Intercellular Concentration Gradients of 3-Phosphoglycerate and Triose-Phosphate Demonstrate Operation of an Energy Shuttle in NAD-Malic Enzyme and Phospho enol ypyruvate Carboxykinase C ₄ Subtypes

In C ₄ photosynthesis, incoming CO ₂ is incorporated in mesophyll cells (MC) into 4-carbon acids that diffuse to bundle sheath cells (BSC) and decarboxylated to generate a high CO ₂ concentration that suppresses the oxygenation reaction of Rubisco. Decarboxylation can occur by NADP-malic enzyme, (NADP-ME), NAD-malic enzyme (NAD-ME) or phospho enol pyruvate carboxykinase (PEPCK). NADP-ME generates NADPH in the BSC chloroplast and species that use it as the major route for decarboxylation typically have dimorphic BSC chloroplasts with little or no photosystem II. They operate an energy shuttle: much of the 3-phosphoglycerate formed in the Calvin-Benson cycle diffuses to the MC, enters the chloroplasts and is reduced to triose phosphates that return to the BSC. In species where carboxylation occurs mainly via NAD-ME or PEPCK, BSC chloroplasts possess photosystem II. Indirect evidence indicates they nevertheless have the capacity to operate an energy shuttle. We show here that NAD-ME and PEPCK species possess large pools of 3PGA and triose phosphates and, for two examples of each subtype, opposed concentration gradients of 3-phosphoglycerate and triose phosphates to drive rapid exchange between the BSC and MC. Reasons for and consequences of the widespread operation of the intercellular energy shuttle in C ₄ plants are discussed.