Sucrose synthase, an important enzyme in carbohydrate metabolism, catalyzes the reversible conversion of sucrose and UDP to UDP-glucose and fructose in vitro. To investigate the in vivo function of sucrose synthase, both the gene (Asus1) and a corresponding cDNA from roots of Arabidopsis were isolated. The Asus1 gene has homologies of 67-72% to sucrose synthase genes from other species. Histochemical GUS analysis of Arabidopsis and tobacco plants transformed with a 1.5 kb Asus1 promoter fragment transcriptionally fused to the beta-glucuronidase reporter gene showed that the Asus1 gene is expressed in the phloem of leaves, and in roots. Induction is found under conditions of limited ATP supply and increased demand for translocation of carbohydrates such as anaerobic or cold treatment. During anaerobiosis the increase in RNA level leads to increased sucrose synthase activity in roots. The expression pattern and regulation of the gene suggest that sucrose synthase is involved in the supply of energy for phloem loading in source tissues, and in metabolization of sucrose in sink tissues after unloading.