Improving loblolly pine somatic embryo maturation: comparison of somatic and zygotic embryo morphology, germination, and gene expression

Clonal production of loblolly pine (Pinus taeda L.) through somatic embryogenesis has the potential to meet the increasing industrial demands for high-quality uniform raw materials. A major barrier to the commercialization of this technology is the low quality of the resulting embryos. Twenty-five newly initiated loblolly pine genotypes were followed through the process of liquid culture establishment, embryo maturation, germination, and retrieval from cryogenic storage. A maturation medium, capable of promoting the development of loblolly pine somatic embryos that can germinate, is presented that combines 1/2 P6 modified salts, 2% maltose, 13% polyethylene glycol 8000 (PEG), 5 mg/l abscisic acid (ABA), and 2.5 g/l Gelrite. A procedure for converting and acclimating germinants, to growth in soil and greenhouse conditions is also described. A set of somatic seedlings, produced from the maturation medium, showed 100% survival when planted in a field setting. Somatic seedlings showed normal yearly growth relative to standard seedlings from natural seed. The quality of the resulting embryos was examined and compared to that of zygotic embryos using such parameters as morphology, dry weight, germination performance, and gene expression. All of the observations that were made support the conclusion that even with the new maturation medium somatic embryos grow approximately only halfway through the normal sequence of development and then prematurely discontinue growth.