A change of developmental program induces the remodeling of the interchromatin domain during microspore embryogenesis in Brassica napus L

After a stress treatment, in vitro-cultured pollen changes its normal gametophytic developmental pathway towards embryogenesis producing multicellular embryos from which, finally, haploid and double haploid plants develop. The architecture of the well-organized nuclear functional domains changes in response to DNA replication, RNA transcription, processing and transport dynamics. A number of subnuclear structures present in the interchromatin region (IR, the nuclear domain between chromosome territories) have been shown as involved, either directly or indirectly, in transcriptional regulation. These structures include the interchromatin granule clusters (IGCs), perichromatin fibrils (PFs), Cajal bodies (CBs) and perichromatin granules (PGs). In this work, we present a cytochemical, immunocytochemical, quantitative and morphometric analysis at the light, confocal and electron microscopy levels to characterize the changes in the functional architecture of the nuclear interchromatin domain during two developmental programs followed by the microspore: differentiation to mature pollen grains (transcriptionally inactive), and microspore embryogenesis involving proliferation in the first stages (highly engaged in transcription). Our results revealed characteristic changes in size, shape and distribution of the different interchromatin structures as a consequence of the reprogramming of the microspore, allowing us to relate the remodeling of the interchromatin domain to the variations in transcriptional activities during proliferation and differentiation events, and suggesting that RNA-associated structures could be a regulatory mechanism in the process. In addition, we document the presence of two structurally different types of CBs, and of IGC and CB-associated regions, similar to those present in animal cells, and not yet described in plants.

Nanoparticles as smart treatment-delivery systems in plants: assessment of different techniques of microscopy for their visualization in plant tissues

BACKGROUND AND AIMS

The great potential of using nanodevices as delivery systems to specific targets in living organisms was first explored for medical uses. In plants, the same principles can be applied for a broad range of uses, in particular to tackle infections. Nanoparticles tagged to agrochemicals or other substances could reduce the damage to other plant tissues and the amount of chemicals released into the environment. To explore the benefits of applying nanotechnology to agriculture, the first stage is to work out the correct penetration and transport of the nanoparticles into plants. This research is aimed (a) to put forward a number of tools for the detection and analysis of core-shell magnetic nanoparticles introduced into plants and (b) to assess the use of such magnetic nanoparticles for their concentration in selected plant tissues by magnetic field gradients.

METHODS

Cucurbita pepo plants were cultivated in vitro and treated with carbon-coated Fe nanoparticles. Different microscopy techniques were used for the detection and analysis of these magnetic nanoparticles, ranging from conventional light microscopy to confocal and electron microscopy.

KEY RESULTS

Penetration and translocation of magnetic nanoparticles in whole living plants and into plant cells were determined. The magnetic character allowed nanoparticles to be positioned in the desired plant tissue by applying a magnetic field gradient there; also the graphitic shell made good visualization possible using different microscopy techniques.

CONCLUSIONS

The results open a wide range of possibilities for using magnetic nanoparticles in general plant research and agronomy. The nanoparticles can be charged with different substances, introduced within the plants and, if necessary, concentrated into localized areas by using magnets. Also simple or more complex microscopical techniques can be used in localization studies.

Nuclear bodies domain changes with microspore reprogramming to embryogenesis

We analysed the presence of nuclear bodies and particularly Cajal bodies during representative stages of gametophytic and haploid embryogenic development in isolated microspore and anther cultures of a model system (Brassica napus cv. Topas) and a recalcitrant species (Capsicum annuum L. var. Yolo Wonder B). The nuclear bodies domain is involved on several important roles on nuclear metabolism, and Cajal bodies are specifically involved on the storage and maturation of both snRNPs and snoRNPs, as well as other splicing factors, necessary for mRNA and pre-rRNA processing, but not directly on the transcription. In this study, immunofluorescence and immunogold labelling with anti-trimethylguanosine antibodies against the specific cap of snRNAs, ultrastructural and cytochemical analysis were performed on cryoprocessed samples at confocal and electron microscopy respectively. Results showed that Cajal bodies increase during the early stages of microspore embryogenic development (young pro-embryos), compared to microspore and pollen development. Our results suggest that Cajal bodies may have a role in the transcriptionally active, proliferative stages that characterise early microspore embryogenic development.

Changes in extranucleolar transcription during actinomycin D-induced apoptosis

Actinomycin D (AMD) inhibits DNA-dependent RNA polymerases and its selectivity depends on the concentration used; at very high concentrations it may also induce apoptosis. This study investigates the effects of different concentrations (0.01 to 1 microg/ml) of AMD on RNA transcription and maturation and on the organization of nuclear ribonucleoproteins (RNPs), and their relationship with apoptosis induction. Human HeLa cells were used as a model system. At the lowest concentration used, AMD induced the segregation of the nucleolar components and impaired r-RNA synthesis, as revealed by the decreased immunopositivity for bromo-uridine incorporation and for DNA/RNA hybrid molecules. The synthesis of pre-mRNAs, on the contrary, was active, while the immunolabeling of snRNP proteins and of the SC-35 splicing factor strongly decreased on perichromatin fibrils (where they are involved in co-transcriptional splicing). This suggests that the post-transcriptional maturation of extranucleolar RNAs was also affected. Moreover, still in the absence of typical late morphological or biochemical signs of apoptosis (i.e. chromatin condensation), these cells displayed the early apoptotic features, i.e. the externalization of phosphatidylserine residues on the plasma membrane and propidium iodide exclusion in vivo. At the highest concentrations of AMD used, apoptosis massively occurred, with the typical morphological events (progressive chromatin condensation, clustering of snRNPs and SC-35 splicing factor, cell blebbing). However, transcription of hnRNAs was maintained in the residual areas of diffuse chromatin up to advanced apoptotic stages. The inhibition of rRNA synthesis and the defective pre-mRNA maturation seem to be part of the apoptotic process induced by AMD.

