Soluble Oligosaccharides and Galactosyl Cyclitols in Maturing Soybean Seeds In Planta and In Vitro

Abstract

Stachyose, sucrose, and other non-reducing soluble carbohydrates are associated with the onset of desiccation tolerance during seed development and with seed storability. Mature soybean [Giycine max (L) Merr.l seeds contain several galactosyl cyclitols in addition to sucrose, stachyose, and raffinose, but except for galactinol, the accumulation of these galactosyl cyclitols has not been reported for developing soybean seeds. Fifteen soluble carbohydrates including members of the raffinose, galactinol, galactopinitol, and fagopyritol series were analyzed in extracts from axis and cotyledon tissues of seeds and zygotic embryos matured in planta and in vitro. Galactopinitol A, galactopinitol B, and fagopyritol B1 accumulate in axis tissues of developing soybean seeds in planta in association with the onset of desiccation tolerance, yellowing of axis tissues, and in parallel with stachyose accumulation. Galactopinitol A, galactopinitol B, fagopyritol B1, and stachyose also accumulate in parallel in cotyledons in planta and in axis tissues during in vitro growth of zygotic embryos at 15 and 25°C. Axes of soybean seeds matured at 25°C contained higher concentrations of sucrose, raffinose, o-pinitol, o-chiro-inositol, fagopyritol B1, and total soluble carbohydrates than axes of seeds matured at 18°C. Soybean seeds accumulate mostly galactosyl pinitols and only small amounts of free pinitol. In future work, It would be of interest to determine if gaiactosyl cyclitols may substitute for the role of stachyose in providing desiccation tolerance and prolonged seed storability.

Soluble Carbohydrates in Soybean

“…myo-Inositol may undergo chemical isomerization after heating under specific conditions (Sasaki et al, 1988;Taguchi et al, 1997). Therefore, seed culture media are sterilized by ultafiltration (Saab & Obendorf, 1989;Obendorf et al, 1990;1998a;1998b;Wettlaufer & Obendorf, 1991). The filtrate of the aqueous ethanolic extracts of seeds or seed parts may be evaporated under a stream of nitrogen gas at room temperature leaving a dry residue for liquid chromatography or for derivatization in preparation for analysis by gas chromatography.…”

“…5). Sucrose and RFO are the major soluble carbohydrates in soybean seeds (Amuti & Pollard, 1977;Kuo et al, 1988;Horbowicz & Obendorf, 1994;Obendorf et al, 1998b). Other soluble carbohydrates include α-galactosyl derivatives of the cyclitols myo-inositol (galactinol and sometimes digalactosyl myo-inositol and trigalactosyl myo-inositol) (Fig.…”

“…13A; Hsu et al, 1973;Amuti & Pollard, 1977;Obendorf et al, 1998b). Upon hydration, stachyose and raffinose concentrations in axis tissues decline to low concentrations before germination (radicle emergence) at 18 hours, followed by an increase in reducing sugars (monosaccharides) (Hsu et al, 1973;Koster & Leopold, 1988).…”

“…Physiological maturity of individual soybean seeds typically occurs at about 50 days after pollination when seed coat color changes from green to yellow (Fig. 14) and seed moisture is about 60% on a fresh weight basis (Obendorf et al, 1980(Obendorf et al, & 1998bFig. 15).…”

“…At the time seed growth ceases, the osmotic potential of embryo tissues is about -1.8 MPa (Saab & Obendorf, 1989;Egli, 1990;Slawinska & Obendorf, 1991). Axis tissues turn yellow and cease growth before cotyledons (Obendorf et al, 1984(Obendorf et al, & 1998b. The pattern of yellowing begins at the radicle tip and progresses up the hypocotyl to the cotyledonary node, whereas cotyledons turn yellow from the edge to the center of each cotyledon (Obendorf et al, 1984(Obendorf et al, & 1998b.…”