Chapter 11 Transport Processes: Connecting the Reactions of C4 Photosynthesis

Abstract

The C4 cycle requires immense metabolite fluxes. The spatial separation of initial carbon fixation by phosphoenolpyruvate carboxylase and entry in the photosynthetic carbon reduction cycle through Rubisco requires metabolites to shuttle not only between cells but also across intracellular membranes. C4 photosynthesis is a highly compartmentalized process. Atmospheric CO2 is fixed into C4 acids (photosynthetic carbon assimilation, PCA) in one tissue and C4 acids donate CO2 to Rubisco in another (photosynthetic carbon reduction, PCR). PCA occurs in chloroplasts and cytosol; PCR occurs in ­chloroplasts, and depending on the subtype of C4 photosynthesis may involve the mitochondria and cytosol. Intercellular transport likely occurs symplastically but the intracellular transport processes across the organellar membranes are at least in part mediated by specific transport proteins. These transport processes are of particular interest because metabolites have to be transported at the rate of carbon assimilation; each carbon which is shuttled as a C4 acid necessitates distinct transport processes as does the C3 acid which returns to recycle the initial carbon acceptor. Currently, it is not fully understood how the organellar membranes accommodate the high volume and velocity of the necessary flux. This chapter will review the different types of C4 cycle reactions and the transport processes required for each sub-type based on the localization of the enzymes involved in the C4 cycle. For each transport process the current knowledge about the transport proteins involved is stated in detail, including discussion of candidate transport proteins characterized in C3 systems. Finally, novel strategies for identifying and characterizing molecular candidates for transport proteins and their importance for engineering a C4 cycle in C3 crop plants are described.