Evidence for the involvement of sucrose phosphate synthase in the pathway of sugar accumulation in sucrose-accumulating tomato fruits

To better understand the mechanism of sugar unloading and sugar concentration in hexose- and sucrose-accumulating tomato fruits (Lycopersicon chmielewskii and L. esculentum, respectively) and to determine the causes of the late accumulation of sucrose present in sucrose-accumulating tomato fruits, the assimilation of [³H](fructosyl)-sucrose was studied. Key enzymes involved in carbohydrate metabolism were also assayed. The results demonstrated that the low level of sucrose present in young fruits accumulates directly without undergoing hydrolysis, suggesting a symplastic pathway for sucrose unloading. By contrast, the large quantity of the sucrose present in ripe sucrose-accumulating fruits originates from hydrolysis and resynthesis, suggesting an apoplastic pathway for sucrose unloading. The increase in sucrose level observed in sucrose-accumulating fruits is associated with a gradual decline in invertase activity and an increase in sucrose phosphate synthase activity. This latter enzyme seems to play a key biochemical role in the accumulation of sucrose and the establishment of a high sugar content in tomato fruits.