Two-Step Maternal-to-Zygotic Transition with Two-Phases of Parental Genome Contributions

The maternal-to-zygotic transition (MZT) is an essential developmental turning point in both plants and animals. In animals, early embryogenesis is maternally controlled and zygotic genome activation (ZGA) starts much later. However, in plants, the timing of MZT and parental contributions to the zygotic transcriptome remain unclear. Here, we demonstrate that plant MZT occurs during the zygote development and is a two-step process, first involving rapid maternal transcript degradation and, second, large-scale de novo transcription. Parental contributions to the zygotic transcriptome are stage-dependent during zygote development; the spherical zygote is characterized by a maternally dominated transcriptome, whereas the elongated zygote genome shows equal parental contributions. Our results indicate that plant MZT is similar to that in animals, showing a typical two-step process; furthermore, we demonstrate that ZGA is required for zygote elongation and division, indicating that de novo transcripts are essential for the establishment of zygote polarity and promotion of embryogenesis.