Developmental profile of storage reserve accumulation in soybean somatic embryos

Soybean somatic embryos were developed as a model for investigating the developmental relationships of lipid biosynthesis and accumulation in this important crop. Batch cultures of embryos grown for 8 wk in liquid culture medium exhibited typical sigmoidal growth kinetics as they passed through characteristic globular, heart, torpedo, and cotyledon stages. Exponential growth occurred for the first 4 wk in culture with net growth terminating when total embryo fresh weight per culture flask reached a maximum of 4-4.5 g at 6 wk. This was followed by a slight decrease in embryo fresh weight (FW) and the onset of apparent tissue senescence as judged by yellowing and browning of embryos. On a FW basis, embryos accumulated up to 4% protein, 2.5% soluble sugars, 1.9% starch, and 1.5% lipid relatively early in development. Levels decreased to 0.8% protein, 0.5% soluble sugars, 0.03% starch, and 0.09% lipid at the end of the culture period. On a mass percent basis, lipid extracts were comprised of approximately 80-90% polar lipid early in embryo development. This shifted to 56% storage lipid (triacylglycerol) and 44% polar lipid after 4-5 wk in culture and then reverted back to 91% and 9% polar vs. storage lipid, respectively, by the end of the 8-wk culture period. On the average, polar and storage lipid fractions were comprised of 24% palmitic acid, 7% stearic acid, 8% oleic acid, 36% linoleic acid, and 26% linolenic acid. However, the amounts of linoleic and linolenic acids declined sharply during embryo senescence at the end of the culture period, with corresponding increases in the combined amounts of palmitic and stearic acids. This is the first report that documents the progress of storage reserve accumulation in soybean somatic embryos in relation to their continuous growth in liquid batch cultures.