Plant fructans in stress environments: emerging concepts and future prospects
Ravi Valluru1 and Wim Van den Ende2,*
Institute for Crop Production and Grassland Research, University of Hohenheim, D-70599 Stuttgart, Germany Laboratory forMolecular Plant Physiology, KU Leuven, Kasteelpark Arenberg 31, B-3001, Leuven, Belgium
Received 3 April 2008; Revised 9 May 2008; Accepted 13 May 2008
Plants are sessile and sensitive organisms known to possess various regulatory mechanisms for defending themselves under stress environments. Fructans are fructose-based polymers synthesized from sucrose by fructosyltransferases(FTs). They have been increasingly recognized as protective agents against abiotic stresses. Using model membranes, numerous in vitro studies have demonstrated that fructans can stabilize membranes by direct H-bonding to the phosphate and choline groups of membrane lipids, resulting in a reduced water outﬂow from the dry membranes. Inulintype fructans are ﬂexible random-coiled structures that canadopt many conformations, allowing them to insert deeply within the membranes. The devitriﬁcation temperature (Tg) can be adjusted by their varying molecular weights. In addition, above Tg their low crystallization rates ensure prolonged membrane protection. Supporting, in vivo studies with transgenic plants expressing FTs showed fructan accumulation and an associated improvement in freezing and/orchilling tolerance. The water-soluble nature of fructans may allow their rapid adaptation as cryoprotectants in order to give optimal membrane protection. One of the emerging concepts for delivering vacuolar fructans to the extracellular space for protecting the plasma membrane is vesicle-mediated, tonoplast-derived exocytosis. It should, however, be noted that natural stress tolerance is a verycomplex process that cannot be explained by the action of a single molecule or mechanism.
Key words: Abiotic stress, freezing tolerance, fructan, inulin, membrane stabilization.
Introduction Plants cannot usually utilize all the carbon skeletons resulting from photosynthesis. Therefore, they store carbon skeletons as short or longer term reserve carbohydrates. Several pathways that link thegeneration, utilization, and storage of various storage carbohydrates dominate plant intracellular metabolism. Fructans are recognized as one of the principal stored forms of energy in 15% of higher plants (Hendry, 1993), as well as in a wide range of bacteria and fungi (Van Hijum et al., 2003; Martinez-Fleites et al., 2005). They are believed to be synthesized from sucrose in the central vacuole ofplants (Frehner et al., 1984), although synthesis in prevacuolar vesicles can not be excluded (Kaeser, 1983). Different types of fructan molecules can be distinguished depending on the linkage type between the fructosyl residues and the position of the glucose residue (Lewis, 1993). Fructans with a terminal glucose residue include the b(2,1) type fructans (inulin, principally occurring in dicots),and the linear b(2,6) (levan) or branched type fructans (graminan) with both b(2,6) and b(2,1) linkages (as occurring in bacteria and monocots). Fructans with an internal glucose residue include the neo-inulin and neolevan types (occurring in monocots such as Lolium, Asparagus, and Allium: Fujishima et al., 2005; Ueno et al., 2005). In dicots, inulin-type fructans accumulate as long-term reservecarbohydrates in underground storage organs such as roots and tubers (Van den Ende and Van Laere, 2007). In grasses, graminan, levan, and neokestose-derived fructans mainly act as short-term storage compounds in stems, tiller bases, leaf sheaths, elongating leaf bases, and to a lesser extent in leaf blades and roots (Maleux and Van den Ende, 2007). However, fructans accumulating in perennial...