Differentiating plant cells switched to proliferation remodel the functional organization of nuclear domains

The immature pollen grain, the microspore, under stress conditions can switch its developmental program towards proliferation and embryogenesis. The comparison between the gametophytic and sporophytic pathways followed by the microspore permitted us to analyse the nuclear changes in plant differentiating cells when switched to proliferation. The nucleus is highly dynamic, the architecture of its well organised functional domains--condensed chromatin, interchromatin region, nuclear bodies and nucleolus--changing in response to DNA replication, RNA transcription, processing and transport. In the present work, the rearrangements of the nuclear domains during the switch to proliferation have been determined by in situ molecular identification methods for the subcellular localization of chromatin at different functional states, rDNA, elements of the nuclear machinery (PCNA, splicing factors), signalling and stress proteins. The study of the changes in the nuclear domains was determined by a correlative approach at confocal and electron microscopy levels. The results showed that the switch of the developmental program and the activation of the proliferative activity affected the functional organization of the nuclear domains, which accordingly changed their architecture and functional state. A redistribution of components, among them various signalling molecules which targeted structures within the interchromatin region upon translocation from the cytoplasm, was also observed.

Visualization of transcription sites at the electron microscope

In order to localize at EM level the sites of transcription of both pre-mRNA and pre-rRNA, we have detected the DNA/RNA hybrid molecules and m3Gcapped structures by means of specific antibodies after short bromo-uridine (BrU) incorporation. In addition, the sections have been stained by a selective RNA stain, terbium citrate. Our data indicate that perichromatin fibrils incorporate BrU and are labeled by the anti-hybrid probe; this supports the idea that they are the pre-mRNA transcription sites. On the contrary, interchromatin granules do not incorporate BrU after short pulses and are not labeled by the anti-hybrid probe. Concerning the nucleolus, anti-hybrid and anti-BrdU antibodies colocalize only on the dense fibrillar component, suggesting that this is the site of rRNA transcription. Interestingly, the dense fibrillar component and the granular component, after specific RNA staining, show remarkable structural similarities, both containing fibrogranular RNA structures.

Hsp70 and Hsp90 change their expression and subcellular localization after microspore embryogenesis induction in Brassica napus L

A stress treatment of 32 degrees C for at least 8h was able to change the gametophytic program of the microspore, switching it to embryogenesis in Brassica napus, an interesting model for studying this process in vitro. After induction, some microspores started symmetric divisions and became haploid embryos after a few days, whereas other microspores, not sensitive to induction, followed their original gametophytic development. In this work the distribution and ultrastructural localization of two heat-shock proteins (Hsp70 and Hsp90) throughout key stages before and after embryogenesis induction were studied. Both Hsp proteins are rapidly induced, localizing in the nucleus and the cytoplasm. Immunogold labeling showed changes in the distribution patterns of these proteins, these changes being assessed by a quantitative analysis. Inside the nucleus, Hsp70 was found in association with RNP structures in the interchromatin region and in the nucleolus, whereas nuclear Hsp90 was mostly found in the interchromatin region. For Hsp70, the accumulation after the inductive treatment was accompanied by a reversible translocation from the cytoplasm to the nucleus, in both induced (embryogenic) and noninduced (gametophytic) microspores. However, the translocation was higher in embryogenic microspores, suggesting a possible additional role for Hsp70 in the switch to embryogenesis. In contrast, Hsp90 increase was similar in all microspores, occurring faster than for Hsp70 and suggesting a more specific role for Hsp90 in the stress response. Hsp70 and Hsp90 colocalized in clusters in the cytoplasm and the nucleus, but not in the nucleolus. Results indicated that stress proteins are involved in the process of microspore embryogenesis induction. The differential appearance and distribution of the two proteins and their association at specific stages have been determined between the two systems coexisting in the same culture: embryogenic development (induced cells) and development of gametes (noninduced cells).

MAPKs entry into the nucleus at specific interchromatin domains in plant differentiation and proliferation processes

Mitogen-activated protein kinases (MAPKs) are involved in the signaling of extracellular stimuli in eukaryotes, including plants. Different MAPKs have recently been shown to be expressed during plant cell proliferation and developmental processes such as pollen development and embryogenesis, but the structural subdomain where these MAPKs are targeted in the nucleus has not yet been characterized. We have determined the changes in the expression and subcellular localization of ERK homologues, proteins belonging to the MAPK family, and MAPK-active forms in two plant developmental processes which involved differentiation (pollen maturation) and proliferation (the initials of pollen embryogenesis). Immunofluorescence and immunogold labeling in the species studied showed that the progression of differentiation and proliferation was accompanied by an increase in the expression of ERKs and MAPK activation together with a translocation to the nucleus. Combining ultrastructural cytochemistry and immunogold for RNA and phosphorylated proteins we have identified the nuclear sites housing these MAPKs in areas of the interchromatin region enriched in RNA and phosphoproteins that include clusters of interchromatin granules. This could suggest a role of these MAPKs in the early events of activation of the transcription and processing machinery, via phosphorylation, which subsequently would be recruited to the transcription sites. The association of the nuclear localization of MAPKs with the progression through the cell cycle and the commitment toward differentiation in the two plant developmental processes can be correlated.

