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BIOENERGETICS AND PHOTOSYNTHESIS

Coordinate Regulation of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxykinase by Light and CO₂ during C₄ Photosynthesis^1,[OA]

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

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 coordinated in response to light intensity and CO₂ concentration during photosynthesis in leaves of the C₄ plant Guinea grass (Panicum maximum). There was a linear, reciprocal relationship between the phosphorylation state of PEPCK and its activation state, determined in a selective assay that distinguishes phosphorylated from nonphosphorylated forms of the enzyme. At high photon flux density and high CO₂ (750 µL L^–1), PEPC was maximally phosphorylated and PEPCK maximally dephosphorylated within 1 h of illumination. The phosphorylation state of both enzymes did not saturate until high light intensities (about 1,400 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 coordinated in vivo, despite being located in the mesophyll and bundle sheath cells, respectively.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Richard C. Leegood (r.leegood{at}shef.ac.uk).

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.093013

^* Corresponding author; e-mail r.leegood{at}shef.ac.uk; fax 44–114–222–0050.

Received November 16, 2006; accepted February 1, 2007; published March 2, 2007.

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