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

Abstract

Summary 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.

Arabidopsis PPC2 is crucial for growth at low CO2 by involvement in photorespiratory metabolism and integration of ABI5

“…Recently, a potential role for PEPC in C 3 plant metabolism at high photorespiratory (low CO 2 /high O 2 ) conditions has been proposed (Abadie and Tcherkez, 2019; Tcherkez and Limami, 2019). Here we also found that PPC2 is involved in seedling development by modulation of photorespiratory metabolism at low CO 2 conditions.…”

“…Recent studies in sunflower have shown that the malate content closely correlates with photorespiration by metabonomics analyses and C 3 PEPC fixation increased at high photorespiratory condition (low CO 2 or high O 2 ) (Abadie et al , 2017; Abadie and Tcherkez, 2019), indicating non-photosynthetic PEPC may still contribute to photorespiration in C 3 plant. The studies in Arabidopsis have reported the novel function of C 3 PEPC in regulating the balance of carbon/nitrogen metabolism (Masumoto et al , 2010; Shi et al , 2015).…”

“…PEPC in C 3 plant is commonly believed to play minor roles in photosynthesis or photorespiration (von Caemmerer, 2013). Recently, NMR analyses of 13 C-fixation in sunflower indicated that PEPC activity and 13 C-fixation was significantly increased as net CO 2 assimilation decreased at high photorespiratory conditions (low CO 2 /O 2 ratio) (Abadie et al , 2017; Abadie and Tcherkez, 2019). Moreover, mutation of OsPPC4 in rice led to high accumulation of photorespiratory intermediates, such as glycine, serine and glycerate (Masumoto et al , 2010).…”

Using photorespiratory oxygen response to analyse leaf mesophyll resistance

“…However, they are ignored here as fractions of these components in R d are generally unknown. There may also be some activity of phosphoenolpyruvate carboxylase (V pepc ) in cytosol (Douthe et al 2012;Abadie and Tcherkez 2019), which would counteract the effect of (F + R d ) on g m,app . But our procedure of estimating R d may have accounted for this, i.e.…”

Is the Kok effect a respiratory phenomenon? Metabolic insight using 13C labeling in Helianthus annuus leaves

“…1 H-NMR analyses of absolute metabolite content were carried out on leaf extracts (10% D 2 O) done with 21 mg lyophilized sample in powder extracted with 1 ml phosphate buffer pH 7 (15 mM), using a 700 MHz NMR spectrometer (Bruker Advance III) with water suppression by excitation sculpting. 13 C-NMR analyses were carried out on leaf extracts with EDTA (12.5 mM) and maleate (standard; 125 µmol per sample) over 12 500 scans as in Abadie & Tcherkez (2019). Metabolic fluxes were estimated using a set of differential equations implemented across four steps of 1200 time increments, as in Cui et al (2020).…”