Summary: Surface views of the shoot apex of Lupinus albus at various developmental stages, referred to as plastochron stages, were photographed by reflected light. A part of the apical surface was shown in a flat montage of surface photomicrographs. A part of the area photographed was then cut paradermally, and cell configurations in surface views were compared with those in sections. The cells derived from individual cells in the surface of the shoot apex of an embryo, referred to as“ initial cells”, remained for a time within the original walls, which were stretched and became thickened. Thus, the surface layer of the shoot apex of a young plant consisted of many packets of cells, “ primary cell groups”, encompassed by the thickened original walls, as revealed by surface photomicrographs and paradermal sections. As the plastochron stages proceeded, these cell groups became enlarged in area due to an increased number of derivatives by cell divisions, until the encompassing walls became stretched and obscure, due to increase in cell number. These cell groups proved to be useful for the study of growth in the shoot apex, and the analytical consideration of surface growth of the shoot apex was made.

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Summary: Several lines of evidence have indicated that plastid and mitochondrial DNAs are preferentially synthesized during the initial phase of cell proliferation, when plant cells undergo multiple cell divisions successively. In the present study, we investigated the conditions necessary for and intracellular processes involved in the preferential activation of organelle DNA synthesis by using BY-2 tobacco(Nicotiana tabacum L.)cells as a model system for plant cell proliferation.When stationary-phase BY-2 cells were transferred to fresh medium, the amounts of plastid and mitochondrial DNA per cell increased immediately after the transfer(during lag to early logphases)and decreased thereafter(during late log-to stationary phases). In vitro DNA synthesis assays using isolated organelle-nuclei demonstrated that the initial increases in the organelle DNA levels were accompanied by transient activation of organelle DNA synthesis. Quantitative Southern hybridization analyses demonstrated that activation of organelle DNA synthesis occurred within 3 to 9 h after renewal of the medium. We found sucrose and auxin to be the most important elements in the fresh culture medium for stimulating organelle DNA synthesis. Experiments with various inhibitors revealed that de novo transcription and translation of cell-nuclear genes were required for the activation of organelle DNA synthesis.

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Summary: Initiation and development of the juice sac of the hespidium were studied in Citrus natsudaidai. The study was carried out with counter stained sections using optical microscopes. In the present study, two types of hairy protrusions were found in the ovary locules in the different stages of fruit development. In a young flower unicellular hairs arose at the funicular base of the ovary and degenerated after the pollination. There is no previous report on this structure, and the term “efunicular hair” is given tentatively. After the pollination, the multicellular juice sacs were initiated at the dorsal side of ovary locule. The epidermis of the endocarp gave rise to the epidermis of the juice sac. The subepidermal cells of the endocarp also underwent cell division to construct the internal structure of the club-shaped juice sac. The internal cells of the juice sac became vacuolated as the fruit developed and degenerated at full maturation. The carpel was occupied with elongated juice sacs that were filled with edible juice.

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Summary: Mitochondrial DNA molecules are folded together by means of the binding of specific proteins to form a mitochondrial nucleoid(mt-nucleoid). In this study, the mt-nucleoid proteins in the yeast Saccharomyces cerevisiae were investigated. One of the major proteins in the isolated mt-nucleoids is Abf2p, a DNA-binding protein that belongs to HMG protein family. In order to identify the protein components specific for the mt-nucleoids, mt-nucleoid proteins were analyzed by means of two-dimensional gel electrophoresis. N-terminal amino acid sequence and immunological evidence showed that α-ketoglutarate dehydrogenase complex is associated with the isolated mt-nucleoids. It was also shown that a 22-kDa protein specific for mt-nucleoids is an unidentified protein encoded by ORF YGL068w. In this study, we identified 16 mt-nucleoid proteins. Our results suggested that the mt-nucleoids are three-dimensionally organized by the association of mtDNA with a specific subset of mitochondrial proteins including matrix enzymes, inner membrane proteins, ribosomal proteins and Abf2p.

