Analysis of a C4 maize pyruvate, orthophosphate dikinase expressed in C3 transgenic Arabidopsis plants

Pyruvate,orthophosphate dikinase (PPDK) catalyzes the formation of phosphoenolpyruvate, the initial acceptor of CO2 in the C4 photosynthetic pathway. Transgenic C3 Arabidopsis plants expressing the maize C4 PPDK gene under the control of either the Arabidopsis rbcS promoter or the cauliflower mosaic virus 35S promoter were studied, The level of PPDK protein was quite low in contrast to the high steady-state level of PPDK transcripts in several transgenic plants. A PPDK polypeptide with a similar size to that in maize was found exclusively in the chloroplasts of transgenic Arabidopsis plants. This result indicates that the transit peptide of C4 PPDK in the C4 monocot maize is functional in the chloroplast protein import system of the C3 dicot Arabidopsis. The activities of PPDK in leaf extracts of the transgenic plants were up to four times higher than those in the control nontransgenic plants and the transgenic plants with the beta-glucuronidase (GUS) gene, although they were still less than 3% of the PPDK activity in maize. The relative PPDK activity per unit PPDK protein in transgenic Arabidopsis was similar to that in maize. These results suggest that the low PPDK activity in transgenic Arabidopsis plants may be attributed to possible regulation at post-transcriptional and/or translational levels. The modestly increased PPDK activity did not influence the activities of ribulose-1,5-bisphosphate carboxylase and other C4-related enzymes (phosphoenolpyruvate carboxylase, NAD(P)-malic enzyme), and photosynthetic CO2-exchange parameters. (C) 1997 Elsevier Science Ireland Ltd.