C 4 acid decarboxylation and photosynthesis in bundle sheath cells of NAD-malic enzyme-type C 4 plants: Mechanism and the role of malate and orthophosphate

Abstract

The mechanism and possible regulation of C 4 acid decarboxylation in NAD-malic enzyme-type C 4 plants was studied using isolated bundle sheath cells and mitochondria from Panicum miliaceum. Rates of C 4 acid-dependent photosynthetic O 2 evolution equalled those observed with saturating NaHCO 3; the rates ranged from 3 to 5 μmol min −1 (mg chlorophyll) −1. C 4 acid-dependent O 2 evolution required the addition of aspartate and 2-oxoglutarate (as a source of oxaloacetate) and also malate and orthophosphate. C 4 acid decarboxylation by both isolated cells and mitochondria, measured as pyruvate production, also required all four of these components. The scheme previously proposed to account for aspartate decarboxylation in NAD-malic enzyme-type C 4 plants does not envisage a role for externally derived malate. However, the mandatory requirement for malate (with orthophosphate), together with the observation that C 4 acid decarboxylation is blocked by an inhibitor of the mitochondrial dicarboxylate transporter, suggests that a net flux of malate from outside the mitochondria is required to sustain this process. Arsenate was found to substitute for orthophosphate favoring a role for orthophosphate in malate transport rather than a metabolic one. The results are discussed in terms of likely mitochondrial metabolite transport mechanisms and regulation of the C 4 acid decarboxylation process.