myo-Inositol, D-chiro-inositol, and D-pinitol synthesis, transport, and galactoside formation in soybean explants

Galactosides of myo-inositol, D-chiro-inositol, and D-pinitol, free cyclitols in soybean [Glycine mar (L.) Merrill] plants, are seed storage products that accumulate during seed maturation and desiccation. There is no evidence for the synthesis of D-chiro-inositol and D-pinitol in embryos. This study was conducted to determine whether D-chiroinositol and D-pinitol are synthesized in maternal tissues and if they are transported to soybean embryos and incorporated into their galactosides, fagopyritol B1 and galactopinitols, respectively. myo-Inositol, D-chiro-inositol, and D-pinitol were exogenously fed to soybean stem-leaf-pod explants. These explants were compared with control explants fed a solution without cyclitols. Galactosyl cyclitol products were extracted from mature seeds and analyzed by gas chromatography. Free cyclitol concentrations were higher in seed coats than in embryonic tissues. After feeding myo-inositol, transient galactinol concentration was 50% higher and D-chiro-inositol concentration was threefold higher in mature embryos than in embryos from the control treatment. After feeding D-chiro-inositol, fagopyritol B1 concentration was 20-fold higher and fagopyritol B2 concentration was 10-fold higher than in embryos from the control treatment. After feeding D-pinitol, galactopinitol concentrations were threefold higher than in embryos from the control treatment. Surgical removal of embryos from seed coats permitted an in planta study of phloem unloading by seed coats of developing soybean. Analysis of downloaded compounds demonstrated that myo-inositol, D-chiro-inositol, and D-pinitol are synthesized in maternal tissues, transported to soybean seeds, and directly unloaded from seed coats to embryos, whereas fagopyritols and galactopinitols are synthesized in embryonic tissues from transported D-chiro-inositol and D-pinitol. Accumulation of fagopyritols and galactopinitols in maturing seeds is limited by the supply of D-chiro-inositol and D-pinitol to the embryo.