Reactive oxygen species and redox regulation in mesophyll and bundle sheath cells of C-4 plants

Redox regulation, antioxidant defence, and reactive oxygen species (ROS) signalling are critical in performing and tuning metabolic activities. However, our concepts have mostly been developed for C-3 plants since Arabidopsis thaliana has been the major model for research. Efforts to convert C-3 plants to C-4 to increase yield (such as IRRI's C-4 Rice Project) entail a better understanding of these processes in C-4 plants. Various photosynthetic enzymes that take part in light reactions and carbon reactions are regulated via redox components, such as thioredoxins as redox transmitters and peroxiredoxins. Hence, understanding redox regulation in the mesophyll and bundle sheath chloroplasts of C-4 plants is of paramount importance: it appears impossible to utilize efficient C-4 photosynthesis without understanding its exact redox needs and the regulation mechanisms used during light reactions. In this review, we discuss current knowledge on redox regulation in C-3 and C-4 plants, with special emphasis on the mesophyll and bundle sheath differences that are found in C-4. In these two cell types in C-4 plants, linear and cyclic electron transport in the chloroplasts operate differentially when compared to C-3 chloroplasts, changing the redox needs of the cell. Therefore, our focus is on photosynthetic light reactions, ROS production dynamics, antioxidant defence, and thiol-based redox regulation, with the aim of providing an overview of our current knowledge.