Evidence for the presence of a sucrose carrier in immature sugar beet tap roots |
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Authors: | Lemoine R Daie J Wyse R |
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Institution: | Department of Soils and Crops, Cook College, Rutgers University, New Brunswick, New Jersey 08903. |
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Abstract: | The objectives of this work were to determine the path of phloem unloading and if a sucrose carrier was present in young sugar beet (Beta vulgaris L.) taproots. The approach was to exploit the characteristics of the sucrose analog, 1'-fluorosucrose (F-sucrose) which is a poor substrate for acid invertase but is a substrate for sucrose synthase. Ten millimolar each of 3H]sucrose and 14C]F-sucrose were applied in a 1:1 ratio to an abraded region of an attached leaf for 6 hours. 14C]F-sucrose was translocated and accumulated in the roots at a higher rate than 3H]sucrose. This was due to 3H]sucrose hydrolysis along the translocation path. Presence of 3H]hexose and 14C]F-sucrose in the root apoplast suggested apoplastic sucrose unloading with its subsequent hydrolysis. Labeled F-sucrose uptake by root tissue discs exhibited biphasic kinetics and was inhibited by unlabeled sucrose, indicating that immature roots have the ability for carrier-mediated sucrose transport from the apoplast. Collectively, in vivo and in vitro data indicate that despite sucrose hydrolysis by the wall-bound invertase, sucrose hydrolysis is not entirely essential for sugar accumulation in this tissue. |
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