Proton/Pyruvate Cotransport into Mesophyll Chloroplasts of C4 Plants

Light-enhanced active pyruvate uptake into mesophyll chloroplastsof C₄ plants was reported to be mimicked by either of the twotypes of cation jump: H⁺-jump in maize and phylogenically relatedspecies (H⁺-type) and Na⁺-jump in all the other C₄ species tested(Na⁺-type) Aoki, N., Ohnishi, J. and Kanai, R. (1992) PlantCell Physiol. 33: 805]. In this study, medium and stromal pH was monitored in the suspensionof C₄ mesophyll chloroplasts. Medium alkalization lasting for5 to 10 seconds after pyruvate addition was detected by a pHelectrode and observed only in the light and only in mesophyllchloroplasts from H⁺-type species, Zea mays L. and Coix lacryma-jobiL., but not in those from Na⁺-type species Panicum miliaceumL., Setaria italica (L.) Beauv. and Panicum maximum Jacq. Theinitial rate of H⁺ consumption showed good correlation with¹⁴C]pyruvate uptake measured by silicone oil filtering centrifugation,both being inhibited by N-ethylmaleimide and 7-chloro-4-nitrobenzo-2-oxa-l,3-diazole to the same degree. The ratio of the rate of H⁺ uptaketo that of pyruvate uptake was always about 1. Pyruvate-inducedacidification of the stroma was observed in maize mesophyllchloroplasts. These results show one to one cotransport of H⁺and pyruvate anion into mesophyll chloroplasts of H⁺-type C₄species in the light. (Received January 5, 1994; Accepted May 6, 1994)