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BIOENERGETICS AND PHOTOSYNTHESIS

The Role of Phosphoenolpyruvate Carboxylase during C₄ Photosynthetic Isotope Exchange and Stomatal Conductance^1,[OA]

Molecular Plant Physiology Group (A.B.C., I.B., M.R.B., A.I., S.v.C) and Australian Research Council Centre of Excellence in Plant Energy Biology (A.B.C., M.R.B.), Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia; Department of Biological Sciences, Lancaster University, Lancaster LA1 4YQ, United Kingdom (P.J.L.); and Department of Animal and Plant Sciences, Robert Hill Institute, University of Sheffield, Sheffield S10 2TN, United Kingdom (R.C.L.)

Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) plays a key role during C₄ photosynthesis and is involved in anaplerotic metabolism, pH regulation, and stomatal opening. Heterozygous (Pp) and homozygous (pp) forms of a PEPC-deficient mutant of the C₄ dicot Amaranthus edulis were used to study the effect of reduced PEPC activity on CO₂ assimilation rates, stomatal conductance, and ¹³CO₂ (¹³C) and C¹⁸OO (¹⁸O) isotope discrimination during leaf gas exchange. PEPC activity was reduced to 42% and 3% and the rates of CO₂ assimilation in air dropped to 78% and 10% of the wild-type values in the Pp and pp mutants, respectively. Stomatal conductance in air (531 µbar CO₂) was similar in the wild-type and Pp mutant but the pp mutant had only 41% of the wild-type steady-state conductance under white light and the stomata opened more slowly in response to increased light or reduced CO₂ partial pressure, suggesting that the C₄ PEPC isoform plays an essential role in stomatal opening. There was little difference in ¹³C between the Pp mutant (3.0 ± 0.4) and wild type (3.3 ± 0.4), indicating that leakiness (), the ratio of CO₂ leak rate out of the bundle sheath to the rate of CO₂ supply by the C₄ cycle, a measure of the coordination of C₄ photosynthesis, was not affected by a 60% reduction in PEPC activity. In the pp mutant ¹³C was 16 ± 3.2, indicative of direct CO₂ fixation by Rubisco in the bundle sheath at ambient CO₂ partial pressure. ¹⁸O measurements indicated that the extent of isotopic equilibrium between leaf water and the CO₂ at the site of oxygen exchange () was low (0.6) in the wild-type and Pp mutant but increased to 0.9 in the pp mutant. We conclude that in vitro carbonic anhydrase activity overestimated as compared to values determined from ¹⁸O in wild-type plants.

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: Peter J. Lea (p.lea{at}lancaster.ac.uk).

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.103390

^* Corresponding author; e-mail asaph.cousins{at}anu.edu.au.

Received June 5, 2007; accepted September 3, 2007; published September 7, 2007.