Antisense inhibition of tomato fruit sucrose synthase decreases fruit setting and the sucrose unloading capacity of young fruit

The role of sucrose synthase (SuSy) in tomato fruit was studied in transgen ic tomato (Lycopersicon esculentum) plants expressing an antisense fragment of fruit-specific SuSy RNA (TOMSSF) under the control of the cauliflower m osaic virus 35S promoter. Constitutive expression of the antisense RNA mark edly inhibited SuSy activity in flowers and fruit pericarp tissues. However , inhibition was only slight in the endosperm and was undetectable in the e mbryo, shoot, petiole, and leaf tissues. The activity of sucrose phosphate synthase decreased in parallel with that of SuSy, but acid invertase activi ty did not increase in response to the reduced SuSy activity. The only effe ct on the carbohydrate content of young fruit was a slight reduction in sta rch accumulation. The in vitro sucrose import capacity of fruits was not re duced by SuSy inhibition at 23 days after anthesis, and the rate of starch synthesized from the imported sucrose was not lessened even when SuSy activ ity was decreased by 98%. However, the sucrose unloading capacity of I-day- old fruit was substantially decreased in lines with low SuSy activity. In a ddition, the SuSy antisense fruit from the first week of flowering had a sl ower growth rate. A reduced fruit set, leading to markedly less fruit per p lant at maturity, was observed for the plants with the least SuSy activity. These results suggest that SuSy participates in the control of sucrose imp ort capacity of young tomato fruit, which is a determinant for fruit set an d development.