Nucleolar activation and vacuolation in embryo radicle cells during early germination

rRNA intermediates coordinate the formation of nucleolar vacuoles in C. elegans

The nucleolus is the most prominent membraneless organelle within the nucleus. How the nucleolar structure is regulated is poorly understood. Here, we identified two types of nucleoli in C. elegans. Type I nucleoli are spherical and do not have visible nucleolar vacuoles (NoVs), and rRNA transcription and processing factors are evenly distributed throughout the nucleolus. Type II nucleoli contain vacuoles, and rRNA transcription and processing factors exclusively accumulate in the periphery rim. The NoV contains nucleoplasmic proteins and is capable of exchanging contents with the nucleoplasm. The high-order structure of the nucleolus is dynamically regulated in C. elegans. Faithful rRNA processing is important to prohibit NoVs. The depletion of 27SA2 rRNA processing factors resulted in NoV formation. The inhibition of RNA polymerase I (RNAPI) transcription and depletion of two conserved nucleolar factors, nucleolin and fibrillarin, prohibits the formation of NoVs. This finding provides a mechanism to coordinate structure maintenance and gene expression.

Molecular dynamics of seed priming at the crossroads between basic and applied research

The potential of seed priming is still not fully exploited. Our limited knowledge of the molecular dynamics of seed pre-germinative metabolism is the main hindrance to more effective new-generation techniques. Climate change and other recent global crises are disrupting food security. To cope with the current demand for increased food, feed, and biofuel production, while preserving sustainability, continuous technological innovation should be provided to the agri-food sector. Seed priming, a pre-sowing technique used to increase seed vigor, has become a valuable tool due to its potential to enhance germination and stress resilience under changing environments. Successful priming protocols result from the ability to properly act on the seed pre-germinative metabolism and stimulate events that are crucial for seed quality. However, the technique still requires constant optimization, and researchers are committed to addressing some key open questions to overcome such drawbacks. In this review, an update of the current scientific and technical knowledge related to seed priming is provided. The rehydration-dehydration cycle associated with priming treatments can be described in terms of metabolic pathways that are triggered, modulated, or turned off, depending on the seed physiological stage. Understanding the ways seed priming affects, either positively or negatively, such metabolic pathways and impacts gene expression and protein/metabolite accumulation/depletion represents an essential step toward the identification of novel seed quality hallmarks. The need to expand the basic knowledge on the molecular mechanisms ruling the seed response to priming is underlined along with the strong potential of applied research on primed seeds as a source of seed quality hallmarks. This route will hasten the implementation of seed priming techniques needed to support sustainable agriculture systems.

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.

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.

Ribonucleoprotein components of root meristematic cell nuclei of the tomato characterized by application of mild loosening and immunocytochemistry

Immunocytochemistry and hypotonic-formaldehyde fixation have been used to study the extranucleolar ribonucleoprotein (RNP) constituents of the nucleus of tomato root meristematic cells. The study of the distribution of small nuclear uridine-rich RNPs (snRNP) by means of a monoclonal anti-Sm antibody recognizing a 29-kDa protein in plants, after standard fixation, shows a preferential labeling of the perichromatin region and a lower labeling of the interchromatin space. These results suggest that in the tomato there is a perichromatin region similar to that of animal cells, in which much of the nonnucleolar transcription and splicing takes place. In hypotonic-formaldehyde-detergent-fixed nuclei, fibrogranular polyparticles have been visualized reacting with anti-snRNP antibody. These structures are frequently associated with filaments of extended chromatin characterized by their reaction with an anti-DNA monoclonal antibody.

Postmitotic 'isodiametric' cell growth in the maize root apex

The onset of rapid cell elongation occurred at different distances from the apex in various tissues of the primary root of maize (Zea mays L.). Furthermore, the comparison of these distances with those determined for the cessation of mitotic divisions revealed a considerable discrepancy. The onset of rapid cell elongation was realized much farther from the root apex than the cessation of cell divisions and therefore a distinct region could be distinguished in every examined maize root tissue. This region was denoted the region of postmitotic 'isodiametric' cell growth. Cells in this region grew in width as well as in length and obtained approximately a square-isodiametric shape. They were also characterized, as are cells in the meristem, by intense nucleic-acid metabolism. This prominent postmitotic 'isodiametric' cell growth was observed in both polyploid and diploid tissues, and indicates that postmitotic 'isodiametric' cell growth, like mitotic division and cell elongation growth, represents an important developmental stage in plant cell ontogeny.

Effects of isolation and high helium pressure on the nucleolus of sympathetic neurons in the rat superior cervical ganglion

In prokaryotes, unicellular eukaryotes and cell-free systems, pressure is known to exert an inhibitory effect on protein synthesis and RNA metabolism, the mechanism(s) of which remain to be investigated in detail. The purpose of the present in vitro study was to compare ultrastructural and quantitative changes of the nucleolus, which is the site of ribosome biogenesis, in sympathetic neurons of rat superior cervical ganglia (SCG) maintained for 2, 3 and 5 h in NCTC 109 medium and subjected to pressure or not. In control SCG (left) the nucleolus greatly increased in volume (+ 33%) 2 h after excision, in comparison with SCG fixed immediately. This overall enlargement was found to reflect a marked increase in all nucleolar components (from 16 to 87%). After 5 h, volumes of nucleolus, fibrillar centers and vacuolar component returned to control values, whereas dense fibrillar and granular components remained affected. Such early and transient changes are regarded as reflecting basic metabolic changes associated with increased nucleolar RNA that should be of primary concern to experiments using SCG transplanted in culture media. Compression under helium up to 180 atmospheric pressure for 1 h of right SCG maintained for 2 h in culture medium, was shown to induce, on the contrary, a marked decrease in nucleolar volume (-39%) and in volumes of all nucleolar components (from -36 to -51%). When they were kept at constant high pressure for 1 and 3 h a progressive recovery of volumes of nucleoli and nucleolar components was observed. Consequently, compression was shown to exert opposite effects to those of isolation of SCG. Present data are interpreted as an inhibitory effect of pressure on ribosome biogenesis. Such observations on a vertebrate neuron might open a new field in the search for cellular mechanisms underlying the effects of pressure on living organisms and especially on the nervous system.