Non-isotopic mapping of ribosomal RNA synthesis and processing in the nucleolus

The precise location of ribosomal RNA (rRNA) synthesis within the nucleolus is the subject of recent controversy; some investigators have detected nascent RNA in the dense fibrillar components (DFCs) while others have localized transcription to the fibrillar centers (FCs). We endeavored to resolve this controversy by applying a new technique for non-isotopic labeling of RNA and examined the synthesis and movement of non-isotopically labeled rRNA within the nucleolus. We found that rRNA is synthesized only in a restricted area of DFCs, also involving the boundary region with FCs. We traced a movement of RNA from transcription sites through DFCs to granular components. Our results indicate functional compartmentalization of DFCs with respect to the synthesis and processing of precursor rRNA. In situ mapping of the 5' leader sequence of the 5' external transcribed spacer together with transcription labeling indicated that transcription and the first steps in processing of precursor rRNA are spatially separated. Surprisingly, the results also pointed to a partially extended conformation of newly synthesized precursor rRNA transcripts.

The protein kinases AtMAP3Kepsilon1 and BnMAP3Kepsilon1 are functional homologues of S. pombe cdc7p and may be involved in cell division

We identified an Arabidopsis thaliana gene, AtMAP3Kepsilon1, and a Brassica napus cDNA, BnMAP3Kepsilon1, encoding functional protein serine/threonine kinases closely related to cdc7p and Cdc15p from Schizosaccharomyces pombe and Saccharomyces cerevisiae, respectively. This is the first report of cdc7-related genes in non-fungal eukaryotes; no such genes have as yet been identified in Metazoans. The B. napus protein is able to partially complement a cdc7 loss of function mutation in S. pombe. RT-PCR and in situ hybridisation revealed that the A. thaliana and B. napus genes are expressed in both the sporophytic and the gametophytic tissues of the respective plant species and revealed further that expression is highest in dividing cells. Moreover, AtMAP3Kepsilon1 gene expression is cell cycle-regulated, with higher expression in G2-M phases. Our results strongly suggest that the plant cdc7p-related protein kinases are involved in a signal transduction pathway similar to the SIN pathway, which positively regulates cytokinesis in S. pombe.

Genes normally expressed in the endosperm are expressed at early stages of microspore embryogenesis in maize

Reproduction in flowering plants is characterized by double fertilization and the resulting formation of both the zygotic embryo and the associated endosperm. In many species it is possible to experimentally deviate pollen development towards an embryogenic pathway. This developmental switch, referred to as microspore embryogenesis or androgenesis, leads to the formation of embryos similar to zygotic embryos. In a screen for genes specifically expressed during early androgenesis, two maize genes were isolated by mRNA differential display. Both genes represent new molecular markers expressed at a very young stage of androgenic embryogenesis. When their expression pattern was studied during normal reproductive development, both showed early endosperm-specific expression. Investigation of the cytological features of young androgenic embryos revealed that they present a partially coenocytic organization similar to that of early endosperm. These findings suggest that maize androgenesis may possibly involve both embryogenesis and the establishment of endosperm-like components.

Immunoelectron microscopy of PCNA as an efficient marker for studying replication times and sites during pollen development

Here we report for the first time the ultrastructural localization of DNA replication sites in the nucleus of plant cells and the timing of replication through the pollen developmental programme by proliferating cell nuclear antigen (PCNA) immunogold labelling. Replication sites were identified by labelling with anti-PCNA antibodies in fibrils of the interchromatin region close to the condensed chromatin, defining a perichromatin subdomain in the interchromatin space where DNA replication takes place. The same nuclear structures are decorated by anti-BrdU (5-bromo-2'-deoxyuridine) immunogold after short pulses of BrdU labelling. Double immunogold labelling for PCNA and DNA show colocalization on these perichromatin structures. PCNA immunoelectron microscopy also allows correlation of replicative activity with the dynamics of chromatin condensation. DNA replication was also monitored at different phases during pollen development by PCNA immunoelectron microscopy, revealing two peaks of DNA synthesis, at the beginning (early tetrad), and the end (late vacuolate), of microspore interphase. High-resolution autoradiography after [3H]thymidine incorporation also showed high replicative activity at the same two periods of microspore interphase. In the bicellular pollen grain, PCNA immunogold labelling revealed that DNA replication in the generative cell starts at an intermediate stage of pollen maturation, whereas the vegetative nucleus does not replicate and is arrested in G1. The use of anti-PCNA antibodies at the ultrastructural level is an easier, faster and more feasible method than the detection of in vivo-incorporated nucleotides, especially in plant systems with long cell cycles. PCNA immunogold labelling is, therefore, proposed as an efficient marker for mapping the sites and timing of replication at the electron microscopy level.

