THE PLANT CELL, Vol 6, Issue 4 561-570, Copyright © 1994 by American Society of Plant Biologists
Fructan as a New Carbohydrate Sink in Transgenic Potato Plants
I. M. van der Meer, MJM. Ebskamp, RGF. Visser, P. J. Weisbeek and SCM. Smeekens
Department of Molecular Cell Biology, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands
Fructans are polyfructose molecules that function as nonstructural storage
carbohydrates in several plant species that are important crops. We have
been studying plants for their ability to synthesize and degrade 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 plants. The normally
non-fructan-storing potato plant was modified by introducing the microbial
fructosyltransferase genes so that it could accumulate fructans. Constructs
were created so that the fructosyltransferase genes of either Bacillus
subtilis (sacB) or Streptococcus mutans (ftf) were fused to the vacuolar
targeting sequence of the yeast carboxypeptidase Y (cpy) gene. These
constructs were placed under the control of the constitutive cauliflower
mosaic virus 35S promoter and introduced into potato tissue. The
regenerated potato plants accumulated high molecular mass (>5 [times]
106 D) fructan molecules in which the degree of polymerization of fructose
units exceeded 25,000. Fructan accumulation was detected in every plant
tissue tested. The fructan content in the transgenic potato plants tested
varied between 1 and 30% of dry weight in leaves and 1 and 7% of dry weight
in microtubers. Total nonstructural neutral carbohydrate content in leaves
of soil-grown plants increased dramatically from 7% in the wild type to 35%
in transgenic plants. Our results demonstrated that potato plants can be
manipulated to store a foreign carbohydrate by introducing bacterial
fructosyltransferase genes. This modification affected photosynthate
partitioning in microtubers and leaves and increased nonstructural
carbohydrate content in leaves.
