Phosphoenolpyruvate Carboxylase and Pyruvate Kinase Involvement in Protein and Oil Biosynthesis during Soybean Seed Development

Abstract

During soybean [Glycine max (L.) Merr.] seed development, sucrose translocated to the seed is metabolized to precursors for protein and oil, the major components of mature soybean seed. while breeders have had success in selecting for increased seed protein or oil concentration, there is little understanding the metabolic events that regulate the partitioning of carbon into these seed components. Enzyme reactions that may determine carbon partitioning to seed protein and oil were examined in six soybean genotypes exhibiting genotypic differences in concentration of seed protein and oil. Developing seed were assayed for the enzymes phosphoenolpyruvate (PEP) carboxylase, pyruvate kinase, malate dehydrogenase, NADP‐malic enzyme, and aspartate and alanine aminotransferases. Correlations between these data and the protein and oil concentrations of each sample were determined. There was no significant difference between the correlation of pyruvate kinase activity with the oil and that with the protein concentration of the seed over development. The correlation of PEP carboxylase activity with protein was generally higher than with oil, suggesting that much of the oxaloacetate (OAA) produced may be used for synthesis of protein precursors. However, the high r values between PEP carboxylase and oil suggested that OAA may also be converted to malate and then to pyruvate (a precursor for oil) via a transhydrogenase system. Therefore, PEP carboxylase and pyruvate kinase activities appeared to contribute to a complex interaction that regulates the metabolic flow of glycolytic carbon into precursors for both protein and oil biosynthesis during soybean seed development.