Review: The complexities of metabolite transport in C4 photosynthesis
C4 photosynthesis, an adaptive mechanism to spatially concentrate CO2 around Rubisco to enhance carbon fixation, has evolved independently at least 60 times in plants. This process spatially separates the initial carbon fixation by PEPC and carbon reduction by Rubisco, which requires that compounds move in and out of various organelles of mesophyll cells and bundle sheath cells. While scientists have made great strides in bridging the gap in our understanding of photosynthesis evolution, there are still questions that need to be answered in terms of metabolite transport. This comprehensive review by Mattison and Kelly presents our current understanding of these important transporters, highlighting that much of what is known about them is derived from studies in C3 plants. They point to several critical unknowns; for example, the transporter facilitating H+-dependent pyruvate uptake in several species including maize is not yet known. Similarly, the transporters that move malate and pyruvate in the bundle sheath chloroplast of NADP-ME subtype C4 species are also unknown. It is also hypothesized that an unidentified aspartate transporter is involved in malate transport to the chloroplast. There is a great need for more experimental validation of metabolite transporters in C4 plants. With continuous advances in molecular, synthetic, genomic biology and genetic engineering, researchers are working to elucidate the complete metabolite transport mechanisms in C4 crops, which is necessary in order to improve photosynthesis and water-use efficiency in C3 crops. (Summary by Mae Mercado) The Plant Cell 10.1093/plcell/koaf019
