Submitted to: Society for Cryobiology Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/30/2002
Publication Date: 7/20/2002
Citation: Hincha, D.K., Livingston, D.P., Heyer, A.G. 2002. Plant fructans: a novel class of polymeric protectants for membranes during drying.. Society for Cryobiology Meeting. Interpretive Summary: Fructan is a type of sugar which resembles starch except that it is made up of fructose molecules instead of glucose. For many years it has been shown to be important in protecting plants from drought and freezing. However, the mechanism involved in the protection has been controversial. This research has shown that fructan has the ability to stabilize artificial membranes that are exposed to freezing and drying. This is the first evidence of direct interaction of fructan with plant tissues resulting in protection from these important environmental stresses. More detailed research suggests that the size of the fructan may be as important as structure in its ability to protect membranes. This information will provide a basis for physiologists who recommend characteristics to breeders, to improve the ability of plants to withstand environment stress.
Technical Abstract: The fructan (polyfructose)-family of oligo- and polysaccharides is a group of compounds that have for a long time been implicated as protective agents in the drought and freezing tolerance of many plant species. We have shown before that low concentrations (up to 10 mg/mL) of inulins (linear fructose polymers) isolated from chicory roots and dahlia tubers stabilize egg phosphatidylcholine (EPC) large unilamellar vesicles during freeze-drying (Hincha et al., 2000, Eur. J. Biochem. 267, 535-540). We have now investigated the effects of fructans on membrane stability during air-drying. We show that while glucans of increasing chain length (DP=degree of polymerization 2-7) are progressively less able to stabilize liposomes against leakage of aqueous content after rehydration, fructans (DP 2-5) show increased protection with increasing DP. Fourier transform infrared (FTIR) spectroscopy showed a reduction of the gel to liquid-crystalline phase transition temperature (Tm) of freeze-dried EPC by approximately 25 C in the presence of sucrose and mannose (from 42°C to 17 and 14°C, respectively). For the DP 5-sugars, Tm was 9°C lower for the fructan than for the glucan, indicating reduced sugar-membrane interactions for the glucan compared to the fructan. A reduced interaction of the higher DP glucans and an increased interaction of the respective fructans with the phospholipid headgroups in the dry state was also indicated by dramatic differences between samples with glucans and fructans in the phosphate asymmetric stretch region of the infrared spectrum. Similar experiments with branched-chain fructans from oat leaves showed increased lyoprotection for liposomes up to DP 6. Longer chain fructans had reduced protective effects by themselves, but highly synergistic effects in combination with lower DP fructans. Comparison with membrane fusion experiments suggests that the long-chain oat fructans are less able to inhibit fusion and might therefore be extremely efficient at interacting with membrane lipids. We are currently testing this hypothesis by FTIR spectroscopy. We conclude from these data that plant fructans are likely to contribute to membrane stability in plants under stress. In addition, they are an interesting new class of polymers for the preservation of liposomes and possibly cells and tissues during drying and freeze-drying.