Transgenic potato (Solanum tuberosum) tubers synthesize the full spectrum of inulin molecules naturally occurring in globe artichoke (Cynara scolymus) roots

Abstract

The ability to synthesize high molecular weight inulin was transferred to potato plants via constitutive expression of the 1-SST (sucrose:sucrose 1-fructosyltransferase) and the 1-FFT (fructan: fructan 1-fructosyltransferase) genes of globe artichoke (Cynara scolymus). The fructan pattern of tubers from transgenic potato plants represents the full spectrum of inulin molecules present in artichoke roots as shown by high-performance anion exchange chromatography, as well as size exclusion chromatography. These results demonstrate in planta that the enzymes sucrose:sucrose 1-fructosyltransferase and fructan:fructan 1-fructosyltransferase are sufficient to synthesize inulin molecules of all chain lengths naturally occurring in a given plant species. Inulin made up 5% of the dry weight of transgenic tubers, and a low level of fructan production also was observed in fully expanded leaves. Although inulin accumulation did not influence the sucrose concentration in leaves or tubers, a reduction in starch content occurred in transgenic tubers, indicating that inulin synthesis did not increase the storage capacity of the tubers.

Figure 1

Oligofructan content of leaves and tubers of 35S-SST-transformed potato plants in comparison to nontransformed wild-type plants. The oligosaccharides from extracts of the transgenic potato line 35S-SST 36 (a) and wild-type potato plants (b) were separated by HPAEC. The numbers indicate the DP of the sugars: 2 = sucrose, 3 = 1-kestose, 4 = nystose, 5–7 = oligofructans.

Figure 2

Inulin content of leaves and tubers of potato plants harboring the 1-SST and the 1-FFT gene of C. scolymus. The HPAEC profile of the fructan pattern of leaves of a transgenic potato plant (a) was compared with that of a wild-type potato plant (b). Additionally, the inulin from transgenic potato tubers (d) was compared with native inulin from artichoke (C. scolymus) roots (c). The numbers indicate the DP of the sugars as described in the legend to Fig. 1.

Figure 3

SEC profiles of crude extracts from wild-type potato tubers, artichoke roots, and transgenic potato tubers expressing the 1-SST and the 1-FFT genes from artichoke. The eluate from the SEC column was fractionated in aliquots of 110 μl. Fractions between 10 and 18 ml of elution volume were subjected to fructose determination after inulin hydrolysis by oxalic acid. A high molecular weight inulin from A. sydowi (M_r 10⁶ g/mol) was added as a standard to all extracts and eluted with a peak in fraction 7–10.