Microbial fructan production in transgenic potato plants and tubers.

Fructans (fructose polymers) derived from plants usually have a very low degree of polymerisation (DP) and this limits the technical application of this versatile carbohydrate polymer. Previously we showed that the expression of bacterial fructosyltransferase genes in transgenic plants results in the accumulation of high molecular weight fructans with a DP of over 25,000. Here we report on our progress in accumulating such high DP fructans in potato plants and tubers. In these plants growth, tuber formation and carbohydrate partitioning were analyzed. Young plants showed no difference in growth between wildtype and fructan-producing (KP) plants. However, at a certain timepoint growth in KP plants is reduced and photosynthate starts to accumulate in source leaves. In these plants no diurnal turnover of starch and sucrose is observed. At harvest, the sink organs (roots, tubers) are reduced in weight. Depending on fructan content the KP tubers display a brown phenotype and yield was reduced. The starch content in the KP tubers is inversely correlated with the fructan level, whereas the sucrose, glucose, fructose and protein levels increase substantially, concomitant with fructan concentration. The cellular location of the fructans was investigated via immunofluorescence using monoclonal antibodies against levan and this showed the presence of fructan located along the cell rim instead of in the expected vacuolar location. It is concluded that fructan accumulation in these plants dramatically affects growth and carbohydrate partitioning and possible solutions for these problems are discussed