Although starch is the dominant carbohydrate reserve in many species, it may be almost absent in the storage organs of some plants. Fructans are less familiar reserve carbohydrates but occur in 15% of flowering species that mainly belong to the Asteraceae, Campanulaceae and Boraginaceae (dicots), and Poaceae and Liliaceae (monocots) (Hendry 1993; Ritsema and Smeekens 2003). This type of polysaccharide, whose basic unit is fructose, was found in 1804 in a hot water extract from Inula helenium and later named "inulin." Fructans can replace starch as a reserve carbohydrate in some plants, though in many species both polysaccharides coexist. Moreover, the carbohydrate reserve substances vary among plant species and even within a single plant species throughout the seasons and under various climatic conditions (Pontis and del Campillo 1985). In contrast to starch—which is stored in plastids—fructans appear in vacuoles of different organs: for example, in bulbs (onion), tubers (Jerusalem artichoke, dahlia), taproots (chicory), leaves, and in stems (wheat, barley) (Carpita et al. 1989).
Chemically, fructans are distinguished on the basis of the glycosidic bond that links fructose residues to each other. The biosynthesis of these polysaccharides starts with the incorporation of a fructose moeity to one of the three primary hydroxyl groups of vacuolar sucrose—that is, carbon-1 and carbon-6 in the fructose moeity, and carbon-6 in the glucose moeity (Web Figure 8.14.A). This reaction, catalyzed by the enzyme sucrose:sucrose 1-fructosyl transferase, implies a transglycosylation in which one molecule of sucrose transfers the fructosyl moeity to another. Thus, the addition of β-D-2,1- or β-D-2,6-linked fructofuranosyl units to the fructose moeity of sucrose yields 1-kestose and 6-kestose, respectively, while the addition of a β-D-2,6-linked fructofuranosyl unit to the glucose moeity of sucrose produces neo-kestose.
|Web Figure 8.14.A GF (sucrose) + GF (sucrose) → GFF (kestose) + G (glucose) (Click image to enlarge.)|
Further elongation of the ketose-type trisaccharide proceeds by the action of specific enzymes that transfer, also via transglycosylation, a fructosyl unit from a fructan molecule to positions 1 or 6 in the second fructose moeity of the trisaccharide. Thus, fructan:fructan fructosyl transferases catalyze the elongation of kestoses yielding the linear β-D-2,1-bonds of inulins, mainly in dicots, and the β-D-2,6-linkages in levans of monocots and bacteria.
GFF (kestose) + G(F)m → G(F)3 + G(F)m-1
While up to 100,000 fructose moeities can be linked in a single molecule of bacterial fructan, fructosyl units range from 10 to 200 in plant counterparts. Fructans generally are present as a continuum of oligofructans that differ from each other in one fructose residue: G(F)3, G(F)4, G(F)5, G(F)6, ..., G(F)n. This feature enables fructans to participate in the regulation of the cellular osmotic potential.
In addition to supplying hexose units for demands of cellular energy, the breakdown of the endogenous polysaccharide elevates the concentration of lower members in the oligofructans series, quickly increasing in consequence the osmotic pressure. Two types of fructan hydrolases cleave the glycosidic bond in fructans: exohydrolases that release terminal fructosyl residues while endohydrolases cleave at random within the fructan chains (Van den Ende et al. 2004). Apparently, plants contain only the former type while bacteria and fungi contain both kinds of enzymes. The recent finding of fructan exohydrolases in nonfructan plants not only suggests that these enzymes are additionally involved in other important functions—perhaps defense and signaling—but also highlights the necessity of functional characterization of these enzymes (Van den Ende et al. 2004).