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BIOENERGETICS AND PHOTOSYNTHESIS

Phosphorylation of Phosphoenolpyruvate Carboxylase Is Not Essential for High Photosynthetic Rates in the C₄ Species Flaveria bidentis^1,[OA]

Department of Biological Science, Hiroshima University, Higashihiroshima 739–8526, Japan (T.F); Department of Biotechnological Science, Kinki University, Wakayama 649–6493, Japan (K.I.); Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia (V.Q., S.v.C.); and Commonwealth Scientific and Industrial Research Organization, Division of Plant Industry, Canberra, Australian Capital Territory 2601, Australia (R.T.F.)

Phosphoenolpyruvate carboxylase (PEPC; EC4.1.1.31) 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 (a C₄ dicot) with antisense or RNA interference 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 phosphorylation 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 phosphorylation, showing that phosphorylation of PEPC in the light is not essential for efficient C₄ photosynthesis for plants grown under standard glasshouse conditions. This raises the intriguing question of what role this complexly regulated reversible phosphorylation of PEPC plays in C₄ photosynthesis.

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: Susanne von Caemmerer (susanne.caemmerer{at}anu.edu.au).

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.102541

^* Corresponding author; e-mail susanne.caemmerer{at}anu.edu.au; fax 61–2–61255075.

Received May 17, 2007; accepted June 14, 2007; published June 22, 2007.

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