Effects of overproduction of the maize C4-specific phosphoenolpyruvateÅ@carboxylase on photosynthesis and respiration

Abstract

The effects of overproduction of phosphoenolpyruvate carboxylase (PEPC) on photosynthesis and respiration in a C3 plant were studied using transgenic rice plants carrying the maize C4-specific PEPC gene. When measured under growth conditions, the CO2 assimilation rates of the transformants were slightly lower than that of the non-transformant. Suppression of the CO2 assimilation in transformants was more remarkable in 2% O2 than in 21% O2, probably due to reduced Pi regeneration capacity. Although not always, the decrease in G * (the CO2 compensation point independent of respiration) and the increase in Rd (non-photorespiratory CO2 release under illumination) were observed in the transformants. On the other hand, there was no difference in Rn (CO2 release in darkness) between the transformants and non-transformant. These results suggest that the maize PEPC stimulate the mitocondrial CO2 release under illumination in transformants. These observations suggest that overproduction of the maize PEPC does not significantly affect photosynthesis and respiration of rice plants. The phosphorylation state of PEPC in transgenic rice leaves were examined using an antibody specific to the phosphorylated PEPC. It was found that, reversely to that occurring in maize, the maize PEPC in transgenic rice leaves was dephosphorylated in the daytime and partly phosphorylated in the night. These result indicated that the maize PEPC remained in the less active form in the daytime. It might be possible that a foreign PEPC could alter carbon metabolism of C3 plants, if the introduced PEPC can escape from the activity regulation by the mechanisms intrinsic to C3 plants.