Galactinol Synthase Activity and Soluble Sugars in Developing Seeds of Four Soybean Genotypes

Abstract

Galactinol synthase (UDP-galactose:inositol galactosyltransferase) is the first unique enzyme in the biosynthetic pathway of raffinose saccharides. Its role as a regulator of carbon partitioning between sucrose and raffinose saccharides in developing soybean (Glycine max L. Merrill) seeds was examined. Galactinol synthase activity and concentrations of sucrose, stachyose, and raffinose were compared during seed development between two genotypes that were high and two genotypes that were low in mature seed raffinose saccharide concentration. In all genotypes, sucrose concentration increased as seed development progressed, but in both low raffinose saccharide genotypes, greater increases in sucrose concentration were observed late in seed development. Sucrose to stachyose ratios in mature seeds were 2.3-fold greater in low raffinose saccharide genotypes than in the high raffinose saccharide genotypes. During seed development, higher levels of galactinol synthase activity were observed in the high raffinose saccharide genotypes than in the low raffinose saccharide genotypes. A common linear relationship for all four soybean genotypes was shown to exist between galactinol formed estimated from galactinol synthase activity data and the concentration of galactose present in raffinose saccharides. Results of this study implied that galactinol synthase is an important regulator of carbon partitioning between sucrose and raffinose saccharides in developing soybean seeds.Soluble sugars account for approximately 10% (w/w) of dry weight in mature soybean seeds (8). Sucrose, stachyose, and raffinose constitute greater than 99% (w/w) of the soluble sugars present (9) and occur in a ratio of approximately 8:4:1 (w/w/w), respectively (8). Early in seed development, concentrations of glucose and fructose are high but decline with maturity of the seed (10, 23). In contrast, stachyose and raffinose are absent from the seed until about the midpoint of seed development at which time they begin to accumulate (1, 10, 23). In some species, stachyose is synthesized in the leaves and translocated in the phloem (21, 23). In soybean leaves, no raffinose saccharides are present and only minimal activity ofGS3 (UDP-galactose:inositol

Galactinol and Raffinose Constitute a Novel Function to Protect Plants from Oxidative Damage

“…As shown in Figure 1, RFOs are synthesized from Suc by the subsequent addition of activated Gal moieties donated by galactinol (Peterbauer and Richter, 2001). Galactinol synthase (GolS; EC 2.4.1.123) catalyzes the first committed step in the biosynthesis of RFOs and plays a key regulatory role in the carbon partitioning between Suc and RFOs (Saravitz et al, 1987). In fact, overexpression of Arabidopsis (Arabidopsis thaliana) GolS2 in transgenic Arabidopsis plants caused an increase in the levels of endogenous galactinol and raffinose under normal conditions (Taji et al, 2002).…”

“…RFOs accumulate in the late stages of soybean (Glycine max) seed maturation and desiccation, indicating that they play a role in the desiccation tolerance of seeds as osmoprotectants (Saravitz et al, 1987;Castillo et al, 1990). Furthermore, RFOs protect unilamellar liposomes from dehydration by direct sugar-membrane interactions in vitro (Hincha et al, 2003).…”

Galactinol Synthase is an Extravacuolar Enzyme in Tubers of Japanese Artichoke (Stachys sieboldii)

“…The most obvious one is that of a key enzyme to commit imported sucrose to stachyose synthesis during tuber formation. Even though GS activity seems to be positively correlated with tuber development (9.9, 21.7, and 6.1 milliunits/g fresh weight for small, medium [both growing] and large [dormant] tubers, respectively), it is relatively low compared with published activities for other RFO plants (about 100 milliunits/g fresh weight for developing soybean seeds [1,10] and up to 2500 milliunits/g fresh weight for leaves [4]). Furthermore, preliminary measurements did not show any detectable enzyme activity of the subsequent steps of RFO synthesis, i.e.…”

“…GS activities showed a positive correlation with RFO levels and a negative correlation with sucrose levels in leaves of 20 plant species (4). During plant development, GS activities were also positively correlated with RFO levels in cucumber leaves (8) and with RFO accumulation in soybean seeds (4,7,10). These correlations strongly suggest an important regulatory role of GS in the channeling of carbon into RFO.…”

Identification and Functional Characterization of the GALACTINOL SYNTHASE (MoGolS1) Gene in Melissa officinalis Plants

“…Because the only known function of galactinol is the formation of RFOs [Saravitz et al, 1987;Liu et al, 1995], which is putatively the committed enzyme step in the RFO biosynthetic pathway [Pridham and Hassid, 1965;Saravitz et al, 1987;Smith et al, 1991;Peterbauer and Richter, 2001] from UDPgalactose. Thus GolS catalyses the first step in the biosynthesis of galactinol and RFO from UDP-galactose and plays regulatory roles in carbon partitioning between sucrose and RFOs [Saravitz et al, 1987] under the abiotic stress conditions, and that galactinol and rafinose function as osmoprotectants in droughtstress tolerance of plants.…”