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BIOENERGETICS AND PHOTOSYNTHESIS

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^1,[W]

Institut de Biotechnologie des Plantes (G.T., A.M., P.G., M.H.), Plateforme Métabolisme-Métabolome IFR87 (G.T., C.M.), and Laboratoire d'Écophysiologie Végétale, Ecologie Systématique Evolution (G.C.), Bâtiment 630, Université Paris-Sud 11, 91405 Orsay cedex, France; and Laboratoire de Physiologie Cellulaire Végétale, Commissariat à l'Energie Atomique-Grenoble, 38054 Grenoble cedex 9, France (E.G., R.B.)

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, ¹³C 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.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Guillaume Tcherkez (guillaume.tcherkez{at}u-psud.fr).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.142976

^* Corresponding author; e-mail guillaume.tcherkez{at}u-psud.fr.

Received June 17, 2009; accepted August 10, 2009; published August 12, 2009.