Seed α-D-Galactosides of Selected Vicia Species and Enzymes Involved in Their Biosynthesis

Abstract

We compared the soluble carbohydrate composition of seeds of ten wild and cultivated species of the genus Vicia. In some Vicia species (V. angustifolia, V. grandiflora, V. sativa, V. sepium) they contained only raffinose family oligosaccharides (RFOs) and in others also D-pinitol and its α-D-galactosides. In terms of galactosyl pinitol composition they were divided into three groups: those accumulating small amounts of mono-, di-, tri-galactosyl pinitol A (GPA, ciceritol and TGPA, respectively) and unknown compound (V. sylvatica and V. hirsuta); those accumulating more ciceritol than TGPA (V. tetrasperma and V. villosa); and those accumulating more TGPA than ciceritol (V. cracca and V. tenuifolia). The differences in the activity of galactosyltransferases engaged in RFOs and galactosyl pinitol synthesis confirmed this classification. Seeds of V. angustifolia, naturally accumulating only RFOs, showed an ability to accumulate exogenously applied D-pinitol or D-chiro-inositol and to form the respective α-D-galactosyl cyclitols. Levels of synthesized galactosides depended on the type and concentration of cyclitol in the feeding solution, and seed maturation stage. However, even a high level of D-pinitol or D-chiro-inositol in the feeding solution caused accumulation of only small amounts of mono- and di-galactosyl pinitols, or tri-galactosyl D-chiro-inositol in seeds of V. angustifolia. Enhanced synthesis of galactosyl cyclitols, mainly mono- and di-galactosides of D-chiro-inositol (fagopyritols), clearly reduced production of verbascose. We suggest that exogenously applied free cyclitols inhibit biosynthesis of tri- and di-galactosides and/or cause substrate competition in enzymes of Vicia species.