Somatic and zygotic embryos share common developmental features at the onset of plant embryogenesis

In many vascular plants, zygotic reproduction regularly alternates with different types of asexual reproduction, so embryos can develop not only from fertilized egg cells, but also from induced somatic cells through the process of somatic embryogenesis. Although somatic and zygotic embryogenesis are not directly correlated, their common features are presented, demonstrating that the origin and development of the somatic embryo morphologically and physiologically resemble zygotic embryogenesis at certain points. To initiate embryogenesis, both competent egg and somatic cells require activation either by fertilization or specific environmental signals, respectively. During induction of somatic and zygotic embryogenesis, modulation of DNA methylation, activation of particular hormonal and stress-related mechanisms and changes in cell wall properties are triggered. Here, we give an overview and discuss the most recent research in the field of plant somatic and zygotic embryogenesis, with special attention given to the onset of embryogenesis and early embryo development as well as to embryogenesis-related interconnections between plant hormones, stress responses, DNA methylation and regulatory gene expression.