Abstract
Cyclic electron transport around photosystem I has been proposed to supply additional ATP required for C4 photosynthesis. Two cyclic electron transport pathways have been described, the antimycin A-sensitive pathway involving PGR5 and the NDH pathway. To investigate which pathway of cyclic electron transport is promoted in evolution of C4 photosynthesis, we analyzed the expression of two proteins, PGR5 and NDH-H, a subunit of the NDH complex, in different Flaveria species performing NADP-malic enzyme-type C4 photosynthesis, C3-C4 intermediate photosynthesis, or C3 photosynthesis. Both PGR5 and NDH-H were more expressed in C4 species than in C3 species and C3-C4 intermediate species. Immunohistochemical analysis revealed that, PGR5 and NDH-H mainly accumulated in bundle sheath of C4 Flaveria chloroplasts. Based on P700 oxidation kinetics, we conclude to the existence of a higher activity of cyclic electron transport in C4 Flaveria than in C3 Flaveria and propose that the higher expression of both PGR5 and NDH complex in agranal bundle sheath chloroplasts is responsible for the increased activity of cyclic electron transport in C4 Flaveria.
