Mutual stimulation of temperature and light effects on C < inf > 4 < /inf > phosphoenolpyruvate carboxylase in leaf discs and leaves of Amaranthus hypochondriacus

Abstract

This article reports marked modulation of the activity and regulatory properties of phosphoenolpyruvate carboxylase (PEPC) by temperature and light in leaf discs as well as leaves of Amaranthus hypochondriacus. The activity of PEPC increased by 1.7-fold at 45 °C over 25 °C. Warm temperature also stimulated the photoactivation of PEPC. The activation by light of PEPC was 1.9-fold at 25 °C and increased to 2.2-fold at 45 °C. The sensitivity of PEPC to its inhibitor malate was less and the activation by glucose-6-phosphate (G-6-P) or inorganic phosphate (Pi) was more at 45 °C than that at 25 °C. These effects of temperature were quite pronounced in light. Similar responses were observed when detached leaves were exposed to varying ambient temperature (dry heat). The activity of PEPC increased by 1.6-fold at 45 °C over 25 °C in the dark. The activation of PEPC by light was 2.1-fold at 25 °C and increased to 2.6-fold at 45 °C. Inhibition by malate was less and activation by G-6-P or Pi was more at 45 °C than that at 25 °C. Thus, there was a marked modulation of not only the activity but also the regulatory properties of the enzyme by temperature and light, independently as well as cooperatively with each other. Further experiments suggested that PEPC was able to memorize to a significant extent the changes induced by warm temperature and that these changes were complemented by subsequent illumination. These effects were not due to changes in PEPC protein levels. We conclude that temperature and light can modulate PEPC activity and regulatory properties not only individually but also in a significantly cooperative manner with each other. As significant increases in temperature are common during daytime in tropical or subtropical conditions, we suggest that the synergistic effects of temperature and light are quite relevant in optimizing the activity of PEPC in leaves of C4 plants. © 2007 Elsevier GmbH. All rights reserved.