PROTON PYRUVATE COTRANSPORT INTO MESOPHYLL CHLOROPLASTS OF C-4 PLANTS/

Light-enhanced active pyruvate uptake into mesophyll chloroplasts of C -4 plants was reported to be mimicked by either of the two types of ca tion jump: H+-jump in maize and phylogenically related species (H+-typ e) and Na+-jump in all the other C-4 species tested (Na+-type) [Aoki, N., Ohnishi, J. and Kanai, R. (1992) Plant Cell Physiol. 33:805]. In t his study, medium and stromal pH was monitored in the suspension of C- 4 mesophyll chloroplasts. Medium alkalization lasting for 5 to 10 seco nds after pyruvate addition was detected by a pH electrode and observe d only in the light and only in mesophyll chloroplasts from H+-type sp ecies, Zea mays L. and Coix lacryma-jobi L., but not in those from Na-type species Panicum miliaceum L., Setaria italica (L.) Beauv. and Pa nicum maximum Jacq. The initial rate of H+ consumption showed good cor relation with [C-14]pyruvate uptake measured by silicone oil filtering centrifugation, both being inhibited by N-ethylmaleimide and 7-chloro -4-nitrobenzo-2-oxa-1,3-diazole to the same degree. The ratio of the r ate of H+ uptake to that of pyruvate uptake was always about 1. Pyruva te-induced acidification of the stroma was observed in maize mesophyll chloroplasts. These results show one to one cotransport of H+ and pyr uvate anion into mesophyll chloroplasts of H+-type C-4 species in the light.