Evidence for the presence of a sucrose carrier in immature sugar-beet roots

Unlike in mature sugar-beet roots, sucrose is assumed to be hydrolyzed by a wall-bound invertase prior to uptake by immature roots. To test this hypothesis, they used a sucrose analog, 1'fluorosucrose which is recognized by the carrier but is a poor substrate for invertases. Asymmetrically labeled sucrose (3H-fructose) 1'fluorosucrose (14C-glucose) were applied at 10 mM (3H/14C=1) to an attached source leaf. After 6 h, sugars from plant parts in the translocation path were separated on HPLC. 14C-1'fluorosucrose was translocated and accumulated in the root at a higher rate than 3H-sucrose due to greater metabolism of 3H-sucrose in the shoot (indicated by the presence of 3H in hexose fractions and loss of asymmetry). In the root 25% of the 3H-sucrose was hydrolyzed to hexoses whereas no 14C was detected in hexose fractions. The data indicate that despite high cell-wall invertase and cytoplasmic sucrose synthase activities, young sugar-beet roots import and store sucrose without hydrolysis. Therefore, the function of a group translocator at the tonoplast is unclear