Phosphate Transport across the Plasma Membrane of Wheat Leaf Protoplasts: Characteristics and Inhibitor Specificities

The kinetics and inhibitor specificities of phosphate transport across the plasma membrane of wheat leaf mesophyll protoplasts have been examined. Studies were also carried out on the effects of light and pH on phosphate transport and the plasma membrane electropotential. At pH 5.8 (30°C), protoplasts accumulated phosphate at the rate of 3.9 ± 0.2 nanomoles per milligram protein per hour. Phosphate uptake rates and inhibitor specificities for the leaf cell plasma membrane phosphate transporter were qualitatively similar to those observed with root protoplasts. Neither picrylsulfonic acid, or p-chloromercuribenzene sulfonate affected phosphate uptake significantly at 0.1 millimolar. Of all compounds tested, carbonyl cyanide-p-trifluoromethoxy phenylhydrazone was the most effective inhibitor of phosphate uptake (60% at 0.1 millimolar). Tribenzylphosphate inhibited uptake by 34% while dibenzylphosphate had no effect. The plasma membrane electropotential was found to be −37 ± 3 millivolts. Initiation of photosynthesis lowered the membrane potential to −39 ± 3 millivolts. Inhibition of phosphate uptake by 34% with the substrate analog tribenzylphosphate resulted in a measured membrane potential of −33 ± 3 millivolts. These changes in potential were not significant at the 5% probability level. Phosphate uptake rates remained constant under photosynthetic and nonphotosynthetic conditions. The utility of tribenzylphosphate as an inhibitor in plant systems is demonstrated.