Abstract
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% CO(2) for growth, and a unique cotton cell line that grows at ambient CO(2) (660 microliters per liter). Photosynthetic characteristics in all four lines were more like C(3) mesophyll leaf cells than the cell suspension cultures previously studied. The pattern of (14)C-labeling reflected the high ratio of ribulosebisphosphate carboxylase to phosphoenolpyruvate carboxylase activity and showed that CO(2) fixation occurred primarily by the C(3) pathway. Photorespiration occurred at 330 microliters per liter CO(2), 21% O(2) as indicated by the synthesis of high levels of (14)C-labeled glycine and serine in a pulse-chase experiment and by oxygen inhibition of CO(2) fixation. Short-term CO(2) fixation in the presence and absence of carbonic anhydrase showed CO(2), not HCO(3) (-), to be the main source of inorganic carbon taken up by the low CO(2)-requiring cotton cells. The cells did not have a CO(2)-concentrating mechanism as indicated by silicone oil centrifugation experiments. Carbonic anhydrase was absent in the low CO(2)-requiring cotton cells, present in the high CO(2)-requiring soybean cell lines, and absent in other high CO(2) 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 CO(2) concentrations.