In plants and animals, embryo development becomes ultimately controlled by zygotic genes, but the timing of zygotic genome activation (ZGA) varies greatly between organisms[1,2]. We recently showed that the transcriptome of young Arabidopsis embryos is dominated by maternal transcripts with a progressive ZGA under the maternal control of epigenetic pathways . In contrast, another study reported that both parental genomes contribute equally to the transcriptome of young embryos, suggesting that ZGA occurs immediately after fertilization. How to explain such dramatic differences?We propose that the discrepancies between these two studies likely reflect genuine biological differences between the two experiments, paving the road towards exciting discoveries on ZGA mechanisms in plants.
In animals, early stages of embryo development are associated with extensive epigenetic reprogramming to coordinate zygotic genome activation (ZGA) . ZGA is typically delayed, although to a varying extent depending on the species, with a gradual loss of the maternal dominance and increase of zygotic influence [1,2]. In flowering plants, maternal effects on seed development have been recognized, yet are difficult to investigate because of the intricate relationships between the embryo, the embryo-nourishing endosperm, and the maternal seed coat . To understand the interaction of parental genomes following fertilization, allele-specific assays were used to distinguish paternal and maternal contributions for selected loci or at the genome-wide level in dissected embryos (reviewed in ), with surprisingly different results. Yet, the diversity of species (Arabidopsis, maize, tobacco) and developmental stages analyzed made it difficult to draw general conclusions. In fact, the observed differences may reflect yet undiscovered biological factors controlling ZGA in flowering plants.