while the smallest was found in those of recovered plants in relation to the amounts of chromatin in the control nucleoli. The condensed nucleolar chromatin, in the form of different-sized and different-shaped clumps, was mainly located in fibrillar centers. One can suppose that changes of condensed rDNA chromatin amounts might be a mechanism controlling the number of transcriptionally active rDNA genes as the nucleoli of plants grown under these experimental conditions show different transcriptional activity and morphology.

Levels of DNA methylation and histone methylation and acetylation change in root tip cells of soybean seedlings grown at different temperatures

In order to check whether changes in DNA and histone modifications occur in the nuclei of root tip cells of soybean seedlings grown 1) under control conditions (25 °C), 2) subjected to chilling stress (10 °C) and 3) recovered (25 °C) after chilling, measurements of fluorescence intensity with the use of antibodies to heterochromatin as well as to euchromatin markers were carried out. Moreover, the number and sizes of chromocentres were analyzed. The studies showed that during chilling stress the fluorescence intensity for the markers characteristic of heterochromatin increased while for the markers of euchromatin decreased in comparison to the control. After the recovery the converse situation was observed, i.e. increase in fluorescence intensity for euchromatin markers and decrease in heterochromatin markers. The number of chromocentres remained unchanged in the nuclei of all three studied variants. However, differences in the sizes of chromocentres were observed - the highest number of big chromocentres and simultaneously the lowest number of small chromocentres were in the nuclei of stressed plants. Conversely - in the nuclei of recovered plants there were the lowest number of big chromocentres and the highest number of small ones. The treatment of seedlings with the inhibitors of DNA methylation (5-aza-dC) and histone deacetylation (NaBu) also caused changes in fluorescence intensity and chromocentre sizes in soybean nuclei. These results suggest that DNA and histone modification patterns can be altered in soybean nuclei by different growth temperatures and by appropriate inhibitors influencing epigenetic chromatic modifications.

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.

The reaction of Lupinus angustifolius L. root meristematic cell nucleoli to lead

The effect of 2-48 h treatment of Lupinus angustifolius L. roots with lead nitrate at the concentration of 10(-4) M on the nucleoli in meristematic cells was investigated. In the lead presence the number of ring-shaped as well as segregated nucleoli increased especially after 12-48 h of treatment, while spindle-shaped nucleoli appeared after 24 h and 48 h. Lead presence also increased the frequency of cells with silver-stained particles in the nucleus and the number of these particles especially from the 12th hour of treatment. It was accompanied by significant decline of nucleolar area. Analysis of these cells in transmission electron microscope confirmed the presence of ring-shaped and segregated nucleoli. Moreover, electron microscopy revealed compact structure nucleoli without granular component. Additionally, one to three oval-shaped fibrillar structures attached to nucleolus or lying free in the nucleoplasm were visible. The possible mechanism of lead toxicity to the nucleolus is briefly discussed.