Transcriptional dynamics during karyogamy in rice zygotes

Upon fertilization, male and female nuclei fuse to form the zygotic nucleus in angiosperms. Karyogamy is considered to be essential for proper embryogenesis; however, the transcriptional dynamics during karyogamy in plant zygotes remain unclear. In this study, we performed a single-cell transcriptome analysis of rice zygotes at six early developmental stages (15 min, 30 min, 1 h, 2 h, 4 h, and 6 h after gamete fusion) to reveal gene expression profiles during karyogamy in plant zygotes. The time-series RNA-seq analysis detected possible de novo and altered gene expression in zygotes from 15 min post-fertilization. Fertilization-induced transcription during karyogamy was characterized by protein interaction database and gene ontology analyses. Furthermore, paternal allele transcription was initiated approximately 30 min-1 h after gamete fusion, when nuclear fusion begins in the zygote. Some transcripts preferentially expressed in egg cells were downregulated after gamete fusion. Moreover, a dynamic shift from maternal-biased transcripts to bi-parental expression occurred during early zygotic development. These results suggest that transcriptional dynamics during karyogamy plays initial role in proper and sequential zygotic development and embryogenesis.