Integration and expression of Sorghum C4 phosphoenolpyruvate carboxylase and chloroplastic NADP-malate dehydrogenase separately or together in C3 potato plants

We have integrated two cDNAs expressing Sorghum photosynthetic phosphoenolpyruvate carboxylase (C₄-PEPC) and NADP-malate dehydrogenase (cpMDH), two key enzymes involved in the primary carbon fixation pathway of NADP-malic enzyme-type C₄ plants, separately or together into a C₃ plant (potato). Analysis of the transgenic plants showed a 1.5-fold increase in PEPC and cpMDH activities compared to untransformed plants. Immunolocalization confirmed an increase at the protein level of these two enzymes in the transgenic plants and indicated that the Sorghum cpMDH was specifically addressed to the chloroplasts of potato mesophyll cells. However, integration of either or both of the cDNAs into the potato genome did not appear to significantly modify either tuber starch grain content or the rate of photosynthetic O₂ production compared to control untransformed plants. The low level of transgene expression probably explains the lack of influence on carbon metabolism and photosynthetic rates. This general observation suggests that some complex mechanism may regulate the level of production of foreign C₄ metabolism enzymes in C₃ plants.