Integration and expression of Sorghum C-4 phosphoenolpyruvate carboxylase and chloroplastic NADP(+)-malate dehydrogenase separately or together in C-3 potato plants

We have integrated two cDNAs expressing Sorghum photosynthetic phosphoenolp yruvate carboxylase (C-4-PEPC) and NADP-malate dehydrogenase (cpMDH), two k ey enzymes involved in the primary carbon fixation pathway of NADP-malic en zyme-type C-4 plants, separately or together into a C-3 plant (potato). Ana lysis of the transgenic plants showed a 1.5-fold increase in PEPC and cpMDN activities compared to untransformed plants. Immunolocalization confirmed an increase at the protein level of these two enzymes in the transgenic pla nts and indicated that the Sorghum cpMDH was specifically addressed to the chloroplasts of potato mesophyll cells. However, integration of either or b oth of the cDNAs into the potato genome did not appear to significantly mod ify either tuber starch grain content or the rate of photosynthetic O-2 pro duction compared to control untransformed plants. The low level of transgen e expression probably explains the lack of influence on carbon metabolism a nd photosynthetic rates. This general observation suggests that some comple x mechanism may regulate the level of production of foreign C-4 metabolism enzymes in C-3 plants. (C) 2001 Elsevier Science Ireland Ltd. All rights re served.