Summary:As the results of the isolation of considerable number of tagged mutants defbctive in meiosis from Arabidopsis and other plants, and the introduction of a variety of techniques to analyze chromosomes and proteins in meiocytes into research of these plants, it has become possible to study meiosis at molecular level in plants. In our lab too, Arabidopsis has been used as experimental material and we studied the pairing process of homologous chromosome. We isolated a mutant, solo dancers(sds), which failed in bivalent formation at meiosis and produced sterile pollens. sds gene was proved to encode cyclin protein and to be expressed only in meiocytes. These results indicate that function of meiosis-specific cyclin is necessary for proper bivalent formation in Arabidopsis. Analysis of chromosome behavior during meiosis using fluorescent in situ hybridization(FISH)showed that centromeres attached each other when homologous chromosomes start paring and that at the last stage of metaphase I chromosomes were connected with their partners through their terminals, which suggested that centromeres and telomeres were playing still unknown roles at those stages.

View full abstract

Summary: Cellular differentiation of a generative and a vegetative cell during pollen development is requisite for sexual plant reproduction and is initiated by an unequal(asymmetric)cell division. The smaller, generative cell produces two sperm cells, or male gametes, whereas the larger, vegetative cell produces an elongated pollen tube, agametophytic cell, to deliver the male gametes to the embryo sac. The distinct differences between two kinds of cell are found in organelle composition and cytoskeletal organization in their cytoplasm. Recent transcriptome analysis also revealed the difference in gene expression profiles between both cells. We pay attention to the difference of theirchromatin configuration, that is assumed to be associated with the difference in gene activity between the two types of nucleus. Namely, the generative nucleus has highly condensed chromatin, as do the nuclei of animal sperm, whereas the vegetative nucleus contains diffuse chromatin. For this reason, by developing an efficient method to isolate the generative and vegetative nuclei from lily pollen protoplasts, their histone composition was investigated biochemically and immunocytochemically. Expression analysis of isolated histone genes was also done. As a result, we detected, in addition to 1)replication-dependent, somatic types of histones common to both nuclei, 2)replication-independent, generative and sperm nuclei-specific histone variants(gH 2 A, gH 2 B and gH 3)and 3)replication-independent, vegetative nucleus-specific histone variant(H 3. 3). The significance of this discovery will be discussed with respect to the function and developmental fates of both cells in the process of sexual plant reproduction.

View full abstract

Summary: The ultra-structural changes and behavior of vacuoles and related organelles(rER and Golgi bodies)during pollen development and maturation of Arabidopsis thaliana, and pollen tube elongation of Tradescantia reflexa will be described. In microspores just before generative cell formation, a large vacuole appeared which was made by fusion of pre-existing vacuoles and probably absorption of solutions. In the young pollen grain after the first mitosis, a large vacuole was divided into small vacuoles. The manner of division was not by binary fission and centripetally, but by the invagination of tonoplasts from one side to the opposite side of a vacuole. After the second mitosis, somatic typed vacuoles disappeared. In mature pollen grains just before germination, membrane-bound structures containing fine fibrillar substances(MBFs)appeared.Because of the expression of the pB vacuolar processing enzyme gene in these mature pollen grains, the MBFs were considered as storage vacuoles. In pollen grains from flowers in bloom, MBFschanged to lysosomal structures with acid phosphatases(lytic vacuole). They gradually increased in number andvolume, and decomposed the cytoplasm. A large vacuole at the base of elongating pollen tube may develope by membrane supply controlled with the ER and lipid granules, in addition to the absorption of water.

View full abstract

Summary: The pollen-tube attractant was first proposed in the late nineteenth century. Pollen tubes were observed to grow toward excised pistil tissues on the medium. Since then, for about 150 years, many plant biologists tried but failedto identify the attractant. Such difficulty also made some plant biologists to cast doubt on the existence of the attractant. In this decade, however, it was shown that the diffusible pollen-tube attractant navigated the directional growth of the pollen tube. Here we review the characteristics of the pollen-tube attractant to figure out the necessary and sufficient conditions as the true chemoattractant of the pollen tube.

View full abstract

Summary: Giant mitochondria are temporarily formed, probably by fusion of smaller mitochondria, in Chlamydomonas reinhardi cells at an intermediate stage in the growth phase of the cell cycle. The formation of giant mitochondria is accompanied by a marked decrease in the oxygen-uptake activity of the cells and the division of giant mitochondria into smaller forms by a re-increase in the activity. Changes in the morphology of mitochondria of Euglena gracilis Z cells were also followed with an electron microscope during the cell cycle in a synchronous culture under photoautotrophic conditions. Giant mitochondria were temporarily formed, most probably by fusion of smaller forms, in the cells at an intermediate stage in the growth phase of the cell cycle. Furthermore, we found for the first time such phenomena in chloroplasts of Euglena gracilis Z, called?ggiant chloroplast?h, and we clarified their arrangement in the cell cycle. Changes in morphology of the pyrenoid and those in distribution of RuBisCO in chloroplasts were followed by immunoelectron microscopy during the growth and division phases of synchronized cells of Euglena.The immuno-reactive protein were densely localized in the pyrenoid, and thinly distributed in the stroma during the growth phase. During the division phase, the pyrenoid could not be detected and the gold particles were dispersed throughout the stroma. From a comparison of photosynthetic CO2-fixation with the total carboxylase activity of RuBisCO extracted from Euglena cells in the growth phase, it is suggested that the carboxylase in the pyrenoid functions in CO2-fixation in photosynthesis.

