Sperm Entry into the Egg Cell Induces the Progression of Karyogamy in Rice Zygotes

The Feeder Effects of Cultured Rice Cells on the Early Development of Rice Zygotes

Feeder cells and the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) in a culture medium promote mitosis and cell division in cultured cells. These are also added to nutrient medium for the cultivation of highly active in mitosis and dividing zygotes, produced in vitro or isolated from pollinated ovaries. In the study, an in vitro fertilization (IVF) system was used to study the precise effects of feeder cells and 2,4-D on the growth and development of rice (Oryza sativa L.) zygote. The elimination of 2,4-D from the culture medium did not affect the early developmental profiles of the zygotes, but decreased the division rates of multicellular embryos. The omission of feeder cells resulted in defective karyogamy, fusion between male and female nuclei, and the subsequent first division of the cultured zygotes. The culture of zygotes in a conditioned medium corrected developmental disorders. Proteome analyses of the conditioned medium revealed the presence of abundant hydrolases possibly released from the feeder cells. Exogenously applied α-amylase ameliorated karyogamy and promoted zygote development. It is suggested that hydrolytic enzymes, including α-amylase, released from feeder cells may be involved in the progression of zygotic development.

50 years of sperm cell isolations: from structural to omic studies

The fusion of male and female gametes is a fundamental process in the perpetuation and diversification of species. During the last 50 years, significant efforts have been made to isolate and characterize sperm cells from flowering plants, and to identify how these cells interact with female gametes to achieve double fertilization. The first techniques and analytical approaches not only provided structural and biochemical characterizations of plant sperm cells but also paved the way for in vitro fertilization studies. Further technological advances then led to unique insights into sperm biology at transcriptomic, proteomic and epigenetic level. Starting with a historical overview of sperm cell isolation techniques, we provide examples of how these contributed to create our current knowledge of sperm cell biology, and point out remaining challenges.

In Vitro Fertilization System Using Wheat Gametes by Electric Fusion

In vitro fertilization (IVF) systems using isolated gametes have been used to dissect post-fertilization events in angiosperms, as female plant gametophytes are deeply embedded within the ovaries. In addition, hybrid and polyploid zygotes can be produced by using IVF systems. Complete IVF systems of maize and rice, two out of three major energy-providing crops, have been established in order to acquire detailed knowledge of mechanisms of fertilization and early embryogenesis. Following in the footsteps of previous success, a wheat IVF system was developed to introduce the advantages of this technology to wheat research. Fusion of gametes was performed via a modified electrofusion method, and the zygote formed a cell wall and two nucleoli. The zygotes divided into symmetric two-celled embryos, globular-like embryos and multicellular club-shaped embryos which are mostly consistent with those in the embryos in planta. IVF-produced club-shaped embryos developed into compact embryonic calli and subsequently regenerated into fertile plants. In this chapter, we provide a detailed description of wheat IVF system that might become an important technique for generating new genotypes of wheat and/or new hybrids as well as for investigating fertilization-induced events in wheat.

Development and regeneration of wheat-rice hybrid zygotes produced by in vitro fertilization system

Hybridization plays a decisive role in the evolution and diversification of angiosperms. However, the mechanisms of wide hybridization remain open because pre- and post-fertilization barriers limit the production and development of inter-subfamily/intergeneric zygotes, respectively. We examined hybridization between wheat and rice using an in vitro fertilization (IVF) system to bypass these barriers. Several gamete combinations of allopolyploid wheat-rice hybrid zygotes were successfully produced, and the developmental profiles of hybrid zygotes were analyzed. Hybrid zygotes derived from one rice egg cell and one wheat sperm cell ceased at the multicellular embryo-like structure stage. This developmental barrier was overcome by adding one wheat egg cell to the wheat-rice hybrid zygote. In the reciprocal combination, one wheat egg and one rice sperm cell, the resulting hybrid zygotes failed to divide. However, doubling the dosage of rice sperm cell allowed the hybrid zygotes to develop into plantlets. Rice chromosomes appeared to be progressively eliminated during the early developmental stage of these hybrid embryos, and c. 20% of regenerated plants showed abnormal morphology. These results suggest that hybrid breakdown can be overcome through optimization of gamete combinations, and the present hybrid will provide a new horizon for utilization of inter-subfamily genetic resources.

