Secondary Somatic Embryogenesis in Plants: From Cellular Mechanisms to Biotechnological Potential.

Secondary somatic embryogenesis (SSE) is a powerful tool in plant biotechnology, enabling the continuous production of embryos from primary somatic embryos (PSEs) and offering broad applications across agriculture, forestry, horticulture, and pharmaceutical industries. Depending on culture conditions, SSE may proceed directly from the surface of PSEs or indirectly via callus formation, with the outcome strongly influenced by exogenous plant growth regulators (PGRs). A key advantage of SSE is its cyclic nature, which offers a valuable strategy to maintain embryogenic potential over extended culture periods, generating true-to-type embryos without reliance on the original explant, while significantly increasing the multiplication rate, often making SSE more productive than PSE in many species. This review explores in detail the cellular origin and developmental pathways of secondary embryos, the maintenance of embryogenic competence through cyclic embryogenesis, as well as genetic and epigenetic aspects and the biotechnological applications of this process. Moreover, it addresses challenges regarding strong genotype dependence, variability in embryo quality and morphology, limitations in maturation and conversion potential, and the gradual decline of embryogenic competence with successive cycles, all of which need to be overcome to ensure the stability and reproducibility of SSE and maximize its impact.