R. Bronchart

Cytological study on water stress during germination of Zea mays.

SummaryKernels of Zea mays were subjected to dehydration treatment at various times during germination. Embryos from kernels dehydrated during the first 36 h of germination are resistant to dehydration and subsequently germinate earlier than controls. Dehydration of kernels germinated during 72h leads to an irreversible arrest of growth of the embryos. However, autoradiographic observations showed that these embryos are still able to incorporate [3H] uridine and probably [4-5-3H] lysine. Incorporation of [3H] thymidine does not occur. The effect of dehydration on root ultrastructure was studied. In embryos dehydrated after 24 h and 72 h of germination, condensation of chromatin is seen and association of elements of rough endoplasmic reticulum with vacuoles and glyoxysomes can be noted. These changes are reversible in drought-resistant embryos and irreversible in drought-sensitive embryos. However, more notable changes than those seen after 24 h can be observed in embryos dehydrated after 72 h of germination: mitochondria and proplastids can not be distinguished with certainty, glyoxysomes fuse and preferably dispose at the periphery of the cell. Rehydration of drought-sensitive embryos causes breakdown in plasma and nuclear membranes, which leads to the loss of cellular compartimentalization. Moreover, the chromatin remains definitively condensed and has lost its function of genetic regulation.

Root cell ultrastructure of Zea mays embryo during early stages of germination.

The ultrastructure of root cells of the germinating corn embryo has been studied during the first 72 hours of soaking. The most spectacular ultrastructural modifications occur in the nucleus. In the dry seed, the chromatin is heavily condensed and complete dispersion occurs during the first 8 hr of germination. The nucleolus appears as a compact structure in the dormant embryo, and as a uniform granular structure after 3 hr. At the 8th hour, large nucleolar vacuoles appear filled with material structurally similar to chromatin. Later on, the nucleolus is composed of a central, fibrillo-granular region surrounded by a thin, peripheral, granular region and fewer nucleolar vacuoles are found.In a previous autoradiographic study (Deltour, 1970), it was shown that the onset of RNA synthesis in these cells occurs 4 hr after soaking. From that time to the 8th hour, uridine-(3)H is incorporated exclusively into the chromatin. Incorporation of radioactive uridine into the nucleolus begins only after the 8th hour.It is interesting that the onset of RNA synthesis in the chromatin occurs simultaneously with the dispersion of this cell component, and that the appearance of vacuoles in the nucleolus is correlated with the beginning if uridine incorporation into this organelle.The following ultrastructural changes take place in the cytoplasm; (a) the lamellae system of proplastids increases slightly; (b) phytoferritin granules present in the proplastids of the dry seed disappear very rapidly; (c) polysomes appear 72 hr after soaking; (d) the spherosomes which are essentially localized in the vicinity of the wall in the dormant embryo become uniformly distributed throughout the cytoplasm at the 72nd hr.SummaryThe ultrastructure of root cells of the germinating corn embryo has been studied during the first 72 hours of soaking. The most spectacular ultrastructural modifications occur in the nucleus. In the dry seed, the chromatin is heavily condensed and complete dispersion occurs during the first 8 hr of germination. The nucleolus appears as a compact structure in the dormant embryo, and as a uniform granular structure after 3 hr. At the 8th hour, large nucleolar vacuoles appear filled with material structurally similar to chromatin. Later on, the nucleolus is composed of a central, fibrillo-granular region surrounded by a thin, peripheral, granular region and fewer nucleolar vacuoles are found.In a previous autoradiographic study (Deltour, 1970), it was shown that the onset of RNA synthesis in these cells occurs 4 hr after soaking. From that time to the 8th hour, uridine-3H is incorporated exclusively into the chromatin. Incorporation of radioactive uridine into the nucleolus begins only after the 8th hour.It is interesting that the onset of RNA synthesis in the chromatin occurs simultaneously with the dispersion of this cell component, and that the appearance of vacuoles in the nucleolus is correlated with the beginning if uridine incorporation into this organelle.The following ultrastructural changes take place in the cytoplasm; (a) the lamellae system of proplastids increases slightly; (b) phytoferritin granules present in the proplastids of the dry seed disappear very rapidly; (c) polysomes appear 72 hr after soaking; (d) the spherosomes which are essentially localized in the vicinity of the wall in the dormant embryo become uniformly distributed throughout the cytoplasm at the 72nd hr.

RNA synthesis in the cells of the apical meristem of Sinapis alba during transition from the vegetative to the reproductive condition

SummaryVegetative plants of Sinapis alba, a long-day species, were induced to flower by exposure to a single 20-hr long day. RNA synthesis in the apical meristem of vegetative (control) and induced plants was investigated by using 3H-uridine and autoradiography of sections.Light-microscope autoradiographs showed a sharp increase in total RNA synthesis per cell in induced meristems. This increase occurred as early as 18 hr after the start of the long day, i.e. at the presumed time of the arrival of the floral stimulus at the meristem. At the same time, electron-microscope autoradiographs showed that there were changes in the pattern of RNA synthesis in the meristematic cells. The ratio of the number of grains in the nucleus to that in the cytoplasm slightly decreased and the ratio of the number of grains in the chromatin to that in the nucleolus greatly increased.Experiments with 2-thiouracil (2-TU), a pyrimidine analogue which was shown to inhibit RNA synthesis in Sinapis, indicated that this compound was most inhibitory to floral induction between the 12th and the 20th hour after the start of the long day, i.e. at the same time as important quantitative and qualitative changes in RNA synthesis were detected in induced meristems by autoradiographic methods. It was thus assumed that 2-TU inhibits floral induction via its effect on these (or on one of these) changes.

