In folio respiratory fluxomics revealed by 13C isotopic labeling and H/D isotope effects highlight the noncyclic nature of the tricarboxylic acid "cycle" in illuminated leaves.
While the possible importance of the tricarboxylic acid (TCA) cycle reactions for leaf photosynthesis operation has been recognized, many uncertainties remain on whether TCA cycle biochemistry is similar in the light compared with the dark. It is widely accepted that leaf day respiration and the metabolic commitment to TCA decarboxylation are down-regulated in illuminated leaves. However, the metabolic basis (i.e. the limiting steps involved in such a down-regulation) is not well known. Here, we investigated the in vivo metabolic fluxes of individual reactions of the TCA cycle by developing two isotopic methods, (13)C tracing and fluxomics and the use of H/D isotope effects, with Xanthium strumarium leaves. We provide evidence that the TCA "cycle" does not work in the forward direction like a proper cycle but, rather, operates in both the reverse and forward directions to produce fumarate and glutamate, respectively. Such a functional division of the cycle plausibly reflects the compromise between two contrasted forces: (1) the feedback inhibition by NADH and ATP on TCA enzymes in the light, and (2) the need to provide pH-buffering organic acids and carbon skeletons for nitrate absorption and assimilation.
Tcherkez
Guillaume
G
Institut de Biotechnologie des Plantes, Plateforme Métabolisme-Métabolome IFR87, Bâtiment 630, Université Paris-Sud 11, 91405 Orsay cedex, France. guillaume.tcherkez@u-psud.fr
Mahé
Aline
A
Gauthier
Paul
P
Mauve
Caroline
C
Gout
Elizabeth
E
Bligny
Richard
R
Cornic
Gabriel
G
Hodges
Michael
M
D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes; D019069:Cell Respiration / Q000528:radiation effects; D002952:Citric Acid Cycle / Q000528:radiation effects*; D003653:Decarboxylation / Q000528:radiation effects; D003903:Deuterium; D005650:Fumarates / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D007553:Isotope Labeling; D008027:Light; D010788:Photosynthesis / Q000528:radiation effects; D018515:Plant Leaves / Q000378:metabolism* / Q000528:radiation effects*; D018526:Plant Transpiration / Q000528:radiation effects; D019289:Pyruvic Acid / Q000378:metabolism; D019802:Succinic Acid / Q000378:metabolism; D031210:Xanthium / Q000378:metabolism* / Q000528:radiation effects*
D002247:Carbon Isotopes; D005650:Fumarates; D002245:Carbon Dioxide; D019289:Pyruvic Acid; D019802:Succinic Acid; D003903:Deuterium; D005947:Glucose
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