Elevated Expression of PGR 5 and NDH-H in Bundle Sheath Chloroplasts in C 4 Flaveria Species

C 4 photosynthesis requires the coordinated functioning of two cell types, namely mesophyll and bundle sheath cells, with individual functions (Hatch 1987 , Sage 1999). Although C 4 pho-tosynthesis suppress photorespiration by a CO 2-concentrating mechanism, it increases the energetic cost of CO 2 assimilation in comparison with C 3 photosynthesis. Consequently, two extra ATP molecules are required for each CO 2 molecule fi xed to drive C 4 photosynthesis (Kanai and Edwards 1999). The extra ATP needed for C 4 photosynthesis was suggested to be produced by PSI cyclic electron transport activity, which contributes to generation of ∆ pH across the thylakoid membrane (Kanai and Edwards 1999). Increased PSI cyclic electron transport compared with C 3 plants has been reported in a number of C 4 plants, including Sorghum bicolor and Zea mays (Herbert et al. 1990 , Asada et al. 1993). Two cyclic pathways around PSI have been identifi ed in C 3 plants; the fi rst pathway involves a plastidial NDH [NAD(P)H dehydrogenase] complex that is able to reduce plastoquinones from stromal NAD(P)H donors (Horvath et al. 2000) and the second pathway is an antimycin A-sensitive pathway involving PROTON GRADIENT REGULATION 5 (PGR5), which is localized in the chloroplast and considered to be a factor for major cyclic electron transport activity in C 3 plants (Munekage et al. 2002 , Munekage et al. 2004 , Munekage et al. 2008). The contribution of NDH-dependent electron transport to C 4 photosyn-thesis was suggested by its expression profi le correlating with predicted ATP requirement in different cell types (Kubicki et al. 1996 , Takabayashi et al. 2005). The contribution of PGR5-related electron transport to C 4 photosynthesis is unclear. However, Ivanov et al. (2007) reported that the oxidation level of P700 in bundle sheath strands isolated from Z. mays was dependent on antimycin A. With the aim of investigating the nature of cyclic electron pathways involved in C 4 photosynthesis, we analyzed expression of PGR5 and NDH-H, a subunit of the NDH complex, in the dicot genus Flaveria (Asteraceae), which contains closely related C 3 , C 3 –C 4 intermediate and NADP-malic enzyme (ME)-type C 4 species and is a widely used model system for studying the C 4 photosynthesis evolutionary process (Sage 2004). The relative abundances of PGR5 and NDH-H were analyzed by immunoblotting in a C 3-type species, F. pringlei , a C 3 –C 4 intermediate …