Poster: Photosynthesis

Abs # 322: Functional Analysis of Chloroplastic Dicarboxylate Transporters in Maize

Presenter:	Taniguchi, Yojiro , yo-jiro@mbox.media.nagoya-u.ac.jp
Authors	Taniguchi, Yojiro  (A)   Taniguchi, Mitsutaka  (A)   Nagasaki, Junko  (A)   Kawasaki, Michio  (A)   Miyake, Hiroshi  (A)   Sugiyama, Tatsuo  (B)
Affiliations:	(A): Grad. Sch. Bioagric. Sci., Nagoya Univ. (B): Plant Science Center, RIKEN

In NADP-malic enzyme-type C₄ plants such as maize, the carboxylation product, oxaloacetate, is transported to mesophyll cell (MC) chloroplasts, where it is reduced to malate. The produced malate is transported to bundle sheath cell (BSC) chloroplasts. Some transporters which facilitate transport of oxaloacetate and malate across the inner chloroplastic envelope membranes were biochemically characterized but their genes have not identified yet. We isolated three maize cDNAs, ZmpOMT1, ZmpDCT1 and ZmpDCT2, encoding candidates for these transporters. Their transcripts were upregulated by light in a cell-specific manner; ZmpOMT1 and ZmpDCT1 were expressed in MC and ZmpDCT2 was expressed in BSC, respectively. The yeast-expressed recombinant ZmpOMT1 protein transported malate and 2-oxoglutarate but not glutamate. By contract, the recombinant ZmpDCT1 and 2 proteins transported 2-oxoglutamate and glutamate at similar affinities in exchange for malate. The recombinant proteins could also transport oxaloacetate at the same binding sites for dicarboxylates. In particular, the ZmpOMT1 transported oxaloacetate at higher efficiency than those for malate and 2-oxoglutarate. We also compared the activities of oxaloacetate transport between MC and BSC chloroplasts. The Km value for oxaloacetate in MC chloroplasts was one order of magnitude lower than that in BSC chloroplasts, and was close to that determined with the recombinant ZmpOMT1 protein. Southern analysis revealed that maize has one OMT gene. These findings suggest that ZmpOMT1 may participate in uptaking oxaloacetate as the C₄ photosynthetic intermediate into MC chloroplasts. In BSC chloroplasts, ZmpDCT2 was expected to function in malate uptake.