SUGAR UPTAKE BY THE DERMAL TRANSFER CELLS OF DEVELOPING COTYLEDONS OFVICIA-FABA L - MECHANISM OF ENERGY COUPLING

The mechanism of carrier-mediated sucrose uptake by the dermal transfe r cells of developing Vicia faba L. cotyledons was studied using excis ed cotyledons and isolated transfer cell protoplasts. Addition of sucr ose resulted in a transitory alkalinization of the bathing solution wh ereas additions of glucose, fructose or raffinose had no effect. Dissi pating the proton motive force by exposing cotyledons and isolated tra nsfer cell protoplasts to an alkaline pH, carbonylcyanide m-chlorophen ylhydrazone, weak acids (propionic acid and 5,5'-dimethyl-oxazolidine- 2,4-dione) or tetraphenylphosphonium ion resulted in a significant red uction of sucrose uptake. The ATPase inhibitors, erythrosin B (EB), di ethylstilbestrol (DES) and N,N'-dicyclohexylcarbodiimide (DCCD) were f ound to abolish the sucrose-induced medium alkanization as well as red uce sucrose uptake. Cytochemical localization of the ATPase, based on lead precipitation, demonstrated that the highest activity was present in the plasma membranes located in wall ingrowth regions of the derma l transfer cells. The presence of a transplasma-membrane redox system was detected by the extracellular reduction of the electron acceptor, hexacyanoferrate III. The reduction of the ferric ion was coupled to a release of protons. The redox-induced proton extrusion was abolished by the ATPase inhibitors EB, DES and DCCD suggesting that proton extru sion was solely through the H+-ATPase. Based on these findings, it is postulated that cotyledonary dermal transfer cells take up sucrose by a proton symport mechanism with the proton motive force being generate d by a H+-ATPase. Sucrose uptake by the storage parenchyma and inner e pidermal cells of the cotyledons did not exhibit characteristics consi stent with sucrose-proton symport.