NO(3) Enhances the Kinase Activity for Phosphorylation of Phosphoenolpyruvate Carboxylase and Sucrose Phosphate Synthase Proteins in Wheat Leaves: Evidence from the Effects of Mannose and Okadaic Acid

The aim of this work was to determine which of the two reactions (i.e. phosphorylation or dephosphorylation) involved in the establishment of the phosphorylated status of the wheat leaf phosphoenolpyruvate carboxylase and sucrose phosphate synthase protein responds in vivo to NO(3) (-) uptake and assimilation. Detached mature leaves of wheat (Triticum aestivum L. cv Fidel) were fed with N-free (low-NO(3) (-) leaves) or 40 mm NO(3) (-) solution (high-NO(3) (-) leaves). The specific inhibition of the enzyme-protein kinase or phosphatase activities was obtained in vivo by addition of mannose or okadaic acid, respectively, in the uptake solution. Mannose at 50 mm, by blocking the kinase reaction, inhibited the processes of NO(3) (-)-dependent phosphoenolpyruvate carboxylase activation and sucrose phosphate synthase deactivation. Following the addition of mannose, the deactivation of phosphoenolpyruvate carboxylase and the activation of sucrose phosphate synthase, both due to the enzyme-protein dephosphorylation, were at the same rate in low-NO(3) (-) and high-NO(3) (-) leaves, indicating that NO(3) (-) had no effect per se on the enzyme-protein phosphatase activity. Upon treatment with okadaic acid, the higher increase of phosphoenolpyruvate carboxylase and decrease of sucrose phosphate synthase activities observed in high NO(3) (-) compared with low NO(3) (-) leaves showed evidence that NO(3) (-) enhanced the protein kinase activity. These results support the concept that NO(3) (-), or a product of its metabolism, favors the activation of phosphoenolpyruvate carboxylase and deactivation of sucrose phosphate synthase in wheat leaves by promoting the light activation of the enzyme-protein kinase(s) without affecting the phosphatase(s).