Effect of Light and NO3− on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity

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Abstract
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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