Zygote Activation: The Start of the New Generation in Angiosperms

In angiosperms, after the egg fuses with the sperm, many structural, physiological, and molecular biological changes occur in the fertilized egg. All of these changes facilitate the conversion of the haploid egg into the diploid zygote, a process known as the maternal-to-zygotic transition (MZT). In the egg, fertilized egg, and zygote, changes occur at each stage under the control of exact spatio-temporal regulation mechanisms. This review focuses on the molecular biological changes that occur during zygote activation in higher plants including the following: maturation and activation of intrinsic parental transcription; zygote genome activation (ZGA), changes in the expression levels of genes from the zygotic genome; the effect of parental genomic dosage; and cellular determination of zygotic asymmetrical division. It is these exact spatio-temporal regulation mechanisms that allow the egg to convert into the zygote, undergo asymmetrical cell division, and initiate embryogenesis. The results of recent studies have shown that the regulation of zygotic division is a complex process occurring in the cell (egg, fertilized egg, and zygote). The results so far have revealed just the tip of the iceberg of zygote activation. More research is required to explore the regulation of zygote activation.