Elevated sucrose-phosphate synthase activity in

Constitutive over-expression of a maize sucrosephosphate synthase (SPS) gene in tobacco (Nicotiana tabacum) had major effects on leaf carbohydrate budgets with consequences for whole plant development. Transgenic tobacco plants ̄owered earlier and had greater ̄ower numbers than wild-type plants. These changes were not linked to modi®ed source leaf carbon assimilation or carbon export, although sucrose to starch ratios were signi®cantly higher in leaves expressing the transgene. The youngest and oldest leaves of plants over-expressing SPS had up to 10-fold wild-type maximal extractable SPS activity, but source leaf SPS activities were only 2±3 times greater in these lines than in the wild type. In the oldest leaves, where the expression of the transgene led to the most marked enhancement in SPS activity, photosynthesis was also increased. It was concluded that these increases in the capacity for sucrose synthesis and carbon assimilation, particularly in older leaves, accelerate the whole plant development and increase the abundance of ̄owers without substantial changes in the overall shoot