Substrate cycles in the central metabolism of maize root tips under hypoxia

• Published in: Phytochemistry (Oxford) Vol. 68; no. 16; pp. 2222 - 2231
• Main Authors: Alonso, Ana Paula, Raymond, Philippe, Rolin, Dominique, Dieuaide-Noubhani, Martine
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.08.2007; Elsevier
• Subjects: adenosine triphosphate; Adenosine Triphosphate - metabolism; alanine; biochemical pathways; carbohydrate metabolism; Carbon Isotopes; Carbon Radioisotopes; Cell Hypoxia; cell respiration; Citric Acid Cycle; corn; glucose; Glucose - metabolism; Glycolysis; Hypoxia; isotope labeling; Isotopic labeling; Life Sciences; Maize root; Metabolic flux analysis; Models, Biological; NMR; Nuclear Magnetic Resonance, Biomolecular; oxygen; Phytopathology and phytopharmacy; plant biochemistry; Plant Roots - metabolism; root tips; spectral analysis; stable isotopes; starch; Substrate cycle; Substrate Cycling; sucrose; sugars; Vegetal Biology; Zea mays; Zea mays - enzymology; Zea mays - metabolism; Maize root; Hypoxia; Substrate cycle; NMR; Metabolic flux analysis; Isotopic labeling; FLUX METABOLIQUE; MODELISATION; CYCLE DE SUBSTRAT; BIOLOGIE VEGETALE; HYPOXIE; ATP; METABOLISME; GLYCOLYSE
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2007.04.022

This work describes a metabolic flux analysis (C13 and C14 labeling experiments for flux quantification) performed to study the response of maize root tips to hypoxia. ATP production was severely reduced, and the flux through the substrate cycles all decreased. However, substrate cycles remain important in terms of ATP consumption (50% of the produced ATP) and their importance was discussed. Substrate cycles, also called “futile” cycles, are ubiquitous and lead to a net consumption of ATP which, in the normoxic maize root, have been estimated at about 50% of the total ATP produced [Alonso, A.P., Vigeolas, H., Raymond, P., Rolin, D., Dieuaide-Noubhani, M., 2005. A new substrate cycle in plants. Evidence for a high glucose-phosphate-to-glucose turnover from in vivo steady-state and pulse-labeling experiments with [ 13C] glucose and [ 14C] glucose. Plant Physiol. 138, 2220–2232]. To evaluate their role we studied the substrate cycles of maize root tips under an oxygen limitation of respiration (3% O 2). Short-time labeling experiments with [U- 14C]-Glc were performed to quantify the fluxes through sucrose and starch cycles of synthesis and degradation. Steady-state labeling with [1- 13C]-Glc followed by 1H NMR and 13C NMR analysis of sugars and free alanine was used to quantify fluxes in the central metabolic pathways, including the Glc-P/Glc cycle and the fructose-P/triose-P cycle of glycolysis. Comparison with results previously obtained in normoxia [Alonso et al., as mentioned above] showed that 3% O 2 induced fermentation and reduced respiration, which led to a lesser amount of ATP produced. The rates of Glc consumption, glycolytic flux and all substrate cycles were lower, but the proportion of ATP consumed in the substrate cycles remained unchanged. These findings suggest that substrate cycles are not a luxury but an integral part of the organization of the plant central metabolism.