Exploring the diversity of the CO 2 -concentrating mechanism (CCM) in different C 4 subtypes
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C 4 plants have traditionally been classified into NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME) and PEP carboxykinase (PEPCK) subtypes based on the predominant C 4 -acid decarboxylating enzyme. To investigate the relative contributions of malate and aspartate to C 4 -pathway fluxes in each subtype, we performed 13 CO 2 pulse and pulse-chase labelling experiments on four C 4 grass species: Zea mays and Setaria viridis (NADP-ME), Panicum miliaceum (NAD-ME) and Megathyrsus maximus (PEPCK). Only a proportion (8-50%) of the total malate pool in the leaves is photosynthetically active whereas essentially all of the aspartate pool is photosynthetically active. Estimates of metabolic fluxes indicate that approximately two thirds of the C 4 pathway flux is via malate in Z. mays and the remaining third via aspartate, while in S. viridis 50% of the flux is via malate and 50% via aspartate. In P. miliaceum and M. maximus , 91% and 85% of the flux is via aspartate and the remaining 5% and 15% via malate, respectively. The results reveal greater complexity of C 4 pathway fluxes than is usually represented in textbook diagrams, and demonstrate the feasibility of using non-radioactive 13 CO 2 in pulse-chase labelling experiments to study C 4 photosynthesis and to detect C 4 pathway fluxes in C 3 plants engineered to perform C 4 photosynthesis.
Highlight Statement
Photosynthetic fluxes in C 4 species are more complex than most textbook models show, with malate and aspartate both carrying C 4 cycle fluxes in all three subtypes (NADP-ME, NAD-ME and PEPCK).
