Photoautotrophic suspension-culture cells of Chenopodium rubrum that were shifted to mixotrophic growth by adding glucose were used as model system to investigate the influence of the source-sink transition in higher plants on the expression and enzyme activities of intracellular and extracellular invertases. The complete cDNA coding for an extracellular invertase was cloned and sequenced from C. rubrum, and its identity has been proven by heterologous expression in Saccharomyces cerevisiae. The higher activity of extracellular invertase after preincubation in the presence of glucose was paralleled by an increased expression of the corresponding gene. The induction by glucose could be mimicked by the nonmetabolizable glucose analog 6-deoxyglucose. Both enzyme activity and mRNA level of extracellular invertase showed a sink-tissue-specific distribution in plants. The activity of neutral and acidic intracellular invertases were not affected by preincubation of autotrophic tissue cultures with sugars, nor did they show a tissue-specific distribution in plants. The data suggest that apoplastic invertase not only has an important function in phloem unloading and carbohydrate partitioning between source and sink tissues but may also have a role in establishing metabolic sinks.