In vivo phosphoenolpyruvate carboxylase activity is controlled by CO2 and O2 mole fractions and represents a major flux at high photorespiration rates

Abstract

Phosphenolpyruvate carboxylase (PEPC)-catalysed fixation of bicarbonate to C₄ acids is commonly believed to represent a rather small flux in illuminated leaves. In addition, its potential variation with O₂ and CO₂ is not documented and thus is usually neglected in gas-exchange studies. Here, we used quantitative NMR analysis of sunflower leaves labelled with ¹³ CO₂ (99% ¹³ C) under controlled conditions and measured the amount of ¹³ C found in the four C-atom positions in malate, the major product of PEPC activity. We found that amongst malate ¹³ C-isotopomers present after labelling, most molecules were labelled at both C-1 and C-4, showing the incorporation of ¹³ C at C-4 by PEPC fixation and subsequent redistribution to C-1 by fumarase (malate-fumarate equilibrium). In addition, absolute quantification of ¹³ C content showed that PEPC fixation increased at low CO₂ or high O₂ , and represented up to 1.8 μmol m^-2 s^-1 , that is, 40% of net assimilation measured by gas exchange under high O₂ /CO₂ conditions. Our results show that PEPC fixation represents a quantitatively important CO₂ -fixing activity that varies with O₂ and/or CO₂ mole fraction and this challenges the common interpretation of net assimilation in C₃ plants, where PEPC activity is often disregarded or considered to be constant at a very low rate.

Keywords: CO 2; PEPC; flux; photorespiration; photosynthesis.