Temperature Modulation of the Activity and Malate Inhibition of the Phosphoenolpyruvate Carboxylase from Leaves of Alternanthera pungens, Compared to that of Lycopersicom esculentum

Abstract

Temperature caused marked modulation of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in leaf discs of Alternanthera pungens (C4 plant) as well as Lycopersicon esculentum (C3 species). The optimal incubation temperature for PEPC activity in A. pungens was 45 °C compared to 30 °C in L. esculentum. A. pungens lost nearly 61% of PEPC activity on exposure to a low temperature of 15 °C, compared to only about a 33% loss in the case of L. esculentum. The C4 enzyme was less sensitive to supra-optimal temperature and more sensitive to sub-optimal temperature than that of the C3 species. Further as the temperature was raised from 15 °C to 50 °C, there was a sharp decrease in malate sensitivity of PEPC. The extent of such a decrease in C4 plants was more than that in C3 species. Arrhenius plots that were constructed by plotting the activity of PEPC against the reciprocal of temperature in the absence or presence of malate exhibited abrupt changes or “break-points” at only one point of 17oC in A. pungens while at two points corresponding 17oC and 27oC in case of L. esculentum. The activation energy of PEPC from A. pungens was less compared to that of L. esculentum in the temperature range of 10 to 27oC. However, the activation energy of PEPC from A. pungens was less than that of L. esculentum above the temperature of 27oC. The activation energy increased by 2 to 4 fold at temperatures below 17oC, in case of both A. pungens and L. esculentum. Thus, our results show the activity and malate sensitivity of PEPC can be influenced in relation to high temperature tolerance of C4 plants, which can be physiologically significant.