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Metabolism and Enzymology

Effect of Light and NO₃^– on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity

Evidence for Covalent Modulation of the C₃ Enzyme

Photosynthèse et Métabolisme (U.R.A. 1128), Bâtiment 430, Université Paris-Sud, F-91405 Orsay cedex, France, Métabolisme et Nutrition des Plantes, Institut National de la Recherche Agronomique, F-78000 Versailles, France

Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO₃^– leaves) or 40 millimolar KNO₃ (high-NO₃^– leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO₃^–. In contrast, the lower rate of increase recorded for low-NO₃^– leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO₃^– effect increased linearly with the level of NO₃^– uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO₃^– leaves, illuminated high-NO₃^– leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, ³²P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO₃^– conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO₃^–, an interpretation of the role of NO₃^– as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO₃^–, the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO₃^– modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis.

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