EXPRESSION OF AN ARABIDOPSIS SUCROSE SYNTHASE GENE INDICATES A ROLE IN METABOLIZATION OF SUCROSE BOTH DURING PHLOEM LOADING AND IN SINK ORGANS

Sucrose synthase, an important enzyme in carbohydrate metabolism, cata lyzes the reversible conversion of sucrose and UDP to UDP-glucose and fructose in vitro. To investigate the in vivo function of sucrose synt hase, both the gene (Asus1) and a corresponding cDNA from roots of Ara bidopsis were isolated. The Asus1 gene has homologies of 67-72% to suc rose synthase genes from other species. Histochemical GUS analysis of Arabidopsis and tobacco plants transformed with a 1.5 kb Asus1 promote r 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 suppl y and increased demand for translocation of carbohydrates such as anae robic 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 i nvolved in the supply of energy for phloem loading in source tissues, and in metabolization of sucrose in sink tissues after unloading.