Cytochemistry and immunocytochemistry of nucleolar chromatin in plants

Expression of Lipoxygenase During Organogenic Nodule Formation from Hop Internodes

Study of lipoxygenase expression (LOX; EC 1.13.11.12) during organogenic nodule formation in hop ( Humulus lupulus var. Nugget) showed that LOXs are developmentally regulated throughout the process, suggesting their involvement in the response of internodes to wounding, nodule formation, and plantlet regeneration from these nodules. LOX activity and lipid peroxides exhibited a huge increase during the first week of culture, which may indicate a role for LOX and LOX products in response to wounding in hop, as reported for other systems. Western blotting analysis showed a de novo synthesis of LOX isoenzymes in response to wounding and the detection of three different isoenzymes. Confocal analysis of LOX immunofluorescence revealed the presence of the enzyme in cortical cells of induced internodes and in prenodular cells, mostly appearing as cytoplasmic spots. Some of them were identified as lipid bodies by cytochemical and double immunofluorescence assays, suggesting the involvement of a lipid body LOX during nodule formation. Immunogold labeling detected LOX in peroxisomes, lipid bodies, and plastids of nodular cells. Quantification of the labeling density provided statistical significance for the localization of LOX (three different isoenzymes) in the three compartments, which suggested a possible involvement of LOX in metabolic functions of these organelles during organogenic nodule formation and plantlet regeneration.

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.

Changes in DNA methylation levels and nuclear distribution patterns after microspore reprogramming to embryogenesis in barley

Under specific stress treatments, the microspore can be induced in vitro to deviate from its gametophytic development and to reprogram towards embryogenesis, becoming a totipotent cell and forming haploid embryos. These can further regenerate homozygous plants for production of new isogenic lines, an important biotechnological tool for crop breeding. DNA methylation constitutes a prominent epigenetic modification of the chromatin fiber which regulates gene expression. Changes in DNA methylation accompany the reorganization of the nuclear architecture during plant cell differentiation and proliferation; however, the relationship between global DNA methylation and genome-wide expression patterns is still poorly understood. In this work, the dynamics of global DNA methylation levels and distribution patterns were analyzed during microspore reprogramming to embryogenesis and during pollen development in Hordeum vulgare. Quantification of global DNA methylation levels and 5-methyl-deoxycytidine (5mdC) immunofluorescence were conducted at specific stages of pollen development and after reprogramming to embryogenesis to analyze the epigenetic changes that accompany the change of developmental program and cell fate. The results showed low DNA methylation levels in microspores and a high increase along pollen development and maturation; an intense 5mdC signal was concentrated in the generative and sperm nuclei whereas the vegetative nucleus exhibited a weaker DNA methylation signal. After inductive stress treatment, low methylation levels and faint 5mdC signals were observed in nuclei of reprogrammed microspores and 2-4-cell proembryos. This data revealed a global DNA hypomethylation during the change of the developmental program and first embryogenic divisions. This is in contrast with the hypermethylation of generative and sperm cells of the male germline during pollen maturation, suggesting an epigenetic regulation after induction of microspore embryogenesis. At later embryogenesis stages, global DNA methylation progressively increased, accompanying embryo development and differentiation events like in zygotic embryos, corroborating that DNA methylation is critical for the regulation of gene expression in microspore embryogenesis.

High-resolution microscopy for biological specimens via cathodoluminescence of Eu- and Zn-doped Y2O3 nanophosphors

High-resolution microscopy for biological specimens was performed using cathodoluminescence (CL) of Y(2)O(3):Eu, Zn nanophosphors, which have high CL intensity due to the incorporation of Zn. The intensity of Y(2)O(3):Eu nanophosphors at low acceleration voltage (3 kV) was increased by adding Zn. The CL intensity was high enough for imaging even with a phosphor size as small as about 30 nm. The results show the possibility of using CL microscopy for biological specimens at single-protein-scale resolution. CL imaging of HeLa cells containing laser-ablated Y(2)O(3):Eu, Zn nanophosphors achieved a spatial resolution of a few tens of nanometers. Y(2)O(3):Eu, Zn nanophosphors in HeLa cells were also imaged with 254 nm ultraviolet light excitation. The results suggest that correlative microscopy using CL, secondary electrons and fluorescence imaging could enable multi-scale investigation of molecular localization from the nanoscale to the microscale.

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.

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.

The pea END1 promoter drives anther-specific gene expression in different plant species

END1 was isolated by an immunosubtractive approach intended to identify specific proteins present in the different pea (Pisum sativum L.) floral organs and the genes encoding them. Following this strategy we obtained a monoclonal antibody (mAbA1) that specifically recognized a 26-kDa protein (END1) only detected in anther tissues. Northern blot assays showed that END1 is expressed specifically in the anther. In situ hybridization and immunolocalization assays corroborated the specific expression of END1 in the epidermis, connective, endothecium and middle layer cells during the different stages of anther development. END1 is the first anther-specific gene isolated from pea. The absence of a practicable pea transformation method together with the fact that no END1 homologue gene exists in Arabidopsis prevented us from carrying out END1 functional studies. However, we designed functional studies with the END1 promoter in different dicot species, as the specific spatial and temporal expression pattern of END1 suggested, among other things, the possibility of using its promoter region for biotechnological applications. Using different constructs to drive the uidA (beta-glucuronidase) gene controlled by the 2.7-kb isolated promoter sequence we have proven that the END1 promoter is fully functional in the anthers of transgenic Arabidopsis thaliana (L.) Heynh., Nicotiana tabacum L. (tobacco) and Lycopersicon esculentum Mill. (tomato) plants. The presence in the -330-bp region of the promoter sequence of three putative CArG boxes also suggests that END1 could be a target gene of MADS-box proteins and that, subsequently, it would be activated by genes controlling floral organ identity.