Defined nuclear changes accompany the reprogramming of the microspore to embryogenesis

The switch of the gametophytic developmental program toward pollen embryogenesis to form a haploid plant represents an important alternative for plant breeding. In the present study, the switch of the gametophytic developmental program toward a sporophytic pathway, "embryogenesis," has been studied in three different plant species, Brassica, tobacco, and pepper. The switch has been induced by stress (heat shock) at the very responsive stage of the microspore, which is the vacuolate period. As a result, the cell nucleus undergoes striking structural changes with regard to late gametophytic development, including alterations of biosynthetic activities and proliferative activity. An enrichment in HSP70 heat-shock protein and in the presence of Ntf6-MAP kinase was observed after inductive treatment in the nuclei during early embryogenesis. This apparently reflected the possible roles of these proteins, specifically the protective role of HSP70 for the nuclear machinery, and signal transduction of Ntf6-MAPK for the entry of cells into proliferation. Importantly, the observed nuclear changes were similar in the three species investigated and represented convenient markers for early monitoring of embryogenesis and selection purposes for obtaining double-haploid plants in plant breeding.

Ultrastructural distribution of a MAP kinase and transcripts in quiescent and cycling plant cells and pollen grains

Mitogen-activated protein kinases (MAPKs) are components of a kinase module that plays a central role in the transduction of diverse extracellular stimuli, including mitogens, specific differentiation and developmental signals and stress treatments. This shows that reversible protein phosphorylation cascades play a pivotal role in signal transduction in animal cells and yeast, particularly the entry into mitosis of arrested cells. Homologues of MAPKs have been found and cloned in various plant species, but there have been no data about their in situ localization at the subcellular level and their expression in plant cells so far. In the present paper we report the first data on the ultrastructural in situ localization of MAPK and their mRNAs in various plant cells. Proliferating and quiescent meristematic plant cells were studied to evaluate whether changes in MAPK presence, distribution and expression accompany the entry into proliferation of dormant cells. Moreover, MAPK localization was analyzed in vacuolate microspores. Polyclonal antibodies against the deduced MAPK from the tobacco Ntf6 clone were able to recognize homologue epitopes by immunocytochemical techniques in the cell types studied. The pattern of protein distribution is similar in all the cases studied: it is localized in the cytoplasm and in the nucleus, mainly in the interchromatin region. The quantitative study of the density showed that MAPK labelling is more abundant in cycling than in quiescent cells, also suggesting that, in plants, MAPK pathways might play a role in cell proliferation. RNA probes for conserved regions of the catalytic domain of plant MAPK homologue genes were used to study MAPK expression in those plant cells. In situ hybridization (ISH) showed the presence of MAPK transcripts in the three plant cell types studied, but levels were very low in quiescent cells compared to those in cycling cells. The quantification of labelling density of ISH signals strongly suggests a higher level of MAPK expression in proliferating cells, but also some basal messenger presence and/or expression in the quiescent ones. Immunogold and ISH results show the presence and distribution of MAPK proteins and mRNAs in vacuolate microspores. This represents a very dynamic stage during pollen development in which the cell nucleus is being prepared for an asymmetrical mitotic division, giving rise to both the generative and the vegetative nuclei of the bicellular pollen grain. Taken together, the data indicate a role played by MAPK in the re-entry into proliferation in plant cells.

Histones and DNA ultrastructural distribution in plant cell nucleus: a combination of immunogold and cytochemical methods

In this work we report for the first time the ultrastructural distribution of histones and DNA in the nuclear compartments in two different plant cell types: Allium cepa L. root meristems and Capsicum annuum L. microspores and pollen grains, by using antibodies against histones H2B and H4 and anti-DNA. Immunolocalizations were combined with ultrastructural cytochemistry for nucleic acids (methylation-acetylation method), DNA (NAMA-Ur) and RNPs (EDTA), to relate the subcellular location of histones and DNA with the chemical subcompartmentalization of the cell nucleus. This is particularly interesting concerning the presence of histones or not on fibers of the interchromatin region and on the fibrillar components of the nucleolus, nuclear subcompartments where transcription has been shown to take place at some regions. Our methodological approach permitted to define precisely the structures where histones were detected in relation to the ultrastructural localization of chromatin in various structural condensation levels. Concerning the localization of DNA and histones on the different components of the nucleolus, the combination of immunogold labeling with the methylation-acetylation cytochemical method, developed in our laboratory, was very useful, thus permitting a clear recognition of the nucleolar components and a correct assignment of labeling, which is not always evident on uranyl-lead-stained Lowicryl sections. Double immunogold assays were also done for a simultaneous visualization of histones and DNA. Our results show a coincident distribution of histones and DNA on the same nuclear compartments revealing the presence of both antigens on condensed chromatin, fibers of the interchromatin region, principally located at the periphery of the condensed chromatin, and in the fibrillar components of the nucleolus.

