Cold tolerance triggered by soluble sugars: a multifaceted countermeasure

Citation: Tarkowski ŁP and Van den Ende W (2015) Cold tolerance triggered by soluble sugars: a multifaceted countermeasure. In their natural habitat, plants are continuously challenged by adverse environmental conditions. Among them, cold stress (CS) is a major environmental factor limiting agricultural productivity and geographic distribution (Chinnusamy et al., 2007). In this respect, chilling (15-0 • C) and freezing (<0 • C) stress should be distinguished (Thomashow, 2010). 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). Clearly, these modifications are mainly linked to the onset of tolerance mechanisms, which ultimately lead to acclimation. Several metabolites are known to contribute to this process, including amino acids, polyamines, polyols, and soluble sugars (Krasensky and Jonak, 2012 and references therein). Among them, particular focus was recently given to understand the multifunctional role of soluble sugars in enhancing cold tolerance (Nägele and Heyer, 2013). Accumulation 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). 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). 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). Different saccharides are capable to directly stabilize biological membranes under stress conditions. Sucrose (Suc) can directly protect cell membranes by interacting with the phosphate in their lipid headgroups, decreasing membrane permeabil-ity (Strauss and Hauser, 1986). 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). Fructans are localized in the vacuole, suggesting that their contribution to membrane stabilization may be restricted to the tonoplast. 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). This scenario seems to be different for RFO. Despite their cytosolic biosyn-thesis, their protective action may be restricted to chloroplast inner membranes, as …