Taken together, these works indicate that the activity and/or expression of sugars transporters may be regulated by sugar signaling, affecting the subcellular distribution of sugars and overall cellular sugar homeostasis, which may be tightly linked to the cellular redox homeostasis (see next paragraph). These authors suggested that the Fru-specific transport features of this carrier may be mediated by a Fru-specific signaling pathway. The fructose (Fru)-specific transporter AtSWEET17 plays a primary role in Fru homeostasis following 1-week 4☌ treatment (Guo et al., 2014b). Cold-stressed AtSWEET16 overexpression lines showed increased freezing tolerance and increased glucose (Glc) and Suc levels (Klemens et al., 2013). The regulation of the activity and/or expression of soluble sugar transporters, especially those involved in chloroplast and Tonoplast Monosaccharide Transporters (TMTs) (Wormit et al., 2006) and Sugars Will Eventually Be Exported Transporters (SWEETs) (Klemens et al., 2013), may play a central role in such processes. Thus, specific changes in subcellular concentrations of potential stress protectants may greatly influence successful responses (Lunn, 2007) (Figure (Figure1). Despite their cytosolic biosynthesis, their protective action may be restricted to chloroplast inner membranes, as suggested by research on Arabidopsis thaliana (Nägele and Heyer, 2013 and references therein). This scenario seems to be different for RFO.
However, their detection in the apoplast of cold-stressed plants also suggests a role in the protection of the plasma membrane, where they can be delivered by a vesicle-mediated transport (Valluru et al., 2008). Fructans are localized in the vacuole, suggesting that their contribution to membrane stabilization may be restricted to the tonoplast. Fructans, fructose-based oligo- and polysaccharides, and RFO can increase stability of phospholipidic mono- and bilayers by direct insertion between polar headgroups (Vereyken et al., 2001 Hincha et al., 2003). Sucrose (Suc) can directly protect cell membranes by interacting with the phosphate in their lipid headgroups, decreasing membrane permeability (Strauss and Hauser, 1986). Different saccharides are capable to directly stabilize biological membranes under stress conditions. One of the major factors affecting overall cellular stability under CS is membrane phospholipid composition regulating membrane fluidity (Ruelland and Collin, 2011 and references therein) associated with cold stimulus perception, as suggested by the protein kinases cascade activation triggered by dimethyl sulfoxide (DMSO)-mediated membrane rigidification (Furuya et al., 2014). Despite this well-known correlation, more recent investigations shed light on the potential underlying biological mechanisms involved (Valluru et al., 2008 Sicher, 2011 Peng et al., 2014). Among them, particular focus was recently given to understand the multifunctional role of soluble sugars in enhancing cold tolerance (Nägele and Heyer, 2013).Īccumulation of soluble sugars following CS is known since long (Levitt, 1958), including studies on their potential roles in stabilizing biological components, particularly for Raffinose Family Oligosaccharides (RFO) (Santarius, 1973). Several metabolites are known to contribute to this process, including amino acids, polyamines, polyols, and soluble sugars (Krasensky and Jonak, 2012 and references therein). Clearly, these modifications are mainly linked to the onset of tolerance mechanisms, which ultimately lead to acclimation. CS responses at the cellular level are characterized by an extensive reprogramming of gene expression and metabolic fluxes (Stitt and Hurry, 2002 Miura and Furumoto, 2013).
In this respect, chilling (15-0☌) and freezing (<0☌) stress should be distinguished (Thomashow, 2010). Among them, cold stress (CS) is a major environmental factor limiting agricultural productivity and geographic distribution (Chinnusamy et al., 2007). In their natural habitat, plants are continuously challenged by adverse environmental conditions.
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