New in situ approaches to study the induction of pollen embryogenesis in Capsicum annuum L

The induction of pollen embryogenesis in Capsicum annuum L. has been studied at the cellular level using various in situ approaches with several molecular probes for DNA, RNA and proteins. The late vacuolated microspore and the young bicellular pollen grain are stages of gametophytic development in which embryogenesis can be induced. Our results show that the late vacuolated microspore stage is most responsive to embryogenesis induction. The proliferating cell nuclear antigen (PCNA) has been immunolocalized at the electron microscopy level, in order to map replication sites in relation to the fine structure of chromatin. It shows different patterns of labelling at both developmental stages studied, revealing that the late vacuolated microspore is in a period of replication. Other in situ studies have been performed to characterize the state of nuclear activity at the specific developmental stages in which the embryogenic induction can occur. The modern in situ terminal-deoxy-nucleotidyl transferase (TdT) reaction for DNA, the immunolocalization of various nuclear antigens (as snRNPs, fibrillarin, RNA) and the ultrastructural in situ hybridization using 18S and 25S ribosomal probes provided valuable data bout the specific features displayed by the functional nuclear compartments of the microspore, and the young vegetative and generative cells. They are related not only to the state of gene activity but also with probably the ability to switch to the sporophytic pathway at specific developmental times of their gametophytic program.

Ultrastructural nonisotopic mapping of nucleolar transcription sites in onion protoplasts

The post- and preembedding ultrastructural localization of transcribing rRNA genes has been carried out in nucleoli of permeabilized onion growing root tip protoplasts by means of the nonisotopic bromouridine method. By means of both post- and preembedding approaches, major synthetic sites were identified with morphologically distinct subdomains of dense fibrillar components, with some signal also being associated with nucleolar fibrillar centers and vacuoles. Moreover, labeled medusoid fibrils within distinct domains seen in Lowicryl thin sections likely represent the morphological correlate of transcribing nucleolar genes.

The immunolocalization of nuclear antigens during the pollen developmental program and the induction of pollen embryogenesis

The immunolocalization of nuclear antigens, combined with cytochemical procedures as well as in situ hybridization and recent in situ molecular methods, has been applied at different steps of pollen development to characterize the functional organization of the nucleus during the formation of the male gametophyte in an agronomically interesting plant, Capsicum annuum L. Pollen embryogenesis has been induced in pepper and the first stages of the process have been studied at the cellular level. Low temperature processing methods including cryosections and Lowicryl sections were very convenient for performing the various in situ techniques used in the pollen grains. Different molecular probes for localizing DNA, RNA, snRNPs, specific nucleolar proteins, various rRNA species, and DNA/RNA hybrids provided positive results in the pollen nuclei. The data obtained, and the changes observed in the organization of the nuclear compartments during pollen development, are related to the variations in gene activity undergone by the male gametophyte. The methodology used is proposed as a very convenient approach to localize molecules and events involved in the nuclear function in both gametophytic and sporophytic pollen development.

The methylation-acetylation method: an ultrastructural cytochemistry for nucleic acids compatible with immunogold studies

Methylation-acetylation (MA) is an easy and reproducible ultrastructural cytochemical method which gives preferential contrast to nucleic acid containing structures. When performed en bloc before Lowicryl embedding it does not affect the main antigenic and chemical properties of the sample and is compatible with a large variety of modern immunogold methods permitting a better assignment of the labelling to the well-defined nuclear structures. DNA, RNA, and nuclear proteins, with different chemical nature, nuclear localization, and amount, can be immunolocalized on MA-treated samples. Ultrastructural in situ hybridization and other approaches for studying the functional regions of chromatin, the terminal deoxynucleotidyl transferase and the bromodeoxyuridine methods, are also compatible with the MA procedure. It can be also performed on ultrathin cryosections and Lowicryl sections. A much better visualization of the nuclear structures is obtained, enhancing the distinction between the nucleolar granular and dense fibrillar components. Moreover, the combination of the MA procedure with EDTA regressive staining gives preferential contrast to the RNA-rich structures. It is proposed as a useful approach which can be regularly used for in situ studies of the functional organization of the nucleus in both plant and animal cells.

Does the synthesis of ribosomal RNA take place within nucleolar fibrillar centers or dense fibrillar components? A critical appraisal

The localization of transcribing rRNA genes within nucleoli of mammalian cells, although intensively studied, has not been established. Most published papers on this topic situate transcribing ribosomal genes either to nucleolar fibrillar centers or to nucleolar dense fibrillar components. To clarify this point, we have generated the electron microscopic affinity cytochemistry picture of the nucleolus of cultured mammalian cells. Three kinds of affinity probes have been used: (1) probes to nucleolar chromatin, including rDNA sequences; (2) probes to a number of macromolecules (such as RNA polymerase I) which are directly, or indirectly, involved in the synthesis and processing of rRNA and formation of preribosomes; (3) antibodies to bromouridine for a recently standardized nonisotopical method depicting incorporated bromouridine within RNA. The results suggest the localization of transcription sites not only to dense fibrillar components but also to the border region between these components and fibrillar centers. Our data support a hypothesis that in metabolically active mammalian nucleoli, fibrillar centers and dense fibrillar components form a single functional domain for the transcription of rRNA genes, with nascent transcripts generating "automatically" dense fibrillar components. Through the active process of transcription, individual rRNA genes thus become engulfed within dense fibrillar components.

