Fructans: from natural sources to transgenic plants

In nature, fructans are synthesized from sucrose by various microbial and plant species. Depending on the source, fructans can contain from two to more than a hundred thousand fructose units linked by β-(2→1) (inulin-type) or β(2→6) (levan-type) glycosidic bonds. Bacteria produce levan or inulin polymers via a one-enzyme process aimed to confer competitiveness in the interaction with the host plant or animal, protection against abiotic and biotic stress, and a circumstantial energy source. Plant fructans are shorter and have diverse structures. They are synthesized in the cell vacuoles as reserve carbohydrates by the concerted action of at least two enzymes with distinct substrate specificities. Plant fructosyltransferases evolved from vacuolar invertases, a process likely connected with the independent adaptation of unrelated families to cold and arid environments. Fructans of short and medium sizes are prebiotics with increasing demand in the functional food market. Large polyfructans also have potential applications in the non-food industry. Current production systems are restricted either to the recovery of linear inulin stored in the roots of the low-yielding chicory plant or to the more costly industrial conversion of sucrose into fructooligosaccharides using immobilized fungal enzymes. The introduction of the appropriate fructosyltransferase genes in agro-industrial productive crops that naturally store high concentrations of sucrose, such as sugar beet or sugar cane, offers a plausible alternative to produce different types of fructans at lower production costs.