Regulatory functions of ROS dynamics via glutathione metabolism and glutathione peroxidase activity in developing rice zygote

Reactive oxygen species (ROS) play essential roles in plant development and environmental stress responses. In this study, ROS dynamics, the glutathione redox status, the expression and subcellular localization of glutathione peroxidases (GPXs), and the effects of inhibitors of ROS-mediated metabolism were investigated along with fertilization and early zygotic embryogenesis in rice (Oryza sativa). Zygotes and early embryos exhibited developmental arrest upon inhibition of ROS production. Egg cells accumulated high ROS levels, and, after fertilization, intracellular ROS levels progressively declined in zygotes in which de novo expression of GPX1 and 3 was observed through upregulation of the genes. In addition to inhibition of GPX activity, depletion of glutathione impeded early embryonic development and led to failure of the zygote to appropriately decrease H2 O2 levels. Moreover, through monitoring of the glutathione redox status, the developing zygotes exhibited a progressive glutathione oxidation, which became extremely delayed under inhibited GPX activity. Our results provide insights into the importance of ROS dynamics, GPX antioxidant activity, and glutathione redox metabolism during zygotic/embryonic development.

Comparative Embryogenesis in Angiosperms: Activation and Patterning of Embryonic Cell Lineages

Following fertilization in flowering plants (angiosperms), egg and sperm cells unite to form the zygote, which generates an entire new organism through a process called embryogenesis. In this review, we provide a comparative perspective on early zygotic embryogenesis in flowering plants by using the Poaceae maize and rice as monocot grass and crop models as well as Arabidopsis as a eudicot model of the Brassicaceae family. Beginning with the activation of the egg cell, we summarize and discuss the process of maternal-to-zygotic transition in plants, also taking recent work on parthenogenesis and haploid induction into consideration. Aspects like imprinting, which is mainly associated with endosperm development and somatic embryogenesis, are not considered. Controversial findings about the timing of zygotic genome activation as well as maternal versus paternal contribution to zygote and early embryo development are highlighted. The establishment of zygotic polarity, asymmetric division, and apical and basal cell lineages represents another chapter in which we also examine and compare the role of major signaling pathways, cell fate genes, and hormones in early embryogenesis. Except for the model Arabidopsis, little is known about embryopatterning and the establishment of the basic body plan in angiosperms. Using available in situ hybridization, RNA-sequencing, and marker data, we try to compare how and when stem cell niches are established. Finally, evolutionary aspects of plant embryo development are discussed.

Plasmogamic Paternal Contributions to Early Zygotic Development in Flowering Plants

Flowering plant zygotes possess complete developmental potency, and the mixture of male and female genetic and cytosolic materials in the zygote is a trigger to initiate embryo development. Plasmogamy, the fusion of the gamete cytoplasms, facilitates the cellular dynamics of the zygote. In the last decade, mutant analyses, live cell imaging-based observations, and direct observations of fertilized egg cells by in vitro fusion of isolated gametes have accelerated our understanding of the post-plasmogamic events in flowering plants including cell wall formation, gamete nuclear migration and fusion, and zygotic cell elongation and asymmetric division. Especially, it has become more evident that paternal parent-of-origin effects, via sperm cytoplasm contents, not only control canonical early zygotic development, but also activate a biparental signaling pathway critical for cell fate determination after the first cell division. Here, we summarize the plasmogamic paternal contributions via the entry of sperm contents during/after fertilization in flowering plants.

Transcriptional Activation of Arabidopsis Zygotes Is Required for Initial Cell Divisions

Commonly referred to as the maternal-to-zygotic transition, the shift of developmental control from maternal-to-zygotic genomes is a key event during animal and plant embryogenesis. Together with the degradation of parental gene products, the increased transcriptional activities of the zygotic genome remodels the early embryonic transcriptome during this transition. Although evidence from multiple flowering plants suggests that zygotes become transcriptionally active soon after fertilization, the timing and developmental requirements of zygotic genome activation in Arabidopsis thaliana (Arabidopsis) remained a matter of debate until recently. In this report, we optimized an expansion microscopy technique for robust immunostaining of Arabidopsis ovules and seeds. This enabled the detection of marks indicative of active transcription in zygotes before the first cell division. Moreover, we employed a live-imaging culture system together with transcriptional inhibitors to demonstrate that such active transcription is physiologically required in zygotes and early embryos. Our results indicate that zygotic genome activation occurs soon after fertilization and is required for the initial zygotic divisions in Arabidopsis.