Soluble Carbohydrates in Soybean

Soybean seeds accumulate soluble carbohydrates throughout development and maturation. Soluble carbohydrates may have important roles in seed germination, seed desiccation tolerance, and cold stress tolerance. The analysis of soluble carbohydrates in soybean seeds and other plant parts at various growth stages (Obendorf et al., 1998b & 2009; Kosina et al., 2009), under environmental stresses (Blackman et al., 1992; Buitink et al., 2004; Caffrey et al., 1988; Koster & Leopold, 1988; Obendorf et al., 1997; Rosnoblet et al., 2007; Obendorf et al., 2008b) and in mutant lines (Sebastain et al., 2000; Hitz et al., 2002; Obendorf et al., 2008b & 2009) has provided valuable information about the potential roles of these compounds. The methods for analysis of soluble carbohydrates involve extraction of compounds and quantification by HPLC or high resolution gas chromatography. A special method for measuring transport unloading from the seed coat to the embryo has been developed (Thorne & Rainbird, 1983; Rainbird et al., 1984; Ellis & Spanswick, 1987; Gomes et al., 2005; Kosina et al., 2009 & 2010). In the leaves of soybean plants, glucose is converted to glucose-6phosphate and then to myo-inositol. Maternally, myo-inositol is converted to D-pinitol (1D-3O-methyl-chiro-inositol) through D-ononitol (1D-4-O-methyl-myo-inositol) as an intermediate; myo-inositol also is converted to D-chiro-inositol. Three cyclitols (myo-inositol, D-pinitol, and D-chiro-inositol) along with sucrose are transported to developing seeds via the phloem where they are unloaded from the seed coat into the apoplastic space surrounding the embryo. During seed maturation the transported free cyclitols accumulate as galactosyl cyclitols, digalactosyl cyclitols, or trigalactosyl cyclitols in the axis and cotyledons of the maturing embryos. Sucrose accumulates as sucrose and as raffinose family oligosaccharides (RFO; raffinose, stachyose and verbascose), myo-inositol accumulates as galactinol series oligosaccharides (galactinol, digalactosyl myo-inositol and trigalactosyl myoinositol), D-chiro-inositol accumulates as fagopyritols (fagopyritol B1, fagopyritol B2, and fagopyritol B3), and D-pinitol accumulates as galactopinitols (galactopinitol A, galactopinitol B, ciceritol, and trigalactopinitol A). In the seed, myo-inositol also is converted to phytic acid (myo-inositol hexakisphosphate) which is stored in seed protein bodies as phytin (the potassium, sodium, and magnesium salts of phytic acid), a major source of phosphorus and cation chelation. Raffinose family oligosaccharides and phytin can result in reduction of the digestibility and the economic, environmental and dietary value of soybean seed. Consumption of RFO from mature seed products results in flatulence in humans and nonruminants in addition to reduced digestibility in chickens and pigs (Sebastian et al., 2000). Reducing raffinose and stachyose accumulation in soybean seeds results in increases in metabolizable energy in soybean feed (Sebastian et al., 2000) and reduces flatulence in