Improving Light Use Efficiency in C₄ Plants by Increasing Electron Transport Rate ^†

Abstract

C₄ plants play a key role in world agriculture and strategies to manipulate and enhance C₄ photosynthesis have the potential for major agricultural impacts. The C₄ photosynthetic pathway is a biochemical CO₂ concentrating mechanism that requires the coordinated functioning of mesophyll and bundle sheath cells of leaves. Chloroplast electron transport in C₄ plants is shared between the two cell types; it provides resources for CO₂ fixation therefore underpinning the efficiency of photosynthesis. Using the model monocot C₄ species Setaria viridis (green foxtail millet) we demonstrated that the Cytochrome (Cyt) b₆f complex regulates the electron transport capacity and thus the rate of CO₂ assimilation at high light and saturating CO₂. Overexpression of the Cyt b₆f in both mesophyll and bundle sheath cells results in a higher electron throughput and allows better light conversion efficiency in both photosystems. Importantly, increased Cyt b₆f abundance in leaves provides higher rates of C₄ photosynthesis without marked changes in Rubisco or chlorophyll content. Our results demonstrate that increasing the rate of electron transport is a viable strategy for improving the light conversion efficiency in C₄ crop species like maize and sorghum.