Fructans, But Not the Sucrosyl-Galactosides, Raffinose and Loliose, Are Affected by Drought Stress in Perennial Ryegrass

Abstract

The aim of this study was to evaluate the putative role of the sucrosyl-galactosides, loliose [␣-d-Gal (1,3) ␣-d-Glc (1,2) ␤-d-Fru] and raffinose 6) ␣-d-Glc (1,2) ␤-d-Fru], in drought tolerance of perennial ryegrass and to compare it with that of fructans. To that end, the loliose biosynthetic pathway was first established and shown to operate by a UDP-Gal: sucrose (Suc) 3-galactosyltransferase, tentatively termed loliose synthase. Drought stress increased neither the concentrations of loliose and raffinose nor the activities of loliose synthase and raffinose synthase (EC 2.4.1.82). Moreover, the concentrations of the raffinose precursors, myoinositol and galactinol, as well as the gene expressions of myoinositol 1-phosphate synthase (EC 5.5.1.4) and galactinol synthase (EC 2.4.1.123) were either decreased or unaffected by drought stress. Taken together, these data are not in favor of an obvious role of sucrosyl-galactosides in drought tolerance of perennial ryegrass at the vegetative stage. By contrast, drought stress caused fructans to accumulate in leaf tissues, mainly in leaf sheaths and elongating leaf bases. This increase was mainly due to the accumulation of long-chain fructans (degree of polymerization Ͼ 8) and was not accompanied by a Suc increase. Interestingly, Suc but not fructan concentrations greatly increased in drought-stressed roots. Putative roles of fructans and sucrosyl-galactosides are discussed in relation to the acquisition of stress tolerance.One of the strategies employed by plants to survive drought stress includes the synthesis of protective compounds, which may act by stabilizing membranes and proteins or mediating osmotic adjustment (Bohnert et al., 1995; Hare et al., 1998; Hoekstra et al., 2001). Included among these protective compounds are the water-soluble carbohydrates (WSCs), Glc, Suc, raffinose, myoinositol, and fructans.Raffinose family oligosaccharides (RFOs) such as raffinose and stachyose accumulate during seed development and are thought to play a role in the desiccation tolerance of seeds (Blackman et al., 1992; Brenac et al., 1997). Raffinose also accumulates in vegetative tissues under drought stress (Taji et al., 2002). RFO biosynthesis requires the presence of galactinol, which is formed by galactinol synthase (GolS; EC 2.4.1.123) from UDP-Gal and myoinositol. Galactinol is the galactosyl donor for the biosynthesis of raffinose from Suc by raffinose synthase (RafS; EC 2.4.1.82). Because galactinol has not been assigned any function in plants other than acting as galactosyl donor for RFOs synthesis, GolS potentially catalyzes a metabolic key step for RFO synthesis. In a recent study, two drought-responsive GolS genes were identified among seven in Arabidopsis (Taji et al., 2002). Overexpression of one of them caused an increase in endogenous galactinol and raffinose as well as an improvement in drought tolerance.In addition to GolS, myoinositol 1-phosphate synthase (INPS; EC 5.5.1.4) is another enzyme that may control the levels of galactinol and raffinose. It repr...

Changes in soluble carbohydrates in polar Caryophyllaceae and Poaceae plants in response to chilling

“…vivipara contained significantly lower amounts of 1-kestose. The accumulation of 1-kestose under stress could be a defense response to low temperatures, but it was demonstrated only in studies of grasses exposed to drought (Amiard et al 2003) and cold (Pontis 1989;Del Viso et al 2011). Kestose belonging to the inulin-type fructans can protect structure of membrane under abiotic stresses (Valluru and Van den Ende 2008).…”

Soluble carbohydrates in developing and mature diaspores of polar Caryophyllaceae and Poaceae

“…Fructans contribute to plant resistance to low temperature and soil drought (Livingston et al 2009;Sandve et al 2011). In the generative phase and during kernel development, fructans synthesized and accumulated in shoots are remobilized to supply kernels with additional sucrose (Amiard et al 2003;Zhang et al 2015). Fructans (like 1-kestose) present in immature kernels are used up during starch synthesis during reserve accumulation (Verspreet et al 2013;Peukert et al 2014).…”

An Investigation of Boron Toxicity in Barley Using Metabolomics

“…(Amiard et al, 2003;Wang et al, 2003). A detailed fructan analysis in both Clipper and Sahara leaves may clarify whether these compounds have any direct involvement in B tolerance.…”