Are Raffinose and Stachyose Unloaded from Soybean Seed Coats to Developing Embryos?

During soybean [Glycine max (L.) Merrill] seed development, seed coat tissues contain sucrose, myo-inositol, D-chiro-inositol, D-pinitol and low concentrations of galactinol. Low concentrations of fagopyritol B1, galactopinitols, and raffinose also accumulate in seed coats during mid-maturation and stachyose accumulates late in maturation. Traces of raffinose can be detected in cotyledons of young seeds (24 days after pollination) and infrequently in seed coat cup exudates at mid-seed fill. On gas chromatograms, questionable peaks corresponding to the retention time of raffinose may be observed in seed coat cup exudates. To determine if raffinose and stachyose can be unloaded from seed coats into the free space surrounding developing seeds, soybean stem-leaf-pod explants from plants with low-raffinose, low-stachyose seeds (LRS) or normal raffinose and stachyose seeds (CHECK) were fed solutions containing 10 mM raffinose or 10 mM stachyose via the cut stem for 3 days. Raffinose was present in leaf, pod and seed coat tissues after feeding raffinose or stachyose to explants. Small amounts of raffinose were unloaded into seed coat cups. Stachyose accumulated in leaf and pod tissues after feeding stachyose to explants, but stachyose was detected in only one of the 32 seed coat exudates assayed. Soybean seed coats unloaded raffinose in very small amounts that may explain the presence of trace amounts of raffinose in embryo tissues of young seeds.