Cyclitols affect accumulation of α-D-galactosides in developing Vicia seeds

The mechanism preferentially regulating accumulation of raffinose family oligosaccharides (RFOs) or galactosyl cyclitols in legume seeds still remains unknown. The broad range of raffinose family oligosaccharides and galactosyl pinitols in the composition of seeds of Vicia genus gives researchers an exceptional opportunity for investigations on relationships in biosynthesis of both types of alpha-D-galactosides. Feeding explants of Vicia species radically different in the composition of RFOs and galactosyl pinitols with basic galactose acceptors, sucrose (for RFOs) or cyclitols (for galactosyl cyclitols) can be a helpful method for assessment of their regulatory role in accumulation of alpha-D-galactosides in seeds. Garden vetch (Vicia sativa L.) seeds, naturally accumulating RFOs, demonstrated an ability to take up and use exogenously applied D-pinitol and D-chiro-inositol for synthesis of their mono-, di- and tri-galactosides. Together with the accumulation of new galactosides, the concentration of RFOs decreased. In fine-leaved (Vicia tenuifolia Roth.) vetch seeds such a remarkably high concentration of galactosyl pinitols (GPs) was discovered that they nearly replaced RFOs, which is unique among legumes. If the accumulation of both types of galactosides is correlated with concentration of galactose acceptors, elevated levels of sucrose or myo-inositol should promote accumulation of RFOs, instead of GPs. Unexpectedly, feeding fine-leaved vetch raceme explants with myo-inositol or sucrose promoted accumulation of GPs, but not of RFOs. Our comparison of accumulation and biosynthesis of both types of galactosides (RFOs and GPs) throughout development and maturation of seeds from fine-leaved vetch has indicated that preferential accumulation of GPs is associated with the drying of seeds during maturation. Different patterns in activities of enzymes engaged in RFOs' biosynthetic pathway and galactosyltransferases involved in biosynthesis of GPs indicated that distinct forms of enzymes can operate in both pathways. The feeding of explants with D-chiro-inositol causes accumulation of fagopyritols B1 in seeds of both Vicia species, which suggests presence of the same or a similar form of galactinol synthase. Accumulation of fagopyritols in fine-leaved vetch seeds did not affect accumulation of RFOs or galactosyl pinitols.