Influence of carbon to nitrogen ratios on soybean somatic embryo (cv. Jack) growth and composition.

Soybean [Glycine max (L.) Merr.] seed are valued for their protein and oil content. Soybean somatic embryos cultured in Soybean Histodifferentiation and Maturation (SHaM) medium were examined for their suitability as a model system for developing an understanding of assimilate partitioning and metabolic control points for protein and oil biosynthesis in soybean seed. This report describes the growth dynamics and compositional changes of SHaM embryos in response to change in the carbon to nitrogen ratio of the medium. It was postulated that at media compositions that were sufficient to support maximal growth rates, changes in the C:N ratio are likely to influence the partitioning of resources between the various storage products, especially protein and oil. As postulated, at steady-state growth rates, embryo protein content was strongly correlated with decreasing C:N ratios and increasing glutamine consumption rates. However, oil content remained relatively unchanged across the C:N ratio range tested, and resources were instead directed towards the starch and residual biomass (estimated by mass balance) pools in response to increasing C:N ratios. Protein and oil were inversely related only at concentrations of sucrose in the medium <88 mM, where carbon limited growth and no starch was found to accumulate in the tissues. These observations and the high reproducibility in the data indicate that SHaM embryos are an ideal model system for the application of metabolic flux analysis studies designed to test hypotheses regarding assimilate partitioning in developing soybean seeds.