View full abstract

Summary: Nucleation of cortical microtubules is essential for morphogenesis of plants, in which arrays of corticalmicrotubules regulate direction of cell expansion. We have found that the major pathway of microtubule nucleation in the cortical array is nucleation on pre-existing microtubulesas branches. The nucleation on existing microtubules is inducedby binding of cytosolic γ-tubulin onto existing microtubules, followed by nucleation of new microtubules by the bound γ -tubulin . It is plausible that the branching-type microtubule nucleation plays a role in formation of other microtubulearrays in higher plant cells.

View full abstract

Summary: The reproduction process in angiosperms is extremely complicated, compared with that of animals. All theprocess, from gametogenesis to fertilization moment, gradually takes place within crowded gametophytic tissues(pollenand embryo sac). From such a condition above, nothing has been known about gamete interaction moment and relatedmolecular factors, so far. Recently, I succeeded in identifying a novel transmembrane protein GCS1(CENERATIVEC ELL SPECIFIC1), expressed exclusively in male gametes, using generative cells isolated from Lilium longiflorumpollen. In GCS 1 function analyses based on an Arabidopsis GCS1mutant line, GCS1was found to be an importantfa ctor necessary to gamete attachment and fusion. Besides, it was found that GCS1homologues are conserved in slimemolds, algae and parasites, suggesting that they are related not only to angiosperm reproduction, but also to eukaryoticmating, widely. This finding must be an important clue to understanding the angiosperm fertilization moment that hadnot given any molecular biological knowledge, and may greatly contribute to clarification of basic mechanisms in thesexual reproduction.

View full abstract

Summary: Determinate growth of leaves means that the number and size of their constituent cells must be preciselycontrolled in order to develop to a reproducible size. In the past few years, many mutants and genes involved in the control of cell proliferation and cell expansion in leaves have been identified. In addition, several mutants show an intriguing phenomenon, known as compensated cell enlargement where the reduced number of leaf cells triggers excess cell expansion in leaves. This phenomenon suggests that there is a regulatory system that coordinate cell proliferation and cell expansion. In this review we summarize current knowledge concerning the regulation of cell proliferation and cell expansion and discuss possible mechanisms for compensated cell enlargement. Arabidopsis thaliana, Cell expansion, Cell proliferation, Compensated cell enlargement

View full abstract

Summary: In leaves of some succulent crassulacean-acid-metabolism(CAM)plants, including Kalanchoe species, we previously discovered the phenomenon of chloroplast clumping. A combination of light and water stress induced chloroplasts to form spherical bodies within mesophyll cells. In this study we focused on the positioning of nuclei and mitochondria with clumping of chloroplasts in mesophyll cells in leaves of the succulent CAM plant Kalanchoe blossfeldiana using fluorescence microscopy. Under both well-watered and water-stressed conditions, there were several mitochondria around each chloroplast. When subjected to water-stress, chloroplasts gathered around a nucleus and formed a clumped chloroplasts. Three-dimensional analysis of the clump showed that the nucleus was localized within the clump. Our findings suggest that the chloroplast movement in succulent CAM plants is closely connected with the nucleus. We discuss the significance of the organelle interactions in the leaves of succulent CAM plant under drought conditions.

View full abstract

Summary: Pungency in Capsicum fruits is due to the accumulation of capsaicin and dihydrocapsaicin . Capsicumchinense cv. Habanero is the most pungent peppers in the world. Capsaicinoids are accumulated in Habanero at least10times higher levels than in C. annuum cv. Takanotsume. This variation in capsaicinoid content might result fromdifference in biosynthesis activity, number of biosynthesis cells, or both. We estimated capsaicinoid biosynthesis activityas capsaicinoid content per tissue volume which biosynthesize capsaicinoids. In-situ hybridization analysis of keto acylsynthase(Kas)demonstrated that capsaicinoids were synthesized in the epidermal cells on the placental dissepiment . Then, the total surface area of the placental dissepiment was calculated by3-dimentional images of Capsicum fruits. Biosynthesis activity of capsaicinoids per surface area of the placental dissepiment was calculated in three types of Capsicum fruits. Biosynthesis activity was almost same levels. Difference in pungency would result from the amount of cells which synthesized capsaicinoids.

View full abstract