Biosynthesis of raffinose family oligosaccharides and galactosyl pinitols in developing and maturing seeds of winter vetch (Vicia vlllosa Roth.)

Abstract

Changes in the accumulation of two types of α-D-galactosides: raffinose family oligosaccharides and galactosyl pinitols were compared with changes in the activities of galactosyltransferases during winter vetch (Vicia villosa Roth.) seed development and maturation. Occurrence of galactinol and raffinose in young seeds and changes in activities of galactinol synthase and raffinose synthase during seed development indicated that formation of raffinose oligosaccharides (RFOs) preceded synthesis of galactopinitols. Although transfer of galactose residues into raffinose oligosaccharides increased as seeds were maturing, at late stages of seed maturation the accumulation of galactopinitols was preferred to that of RFOs. In the present study, activities of enzymes transferring galactose moieties from galactinol to D-pinitol forming galactopinitol A, and further transfer of galactose moieties from galactinol to mono- and di-galactopinitol A were detected throughout seed development and maturation. This is a new observation, indicating biological potential of winter vetch seeds to synthesize mono-, di- and tri-galactosides of D-pinitol in a pathway similar to RFOs. The pattern of changes in activities of stachyose synthase and enzymes synthesizing galactopinitols (named galactopinitol A synthase and ciceritol synthase) suggests that formation of stachyose, mono- and di-galactopinitol A (ciceritol) is catalyzed by one enzyme. High correlation between activities of verbascose synthase and enzyme catalyzing synthesis of tri-galactopinitol A from galactinol and ciceritol (named tri-galactopinitol A synthase) also suggests that biosynthesis of both types of tri-galactosides was catalyzed by one enzyme, but distinct from stachyose synthase. Changes in concentrations of galactosyl acceptors (sucrose and D-pinitol) can be a factor which regulates splitting of galactose moieties between both types of galactosides in winter vetch seeds.