229 Chapter 17 The Use of Zygotic Embryos as Explants for In Vitro Propagation: An Overview Mohamed Elhiti and Claudio Stasolla Abstract Plant propagation in vitro via somatic embryogenesis or organogenesis is a complicated process requiring the proper execution of several steps, which are affected by culture conditions and environment. A key element for a successful outcome is the choice of the explants. Several studies have shown that factors such as age, ontogenic and physiological conditions, and degree of differentiation affect the response of the explants to culture conditions. As a general rule, younger tissues, such as zygotic embryos, are the preferred choice for tissue culturists as they have better potential and competence to produce embryos and organs compared to more differentiated and mature tissues. This chapter focuses on how compe- tence and commitment to regenerate embryos and organs in cultures are acquired by somatic cells and why zygotic embryos are so often utilized for propagation practices. Key words: Embryo, Organogenesis, Plant growth regulators, Propagation, Somatic embryogenesis The utilization of in vitro techniques to regenerate plants in vitro has been largely used as a “propagation” tool as well as a model system to understand basic aspects of plant development. Plant regeneration in culture can be achieved either through embryo- genesis, that is the formation of bipolar structures, i.e., embryos, or through organogenesis, the generation of a specific organs, i.e., shoots or roots. Regeneration through either process can occur directly or indirectly, depending on whether an intervening proliferation of undifferentiated tissue (callus) is required. Embryogenesis represents an important event in the plant life cycle which is initiated with double fertilization, followed by the formation of embryos which are characterized by lateral, radial, 1. Introduction 1.1. Embryogenesis Trevor A. Thorpe and Edward C. Yeung (eds.), Plant Embryo Culture: Methods and Protocols, Methods in Molecular Biology, vol. 710, DOI 10.1007/978-1-61737-988-8_17, © Springer Science+Business Media, LLC 2011