Pozueta, Diego , González, Pedro , Pozueta-Romero, Javier , Etxeberria, Ed .
The linear phase of sucrose uptake concentration curve in sink organs is largely mediated by fluid phase endocytosis.
Biochemical studies have demonstrated that sucrose uptake kinetics in sink cells consist of multiple components collectively characterized by a bi-phasic curve. Whereas the hyperbolic phase at low external sugar concentration is believed to represent a high-affinity, membrane-bound, carrier-mediated component, the linear non-saturable phase at higher concentrations denotes facilitated diffusion, presumably mediated by a sucrose binding protein. Recent demonstrations of endocytic sucrose uptake in various heterotrophic cells and observations that fluid-phase endocytosis (FPE) in celery parenchyma was only induced at high external mannitol concentrations, prompted us to re-examine the possible role of FPE and membrane-carrier transport within both phases of the characteristic concentration uptake curve. At low external concentration (5 mM), sucrose uptake into turnip (Brassica campestris) hypocotyl discs was inhibited by the sucrose carrier inhibitor phloridzin (2 mM) but largely unaffected by the endocytic inhibitor latrunculin-B (10 uM). When sucrose concentration was increased to 100 mM, transport was significantly reduced by both phloridzin and latrunculin-B. Uptake of the endocytic marker Alexa-488 was strongly inhibited by latrunculin-B at both external sucrose concentrations, with no effect noted in the presence of phloridzin. Analyses of the data, double fluorescence confocal imaging, and results of Alexa-488/sucrose ‘specific activity’, indicate that: a) both carrier-mediated and endocytic uptake of sucrose take place at any external sucrose concentration, b) within the hyperbolic phase of the sucrose accumulation curve most of sucrose accumulating in the cell enters via plasmalemma-bound carrier(s), and c) within the linear phase both plasmalemma bound carriers and endocytosis are determinants of sucrose accumulation, the involvement of endocytosis increasing parallel to external sucrose.
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1 - University of Florida, Citrus Research Center, 700 Experiment Station Road, Lake Alfred, FL, 33850, USA
2 - Agrobioteknologiako Instituta, Consejo Superior de Investigaciones Cientificas, Mutiloako etorbidea zembaki gabe, Mutiloabeti, Nafarroa, 31192, Spain
Presentation Type: Poster:Posters for Sections
Location: Ball Room & Party Room/SUB
Date: Monday, July 28th, 2008
Time: 12:30 PM