Reappraisal of the Role of Sodium in the Light-Dependent Active Transport of Pyruvate into Mesophyll Chloroplasts of C4 Plants

The mechanism of light-dependent active transport of pyruvate in C4 mesophyll chloroplasts has not been clarified, particularly in Na+-type C4 species, in which the pyruvate uptake into mesophyll chloroplasts is enhanced by illumination or by making a Na+ gradient (Na+-jump) across the envelope in the dark. We re-investigated here the effect of Na+ on the active transport of pyruvate in mesophyll chloroplasts of Panicum miliaceum, a Na+-type C4 species, by comparing the rate of pyruvate uptake at various external pHs under four conditions; in the light and dark together with/without Na+-jump: (1) At neutral pH, the rate of pyruvate uptake in the dark was enhanced by Na+-jump but scarcely by illumination. (2) While the enhancement effect by Na+-jump was independent of external pH, that by illumination increased greatly at pH over 7.4, and the effects of light and Na+ at the alkaline pH were synergistic. (3) The light-enhanced pyruvate uptake was related to stromal alkalization induced by illumination. In fact, pyruvate uptake was induced by H+-jump in the medium from pH 8.0 to 6.7. (4) Stromal pH was lowered by the addition of K+-pyruvate and more by Na+-pyruvate into the medium at pH 7.8 in the light. (5) However, the pH and ATP levels in the stroma were not affected by Na+-jump. Thus, we discussed possibility that besides pyruvate/Na+ cotransport at neutral pH in the medium, pyruvate/H+ cotransport enhanced by the presence of Na+ operates in mesophyll chloroplasts of Na+-type C4 species at alkaline medium.