SUGAR-TRANSPORT INTO PLASMA-MEMBRANE VESICLES OF TRANSFORMED TOBACCO CELLS - UPTAKE OF SUCROSE BUT NOT OF HEXOSES IS STIMULATED BY DELTA-PH

In vitro transport experiments with outside-out plasma membrane vesicl es of Agrobacterium tumefaciens-transformed tobacco cells (Nicotiana t abacum cv. Petit Havana SR 1-C58) show that despite high cell wall inv ertase activity transformed cells express not only distinct hexose tra nsporters for 3-O-methyl-D-glucose (OMG) and D-fructose, but also a su crose transporter. When imposing a Delta pH of 2 units sucrose transpo rt was strongly stimulated indicative of H+/sucrose cotransport wherea s transport of both hexoses was almost unaffected. However, imposing a Delta Psi in addition to Delta pH stimulated initial transport rates of hexoses and sucrose. For comparison, electrophysiological experimen ts were performed. The membrane potential of intact transformed cells recorded with the conventional microelectrode technique reacted with t ransient depolarizations upon addition of sucrose, D-glucose and D-fru ctose indicative of electrogenic transport for all three sugars. The r esults clearly support the existence of a H+/sucrose cotransporter whe reas the absence of a Delta pH effect on hexose transport is in appare nt contradiction to the membrane depolarizations found in situ. The im plications of the in vitro and electrophysiological transport data are discussed with respect to reaction mechanism and physiological functi on of the different sugar transporters in transformed cells.