Metabolite pools and carbon flow during C4 photosynthesis in maize: 13CO2 labeling kinetics and cell type fractionation.
Worldwide efforts to engineer C4photosynthesis into C3crops require a deep understanding of how this complex pathway operates. CO2is incorporated into four-carbon metabolites in the mesophyll, which move to the bundle sheath where they are decarboxylated to concentrate CO2around RuBisCO. We performed dynamic13CO2labeling in maize to analyze C flow in C4photosynthesis. The overall labeling kinetics reflected the topology of C4photosynthesis. Analyses of cell-specific labeling patterns after fractionation to enrich bundle sheath and mesophyll cells revealed concentration gradients to drive intercellular diffusion of malate, but not pyruvate, in the major CO2-concentrating shuttle. They also revealed intercellular concentration gradients of aspartate, alanine, and phosphenolpyruvate to drive a second phosphoenolpyruvate carboxykinase (PEPCK)-type shuttle, which carries 10-14% of the carbon into the bundle sheath. Gradients also exist to drive intercellular exchange of 3-phosphoglycerate and triose-phosphate. There is rapid carbon exchange between the Calvin-Benson cycle and the CO2-concentrating shuttle, equivalent to ~10% of carbon gain. In contrast, very little C leaks from the large pools of metabolites in the C concentration shuttle into respiratory metabolism. We postulate that the presence of multiple shuttles, alongside carbon transfer between them and the Calvin-Benson cycle, confers great flexibility in C4photosynthesis.
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany arrivault@mpimp-golm.mpg.de.
Stéphanie
S
Arrivault
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Toshihiro
T
Obata
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Marek
M
Szecówka
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Virginie
V
Mengin
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Manuela
M
Guenther
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Melanie
M
Hoehne
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Alisdair R
AR
Fernie
Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, D-14476 Potsdam-Golm, Germany.
Mark
M
Stitt
D017360:Arabidopsis / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000378:metabolism; D002469:Cell Separation; D007700:Kinetics; D058503:Mesophyll Cells / Q000378:metabolism; D010100:Oxygen; D010788:Photosynthesis; D018515:Plant Leaves / Q000166:cytology / Q000378:metabolism*; D003313:Zea mays / Q000166:cytology / Q000378:metabolism*
D002247:Carbon Isotopes; D002245:Carbon Dioxide; D010100:Oxygen

13C labeling; C4 photosynthesis; CO2-concentrating shuttle; carbon flow; maize.