Resource allocation is a major determinant of plant fitness and is influenced by external as well as internal stimuli. We have investigated the effect of cell wall invertase activity on the transition from vegetative to reproductive growth, inflorescence architecture, and reproductive output, i.e. seed production, in the model plant Arabidopsis thaliana by expressing a cell wall invertase under a meristem-specific promoter. Increased cell wall invertase activity causes accelerated flowering and an increase in seed yield by nearly 30%. This increase is caused by an elevation of the number of siliques, which results from enhanced branching of the inflorescence. On the contrary, as cytosolic enzyme, the invertase causes delayed flowering, reduced seed yield, and branching. This demonstrates that invertases not only are important in determining sink strength of storage organs but also play a role in regulating developmental processes.