FRUCTAN AS A NEW CARBOHYDRATE SINK IN TRANSGENIC POTATO PLANTS

Fructans are polyfructose molecules that function as nonstructural sto rage carbohydrates in several plant species that are important crops. We have been studying plants for their ability to synthesize and degra de fructans to determine if this ability is advantageous. We have also been analyzing the ability to synthesize fructan in relation to other nonstructural carbohydrate storage forms like starch. To study this, we induced fructan accumulation in normally non-fructan-storing plants and analyzed the metabolic and physiological properties of such plant s. The normally non-fructan-storing potato plant was modified by intro ducing the microbial fructosyltransferase genes so that it could accum ulate fructans. Constructs were created so that the fructosyltransfera se genes of either Bacillus subtilis (sacB) or Streptococcus mutans (f tf) were fused to the vacuolar targeting sequence of the yeast carboxy peptidase Y (cpy) gene. These constructs were placed under the control of the constitutive cauliflower mosaic virus 35S promoter and introdu ced into potato tissue. The regenerated potato plants accumulated high molecular mass(>5 x 10(6) D) fructan molecules in which the degree of polymerization of fructose units exceeded 25,000. Fructan accumulatio n was detected in every plant tissue tested. The fructan content in th e transgenic potato plants tested varied between 1 and 30% of dry weig ht in leaves and 1 and 7% of dry weight in microtubers. Total nonstruc tural neutral carbohydrate content in leaves of soil-grown plants incr eased dramatically from 7% in the wild type to 35% in transgenic plant s. Our results demonstrated that potato plants can be manipulated to s tore a foreign carbohydrate by introducing bacterial fructosyltransfer ase genes. This modification affected photosynthate partitioning in mi crotubers and leaves and increased nonstructural carbohydrate content in leaves.