Linear sucrose transport in protoplasts from developing soybean cotyledons

Previous studies with isolated soybean cotyledon protoplasts revealed the presence of a saturable, simple diffusion, and nonsaturating carrier-mediated uptake of sucrose into soybean cotyledon cells. A proton/sucrose cotransport may be involved in the saturable sucrose uptake (Lin et al. 1984 Plant Physiol 75: 936-940 and Schmitt et al. 1984 Plant Physiol 75: 941-946). In this study, we investigated the linear sucrose uptake mechanism by treating isolated protoplasts with 15 micromolar p-trifluoromethoxy-carbonylcyanide phenylhydrazone (FCCP) or 100 micromolar p-chloromecuribenzenesulfonic acid to eliminate the saturable uptake. We found: (a) increasing external pH decreases the linear sucrose uptake; (b) fusicoccin at 20 micromolar stimulates and FCCP at 15 micromolar inhibits this linear sucrose uptake; and (c) the ratio of the initial influx of proton to sucrose is close to one in both saturable and nondiffusive linear (difference between the total linear and diffusive components) uptakes. The results suggest that a proton/sucrose cotransport is also involved in the nondiffusive linear sucrose uptake into soybean cotyledon cells.