Fructan: more than a reserve carbohydrate?

Most plants store starch or Suc as reserve carbohydrates, but about 15% of all flowering plant species store fructans, which are linear and branched polymers of Fru. Among the plants that store fructans are many of significant economic importance, such as cereals (e.g. barley, wheat, and oat), vegetables (e.g. chicory, onion, and lettuce), ornamentals (e.g. dahlia and tulip), and forage grasses (e.g. Lolium and Festuca) (Hendry and Wallace, 1993). Fructans isolated from these plants have a variety of applications. Small fructans have a sweet taste, whereas longer fructan chains form emulsions with a fat-like texture and a neutral taste. The human digestive tract does not contain enzymes able to degrade fructans; therefore, there is strong interest from the food industry to use them as low-calorie food ingredients. In plants, fructans may have functions other than carbon storage; they have been implicated in protecting plants against water deficit caused by drought or low temperatures (Hendry and Wallace, 1993; Pilon-Smits et al., 1995). The substrate for fructan synthesis is Suc, and like Suc, fructans are stored in the vacuole. Although Suc is synthesized in the cytoplasm, fructans are produced in the vacuole by the action of specific enzymes (fructosyltransferases) that transfer Fru from Suc to the growing fructan chain. Fructan synthesis is modulated by light, which changes the availability of Suc in the cell (Fig. 1). The biosynthetic enzymes are evolutionarily related to invertases, enzymes that hydrolyze Suc. The biochemistry of fructan synthesis has been determined, and the first genes encoding these biosynthetic enzymes have recently been cloned, opening new biotechnological opportunities for the use of fructans. Until now the major obstacles have been the limited availability of long-chain fructans and the heterogeneity of harvested fructans. It will now be possible to genetically engineer plants to produce large quantities of fructans of defined structure and size. Furthermore, fructan accumulation in plants that normally do not produce them may contribute to protection from water stress in these plants. A number of research groups have studied fructan accumulation in plants in an attempt to understand fructan synthesis and the physiological role of fructan accumulation in plants and to improve the commercial availability of fructans. In this Update we give an overview of these attempts and discuss their impact on our insight into fructan production in plants. First, a few words on fructan synthesis in bacteria, which is simpler than plant fructan biosynthesis because only a single biosynthetic enzyme is involved.