Three-dimensional electron microscopy of the nucleolus and nucleolus-associated chromatin (NAC) during early germination of Zea mays L.

Nucleoli and nucleolus‐associated chromatin (NAC) of radicle cells have been three‐dimensionally reconstructed from serial ultrathin sections during early germination of Zea mays. As a preliminary, the effect of 5 methods of fixation on the ultrastructure of the active NAC were tested qualitatively and quantitatively. It appeared that paraformaldehyde best preserved the fibrillar centres (FCs) and was consequently used for the 3‐D reconstructions. In quiescent cells, the NAC forms either 2 short internal strands about 0.7 μm thick running within the nucleolus or 2 peripheral knobs of the same diameter. Whatever its morphology, the NAC was composed of one clear zone, i.e., secondary constriction (SC) of the nucleolar organizer region (NOR), and one electron‐opaque zone, i.e., heterochromatic segment (HS). During germination the NAC was always connected to the nuclear envelope (NE) by a bridge of dense chromatin. The NAC strands or knobs of the quiescent cells are likely to be the counterpart of the 2 NORs of this species. 10–12 hr after onset of germination, one or several networks of nucleolar vacuoles were formed within which the whole NAC was located. Chromatin fibers about 12 nm thick emerged from unfolding portions of the NAC within these “nucleolar chromatin dispersal vacuoles” (NCDV). At 24 hr, the NAC appeared as 2 dichotomous strands. Seventy‐two hr after germination both the stretching out and branching of the NAC were more pronounced. After 120 hr, the transcribing ribosomal genes for each NAC strand together with the newly synthesized RNP transcripts formed a layer of dense fibrillar component surrounding a thin axis which was composed mainly of pale material. Together these formed a typical plant nucleolonema.

Quantitative freeze-fracture study of plasmalemma and nuclear envelope of Zea mays root cells during early germination.

Descriptive and quantitative observations of the plasmalemma and nuclear envelope of freeze-fractured root cells are reported for quiescent and early germinating maize embryos. The numerical density of particles exposed on the extracellular fracture faces of the plasmalemma increases significantly between 12 and 24 hr of germination while it remains unchanged on the protoplasmic fracture faces. The plasmalemma is significantly thicker after 24 hr of germination probably due partially to its enrichment in particles. Between 24 and 72 hr of germination there is a significant increase in the density of nucleopores. The fine structure of the nucleopores changes between 4 and 24 hr of germination. The first 24 hr of germination are also characterized by a spectacular rise in the density of particles of the two nuclear membranes. The results are discussed with respect to germination, particularly that of the maize embryo in which several cellular and molecular aspects have already been studied.

Phenotypic and genotypic changes induced in eucaryotic cells by protein inhibitors

Abstract Component M of Virginiamycin blocks chloroplast development and chlorophyll formation in Euglena, thus producing a reversible phenotypic change (algae are temporarely bleached). The S component of Virginiamycin alone does not produce any apparent modification of the phenotype, but potentiates the inhibitory activity of M factor and renders it permanent. Cytoplasmic mutations with an efficiency of almost 100 % are produced in the presence of the two virginiamycin components. The modifications of chloroplast structure observed at the electron microscope explain the biochemical and genetical alterations produced by the antibiotic.

Ultrastructural localization of argyrophilic proteins in nucleoli of Zea mays by two silver staining techniques

Ag staining was applied on interphasic nucleoli of Zea mays root cells 120h after germination. We applied the two‐step Ag‐NOR staining technique to small root fragments and the one‐step technique to sections of Lowicryl‐embedded tissue. The small‐sized silver grains were mainly located in the dense fibrillar component (DFC). The unstained fibrillar centers (FCs) differed in their proteinic contents from the NOR (which is positively silver stained) and were not the interphasic NOR counterpart.

Ultrastructural and histoautoradiographic study of benomyl action on microconidia of Fusarium oxysporum in germination

Abstract Ultrastructural changes produced by benomyl, a benzimidazole derivative, during the first 48 hr of germination of Fusarium oxysporum f. sp. melonis conidia are investigated; an attempt to localize 3 H-benomyl by autoradiography was carried out. Benomyl has fungicidal and fungistatic activity. Indeed it causes the lysis of 10% of the germinating microconidia and reduces drastically the growth of the germ tube in the other cells. In this latter case, cell wall thickening and anomalous morphology have been observed. Lysosome-like vesicles appear in the cytoplasm. It is proposed that benomyl acts on membrane integrity.

Mise en évidence par le cryodécapage de lomasomes dans la basidiospore de Hypholoma fasciculare (Huds. ex Fr.) Kummer

SummaryLomasomes were shown to be present in mature basidiospores of Hypholoma fasciculare using the freeze-etching technique. Grooves in the plasmalemma were also generally observed. It is suggested that lomasomes are related to the elaboration of the chitinous spore wall and, in accordance with the views of Campbell, plasmalemma grooves are connected with the melanisation of its outer parts.Lomasomes were shown to be present in mature basidiospores of Hypholoma fasciculare using the freeze-etching technique. Grooves in the plasmalemma were also generally observed. It is suggested that lomasomes are related to the elaboration of the chitinous spore wall and, in accordance with the views of Campbell, plasmalemma grooves are connected with the melanisation of its outer parts.

Ultrastructural study of the brown core of Witloof chicory chicon

Abstract A study of the ultrastructure of the chicon axis suffering from “brown core” has revealed a gradual cell wall weakening. It began in the middle lamella and led to cells collapsing. Breaks of the plasmalemma and of the tonoplast were also noted. The severe deterioration of the cell compartmentation finally led to the formation of dark precipitates in the central vacuole. These observations are consistent with the assumption that the brown core results from a Ca-deficiency.