Phosphate Translocator of Isolated Guard-Cell Chloroplasts from Pisum sativum L. Transports Glucose-6-Phosphate

Chloroplasts were isolated from ruptured guard-cell protoplasts of the Argenteum mutant of Pisum sativum L. and purified by centrifugation through a Percoll layer. The combined volume of the intact plastids and the uptake of phosphate were determined by silicone oil-filtering centrifugation, using tritiated water and [14C]sorbitol as membrane-permeating and nonpermeating markers and [32P]phosphate as tracer for phosphate. The affinities of the phosphate translocator for organic phosphates were assessed by competition with inorganic phosphate. The affinities for dihydroxyacetone phosphate, 3-phosphoglycerate (PGA), and phosphoenolpyruvate were in the same order as those reported for mesophyll chloroplasts of several species. However, the guard-cell phosphate translocator had an affinity for glucose-6-phosphate that was as high as that for PGA. Guard-cell chloroplasts share this property with amyloplasts from the root of pea (H.W. Heldt, U.I. Flugge, S. Borchert [1991] Plant Physiol 95: 341-343). An ability to import glucose-6-phosphate enables guard-cell chloroplasts to synthesize starch despite the reported absence of a fructose-1,6-bisphosphatase activity in the plastids, which would be required if only C3 phosphates could enter through the translocator.