Overexpression of the Rieske FeS protein of the Cytochrome b6f complex increases C4 photosynthesis

Abstract

C4 plants contribute 20% to the global primary productivity despite representing only 4% of higher plant species. Their CO2 concentrating mechanism operating between mesophyll and bundle sheath cells increases CO2 partial pressure at the site of Rubisco and hence photosynthetic efficiency. Electron transport chains in both cell types supply ATP and NADPH for C4 photosynthesis. Since Cytochrome b6f is a key point of control of electron transport in C3 plants, we constitutively overexpressed the Rieske FeS subunit in Setaria viridis to study the effects on C4 photosynthesis. Rieske FeS overexpression resulted in a higher content of Cytochrome b6f in both mesophyll and bundle sheath cells without marked changes in abundances of other photosynthetic complexes and Rubisco. Plants with higher Cytochrome b6f abundance showed better light conversion efficiency in both Photosystems and could generate higher proton-motive force across the thylakoid membrane. Rieske FeS abundance correlated with CO2 assimilation rate and plants with a 10% increase in Rieske FeS content showed a 10% increase in CO2 assimilation rate at ambient and saturating CO2 and high light. Our results demonstrate that Cytochrome b6f controls the rate of electron transport in C4 plants and that removing electron transport limitations can increase the rate of C4 photosynthesis.