Patterns of storage protein and triacylglycerol accumulation during loblolly pine somatic embryo maturation

Conifer somatic embryo germination and early seedling growth are fundamentally different than in their zygotic counterparts in that the living maternal megagametophyte tissue surrounding the embryo is absent. The megagametophyte contains the majority of the seed storage reserves in loblolly pine and the lack of the megagametophyte tissue poses a significant challenge to somatic embryo germination and growth. We investigated the differences in seed storage reserves between loblolly pine mature zygotic embryos and somatic embryos that were capable of germination and early seedling growth. Somatic embryos utilized in this study contained significantly lower levels of triacylglycerol and higher levels of storage proteins relative to zygotic embryos. A shift in the ratio of soluble to insoluble protein present was also observed. Mature zygotic embryos had roughly a 3:2 ratio of soluble to insoluble protein whereas the somatic embryos contained over 5-fold more soluble protein compared to insoluble protein. This indicates that the somatic embryos are not only producing more protein overall, but that this protein is biased more heavily towards soluble protein, indicating possible differences in metabolic activity at the time of desiccation.