Recent advances in conifer somatic embryogenesis: improving somatic embryo quality

Abstract

Somatic embryogenesis of coniferous species was first reported more than 20 years ago. Since then, there has been an explosion of research aimed at developing and optimizing protocols for efficient regeneration of plantlets. Although routinely used both as a means of propagation, as well as a valuable model system for investigating the structural, physiological, and molecular events occurring during embryo development, in vitro embryogenesis is still problematic for some coniferous species. Major problems include: low number of embryos generated; and low frequency of mature embryos able to convert into viable plantlets. Until recent years, despite the fact that embryogenesis is comprised of a sequence of defined steps which include proliferation of embryogenic tissue, embryo maturation, and germination, attempts at improving the whole procedure have been made almost exclusively during the maturation stage. This strategy was based on the assumption that successful regeneration is related to treatments provided during the development of the embryos. Major optimizations of the maturation medium have involved judicious selections of type and concentration of growth regulators, namely abscisic acid, and adjustments of the osmoticum of the culture medium. Extensive work has been conducted in defining the effects of plasmolysing and non-plasmolysing osmoticum agents during maturation, as well as in improving desiccation techniques required for the completion of the maturation program. In the last 2 years, however, work on spruce has clearly demonstrated that the early events in embryogenesis are crucial for the successful completion of the overall embryogenic program. The use of cell tracking techniques, implemented by physiological and molecular studies, has revealed that manipulations of the culture conditions early in the process can increase both number and quality of embryos produced in culture. Additional manipulations of the germination medium can also enhance germination and conversion frequency of somatic embryos matured in a sub-optimal environment. These new findings, together with the unraveling of molecular mechanisms involved in the control/regulation of embryo development hold considerable promise for clonal propagation in conifers.