A pharmacological study of Arabidopsis cell fusion between the persistent synergid and endosperm

Peptides/receptors signaling during plant fertilization

Double fertilization is a unique and particularly complicated process for the generation alternation of angiosperms. Sperm cells of angiosperms lose the motility compared with that of gymnosperms. The sperm cells are passively carried and transported by the pollen tube for a long journey before targeting the ovule. Two sperm cells are released at the cleft between the egg and the central cell and fused with two female gametes to produce a zygote and endosperm, respectively, to accomplish the so-called double fertilization process. In this process, extensive communication and interaction occur between the male (pollen or pollen tube) and the female (ovule). It is suggested that small peptides and receptor kinases play critical roles in orchestrating this cell-cell communication. Here, we illuminate the understanding of phases in the process, such as pollen-stigma recognition, the hydration and germination of pollen grains, the growth, guidance, and rupture of tubes, the release of sperm cells, and the fusion of gametes, by reviewing increasing data recently. The roles of peptides and receptor kinases in signaling mechanisms underlying cell-cell communication were focused on, and directions of future studies were perspected in this review.

Live-cell imaging reveals the cellular dynamics in seed development

Seed development in flowering plants is highly complex and governed by three genetically distinct tissues: the fertilization products, the diploid embryo and triploid endosperm, as well as the seed coat that has maternal origin. There are diverse cellular dynamics such as nuclear movement in gamete cells for fertilization, cell polarity establishment for embryo development, and multinuclear endosperm formation. These tissues also coordinate and synchronize the developmental timing for proper seed formation through cell-to-cell communications. Live-cell imaging using advanced microscopy techniques enables us to decipher the dynamics of these events. Especially, the establishment of a less-invasive semi-in vivo live-cell imaging approach has allowed us to perform time-lapse analyses for long period observation of Arabidopsis thaliana intact seed development dynamics. Here we highlight the recent trends of live-cell imaging for seed development and discuss where we are heading.

AtNOT1 Is a Novel Regulator of Gene Expression during Pollen Development

Development of pollen, the male gametophyte of flowering plants, is tightly controlled by dynamic changes in gene expression. Recent research to clarify the molecular aspects of pollen development has revealed the involvement of several transcription factors in the induction of gene expression. However, limited information is available about the factors involved in the negative regulation of gene expression to eliminate unnecessary transcripts during pollen development. In this study, we revealed that AtNOT1 is an essential protein for proper pollen development and germination capacity. AtNOT1 is a scaffold protein of the AtCCR4-NOT complex, which includes multiple components related to mRNA turnover control in Arabidopsis. Phenotypic analysis using atnot1 heterozygote mutant pollen showed that the mature mutant pollen failed to germinate and also revealed abnormal localization of nuclei and a specific protein at the tricellular pollen stage. Furthermore, transcriptome analysis of atnot1 heterozygote mutant pollen showed that the downregulation of a large number of transcripts, along with the upregulation of specific transcripts required for pollen tube germination by AtNOT1 during late microgametogenesis, is important for proper pollen development and germination. Overall, our findings provide new insights into the negative regulation of gene expression during pollen development, by showing the severely defective phonotype of atnot1 heterozygote mutant pollen.

How cells fuse

Brukman et al. review cell–cell fusion mechanisms, focusing on the identity of the fusogens that mediate these processes and the regulation of their activities.

Cell–cell fusion remains the least understood type of membrane fusion process. However, the last few years have brought about major advances in understanding fusion between gametes, myoblasts, macrophages, trophoblasts, epithelial, cancer, and other cells in normal development and in diseases. While different cell fusion processes appear to proceed via similar membrane rearrangements, proteins that have been identified as necessary and sufficient for cell fusion (fusogens) use diverse mechanisms. Some fusions are controlled by a single fusogen; other fusions depend on several proteins that either work together throughout the fusion pathway or drive distinct stages. Furthermore, some fusions require fusogens to be present on both fusing membranes, and in other fusions, fusogens have to be on only one of the membranes. Remarkably, some of the proteins that fuse cells also sculpt single cells, repair neurons, promote scission of endocytic vesicles, and seal phagosomes. In this review, we discuss the properties and diversity of the known proteins mediating cell–cell fusion and highlight their different working mechanisms in various contexts.

Gamete Nuclear Migration in Animals and Plants

The migration of male and female gamete nuclei to each other in the fertilized egg is a prerequisite for the blending of genetic materials and the initiation of the next generation. Interestingly, many differences have been found in the mechanism of gamete nuclear movement among animals and plants. Female to male gamete nuclear movement in animals and brown algae relies on microtubules. By contrast, in flowering plants, the male gamete nucleus is carried to the female gamete nucleus by the filamentous actin cytoskeleton. As techniques have developed from light, electron, fluorescence, immunofluorescence, and confocal microscopy to live-cell time-lapse imaging using fluorescently labeled proteins, details of these differences in gamete nuclear migration have emerged in a wide range of eukaryotes. Especially, gamete nuclear migration in flowering plants such as Arabidopsis thaliana, rice, maize, and tobacco has been further investigated, and showed high conservation of the mechanism, yet, with differences among these species. Here, with an emphasis on recent developments in flowering plants, we survey gamete nuclear migration in different eukaryotic groups and highlight the differences and similarities among species.