The effects of three conditions likely to affect soluble carbohydrate pools, namely drought, expression of barley
sucrose: fructan 6-fructosyl transferase (6-SFT, EC 18.104.22.168) and the establishment of the arbuscular mycorrhizal
symbiosis with Glomus mosseae were studied in a multifactorial experiment using tobacco (Nicotiana tabacum).
Tobacco, a plant naturally unable to form fructan, accumulated fructan in leaves, and to a larger extent in the roots,
when transformed with 6-SFT. Under drought conditions, growth was considerably reduced, but neither
expression of 6-SFT nor mycorrhiza formation had an effect on growth rate. However, in response to drought,
carbon partitioning was significantly altered towards accumulation of soluble sugars. In plants exposed to drought,
pools of sucrose were greater than those of unstressed plants, particularly in their roots. In the transgenic plants
expressing 6-SFT, there were also increased contents of the products of 6-SFT, namely fructan, most probably
because of the increased availability of the substrate, sucrose. These effects were the same in the presence or
absence of mycorrhiza. Hexoses (glucose and fructose) also increased in response to drought, primarily in the
leaves. This effect of drought was little affected by the expression of 6-SFT, except that it slightly enhanced
drought-induced glucose accumulation in roots. However, the presence of mycorrhiza led to a considerable
reduction in drought-induced accumulation of hexoses in the leaves. The content of the fungal disaccharide
trehalose was greatly increased in the roots of all mycorrhizal plants upon exposure to drought, particularly in
some of the transgenic plants expressing 6-SFT.