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Published on June 22, 2007; 10.1104/pp.107.102541

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Received May 17, 2007
Accepted June 14, 2007

Phosphorylation of Phosphoenolpyruvate Carboxylase Is not Essential for High Photosynthetic Rates in the C4 Species Flaveria bidentis

Tsuyoshi Furumoto , Katsura Izui , Vanda Quinn , Robert T Furbank , and Susanne von Caemmerer *

Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia; Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, GPO Box 1600, Canberra, Australian Capital Territory 2601Australia; Department of Biological Science, Hiroshima University, Kagamiyama 1-3-1, Higashihiroshima, 739-8526 Japan; Department of Biotechnological Science, Kinki University, Nishimitani Kinokawa 930, Wakayama, 649-6493 Japan

* Corresponding author; email: Susanne.caemmerer{at}anu.edu.au.

Phosphoenolpyruvate carboxylase (EC4.1.1.31; PEPC) plays a key role during C4 photosynthesis. The enzyme is activated by metabolites such as glucose 6-phosphate and inhibited by malate. This metabolite sensitivity is modulated by the reversible phosphorylation of a conserved serine residue near the N-terminus in response to light. The phosphorylation of PEPC is modulated by a protein kinase specific to PEPC (PEPC-PK). To explore the role PEPC-PK plays in the regulation of C4 photosynthetic CO2 fixation we have transformed Flaveria bidentis (L.) Kuntze (a C4 dicot) with antisense or RNAi constructs targeted at the mRNA of this PEPC-PK. We generated several independent transgenic lines where PEPC is not phosphorylated in the light, demonstrating that this PEPC-PK is essential for the phosophorylation of PEPC in vivo. Malate sensitivity of PEPC extracted from these transgenic lines in the light was similar to the malate sensitivity of PEPC extracted from darkened wild type leaves but greater than the malate sensitivity observed in PEPC extracted from wild type leaves in the light, confirming the link between PEPC phosphorylation and the degree of malate inhibition. There were, however, no differences in the CO2 and light response of CO2 assimilation rates between wild type plants and transgenic plants with low PEPC phosophorylation showing that phosphorylation of PEPC in the light is not essential for efficient C4 photosynthesis for plants grown under standard glass house conditions. This raises the intriguing question of what role this complexly regulated reversible phosphorylation of PEPC plays in C4 photosynthesis.




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