Zygotic Genome Activation in Unicellular Plant Zygotes

The transition from a fertilized egg to an embryo is central to the life cycles of animals and plants. In animals, embryonic divisions depend completely upon maternally provided factors until zygotic genome activation (ZGA). In plants, both the timing and parental genome contributions to ZGA remain unresolved, and zygote transcriptomes have not been characterized in detail. Here, we describe transcriptome analysis of isogenic and hybrid rice zygotes at discrete time-stages following fertilization. Zygotes exhibited large-scale transcriptomic changes prior to division, including marked upregulation of S-phase genes, a characteristic of ZGA. Expression of transcription factor families known to promote plant regeneration preceded the burst of S-phase gene expression. We conclude that unlike animals, ZGA is initiated in unicellular zygotes. The parental contributions to ZGA were highly asymmetric. Upregulated genes for general cellular processes, including DNA replication, were primarily or exclusively from the maternal genome. The very restricted paternal genome contribution unexpectedly included the putative pluripotency factors expressed at the onset of ZGA. Thus, distinct roles appear to be played by the parental genomes for the initiation of embryogenesis, that presumptively derive from divergent pre-zygotically established states.