The effect of osmoticum on ascorbate and glutathione metabolism during white spruce (Picea glauca) somatic embryo development.

Water stress is an important factor which regulates organized development of both zygotic and somatic embryos. Somatic embryos of white spruce were cultured in the presence of polyethylene glycol (PEG), a non-plasmolyzing agent which increases embryo quality and number, and mannitol, a plasmolyzing agent. The effects of these two compounds on both ascorbate and glutathione metabolism were investigated at different stages of embryo development. Compared to control and mannitol-treated embryos, embryos treated with PEG accumulated higher levels of endogenous ascorbate (ASC) in its reduced form, especially during the first half of the maturation period. This increase, also observed in immature seeds, was mainly the result of two different processes: activation of the de novo ASC machinery, and recycling of ASC from ascorbate free radicals (AFR) which was modulated by the activity of ascorbate free radical reductase (AFRR, EC. 1.6.5.4). The activity of this enzyme increased during the early phases of development in both PEG-treated somatic embryos and seeds. Compared to control somatic embryos, mannitol and PEG were shown to change the levels of reduced (GSH) and oxidized glutathione (GSSG). In particular, a constant decline in the GSH/GSSG ratio was observed in the presence of PEG. This pattern was also observed in maturing white spruce seeds. Overall, these data indicate that applications of non-plasmolyzing agents in the culture medium of spruce somatic embryos result in seed-like fluctuations of the ascorbate-glutathione metabolism, which may have a positive effect on embryo yield.