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Metabolism and Enzymology

Photosynthetic Carbon Metabolism in Photoautotrophic Cell Suspension Cultures Grown at Low and High CO₂¹

Department of Plant Biology, University of Illinois at Urbana-Champaign, Illinois 61801, Department of Agronomy, University of Illinois at Urbana-Champaign, Illinois 61801, U.S. Department of Agriculture, Agricultural Research Service, Urbana, Illinois 61801

Photosynthetic carbon metabolism was characterized in four photoautotrophic cell suspension cultures. There was no apparent difference between two soybean (Glycine max) and one cotton (Gossypium hirsutum) cell line which required 5% CO₂ for growth, and a unique cotton cell line that grows at ambient CO₂ (660 microliters per liter). Photosynthetic characteristics in all four lines were more like C₃ mesophyll leaf cells than the cell suspension cultures previously studied. The pattern of ¹⁴C-labeling reflected the high ratio of ribulosebisphosphate carboxylase to phosphoenolpyruvate carboxylase activity and showed that CO₂ fixation occurred primarily by the C₃ pathway. Photorespiration occurred at 330 microliters per liter CO₂, 21% O₂ as indicated by the synthesis of high levels of ¹⁴C-labeled glycine and serine in a pulse-chase experiment and by oxygen inhibition of CO₂ fixation. Short-term CO₂ fixation in the presence and absence of carbonic anhydrase showed CO₂, not HCO₃^–, to be the main source of inorganic carbon taken up by the low CO₂-requiring cotton cells. The cells did not have a CO₂-concentrating mechanism as indicated by silicone oil centrifugation experiments. Carbonic anhydrase was absent in the low CO₂-requiring cotton cells, present in the high CO₂-requiring soybean cell lines, and absent in other high CO₂ cell lines examined. Thus, the presence of carbonic anhydrase is not an essential requirement for photoautotrophy in cell suspension cultures which grow at either high or low CO₂ concentrations.

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