The metabolite transporters of C4 photosynthesis

C4 photosynthesis is a highly efficient form of photosynthesis that utilizes a biochemical pump to concentrate CO2 around rubisco. Although variation in the implementation of this biochemical pump exists between species, each variant of the C4 pathway is critically dependent on metabolite transport between organelles and between cells. Here we review our understanding of metabolite transport in C4 photosynthesis. We discuss how the majority of our knowledge of the metabolite transporters co-opted for use in C4 photosynthesis has been obtained from studying C3 plants and how there is a pressing need for in planta validation of transporter function in C4 species. We further explore the diversity of transport pathways present in disparate C4 lineages and highlight the important gaps in our understanding of metabolite transport in C4 plants. Finally, through integration of functional and transcriptional data from multiple C3 and C4 plants, we propose a molecular blueprint for metabolite transport for NAD-malic enzyme C4 photosynthesis.