The role of sucrose synthase (SuSy) in tomato fruit was studied in transgenic tomato (

) plants expressing an antisense fragment of fruit-specific SuSy RNA ( TOMSSF) under the control of the cauliflower mosaic vi-rus 35S promoter. Constitutive expression of the antisense RNA markedly inhibited SuSy activity in flowers and fruit pericarp tissues. However, inhibition was only slight in the endosperm and was undetectable in the embryo, shoot, pet-iole, and leaf tissues. The activity of sucrose phosphate synthase decreased in parallel with that of SuSy, but acid inver-tase activity did not increase in response to the reduced SuSy activity. The only effect on the carbohydrate content of young fruit was a slight reduction in starch accumulation. The in vitro sucrose import capacity of fruits was not reduced by SuSy inhibition at 23 days after anthesis, and the rate of starch synthesized from the imported sucrose was not less-ened even when SuSy activity was decreased by 98%. However, the sucrose unloading capacity of 7-day-old fruit was substantially decreased in lines with low SuSy activity. In addition, the SuSy antisense fruit from the first week of flow-ering had a slower growth rate. A reduced fruit set, leading to markedly less fruit per plant at maturity, was observed for the plants with the least SuSy activity. These results suggest that SuSy participates in the control of sucrose import ca-pacity of young tomato fruit, which is a determinant for fruit set and development