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 oxidoreductase (FQR)-dependent flows, but their physiological relevance has not been elucidated in detail. Meanwhile, because C 4 photosynthesis requires more ATP than does C3 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 C4 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 C4 photosynthesis.ATP synthesis ͉ ferredoxin:plastoquinone oxidoreductase ͉ NAD(P)H dehydrogenase ͉ PGR5 ͉ plant D uring plant photosynthesis, both ATP and NADPH are produced by the linear electron flow (LEF) using both photosystem (PS)I and PSII. Alternatively, ATP can also be produced by cyclic electron flow (CEF) without production of NADPH. Because the ratio of ATP͞NADPH required for CO 2 fixation changes, depending on activities of photorespiration or nitrate assimilation to glutamate, balancing between CEF and LEF is important to perform C 3 photosynthesis effectively (1-4).There are two known pathways of CEF around PSI; chloroplastic NAD(P)H dehydrogenase (NDH)-dependent flow and ferredoxin:plastoquinone oxidoreductase (FQR)-dependent flow. NDH complex was identified on the basis of high homology with respiratory complex I in bacteria and mitochondria, when plastid genomes of tobacco (5) and liverwort (6) were completely sequenced. To elucidate the function of NDH complex, several laboratories inactivated the tobacco ndh genes by the plastid transformation technique. Although NDH-inactivated tobacco plants showed normal growth under standard conditions (7-10), environmental stress, such as heat, high light, or decreased air humidity, causes retarded growth, suggesting that the absence of the ATP produced by NDH-dependent CEF makes it more difficult to adapt to changing environmental conditions (9,11,12).On the contrary, the molecular identity of FQR had not been revealed. However, a mutant analysis of Arabidopsis identified the protein PGR5, which is essential for FQR-mediated CEF, although PGR5 itself has no known redox motif (13).Involvement of PGR5 in FQR-mediated CEF is supported by the report that ssr2016 in Synechocystis sp. PCC 6803, homologue of Arabidopsis pgr5, is also required for FQR activity (14). Double mutants defective in both NDH-and FQR-depende...

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An Arabidopsis mutant rnr1 , which has a defect in the basic genetic system in chloroplasts, was isolated using the screening of the high chlorophyll fluorescence phenotype. Whereas chlorophyll fluorescence and immunoblot studies showed the mutant had reduced activities of photosystems I and II, molecular characterization of the mutant suggested that a T-DNA insertion impaired the expression of a gene encoding a RNase R family member with a targeting signal to chloroplasts. Since RNase R family members have a 3'-5' exoribonuclease activity, we examined the RNA profile in chloroplasts. In rnr1 the intercistronic cleavage between 23S and 4.5S rRNA was impaired, and a significant reduction in rRNA in chloroplasts was found, suggesting that RNR1 functions in the maturation of chloroplast rRNA. The present results suggest that defects in the genetic system in chloroplasts cause high chlorophyll fluorescence, pale green leaf, and marked reduction in the growth rate, whereas the levels of some chloroplast RNA were higher in rnr1 than in the wild-type.