▲ FIGURE 7-23 Concentration of ions and sucrose by the plant vacuole. The vacuolar membrane contains two types of proton pumps (orange): a V-class H+ ATPase (left) and a pyrophosphate-hydrolyzing proton pump (right) that differs from all other ion-transport proteins and probably is unique to plants. These pumps generate a low luminal pH as well as an inside-positive electric potential across the vacuolar membrane owing to the inward pumping of H+ ions. The inside-positive potential powers the movement of CT and NO3 from the cytosol through separate channel proteins (purple). Proton antiporters (green), powered by the H+ gradient, accumulate Na+, Ca2+, and sucrose inside the vacuole. [After P Rea and D. Sanders, 1987, Physiol. Plant 71:131; J. M. Maathuis and D. Sanders, 1992, Curr. Opin. Cell Biol. 4:661; P A. Rea et al., 1992, Trends Biochem. Sci. 17:348.]

creased sequestration of Na+ in the vacuole. For instance, transgenic tomato plants that overexpress the vacuolar Na+/H+ antiporter have been shown to grow, flower, and produce fruit in the presence of soil NaCl concentrations that kill wild-type plants. Interestingly, although the leaves of these transgenic tomato plants accumulate large amounts of salt, the fruit has a very low salt content. I

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