Differential use of two cyclic electron flows around photosystem I for driving CO2-concentration mechanism in C4 photosynthesis

Whereas linear electron flow (LEF) in photosynthesis produces both ATP and NADPH, the cyclic electron flow (CEF) around photosystem I has been shown to produce only ATP. Two alternative routes have been shown for CEF; NAD(P)H dehydrogenase (NDH) and ferredoxin:plastoquinone oxicloreductase (FQR)-dependent flows, but their physiological relevance has not been elucidated in detail. Meanwhile, because C-4 photosynthesis requires more ATP than does C-3 photosynthesis to concentrate CO2, it has not been clear how the extra ATP is produced. In this study, to elucidate whether CEF contributes to the additional ATP needed in C-4 photosynthesis, we estimated the amounts of PGR5, which participates in FQR-dependent flow, and NDH-H, a subunit of NDH, in four C-4 species. Although the expression profiles of PGR5 did not correlate well with the additional ATP requirement, NDH was greatly expressed in mesophyll cells in the NAD-malic enzyme (ME) species, and in bundle-sheath cells in NADP-ME species, where there is a strong need for ATP in the respective cells. Our results indicate that CEF via NDH plays a central role in driving the CO2-concentrating mechanism in C-4 photosynthesis.