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.

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 methods for nucleic acid detection by immunocytology

In the present review are summarized recent developments in immunocytochemical detection of nucleic acids in biological materials at the ultrastructural level. Not only the approaches using antibodies to natural nucleic acids are described but also the techniques involving the use of antibodies raised against various nucleotide analogs incorporated beforehand into nucleic acids. Special emphasis is placed on each method's potential and limitations. These methods, combined or not with molecular biotechnology, are powerful tools for studying the structure and function of nucleic acids. They can be used to investigate the distribution and topological organization of DNA and RNA molecules or of specialized within these molecules in the cells.

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.

Human autoantibodies identify a protein in dense fibrillar and granular components of the nucleolus

A novel nucleolar protein of an approximate molecular weight of 60 kDa was identified by immunoprecipitation in human cells with an autoimmune sclerodermic serum. It maps at the ultrastructural level to nucleolar granular and dense fibrillar components. This 60 kDa protein could not be demonstrated in Western blots suggesting that the epitope structure is complex and/or is sensitive to the treatment of cells. The immunoprecipitation results indicate that the 60 kDa protein is not a phosphoprotein and is not associated with a nucleolar RNA containing particle. The identified protein represents a new autoimmune marker in the field of systemic connective tissue diseases.

Ultrastructural distribution of DNA within plant meristematic cell nucleoli during activation and the subsequent inactivation by a cold stress

We have investigated the precise location of DNA within the meristematic cell nucleolus of Zea mays root cells and Pisum sativum cotyledonary buds, in the course of their activation and induced inactivation following a subsequent treatment at low temperature. For this purpose, we combined the acetylation method, providing an excellent distinction between the various nucleolar components, with the in situ terminal deoxynucleotidyl transferase-immunogold technique, a highly sensitive method for detecting DNA at the ultrastructural level. In addition to the presence of DNA in the condensed chromatin associated with the nucleolus, we demonstrated that a significant label was detected in the nucleolus of quiescent cells in both plant models. Evident labels were also found in the dense fibrillar component of actived nucleoli. Whereas in inactivated nucleoli no significant label was observed within the dense fibrillar component, an intense label was seen over the large heterogeneous fibrillar centres only during inactivation. The granular component was never significantly labelled. These results appear to indicate that the DNA present in the dense fibrillar component of activated nucleoli withdraws from this structure during its inactivation and becomes incorporated in the large fibrillar centres. These observations suggest that in plant cells inactivation of rRNA genes is clearly accompanied by changes in the conformation of ribosomal chromatin.

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.

The organization of ribosomal RNA processing correlates with the distribution of nucleolar snRNAs

We have analyzed the organization of pre-rRNA processing by confocal microscopy in pea root cell nucleoli using a variety of probes for fluorescence in situ hybridization and immunofluorescence. Our results show that transcript processing within the nucleolus is spatially highly organized. Probes to the 5' external transcribed spacer (ETS) and first internal transcribed spacer (ITS1) showed that the excision of the ETS occurred in a sub-region of the dense fibrillar component (DFC), whereas the excision of ITS1 occurred in the surrounding region, broadly corresponding to the granular component. In situ labelling with probes to the snoRNAs U3 and U14, and immunofluorescence labelling with antibodies to fibrillarin and SSB1 showed a high degree of coincidence with the ETS pattern, confirming that ETS cleavage and 18 S rRNA production occur in the DFC. ETS, U14, fibrillarin and SSB1 showed a fine substructure within the DFC comprising closely packed small foci, whereas U3 appeared more diffuse throughout the DFC. A third snoRNA, 7-2/MRP, was localised to the region surrounding the ETS, in agreement with its suggested role in ITS1 cleavage. All three snoRNAs were also frequently observed in numerous small foci in the nucleolar vacuoles, but none was detectable in coiled bodies. Antibodies to fibrillarin and SSB1 labelled coiled bodies strongly, though neither protein was detected in the nucleolar vacuoles. During mitosis, all the components analyzed, including pre-rRNA, were dispersed through the cell at metaphase, then became concentrated around the periphery of all the chromosomes at anaphase, before being localized to the developing nucleoli at late telophase. Pre-rRNA (ETS and ITS1 probes), U3 and U14 were also concentrated into small bodies, presumed to be pre-nucleolar bodies at anaphase.

Capsid protein of cucumber mosaic virus accumulates in the nuclei and at the periphery of the nucleoli in infected cells

Tobacco leaves infected with two strains and their reciprocal RNA 3 pseudorecombinants of cucumber mosaic virus (CMV) were examined by immunoelectron microscopy. In addition to the regular detection of CMV in the cytoplasm and vacuoles, immunogold-labelled viral proteins occurred commonly in the nuclei and at the periphery of impacted nucleoli in all four samples examined. However, viral protein was present only in the euchromatin region and rare in the heterochromatin region.