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 vesicles of Agrobacterium tumefaciens-transformed tobacco cells (Nicotiana tabacum cv. Petit Havana SR 1-C58) show that despite high cell wall invertase activity transformed cells express not only distinct hexose transporters for 3-O-methyl-D-glucose (OMG) and D-fructose, but also a sucrose transporter. When imposing a Delta pH of 2 units sucrose transport was strongly stimulated indicative of H+/sucrose cotransport whereas 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 experiments were performed. The membrane potential of intact transformed cells recorded with the conventional microelectrode technique reacted with transient depolarizations upon addition of sucrose, D-glucose and D-fructose indicative of electrogenic transport for all three sugars. The results clearly support the existence of a H+/sucrose cotransporter whereas the absence of a Delta pH effect on hexose transport is in apparent contradiction to the membrane depolarizations found in situ. The implications of the in vitro and electrophysiological transport data are discussed with respect to reaction mechanism and physiological function of the different sugar transporters in transformed cells.