The Role of Phosphoenolpyruvate Carboxylase (PEPC) during C4 Photosynthetic Isotope Exchange and Stomatal Conductance

Asaph B. Cousins ^*, Irene Baroli , Murray R. Badger , Alexander Ivakov , Peter J. Lea , Richard C. Leegood , and Susanne von Caemmerer

Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia; ARC Centre of Excellence in Plant Energy Biology, Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory, 2601 Australia; Department of Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, UK; Robert Hill Institute, Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK

^* Corresponding author; email: asaph.cousins{at}anu.edu.au.

Phosphoenolpyruvate carboxylase (EC4.1.1.31; PEPC) plays a keyrole during C₄ photosynthesis and is involved in anapleroticmetabolism, pH regulation and stomatal opening. Heterozygous(Pp) and homozygous (pp) forms of a PEPC-deficient mutant ofthe C₄ dicot Amaranthus edulis were used to study the effectof reduced PEPC activity on CO₂ assimilation rates, stomatalconductance and ¹³CO₂ ( ${Delta}$ ¹³C) and C¹⁸OO ( ${Delta}$ ¹⁸O) isotope discriminationduring leaf gas exchange. PEPC activity was reduced to 42% and3% and the rates of CO₂ assimilation in air dropped to 78% and10% of the wild type values in the Pp and pp mutants, respectively.Stomatal conductance in air (531 µbar CO₂) was similarin the wild type and Pp mutant but the pp mutant had only 41%of the wild type steady state conductance under white lightand the stomata opened more slowly in response to increasedlight or reduced CO₂ partial pressure, suggesting that the C₄PEPC isoform plays an essential role in stomatal opening. Therewas little difference in ${Delta}$ ¹³C between the Pp mutant (3.03 ±0.4 ${per thousand}$ ) and wild type (3.33 ± 0.4 ${per thousand}$ ) indicating that leakiness( ${phi}$ ), the ratio of CO₂ leak rate out of the bundle sheath to therate of CO₂ supply by the C₄ cycle, a measure of the coordinationof C₄ photosynthesis, was not affected by a 60% reduction inPEPC activity. In the pp mutant ${Delta}$ ¹³C was 16±3.2 ${per thousand}$ indicativeof direct CO₂ fixation by Rubisco in the bundle sheath at ambientCO₂ partial pressure. ${Delta}$ ¹⁸O measurements indicated that the extentof isotopic equilibrium between leaf water and the CO₂ at thesite of oxygen exchange ( ${theta}$ ) was low (0.6) in the wild type andPp mutant but increased to 0.9 in the pp mutant. We concludethat in vitro carbonic anhydrase activity overestimated ${theta}$ ascompared to values determined from ${Delta}$ ¹⁸O in wild-type plants.

This article has been cited by other articles:

X.-R. Li, L. Wang, and Y.-L. Ruan
Developmental and molecular physiological evidence for the role of phosphoenolpyruvate carboxylase in rapid cotton fibre elongation
J. Exp. Bot., October 8, 2009; (2009) erp299v1.
[Abstract] [Full Text] [PDF]

A. B. Cousins, M. R. Badger, and S. von Caemmerer
C4 photosynthetic isotope exchange in NAD-ME- and NADP-ME-type grasses
J. Exp. Bot., May 1, 2008; 59(7): 1695 - 1703.
[Abstract] [Full Text] [PDF]