Fructan and hormone connections
Plants rely on “reserve” (stored) carbon (C) for growth and survival when newly synthesized C becomes limited. Besides a classic yet recalcitrant C reserve starch, fructans, a class of sucrosederived soluble fructosyl-oligosaccharides, represent a major store of C in many temperate plant species including the economically important Asteraceae and Poaceae families (Hendry, 1993). Dicots typically accumulate inulin-type fructans as long-term storage (underground organs) whilst grasses and cereals accumulate fructans as short-term reserves in above-ground parts (Pollock and Cairns, 1991; Van Laere and Van den Ende, 2002). Unlike chloroplast-based water-insoluble starch, fructans are semi-soluble, possess flexible structures (Phelps, 1965; Valluru and Van den Ende, 2008), can be synthesized at low temperatures (Pollock and Cairns, 1991), and are degraded by a single type of fructan hydrolases, fructan exohydrolases (FEHs). Unlike starch that store in plastids, fructans store in vacuoles, which is physically less stressful to the active constituents of, and allows more C synthesis by, the photosynthetic cell, whichmay be different in dicots where fructans do not typically accumulate in green parts. Plants synthesize diverse fructan types exhibiting a wide range of functions (for review, see Valluru and Van den Ende, 2008; Van den Ende, 2013). Fructan biosynthetic enzymes, fructosyltransferases (FTs), which evolved from vacuolar-type acid invertases (VIs) (Altenbach et al., 2009), use sucrose (Suc) as a substrate whereby an organ-specific Suc threshold triggers FT genes at the transcriptional level (Lu et al., 2002). Though the regulatory mechanism of Suc signal transduction remains largely elusive, transcription factors (TFs) can be suspected to mediate such inductive processes either by directly binding and stimulating FT genes (e.g., TaMYB13 TF binds to FT genes, 1-SST and 6-SFT; Xue et al., 2011) or by up-regulating vacuolar based proteins (e.g., TaMYB13 TF up-regulates vacuolar processing enzyme, Taγ-VPE1, whose mRNA levels highly correlated with FTs mRNA levels in wheat stems; Kooiker et al., 2013). In addition, protein phosphatases (PP2A; Martínez-Noël et al., 2009) and second messenger Ca2+ (Martínez-Noël et al., 2006) mediate Suc-induction of fructan synthesis in wheat, although the underlying mechanisms remain largely undefined. The cationic role of Ca2+ in fructan synthesis is somewhat counterintuitive because Suc induces a Ca2+ efflux from the vacuole (Furuichi et al., 2001), the site of fructan synthesis. Perhaps Suc might ensure more alkaline (less acidic) vacuolar environment [Suc-induces Slowly activating Vacuolar (SV) ion channel that transiently effluxes vacuolar Ca2+; (Pottosin and Schönknecht, 2007)], favoring fructan synthesis that is thought to be less stable under low pH (Flores-Maltos et al., 2014). Some of the protein mediators involved in Suc-mediated induction of fructan synthesis, including Ca2+ signaling components, calmodulin (CaM), calcineurin B-like (CBL1), and Ca2+–dependent protein kinases (CDPKs), are closely involved in hormone signaling and environmental stress (Ludwig et al., 2004).