Overexpression of sucrose phosphate synthase increases sucrose unloading in transformed tomato fruit

Sucrose unloading and sink activity were examined in tomato plants(Lycopersicon esculentum) overexpression sucrosephosphate synthase (SPS; EC 2.3.1.14). Like the leaves, the fruit of thetransformed tomato plants had elevated (2.4-fold) SPS activity. SPSover-expression in tomato fruit did not significantly change acidinvertase, and only slightly reduced ADPglc ppase activity, but enhancedsucrose synthase activity by 27%. More importantly, the amount of sucroseunloaded into the fruit was considerably increased. Using[³H]- (fructosyl)-sucrose in invitro unloading experiments with harvested 20-d-old fruit, 70%more sucrose was unloaded into the transformed fruits compared to theuntransformed controls. Furthermore, the turnover of the sucrose unloadedinto the fruit of transformed plants was 60% higher than that observed inthe untransformed controls. Taken together, these results demonstrate thatSPS overexpression increases the sink strength of transformed tomatofruit.