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In vivo phosphoenolpyruvate carboxylase activity is controlled by CO2 and O2 mole fractions and represents a major flux at high photorespiration rates
- Abadie, Cyril, Tcherkez, Guillaume
- Thenew phytologist 2019 v.221 no.4 pp. 1843-1852
- C3 plants, Helianthus annuus, bicarbonates, carbon, carbon dioxide, enzyme activity, fumarate hydratase, gas exchange, leaves, malates, nuclear magnetic resonance spectroscopy, oxygen, phosphoenolpyruvate carboxylase, photorespiration, stable isotopes
- 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 ¹³CO₂ (99% ¹³C) under controlled conditions and measured the amount of ¹³C found in the four C‐atom positions in malate, the major product of PEPC activity. We found that amongst malate ¹³C‐isotopomers present after labelling, most molecules were labelled at both C‐1 and C‐4, showing the incorporation of ¹³C at C‐4 by PEPC fixation and subsequent redistribution to C‐1 by fumarase (malate–fumarate equilibrium). In addition, absolute quantification of ¹³C 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.