Localization of arabinogalactan proteins in egg cells, zygotes, and two-celled proembryos and effects of beta-D-glucosyl Yariv reagent on egg cell fertilization and zygote division in Nicotiana tabacum L

Arabinogalactan proteins (AGPs) have been implicated in a variety of plant development processes including sexual plant reproduction. As a crucial developmental event, plant sexual reproduction generally occurs inside an ovule embedded in an ovary. The inaccessibility of the egg cells, zygotes, and embryos has hindered our understanding of the importance of AGPs in the early events involving fertilization, zygotic division, and early embryogenesis. In this study, the well-established in vitro zygote and ovary culture systems, together with immunofluorescence and immunogold labelling techniques, were employed to investigate the role of AGPs in the early events of sexual reproduction in Nicotiana tabacum. Dramatic changes in AGP content during ovule development were evidenced by western blotting. Subcellular localization revealed that AGPs are localized in the plasma membrane, cell wall, and cytoplasm of pre- and post-fertilized egg cells, and cytoplasm and vacuoles of two-celled proembryos. Abundant AGPs were detected in unfertilized egg cells; however, the level of AGPs substantially decreased in fertilized egg cells. Polar distribution of AGPs in elongated zygotes was observed. The early two-celled proembryos just from zygote division displayed accumulation of AGPs at a low level, while in the elongated two-celled proembryos at the late stage, the AGP content clearly increased. Provision of betaGlcY, a synthetic phenylglycoside that specifically binds AGPs, to the in vitro cultures of isolated zygote and fertilized ovaries increased abnormal symmetrical division of zygotes. In the culture of pollinated but unfertilized ovaries, addition of betaGlcY resulted in arrest of fertilization of the egg cells, but had no effect on fertilization of the central cells. The possible roles of AGPs in fertilization, zygotic division, and proembryo development are discussed.     

Changes of calcium distribution in egg cells,zygotes and two-celled proembryos of rice (<Emphasis Type="Italic">Oryza sativa </Emphasis>L.)

Changes in loosely bound calcium of rice egg cells (pre- and post-pollination) and two-celled proembryos were localized using potassium pyroantimonate precipitation. Egg cells and zygotes (6 h after pollination) have few calcium precipitates (ppts); however within 3 h, (9 -12 h after pollination), ppts become locally abundant in all regions. Statistical analysis indicates that ppts in cytoplasm and nucleoplasm increase by 4.4 and 10.5-fold in zygotes at 9 h and 12 h after pollination, respectively. The ppts labeling the nucleolus 9 h after pollination show a 52-fold increase. In two-celled proembryos, ppts declined to a level similar to that of zygotes 6 h after pollination, except in the nucleolus and cell wall. Following pollination, abundant calcium was detected in the apoplast of the embryo sac.     

Low-temperature treatment of tobacco ovaries improves the isolation, viability, and embryogenic potential of two-celled proembryos

The influence of low-temperature treatment of ovaries on the isolation and subsequent culture of two-celled proembryo in vitro was explored in tobacco. The efficiency of isolation of two-celled proembryos was significantly improved, by an average of 23.47?% after treatment of ovaries at 4°C for 5?C24?h. Cell viability and polarity of two-celled proembryos, as assessed by organelle distribution, growth pattern, and cell wall component distribution, were all well maintained after cold treatment. Furthermore, in vitro cultivated two-celled proembryos divided normally, with timing paralleling that of two-celled proembryos isolated from fresh ovaries. Unexpectedly, the division frequency of two-celled proembryos was significantly improved by cold treatment. These results demonstrate that low-temperature treatment of tobacco ovaries was beneficial for two-celled proembryo manipulation in vitro, by not only improving isolation efficiency but also by promoting cell division and improving the potential for further cultivation.     