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Summary: Cyanelles of glaucocystophytes are probably the most primitive plastid known because of their peptidoglycan content and the sequence phylogeny of cyanelle DNA. Cyanelle division involves ingrowth of the septum at the cleavage site with the inner envelope membrane invaginating at the leading edge and the outer envelope membrane invaginating behind the septum. In dividing cyanelles, a single electron-dense ring(cyanelle ring)is present on the stromal face of the inner envelope membrane at the isthmus, but no electron-dense annular structures are detectable on the outer envelope membrane. Thus a single, stromal cyanelle ring such as this is quite unique and also distinct from FtsZ rings, which are not detectable by transmission electron microscopy. These features suggest that the cyanelle division of glaucocystophytes represent an intermediate stage between cyanobacterial and plastid division. If monophyly of all plastids is true, the cyanelle ring and the homologous inner PD-ring might have evolved earlier than the outer PD-ring. In Nannochloropsis oculata(Eustigmatophyta), the outermost membrane of the secondary plastids merges with the outer membrane of the nuclear envelope, forming a Nucleus-Plastid Continuum(NPC). As the true plastids surrounded by the double envelope complete to divide, the inner nuclear envelope divides by binary fission in advance of the outer one. This allows to maintain continuity of the plastidal outermost membrane and the outer membrane of the nuclear envelope throughout the cell division cycle. Finally the closed sac composed of the plastidal outermost membrane and the nuclear outer envelope membrane divides into two halves, giving rise to two daughter sets of the NPC. The NPC may have evolved during the establishment of the eukaryote-eukaryote endosymbiogenesis as a mechanism for division and partitioning of the secondary plastids under control of the secondary host nucleus.

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Summary: The ultrastructure of yeast cells was first studied in 1957 and the techniques used since then have advanced greatly; an overview of the methods is first presented in this minireview. We achieved high-pressure freezing(HPF)fixation for yeast and developed HPF at ultra-low temperature which required using low voltage SEM(ULT-LVSEM)and immunoelectron microscopy. The cell wall biogenesis of the fission yeast Schizosaccharomyces pombe was defined using this technique. The high quality image of HPF. EULT-LVSEM showed an ultrastructure corresponding to a thin sectioned cell and could be observed three dimensionally from the outside and inside of the cell, allowing the initial septum formation to be seen. Using the antibodies of glucans from this yeast, the cells displayed in situ localization of glucans in the cell wall and Golgi apparatus, and their behavior during septum formation. Highly branched. β-1, 6-glucan was not detected in the first step, but all other glucans, . β-1, 3-and.a-1, 6-glucan, appeared throughout the septum until the secondary septum was formed. Accompanying septum formation the actin was located at the tip of the invaginated cell membrane and also accumulated at the contracted ring in the cell. From these facts it is concluded that the actin cytoskeleton governs septum formation during cell division.

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Summary: As chloroplasts(plastid)increase by binary fission, the understanding of the division machinery of the chloroplast is an important theme. Previously we reported on the chloroplast division apparatus, which consists of inner and outer double or triple rings(PD rings). Chloroplasts are assumed to arise from bacterial endosymbionts, while bacterial division is instigated by a bacterial cytokinesis Z ring protein(FtsZ). Here we present immuno-fluorescent and electron microscopic evidence of chloroplast division via complex machinery involving the FtsZ and PD rings in the higher plant Pelargonium zonale Ait. Prior to invagination, the FtsZ protein was attached to a ring at the stromal division site. Following formation of the FtsZ ring, the inner stromal and outer cytosolic PD rings appeared, signifying the initiation of invagination. The FtsZ ring and the PD rings were found at the leading edge of chloroplast constriction throughout division. During chloroplast division, neither the FtsZ nor the inner rings changed width, but the volume of the outer ring gradually increased. We suggest that the FtsZ ring determines the division region, after which the inner and outer PD rings are formed as a lining for the FtsZ ring. With the outerring providing the motivating force, the FtsZ and inner PD rings ultimately decompose to their base components.

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Summary: Eukaryotic cells contain organelles, such as plastids and mitochondria, that must divide during cell division to be inherited by progeny cells. Plastids and mitochondria probably arose by endosymbiosis of bacteria with a host cell. Several works suggested that division apparatuses formed by the host nuclear genome perform plastid and mitochondrial division. To understand the mechanism of plastid and mitochondrial division, we studied the architecture and the behavior of their division apparatuses, using synchronized organelles of the unicellular red alga Cyanidioschyzon merolae. A series of ultrastructural studies showed that division apparatuses of the plastid and the mitochondrion behave in a very similar manner. Further biochemical and immunocytochemical investigation about the plastid division apparatus showed that the FtsZ ring-based system, which originated from a plastid ancestor cyanobacteria, and the plastid-dividing ring-based system, which probably originated from host eukaryotic cells, form a complex and are involved in plastid division by distinct modes. In this reviezw, the mechanism of organellar division is discussed based on our current study and results by several approaches by other groups.

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