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Published on March 2, 2007; 10.1104/pp.106.093013

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Received November 16, 2006
Accepted February 1, 2007

Co-ordinate Regulation of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxykinase by Light and CO2 during C4 Photosynthesis

Karen J Bailey , Julie E Gray , Robert P Walker , and Richard C Leegood *

Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK

* Corresponding author; email: r.leegood{at}shef.ac.uk.

The aim of this study was to investigate the relationship between the phosphorylation and activation states of phosphoenolpyruvate carboxykinase (PEPCK) and to investigate how the phosphorylation states of PEPCK and phosphoenolpyruvate carboxylase (PEPC) are co-ordinated in response to light intensity and CO2 concentration during photosynthesis in leaves of the C4 plant Guinea grass (Panicum maximum L.). There was a linear, reciprocal relationship between the phosphorylation state of PEPCK and its activation state, determined in a selective assay which distinguishes phosphorylated from non-phosphorylated forms of the enzyme. At high PFD and at high CO2 (750 ppm), PEPC was maximally phosphorylated and PEPCK maximally dephosphorylated within 1h of illumination. The phosphorylation state of both enzymes did not saturate until high light intensities (about 1400 µmol.quanta.m-2.s-1) were reached. After illumination at lower light intensities and CO2 concentrations, the overall change in phosphorylation state was smaller and it took longer for the change in phosphorylation state to occur. Phosphorylation states of PEPC and PEPCK showed a strikingly similar, but inverse, pattern in relation to changes in light and CO2. The protein phosphatase inhibitor, okadaic acid promoted the phosphorylation of both enzymes. The protein synthesis inhibitor, cycloheximide, blocked dark phosphorylation of PEPCK. The data show that PEPC and PEPCK phosphorylation states are closely co-ordinated in vivo, despite being located in the mesophyll and bundle sheath cells, respectively.




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