The Zygotic Transition Is Initiated in Unicellular Plant Zygotes with Asymmetric Activation of Parental Genomes

The zygotic transition, from a fertilized egg to an embryo, is central to animal and plant reproduction. Animal embryos depend upon maternally provided factors until zygotic genome activation (ZGA). In plants, the timing and parental genome contributions to ZGA are unresolved. Here, we use the flowering plant Oryza sativa (rice) to characterize transcriptomes of time-staged isogenic and hybrid zygotes following fertilization. Large-scale transcriptomic changes were observed in unicellular zygotes, including upregulation of S-phase genes, a characteristic of ZGA. The parental contributions to ZGA were highly asymmetric. Zygotic transcription was primarily from the maternal genome and included genes for basic cellular processes. Transcription of the paternal genome was highly restricted but unexpectedly included genes encoding putative pluripotency factors expressed at the onset of ZGA. Thus, distinct transcriptional activities are exhibited by the parental genomes during the initiation of embryogenesis, which presumptively derive from divergent pre-zygotic transcriptional states established in the gametes. The transition from maternal to zygotic gene expression is critical to multicellular organism development. Anderson et al. elucidate timing and parental genome contribution to zygotic genome activation (ZGA) in rice. ZGA occurs in the unicellular zygote, with unequal parental contribution where most genes are expressed primarily from the maternal genome.