The yeast trehalose-6-phosphate synthase gene (TPS1) was engineered under the control of the cauliflower mosaic virus regulatory sequences (CaMV35S) for expression in plants. Using Agrobacterium-mediated transfer, the gene was incorporated into the genomic DNA and constitutively expressed in Nicotiana tabacum L. plants. Trehalose was determined in the transformants, by anion-exchange chromatography coupled to pulsed amperometric detection. The non-reducing disaccharide accumulated up to 0.17 mg per g fresh weight in leaf extracts of transgenic plants. Trehaloseaccumulating plants exhibited multiple phenotypic alterations, including stunted growth, lancet-shaped leaves, reduced sucrose content and improved drought tolerance. These pleiotropic effects, and the fact that water loss from detached leaves was not significantly affected by trehalose accumulation, suggest that synthesis of this sugar, rather than leading to an osmoprotectant effect, had altered sugar metabolism and regulatory pathways affecting plant development and stress tolerance.