The Ultracytochemical Localzation of Acid Phosphatase Activity in Wheat During Fertilization

No acid phosphatase activity was observed in the mature embryo sac of wheat (Triticum aestivum) except the chalazal cytoplasm Of the central cell before fertilization. During fertilization, acid phosphataseactivity was observed in the following loci: part of chromatin of the egg nucleus and most of the mitochondria in the egg cytoplasm; the perinuclear spaces of the egg and sperm nuclei at the fusion of the egg and sperm nuclei; the chalazal cytoplasm and some vacuoles of the degenerated synergid; two sperm nuclei within the cytoplasm of female cells; the cell wall of each cell of the embryo sac and that of the nucellar cells surrounding the embryo sac. No acid phosphatase was observed in the two-celled proembryo. Dense enzyme reaction product was localized in the chromatin of the free nuclei at early stage of the endosperm. The characteristic of acid phosphatase distribution during fertilization may be associated with the physiological change of the egg Cell, the reorganization of mitochondria in the egg cell cytoplasm, the degeneration of one of the two synergids, the physiological state of the sperm nuclei and the nuclear membrane fusion of the egg and sperm nuclei.     

Development of polyspermic zygote and possible contribution of polyspermy to polyploid formation in angiosperms

Fertilization is a general feature of eukaryotic uni- and multicellular organisms to restore a diploid genome from female and male gamete haploid genomes. In angiosperms, polyploidization is a common phenomenon, and polyploidy would have played a major role in the long-term diversification and evolutionary success of plants. As for the mechanism of formation of autotetraploid plants, the triploid-bridge pathway, crossing between triploid and diploid plants, is considered as a major pathway. For the emergence of triploid plants, fusion of an unreduced gamete with a reduced gamete is generally accepted. In addition, the possibility of polyspermy has been proposed for maize, wheat and some orchids, although it has been regarded as an uncommon mechanism of triploid formation. One of the reasons why polyspermy is regarded as uncommon is because it is difficult to reproduce the polyspermy situation in zygotes and to analyze the developmental profiles of polyspermic triploid zygotes. Recently, polyspermic rice zygotes were successfully produced by electric fusion of an egg cell with two sperm cells, and their developmental profiles were monitored. Two sperm nuclei and an egg nucleus fused into a zygotic nucleus in the polyspermic zygote, and the triploid zygote divided into a two-celled embryo via mitotic division with a typical bipolar microtubule spindle. The two-celled proembryos further developed and regenerated into triploid plants. These suggest that polyspermic plant zygotes have the potential to form triploid embryos, and that polyspermy in angiosperms might be a pathway for the formation of triploid plants.     

Gamete fusion site on the egg cell and autonomous establishment of cell polarity in the zygote

Gamete fusion activates the egg in animals and plants, and the gamete fusion site on the zygote might provide a possible cue for zygotic development and/or embryonic patterning. In angiosperms, a zygote generally divides into a two-celled proembryo consisting of an apical and a basal cell with different cell fates. This is a putative step in the formation of the apical-basal axis of the proembryo. We observed the positional relationship between the gamete fusion site and the division plane formed by zygotic cleavage using an in vitro fertilization system with rice gametes. There was no relationship between the gamete fusion site and the division plane leading to the two-celled proembryo. Thus, the gamete fusion site on the rice zygote does not appear to function as a determinant for positioning the zygote division plane, and the zygote apparently possesses autonomous potential to establish cell polarity along the apical-basal axis for its first cleavage.Key words: asymmetric division, egg cell, fertilization, gamete fusion, rice, sperm cell, two-celled proembryo, zygote     

The kinetics of cytological events during double fertilization in Zea mays L.

By using a clearing method, the process of double fertilization in Zea mays L. (line A 188) was analysed and the precise sequence of events was determined. The period from pollen tube arrival to gamete fusion was relatively short, possibly less than 1 h. The karyogamy was of premitotic type, and the time from the contact of male and female nuclei to the fusion of male and female nucleoli was about 5 h in the egg cell and 3 h in the central cell. In the central cell, the sperm nucleus fused with either one of the polar nuclei or the secondary nucleus, the latter being observed for the first time in maize. The zygote was in the resting period for 13–16 h before division commenced, changing the cell polarity during karyogamy and the resting period. The primary endosperm nucleus divided immediately after karyogamy was completed in the central cell. The embryo sacs with two-celled proembryos contained four to eight endosperm nuclei. The timetable of fertilization events could be a standard for further studies on in vitro fertilization at the cytological and molecular levels.