Immunoelectron microscopy of RNA combined with nucleic acid cytochemistry in plant nucleoli

The immunoelectron microscopy detection of RNA using anti-RNA monoclonal antibodies has been performed for the first time over different plant cells. The use of the methylation-acetylation (MA) method permits clear distinction among the nuclear and nucleolar compartments and can be combined with the immunogold approach. Cytochemical methods for nucleic acids were performed together with the immunoassays, providing additional data about the different composition of the various nucleolar components. Anti-RNA antibodies highly labeled the ribosome-rich areas of the cytoplasm and the nucleolus. The interchromatin region also is labeled. The labeling was intense in the granular component, lower in the dense fibrillar component, and very scarce in the fibrillar centers. The MA method made possible the statistical evaluation of the labeling density in the various nuclear compartments by permitting the clear assignment of the particles to precise nuclear structures.

A new approach to map transcription sites at the ultrastructural level

We describe a new ultrastructural method for locating transcription on ultra-thin sections. The use of anti-DNA/RNA hybrid antibodies provides specific labeling on precise structures of the nuclear compartments of several cell types. All mammalian and plant material studied (HeLa cells, lymphocytes, onion root meristematic cells) showed the same pattern of labeling: fibrillar structures in the interchromatin region and discrete regions of the dense fibrillar component at the periphery of the fibrillar centers in the nucleolus. The specificity of the immunogold labeling was tested by RNAse H digestion and by pre-blocking the antibody with synthetic DNA/RNA hybrids; in both cases no gold particles were observed. This method has considerable advantages compared with current techniques, constituting a very useful tool to map transcriptionally active loci in a variety of cells.

Cytochemistry and immunocytochemistry of nucleolar chromatin in plants

This review attempts to document the most relevant data currently available on the in situ localization of nucleolar chromatin on plant cells. The data provided by the most powerful and recent in situ techniques, such as DNA specific ultrastructural staining, immunogold labelling, in situ molecular cytochemistry, in situ hybridization or confocal microscopy, are summarized and discussed in the light of the potential and limitations of each individual methodology. The presence of DNA in both fibrillar centres and regions of the dense fibrillar component is extensively documented. Data on the nucleolar distribution of other important macromolecules involved in ribosomal transcription are also shown and referred to with regard to the location of DNA. The comparison with the available data on the animal cell nucleolus points towards models of similar functional organization in both plant and animal nucleoli.

Ultrastructural rRNA localization in plant cell nucleoli. RNA/RNA in situ hybridization, autoradiography and cytochemistry

The distribution of ribosomal transcripts in the plant nucleolus has been studied by non-isotopic in situ hybridization in ultrathin Lowicryl K4M sections and by high-resolution autoradiography after labelling with tritiated uridine. In parallel, cytochemical techniques were applied to localize RNA on different plant nucleolar components of Allium cepa L. root meristematic cells and Capsicum annuum L. pollen grains. For RNA/RNA in situ hybridization, several biotinylated single-stranded ribosomal RNA probes were used for mapping different fragments of the 18 S and the 25 S rRNA gene transcribed regions. Ribosomal RNAs (from pre-rRNAs to mature 18 and 25 S RNAs) were found in the nucleolus, in the dense fibrillar (DFC) and granular components (GC). Hybridization signal was found at the periphery of some fibrillar centres (FCs) with probes recognizing both 18 and 25 S rRNA sequences. A quantitative study was performed to analyze the significance of this labelling. Incorporation of tritiated uridine into roots was carried out and, later, after a long time-exposure, autoradiography revealed the presence of newly synthesized RNA mainly in the DFC and at the periphery of the FCs. The presence of RNA in these areas was also confirmed by the cytochemical techniques used in this study. Taken together, these data favour the hypothesis that transcription can begin at the periphery of the FCs, although we cannot exclude the possibility that the DFC plays a role in this process.

Characterization of the interchromatin region as the nuclear domain containing snRNPs in plant cells. A cytochemical and immunoelectron microscopy study

The combination of electron microscopy (EM) cytochemical with immunocytochemical methods is used to characterize the interchromatin region (IR) of the plant cell nucleus. Cryoprocessing of the sample provides a better ultrastructural preservation and allows the observation of some differences in the fine structure of the IR which shows a denser aspect resulting from the lower extraction of components with low-temperature methods. A complex network of fibrillar structures and isolated or clustered 30 to 50-nm granules are observed in the IR. Anti-DNA antibodies combined with the NAMA-Ur method for DNA or the EDTA staining, preferential for RNPs, allow the detection of chromatin fibers in the IR. Bismuth staining reveals the presence of highly phosphorylated proteins in some interchromatin structures. The spliceosomal snRNPs are immunolocalized on cryosections and Lowicryl sections of plant cells using monoclonal and polyclonal antibodies. They provide a homogeneous immunofluorescence pattern with no speckles. This is in correlation with the labeling at EM, immunogold particles decorate the EDTA-positive fibrillar structures of the IR but no labeling is found over the 30 to 50-nm granules. The presence of the spliceosomal snRNPs, DNA and phosphorylated proteins in the IR indicate that this nuclear domain plays a major role in pre-messenger RNA splicing and, possibly in transcription, in the plant cell nucleus.

