We have shown that feeding 40 mM NOS in the light to N-limited detached leaves from wheat (Triticum aestivum L. cv Fidel) results in the short-term, in the enhancement of light-dependent phosphoenolpyruvate carboxylase (PEPcase) activation and the decrease of net sucrose synthesis. The aim of the research was to determine specifically the N metabolite(s) responsible for this modulation. Detached leaves from wheat were fed for 90 min in the light with (1) NO3-, NH4+, glutamine, glutamate, alanine or aspartate; or (2) NO3- supplemented with sodium tungstate, methionine sulfoximine, azaserine or amino oxyacetate in order to inhibit specifically nitrate reductase, glutamine synthetase, glutamate synthase or amino transferases. Changes in phosphoenolpyruvate carboxylase activity and net sucrose synthesis were measured at the end of each treatment. Both sodium tungstate and methionine sulfoximine which inhibit reduction of NO3- and assimilation of NH4+ respectively, suppressed the NO(3)(-)dependent activation of phosphoenolpyruvate carboxylase. Tungstate restored the rate of sucrose synthesis to the level of the control (absence of nitrate), while methionine sulfoximine enhanced it. Whatever way the glutamine level was increased in tissues (feeding the leaves with glutamine or with azaserine which inhibits glutamate synthase), enhancement of the light-dependent activation of phosphoenolpyruvate carboxylase and reduction of sucrose synthesis were observed similar to feeding the leaves with NO3-. In contrast, glutamate and aspartate suppressed light-dependent phosphoenolpyruvate carboxylase activation. These results indicate that glutamine is the most likely effector for controlling the N-dependent activation of phosphoenolpyruvate carboxylase, and that the glutamine/glutamate ratio might regulate phosphoenolpyruvate carboxylase activation and the rate of sucrose synthesis in wheat leaves. Glutamine and glutamate were shown to be positive and negative effectors respectively of in vitro PEPcase-protein kinase. In addition glutamine and glutamate were shown to control PEPcase by induction and repression of RNA synthesis respectively. Thus, two aspects of PEPcase modulation by these metabolites must be considered, (1) control of the PEPcase gene expression and (2) regulation of the PEPcase-protein kinase.
