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

Sucrose unloading and sink activity were examined in tomato plants (Lycoper sicon esculentum) overexpressing sucrose phosphate synthase (SPS; EC 2.3.1. 14), Like the leaves, the fruit of the transformed tomato plants had elevat ed (2.4-fold) SPS activity, SPS overexpression in tomato fruit did not sign ificantly change acid invertase, and only slightly reduced ADPglc ppase act ivity, but enhanced sucrose synthase activity by 27%, More importantly, the amount of sucrose unloaded into the fruit was considerably increased. Usin g [H-3]- (fructosyl)-sucrose in in vitro unloading experiments with harvest ed 20-d-old fruit, 70% more sucrose was unloaded into the transformed fruit s compared to the untransformed controls, Furthermore, the turnover of the sucrose unloaded into the fruit of transformed plants was 60% higher than t hat observed in the untransformed controls, Taken together, these results d emonstrate that SPS overexpression increases the sink strength of transform ed tomato fruit.