The Hyperbolic and Linear Phases of the Sucrose Accumulation Curve in Turnip Storage Cells Denote Carrier-mediated and Fluid Phase Endocytic Transport, Respectively

Abstract

Sucrose accumulation into sink cells consists of multiple components collectively characterized by a biphasic kinetics curve. Whereas the hyperbolic phase at low external sucrose concentration denotes a membrane-bound, carrier-mediated component, the linear nonsaturable phase at higher concentrations has been suggested to represent facilitated diffusion. We recently demonstrated the occurrence of fluid phase endocytosis (FPE) uptake of sucrose in heterotrophic cells. To investigate the involvement of this process within both phases of the sucrose accumulation curve, we analyzed the effect of phloridzin and latrunculin-B (sucrose/H+ symport and endocytosis inhibitors, respectively) in the accumulation of sucrose and the endocytic marker Alexa-488 in turnip (Brassica campestris L.) storage parenchyma cells. At low external sucrose concentration, phloridzin, but not latrunculin-B, greatly reduced sucrose accumulation. By contrast, at high external sucrose concentration, phloridzin and latrunculin-B significantly inhibited sucrose accumulation. In addition, latrunculin-B exerted a pronounced inhibitory effect on Alexa-488 uptake at any external sucrose concentration. Our results indicate that carrier-mediated and endocytic uptake (at different magnitudes) of sucrose take place at any external sucrose concentration; that within the hyperbolic phase of the sucrose accumulation curve, most of the sucrose accumulating in the cell enters via plasmalemma-bound carrier(s); and that within the linear phase, plasmalemma-bound carriers and FPE are determinants of sucrose accumulation, with the involvement of FPE increasing parallel to external sucrose. Confocal laser scanning microscopy observations confirmed the increased involvement of FPE at higher external sucrose concentrations.