C 4 acid decarboxylation and CO 2 donation to photosynthesis in bundle sheath strands and chloroplasts from species representing three groups of C 4 plants

Abstract

The mechanism of decarboxylation of C 4 acids and carboxyl donation to the Calvin pathway was studied in bundle sheath strands and chloroplasts isolated from leaves of species representing three groups of C 4 plants: Digitaria sanguinalis (NADP-malic enzyme type), Panicum miliaceum (NAD-malic enzyme type), and Eriochloa borumensis (phosphoenolpyruvate-carboxykinase type). High rates of 14CO 2 fixation and HCO 3− dependent O 2 evolution were exhibited by bundle sheath strands and chloroplasts of P. miliaceum and E. borumensis, while those of D. sanguinalis (the NADP-malic enzyme type) required the addition of ribose 5-phosphate and malate for maximum activity. In all three species, 3-(3-4-dichlorophenyl)-1,1-dimethylurea inhibition of 14CO 2 fixation was partially overcome by the addition of malate, suggesting that malate can serve as a source of reductive power for carbon assimilation. Decarboxylation of 4- 14C-labeled C 4 acids was followed in the presence of glyceraldehyde, an inhibitor of the Calvin pathway. A large light-dependent stimulation of malate decarboxylation occurred by the addition of 3-phosphoglycerate (6 m m) with bundle sheath cells of all three groups and with bundle sheath chloroplasts of D. sanguinalis and E. borumensis. Although malate is suggested to be decarboxylated through the respective primary decarboxylase of each species, the 3-phosphoglycerate enhancement suggests that pyridine nucleotide reduction from malate oxidation is used during the reductive phase of the Calvin pathway. Bundle sheath strands from all three species decarboxylated aspartate with severalfold stimulation by the addition of either α-ketoglutarate (10 m m) or pyruvate (1 m m), with the lowest rate in D. sanguinalis. Bundle sheath chloroplasts from either of the three species could not decarboxylate aspartate. A light-independent decarboxylation of aspartate is suggested through NAD-malic enzyme in bundle sheath mitochondria in all three types of C 4 species, the rates being highest in P. miliaceum. A light-dependent portion of aspartate decarboxylation in E. borumensis is suggested to be through phosphoenolpyruvate carboxykinase. C 4. acid decarboxylation in all three species was inhibited by HCO 3− (CO 2 may be the active species). Consistent with the C 4 acid decarboxylation studies, the bundle sheath strands of the three species gave light-dependent O 2 evolution with aspartate or malate, the bundle sheath chloroplasts of D. sanguinalis and E. borumensis exhibited light-dependent O 2 evolution only with malate, and the bundle sheath chloroplasts of P. miliaceum showed no C 4 acid-dependent O 2 evolution. In all three C 4 types, malate decarboxylation, though by different decarboxylases, interacted with the Calvin pathway by providing both CO 2 and reducing power, while aspartate decarboxylation provided only CO 2.