Initiation of Zygotic Genome Activation in the Unicellular Rice Zygote

The fusion of the two terminally differentiated gametes, the egg and sperm cells, to form the totipotent zygote is critical to successful reproduction. Following fertilization, embryos must transition from reliance on maternal transcripts deposited in the mature egg cell before fertilization to transcriptional activation of its own genome. The timing and regulation of this process, called zygotic genome activation, has been difficult to decipher in plants. By transcriptome profiling of isolated time-staged zygotes in rice, we demonstrate large-scale expression changes before the first cell division, including a burst of expression of S phase genes. Hybrid zygote transcriptomes display a strong maternal allele bias, though a small set of genes with paternal or biparental expression unexpectedly includes key embryogenic factors, contrasting with maternal sequestration of animal pluripotency factors. We conclude that in plants, unlike animals, zygotic genome activation initiates within the unicellular zygote, with differential but functionally significant contributions from both parental genomes.