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Published on May 7, 2004; 10.1104/pp.103.036962

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Received November 28, 2003
Returned for revision February 25, 2004
Accepted February 25, 2004

Expression of Phosphoenolpyruvate Carboxylase and Phosphoenolpyruvate Carboxylase Kinase Genes. Implications for Genotypic Capacity and Phenotypic Plasticity in the Expression of Crassulacean Acid Metabolism

Tahar Taybi *, Hugh G. Nimmo , and Anne M. Borland

School of Biology, University of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom (T.T., A.M.B.); and Plant Science Group, Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, United Kingdom (H.G.N.)

* Corresponding author; email: tahar.taybi{at}ncl.ac.uk.

In plants with crassulacean acid metabolism (CAM), dark CO2 uptake is mediated by phosphoenolpyruvate carboxylase (PEPC), an enzyme that can be regulated at transcriptional and posttranslational levels. Reversible phosphorylation of PEPC is catalyzed by a dedicated PEPC kinase, which in turn is regulated at the transcriptional level over the 24-h cycle in CAM plants. PEPC kinase controls the day/night regulation of PEPC during the CAM cycle, thus facilitating plasticity for optimizing CO2 uptake under different environmental conditions. To understand the importance of PEPC kinase in relation to its target PEPC in terms of CAM performance, the expression of the genes encoding the two enzymes was investigated in four species of Clusia that have photosynthetic patterns ranging from C3 photosynthesis to constitutive CAM. By linking changes in the expression of PEPC and PEPC kinase to day/night patterns of leaf gas exchange, organic acid, and soluble sugar contents under different environmental conditions, the genetic and metabolic limitations to CAM plasticity were assessed. The results indicate that PEPC expression is a major factor underpinning the genotypic capacity for CAM and that PEPC kinase expression does not appear to limit CAM. The day/night regulation of Ppck transcript abundance was found to be a consequence of CAM and the day/night cycling of associated metabolites, rather than the primary controlling factor for the temporal separation of carboxylation processes.




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