Overproduction of PGR5 enhances the electron sink downstream of photosystem I in a C₄ plant, Flaveria bidentis

C₄ plants can fix CO₂ efficiently using CO₂-concentrating mechanisms (CCMs), but they require additional ATP. To supply the additional ATP, C₄ plants operate at higher rates of cyclic electron transport around photosystem I (PSI), in which electrons are transferred from ferredoxin to plastoquinone. Recently, it has been reported that the NAD(P)H dehydrogenase-like complex (NDH) accumulated in the thylakoid membrane in leaves of C₄ plants, making it a candidate for the additional synthesis of ATP used in the CCM. In addition, C₄ plants have higher levels of PROTON GRADIENT REGULATION 5 (PGR5) expression, but it has been unknown how PGR5 functions in C₄ photosynthesis. In this study, PGR5 was overexpressed in a C₄ dicot, Flaveria bidentis. In PGR5-overproducing (OP) lines, PGR5 levels were 2.3- to 3.0-fold greater compared with wild-type plants. PGR5-like PHOTOSYNTHETIC PHENOTYPE 1 (PGRL1), which cooperates with PGR5, increased with PGR5. A spectroscopic analysis indicated that in the PGR5-OP lines, the acceptor side limitation of PSI was reduced in response to a rapid increase in photon flux density. Although it did not affect CO₂ assimilation, the overproduction of PGR5 contributed to an enhanced electron sink downstream of PSI.