A specific ultrastructural method to reveal DNA: the NAMA-Ur

We have developed a new, simple, and reproducible cytochemical method to specifically stain DNA at the electron microscopic level: the NAMA-Ur. It is based on the extraction of RNA and phosphate groups from phosphoproteins by a weak alkali hydrolysis (NA) which does not affect DNA, followed by blockage of the amino and carboxyl groups by methylation and acetylation (MA). Finally, sections are stained by uranyl (Ur), which can bind only to DNA. The efficiency of the pre-treatment (NA and MA) was demonstrated by X-ray microanalysis at the transmission electron microscopic level. The NAMA-Ur method has been successfully performed en bloc and on Lowicryl sections in mammalian and plant cells. A specific contrast is observed in the DNA-containing structures after this method, whose sensitivity allows visualization of electron-dense chromatin fibers of 10-12 nm composed of 3-nm DNA fibrils. This staining method has been combined with anti-DNA antibodies, providing complementary information to detect DNA in situ. We propose the NAMA-Ur as an easy method to investigate the chromatin organization in situ at the ultrastructural level.

Tungsten and molybdenum heteropolyacids as staining and contrasting agents: reactivity with epoxyresin-embedded cell and tissue structures

In this work, we carry out a further approach to the knowledge of the reaction mechanism of phosphotungstic and phosphomolybdic acids (PTA and PMA), as well as some derivatives, with cell structures from epoxyresin-embedded materials. Applied on thin sections from glutaraldehyde-fixed tissues, PTA and PMA induced a strong electron contrasting reaction in spermatid acrosomes, goblet cell mucin, callose and plant cell walls, endexine, intine and starch granules. In light microscopy, the localization of heteropolyacids on these structures was achieved by treatments of semithin sections with suitable reducing agents (titanous sulfate, stannous chloride, sodium borohydride, or p-phenylenediamine) to form the mixed-valence heteropolyblues, or with Schiffs's reagent. The use of PTA-dye complexes (pyronin-PTA and Mallory's PTA-hematoxylin) also showed the same staining pattern. Taking into account the chemical characteristics of the PTA- and PMA-reactive tissue elements, the present results indicate that heteropolyacids selectively enter into the highest hydrophilic structures from non-polar epoxy-embedded sections; after brief washing, they appear predominantly retained in tissue structures containing a great amount of carbohydrate components.

"Ag-NOR" proteins are present when transcription is impaired

The use of nucleolar silver staining in plant cells in conditions of impaired transcription, either natural (quiescent cells) or induced (actinomycin D, ethidium bromide and cordycepin treatments), shows that nucleoli with very low transcription rates, compared with controls, contain similar amounts of argyrophilic proteins, as well as the same structural location. This demonstrates the absence of correlation between silver staining and present transcriptional activity. The effects of conditions used on the structure of nucleolar chromatin support our previous suggestion on the involvement of "Ag-NOR" proteins in the process of decondensation of NOR chromatin.

Nucleolar fibrillar centres in plant meristematic cells: ultrastructure, cytochemistry and autoradiography

In plant cells nucleolar fibrillar centres (FCs) undergo ultrastructural changes, depending on the nucleolar activity. We have found two types of FC structure in nucleoli with either high or low activity, to which we have given the conventional names of homogeneous and heterogeneous, respectively. The first type is characterized by the presence of fibres that we describe structurally and cytochemically as decondensed chromatin; the second type, in addition to these fibres, contains a variable number of dense cores made up of condensed chromatin. Moreover, RNA does not appear to be present in FCs, while proteins are a major component. Autoradiography after tritiated uridine incorporation shows that FCs are not the site of transcription, but that this takes place in the fibrillar component; the same result is obtained using the lead acetate fixation technique for detecting orthophosphate ions. This fact leads us to think that FCs are not the whole interphasic counterpart of the mitotic nucleolar organizing region (NOR), as stated by other authors, but only the portion of the NOR that is temporarily inactive in transcription; the transcriptionally active part of the NOR is in the fibrillar component, bound to its earliest product of transcription. Thus, FCs and the fibrillar component constitute a functional unit.

Transmission electron microscopic studies on the mitotic cycle of nucleolar proteins impregnated with silver

Hela cells were impregnated with silver according to Paweletz et al. (1967). In cells in mitosis not only the nucleolar organizer regions (NORs) are strongly impregnated but also part of the nucleolar material, which accumulates in and around the chromosomes. The treatment with adenosine, which in interphase cells spreads the nucleolar material within the nucleus, also distributes the argentophilic material in and around the chromosomes. During the reconstruction phase this material reassembles around the NORs to form parts of the new nucleolus. The silver impregnation technique clearly demonstrates that two main components are responsible for the argentophily of the nucleolus. This is in agreement with the results obtained by Lischwe et al. (1979).

