Intercellular Concentration Gradients of 3-Phosphoglycerate and Triose-Phosphate Demonstrate Operation of an Energy Shuttle in NAD-Malic Enzyme and Phosphoenolypyruvate Carboxykinase C4 Subtypes.

In C4 photosynthesis, incoming CO2 is incorporated in mesophyll cells (MC) into 4-carbon acids that diffuse to bundle sheath cells (BSC) and are decarboxylated to generate a high CO2 concentration that suppresses the oxygenation reaction of Rubisco. Decarboxylation can occur by NADP-malic enzyme, (NADP-ME), NAD-malic enzyme (NAD-ME) or phosphoenolpyruvate 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 by Rubisco diffuses to the MC, enters the chloroplasts and is reduced to triose phosphates that return to the BSC. In species where decarboxylation 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 several NAD-ME and PEPCK species possess large pools of 3PGA and triose phosphates and, for two examples of each subtype, large opposed concentration gradients of 3-phosphoglycerate and triose phosphates to drive rapid exchange between the BSC and MC. Possible reasons for and consequences of the operation of the intercellular energy shuttle in NAD-ME and PEPCK species are discussed.