Oligosaccharide Metabolism and Accumulation in Developing Soybean Seeds

Soluble sugar composition of developing soybean seed (Glycine max L. Merr.) vary with cultivar. This study was conducted to determine the relation between soluble sugar accumulation and associated enzyme activities in developing seed of two soybean cultivars, Williams 82 and Wolverine. Activities of sucrose synthase, invertase, galactinol synthase, and α-galactosidase were compared to soluble sugar content (hexoses, galactinol, myo-inositol, oligosaccharides) at seven different reproductive growth stages. A twofold higher dry matter content in mature seeds of Wolverine corresponded to a larger seed size and longer linear seed fill. Increases in sucrose synthase and alkaline invertase activities in both cultivars corresponded to accumulation of 90% total dry matter. Acid invertase activity remained below 45 nmol glucose seed⁻¹ min⁻¹ during seed development. Initial raffinose and stachyose formation occurred at beginning maturity in Williams 82 and full seed in Wolverine. This change in sugar partitioning during late seed development was accompanied by twofold and threefold increases in galactinol synthase activity and galactinol content, respectively, and a substantial decrease in the substrate for this enzyme, myo-inositol. Highest levels of galactinol synthase activity at the end of linear seed fill in both cultivars, however, preceded the accumulation of the highest stachyose levels by two growth stages. Activity of α-galactosidase increased linearly in both varieties with time. Relatively high activities of sucrose synthase and alkaline invertase during the accumulation of the majority of dry matter in both cultivars suggest that these enzymes may be associated with this process in soybean seeds. Regulation of raffinose oligosaccharide accumulation in developing soybean seeds also may depend on galactosyl transferase activity. Contribution from the USDA-ARS in cooperation with the Agric. Res. Ctr., College of Agriculture and Home Economics, Washington State Univ., Pullman.