The C4-pathway of photosynthesis. Evidence for an intermediate pool of carbon dioxide and the identity of the donor C4-dicarboxylic acid

Abstract

1. Leaves were exposed to (14)CO(2) under steady-state conditions for photosynthesis. The kinetics of entry or loss of label in pools of CO(2) and other compounds was examined during the period of the pulse and a ;chase' with (12)CO(2). 2. With maize the kinetics of labelling of the major CO(2) pool and of depletion of label during a ;chase' was consistent with this pool being derived from the C-4 of malate and being the precursor of the C-1 of 3-phosphoglycerate. 3. Similar results were obtained for Amaranthus leaves except that the C-4 of aspartate rather than malate was apparently the primary source of CO(2). 4. The size and turnover time of the CO(2) and C(4) acid pools was calculated. These results provided the basis for estimating the concentration of CO(2) in the bundle-sheath cells or chloroplasts assuming the pool was largely restricted to one or other of these compartments. 5. These findings are considered in relation to current schemes for the C(4)-pathway and the operation of a CO(2) concentrating mechanism to serve ribulose diphosphate carboxylase.

Isocitrate dehydrogenase and related oxidative decarboxylases

“…is the substrate [69]. This might be considered more consistent with the role of the enzyme in furnishing carbon dioxide from the Cq pathway to ribulose diphosphate carboxylase in the bundle sheath chloroplasts [70], since it has been shown by the kinetic method that CO? is the substrate of this enzyme [71]; moreover carbonic anhydrase activity in C4 plants is relatively low and located in the cytoplasm [72].…”

Phosphoenolpyruvate Carboxykinase Is Involved in the Decarboxylation of Aspartate in the Bundle Sheath of Maize1

“…In maize, substantial amounts of Asp are labeled (Hatch, 1971;Créach et al, 1974;MorotGaudry and Farineau, 1978;Chapman and Hatch, 1981), more so in N-sufficient than in N-deficient maize (Khamis et al, 1992), but measurements of labeling and/or pools do not give any indication of relative fluxes. The relative capacities for Asp and malate decarboxylation would have major energetic consequences in the BS.…”

“…In NADP-ME species such as maize, 14 CO 2 is initially incorporated into the C-4 position of malate and Asp (about 75% and 25%, respectively), and the C-4 of both is subsequently incorporated into other metabolites (Hatch, 1971;Morot-Gaudry and Farineau, 1978). Using isolated BS strands, Chapman and Hatch (1981) showed that BS cells of maize have a significant capacity to decarboxylate Asp, and they assumed that NADP-ME was responsible.…”

“…Other NADP-ME species such as Echinochloa colona, Echinochloa crus-galli, Digitaria sanguinalis, and Paspalum notatum also contain PEPCK, whereas PEPCK protein was not detectable in Sorghum bicolor, Saccharum officinarum, or Flaveria bidentis (Walker et al, 1997). The reason that the occurrence of PEPCK in these plants has been overlooked may be due to difficulties in measuring PEPCK activity (Walker et al, 1997) or to the lack of an antibody.PEPCK-type C 4 species mainly use Asp as the CO 2 donor and decarboxylate the oxaloacetate formed via the following reaction:In NADP-ME species such as maize, 14 CO 2 is initially incorporated into the C-4 position of malate and Asp (about 75% and 25%, respectively), and the C-4 of both is subsequently incorporated into other metabolites (Hatch, 1971; Morot-Gaudry and Farineau, 1978). Using isolated BS strands, Chapman and Hatch (1981) showed that BS cells of maize have a significant capacity to decarboxylate Asp, and they assumed that NADP-ME was responsible.…”

Systems Analysis of a Maize Leaf Developmental Gradient Redefines the Current C4 Model and Provides Candidates for Regulation

“…Older maize leaves, at least, harbor a second decarboxylation enzyme, PEP carboxykinase (PEP-CK), which releases carbon dioxide from OAA, producing PEP (Wingler et al, 1999). Furthermore, approximately one-quarter of radioactively labeled carbon dioxide that was fed to maize leaves was found to be rapidly incorporated into Asp (Hatch, 1971). Such side routes to the canonical NADP-ME C 4 pathway would require alternative transfer metabolites between mesophyll and bundle sheath cells, such as Asp or Ala, and alternative decarboxylation pathways would alter the demands on the remaining enzymes and the intracellular (Brä utigam and Weber, 2011a) and intercellular (Sowínski et al, 2008) transport systems.…”