The aims of this work were, on one hand, to examine the in planta role of sucrose-6-phosphate phosphatase (SPP) in regulation of sucrose synthesis and carbon partitioning in photosynthetically active source tissues and heterotrophic sink tissues; on the other hand, to investigate the potential impact of sucrose-6-phosphate (Suc6P) on plant metabolisms. To this end, the genes encoding SPP were cloned from tobacco and potato and down-regulation of SPP expression was achieved via an RNA interference approach in transgenic tobacco and potato in a constitutive, or tissue-specific, or inducible way. Molecular, biochemical and physiological analyses were subsequently carried out. The results showed that in tobacco leaves SPP can be "lost" up to 80 % before strong effects on sucrose synthesis and plant growth can be observed. Carbon partitioning was redirected to starch synthesis in the constitutively silenced SPP transgenic tobacco plants. Ethanol inducible silencing of SPP in transgenic tobacco plants resulted in rapid phenotypical alterations after ethanol induction. In potato tubers, the results showed that sucrose cycling in growing tubers does not possess any regulatory function in the control of starch accumulation and tuber yield under normal growth conditions. Studies with 35SPPi tubers showed that the levels of Suc6P in the cold-stored tubers were negatively correlated with the cold induction of vacuolar invertase, providing first evidence of a potential signaling function for Suc6P in the regulation of gene expression in plants.