INVOLVEMENT OF NA+ IN ACTIVE UPTAKE OF PYRUVATE IN MESOPHYLL CHLOROPLASTS OF SOME C4 PLANTS - NA+/PYRUVATE COTRANSPORT

An artificial Na+ gradient across the envelope (Na+ jump) enhanced pyruvate uptake in the dark into mesophyll chloroplasts of a C4 plant, Panicum miliaceum (NAO-malic enzyme type) (J Ohnishi, R Kanai [1987] FEBS Lett 219:347). In the present study, 22Na+ and pyruvate uptake were examined in mesophyll chloroplasts of several species of C4 plants. Enhancement of pyruvate uptake by a Na+ jump in the dark was also seen in mesophyll chloroplasts of Urochloa panicoides and Panicum maximum (phosphoeno/pyruvate carboxykinase types) but not in Zea mays or Sorghum bicolor (NADP-malic enzyme types). In mesophyll chloroplasts of P. miliaceum and P. maximum, pyruvate in turn enhanced Na+ uptake in the dark when added together with Na+. When flux of endogenous Na+ was measured in these mesophyll chloroplasts preincubated with 22Na+, pyruvate addition induced Na+ influx, and the extent of the pyruvate-induced Na+ influx positively correlated with that of pyruvate uptake. A Na+/H+ exchange ionophore, monensin, nullified all the above mutual effects of Na+ and pyruvate in mesophyll chloroplasts of P. miliaceum, while it accelerated Na+ uptake and increased equilibrium level of chloroplast 22Na+. Measurements of initial uptake rates of pyruvate and Na+ gave a stoichiometry close to 1:1. These results point to Na+/pyruvate cotransport into mesophyll chloroplasts of some C4 plants.