Non-Zygotic Embryogenesis in Hardwood Species

Hardwood species are valuable biological resources that have an important role in the economy and ecology of ecosystems worldwide. Non-zygotic or somatic embryogenesis (SE) is a powerful tool in plant biotechnology as it is a form of clonal propagation, amenable to cryopreservation of valuable germplasm and genetic transformation including gene editing. The SE process involves five steps and includes somatic embryo induction, proliferation, maturation, plantlet conversion, and subsequent plant acclimatization. This review aims to provide a general overview of these steps in different SE systems developed for hardwood species. Factors that influence the induction stage such as the age of the donor plant, genotype and culture media are discussed. The role of different explant types, i.e. zygotic embryos and non-zygotic tissues, such as roots, flower tissues, nodes, internodes, leaves or shoot apices, in SE induction are especially emphasized. Histological studies of the origin of somatic embryos and the sequence of events leading to their development from initial explants are assessed. Maintenance of embryogenic capacity carried out by subculture of embryogenic inocula on semisolid or liquid media through cell suspension cultures or by temporary immersion systems is described. At present, the main concerns associated with the application of SE for large-scale propagation of elite hardwoods are related to the embryo maturation, germination, and plantlet conversion steps, and these are highlighted in this review. Finally, molecular aspects associated with somatic embryo induction and development are also described. Attempts to overcome the hurdles identified in the embryogenic process, and future lines of research are proposed.