Numerous Transport Proteins Enable Plant Vacuoles to Accumulate Metabolites and Ions

The lumen of plant vacuoles is much more acidic (pH 3 to 6) than is the cytosol (pH 7.5). The acidity of vacuoles is maintained by a V-class ATP-powered proton pump (see Figure 7-6) and by a PPrpowered pump that is unique to plants. Both of these pumps, located in the vacuolar membrane, import H+ ions into the vacuo-lar lumen against a concentration gradient. The vacuolar membrane also contains CP and NO3~ channels that transport these anions from the cytosol into the vacuole. Entry of these anions against their concentration gradients is driven by the inside-positive potential generated by the H+ pumps. The combined operation of these proton pumps and anion channels produces an inside-positive electric potential of about 20 mV across the vacuolar membrane and also a substantial pH gradient (Figure 7-23).

The proton electrochemical gradient across the plant vacuole membrane is used in much the same way as the Na+ electrochemical gradient across the animal-cell plasma membrane: to power the selective uptake or extrusion of ions and small molecules by various antiporters. In the leaf, for example, excess sucrose generated during photosynthesis in the day is stored in the vacuole; during the night the stored sucrose moves into the cytoplasm and is metabolized to CO2 and H2O with concomitant generation of ATP from ADP and P(. A proton/sucrose antiporter in the vacuolar membrane operates to accumulate sucrose in plant vacuoles. The inward movement of sucrose is powered by the outward movement of H+, which is favored by its concentration gradient (lumen > cytosol) and by the cytosolic-negative potential across the vacuolar membrane (see Figure 7-23). Uptake of Ca2+ and Na+ into the vacuole from the cytosol against their concentration gradients is similarly mediated by proton antiporters. I

Understanding of the transport proteins in plant vacuolar membranes has the potential for increasing agricultural production in high-salt (NaCl) soils, which are found throughout the world. Because most agriculturally useful crops cannot grow in such saline soils, agricultural scientists have long sought to develop salttolerant plants by traditional breeding methods. With the availability of the cloned gene encoding the vacuolar Na+/H+ antiporter, researchers can now produce transgenic plants that overexpress this transport protein, leading to in-

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