Fructan Biosynthesis Pathway and Freezing Tolerance
Severe abiotic stresses induce detrimental changes in cellular compounds, and sugars are regarded as one of the metabolites preventing detrimental changes (Valluru and Van den Ende 2008). In particular, long-term stress conditions lead to higher soluble sugar concentrations and lower amounts of starch (Silva and Arrabaca 2004). Fructans, soluble fructosyl polysaccharides, are storage carbohydrates in a large number of higher plants. Fructans accumulating in perennial grasses can be considered as longer-term reserve carbohydrates to survive the winter period (Yoshida et al. 1998). Transgenic perennial ryegrass plants with an increased amount of fructans showed significantly increased levels of freezing tolerance (Hisano et al. 2004). Genetic transformation of two wheat fructan-synthesizing enzymes conferred fructan accumulation and enhanced chilling tolerance in rice (Kawakami et al. 2008). Therefore, fructans play important roles as anti-stress agents in overwintering plants (Kawakami and Yoshida 2005), and are considered to function in membrane stabilization through formation of a fructan-lipid interaction under water stresses such as cold and drought (Valluru and Van den Ende 2008).
Inwheatandbarley, threeenzymefamilies, sucrose:sucrose 1-fructosyltransferrase (1-SST), sucrose:fructan 6-fructosyltransferase (6-SFT) and fructan:fructan 1-fructosyltransferase (1-FFT), synthesize graminian-type fructans consisting of β-2,6 linked fructosyl units with β-2,1 branches (Ritsema and Smeekens 2003). The TaMYB13 transcription factor binds to the promoters of wheat 1-SST and 6-SFT genes and activates fructosyltransferase gene expression (Xue et al. 2011).
Overexpression of TaMYB13 results in upregulation of 1-SST, 6-SFT and 1-FFT and enhances fructan accumulation and yield-related traits under water-limited conditions in transgenic wheat plants (Kooiker et al. 2013). Snow mold resistant cultivars accumulate and maintain higher fructan levels in the crown tissues from autumn to the end of winter (Yoshida et al. 1998). Yoshida et al. (1998) also reported that fructan may increase freezing tolerance, although its efficiency is lower than monoand disaccharides in common wheat. Livingston (1996) suggested that fructan is indirectly involved in freezing tolerance of oat and barley. Therefore, fructans surely play important roles in development of water stress tolerance.
As mentioned above, our transcriptome analysis showed that fructan biosynthesisrelated genes were significantly upregulated during long-term LT treatment in crown tissues of wheat synthetics (Yokota et al. 2015). In fact, fructan accumulation levels also reflected the distinct freezing tolerance levels of two synthetic wheat lines (Yokota et al. 2015). These observations support a significant association of fructan biosynthesis with development of freezing tolerance in common wheat (Fig. 27.1). The relationship between carbohydrate accumulation in crown tissues and wheat freezing tolerance and winter hardiness should be elucidated in more detail in future studies.
Acknowledgments This work was supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Scientific Research (B) Nos. 21380005 and 25292008), and by cooperative research funds from KANEKA Co. Ltd.