Effects of drought, transgenic expression of a fructan synthesizing enzymeand of mycorrhizal symbiosis on growth and soluble carbohydrate pools in tobacco plants

The effects of three conditions likely to affect soluble carbohydrate pools , namely drought, expression of barley sucrose :fructan 6-fructosyl transfe rase (6-SFT, EC 2. 4. 1. 10) and the establishment of the arbuscular mycorr hizal symbiosis with Glomus mosseae were studied in a multifactorial experi ment using tobacco (Nicotiana tabacum). Tobacco, a plant naturally unable t o form fructan, accumulated fructan in leaves, and to a larger extent in th e roots, when transformed with 6-SFT. Under drought conditions, growth was considerably reduced, but neither expression of 6-SFT nor mycorrhiza format ion had an effect on growth rate. However, in response to drought, carbon p artitioning was significantly altered towards accumulation of soluble sugar s. In plants exposed to drought, pools of sucrose were greater than these o f unstressed plants, particularly in their roots. In the transgenic plants expressing 6-SFT, there were also increased contents of the products of 6-S FT, namely fructan, most probably because of the increased availability of the substrate, sucrose. These effects were the same in the presence or abse nce of mycorrhiza. Hexoses (glucose and fructose) also increased in respons e to drought, primarily in the leaves. This effect of drought was little af fected 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 greatl y increased in the roots of all mycorrhizal plants upon exposure to drought , particularly in some of the transgenic plants expressing 6-SFT.