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In the present study, the feeding of stem-flag leaf-ear explants of wheat, triticale and barley with d-chiro-inositol and d-pinitol was used for modification of the composition of soluble carbohydrates in grains without genetic transformation of plants. Maturing grains indicated ability to uptake exogenously applied cyclitols, not occurring naturally in cereal plants, and synthesized their a-d-galactosides. The pattern of changes in soluble carbohydrates during grain maturation and germination was not disturbed by the uptake and accumulation of cyclitols. Both, d-chiro-inositol and d-pinitol as well as their a-d-galactosides can be an additional pool of soluble carbohydrates accumulated by maturing grains, without decreasing seeds viability. This is the first report indicating the possibility of introduction of cyclitols with potentially human health benefits properties into cereal grains.
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.
In the present study we have investigated the effect of exogenous cyclitols on the accumulation of their galactosides and raffinose family oligosaccharides (RFOs), as well as on some enzymes important for their biosynthesis in seeds of tiny vetch (Vicia hirsuta [L.] S.F. Gray). Immature seeds during 6-day incubation with D-chiro-inositol (naturally does not appear in seeds of tiny vetch) were accumulated cyclitol and its galactosides (fagopyritols: B1 and B2). Short 4-hour incubation with D-chiro-inositol, and subsequent slow desiccation process caused accumulation of free cyclitol only, without biosynthesis of its galactosides. Feeding D-chiro-inositol to pods of tiny vetch induced accumulation of high levels of its galactosides (fagopyritol B1, B2 and B3) in maturing seeds. Similarly, feeding D-pinitol increased accumulation of its mono-, di- and tri-galactosides: GPA, GPB, DGPA and TGPA in tiny vetch seed. Accumulation of both cyclitols and their galactosides drastically reduced accumulation of verbascose. Inhibition of RFOs biosynthesis by elevated levels of free cyclitols suggests some competition between formation of both types of galactosides and similarity of both biosynthetic routes in tiny vetch seeds. Galactinol synthase (GolS) from tiny vetch seeds demonstrated ability to utilize D-chiro-inositol as galactosyl acceptor, instead of myo-inositol. Presence of both cyclitols, as substrates for GolS, caused synthesis of their galactosides: fagopyritol B1 and galactinol. However, formation of galactinol was more efficient than fagopyritol B1. D-chiro-Inositol and D-pinitol at concentrations several-fold higher than myo-inositol had inhibitory effect on GolS. Thus, we suggest that a level of free cyclitols can have an influence on the rate of galactinol biosynthesis and further accumulation of RFOs and galactosyl cyclitols in tiny vetch seeds.
In the present study we have investigated the effect of exogenous cyclitols on accumulation of their galactosides and raffinose family oligosaccharides (RFOs) in maturing smooth tare (Vicia tetrasperma [L.] Schreb) seeds. Feeding D-pinitol to pods of smooth tare increased the amount of free D-pinitol and its galactosides: galactopinitol A, galactopinitol B, di- and trigalactopinitol A in seeds. Similarly, feeding D-chiro-inositol, which does not occur naturally in Vicia seeds, resulted in the transport of this cyclitol in the seed, and caused accumulation of high levels of D-chiro-inositol galactosides (fagopyritol B1, B2 and B3). Accumulation of both cyclitols and their galactosides drastically reduced accumulation of verbascose and, to a lesser extent, stachyose and di-galactosyl- myo-inositol. Feeding D-chiro-inositol also decreased accumulation of di- and tri-galactosyl pinitols, naturally occurring in seeds. Inhibition of RFOs accumulation by elevated levels of free cyclitols indicates competition between biosynthesis of both types galactosides, and similarity of both biosynthetic pathways in smooth tare seeds.
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