Evolution of the cytoplasmic organelles during female meiosis in Pisum sativum L

In this paper we have traced the evolution of the cytoplasmic organelles in the female germinal cell of Pisum sativum L., from the beginning of meiosis to the early stages of the maturing megaspore, in order to correlate the morphological changes with the physiological aspects of megasporogenesis.A process of intense cytoplasmic vacuolation takes place in the megaspore mother cell (MMC) during prophase I, probably proceeding from the smooth endoplasmic reticulum and dictyosomes; it results in the formation of big vacuoles, which play a role in MMC polarization. By means of this polarization most plastids and mitochondria are incorporated into the functional megaspore at the end of meiosis.There are plastid and mitochondria cycles which consist of dedifferentiation followed by redifferentiation, During these cycles a transient morphology appears, called a cup-shaped form, which we interpret as an expression of low organelle activity.The wall of the MMC thickens throughout megasporogenesis and loses its plasmodesmata during middle prophase I. The ribosome population is reduced during prophase I and then restored during the early stages of the megaspore maturing process, as shown by the quantitative study that we have carried out. The nucleolar cytoplasmic bodies play a part in this restoring process. These bodies have a special morphology and appear to be originated from the activity of the nucleolar organizing region (NOR) during nucleolar disorganization in prophase I.We think that this cytoplasmic evolution is a response to nuclear genic recombination, in order to provide the most adequate expression of the zygote genome.

Correlation of nucleolar activity and nucleolar vacuolation in plant cells

In root meristematic cells nucleolar structure varies with the cell cycle. Apart from normal meristematic nucleoli one finds nucleoli with a big central vacuole surrounded by a loose cortex with individual fibrillar centres [22] clearly visible within it. There are also intermediate structures between both nucleolar types. In Pisum sativum nuclear tissue, the structure of the vacuolated nucleoli is similar and appears in periods of high metabolic activity during megasporogenesis. In both tissues, vacuolated nucleoli incorporate tritiated uridine more actively than 'normal' nucleoli. In this work the structure of spontaneous nucleolar vacuoles is compared with that induced by drugs such as cordycepin, and FUdR. The vacuolated nucleolus with its increased surface corresponds to a transient structure which not only shows higher metabolic activity but also supplies a storing and/or transporting mechanism for nucleolar products.

Ultrastructural aspects of tetrads with micronuclei in a wheat monosomic

Meiosis of monosomics produces tetrad cells with micronuclei. These micronuclei consist exclusively of one chromosome, the critical chromosome. The electron microscope study of these cells reveals that, in the main nucleus the chromatin is relatively uncondensed with large numbers of perichromatin structures and the nucleolus is almost exclusively fibrillar. This corresponds with a phase of active extranucleolar synthesis and moderate nucleolar activity as reported by other authors using biochemical techniques. The chromatin of the micronucleus is in a more condensed stage than in the main nucleus which, in our opinion, is due to a later beginning of interphase. A nuclear envelope surrounds the micronucleus. In its interior, a patch of nucleolar material can be seen. From the available data on the phylogenetic evolution of this special chromosome which constitutes the micronucleus, we suggest that there must be a partial expression of the nucleolar genes carried by this chromosome due to the absence of the factors which impair their expression in disomic plants.

Localization of the acid phosphatasic activity in plant cell nucleoli

We have made an ultrastructural study of the nuclear acid phosphatase localization in the different tissues of Allium cepa L. anthers, after meiosis. The enzyme is preferentially located in the fibrillar component, within the nucleolus, with a greater reaction density around the nucleolar organizing region (NOR). The specificity of the reaction was proved by contrasting the results with those obtained in two different series of anthers. One of these series was incubated in Gomori's medium with an acid phosphatasic inhibitor, and the other in Gomori's medium without substrate. In both cases the acid phosphatase reaction was negative. Some authors (Hardin and Spicer, 1970; Tandler et al., 1970; Rodríguez-García and Stockert, 1979) have observed an accumulation of inorganic cations (Ca++, Mg++, ... and Na+) within the nucleolar region where the phosphatase acid activity was detected by us. The coincidence of this localization is discussed, suggesting that these cations are probably involved in the phosphatasic activity of these nucleolar zones. Our results agree with the idea that this enzyme contributes to an increase in the amount of nuclear ortophosphate ions (Harris, 1963). On the other hand this enzyme could play a part in the successive triming of the different preribosomal RNA species that occurs during their maturation (Dalgarno and Shine, 1977).

Occurrence of nucleolar material in the cytoplasm of plant cells

This paper deals with the induction of cytoplasmic nucleolar bodies in meristematic Allium cepa L. cells after treatment with drugs which interfere with nucleolar functionality. The drugs which interfere with protein synthesis failed to produce these bodies. The ultrastructure origin and physiological significance of these bodies are discussed here, as well as their relation with the mitotic prenucleolar bodies (Moreno-Díaz de la Espina et al., 1976).

Alteration of nucleolar dispersion in prophase by 5-FUdR treatment

The action of 5-Fluorodeoxyuridine (FUdR) used as an inhibitor of RNA synthesis on the nucleolar evolution during mitosis, has been studied in meristematic cells. Under FUdR treatment the nucleolar dispersion appears as a continuous process, but generally it is not completed and nucleolar remnants remain throughout the whole mitosis. The nucleolar material which was dispersed is transported by the mitotic chromosomes, and in telophase contributed to the formation of the new nucleolus. The non-dispersed part persisted in the cytoplasm during telophase, coexisting with both the prenucleolar bodies and the new nucleolus which was being formed. Our results suggest the necessity of some kind of RNA synthesis, preferentially blocked by FUdR, for nucleolar dispersion to take place.