METABOLITE EFFECTORS FOR SHORT-TERM NITROGEN-DEPENDENT ENHANCEMENT OFPHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITY AND DECREASE OF NET SUCROSE SYNTHESIS IN WHEAT LEAVES

We have shown that feeding 40 mM NOS in the light to N-limited detache d leaves from wheat (Triticum aestivum L. cv Fidel) results in the sho rt-term, in the enhancement of light-dependent phosphoenolpyruvate car boxylase (PEPcase) activation and the decrease of net sucrose synthesi s. The aim of the research was to determine specifically the N metabol ite(s) responsible for this modulation. Detached leaves from wheat wer e fed for 90 min in the light with (1) NO3-, NH4+, glutamine, glutamat e, alanine or aspartate; or (2) NO3- supplemented with sodium tungstat e, methionine sulfoximine, azaserine or amino oxyacetate in order to i nhibit specifically nitrate reductase, glutamine synthetase, glutamate synthase or amino transferases. Changes in phosphoenolpyruvate carbox ylase activity and net sucrose synthesis were measured at the end of e ach treatment. Both sodium tungstate and methionine sulfoximine which inhibit reduction of NO3- and assimilation of NH4+ respectively, suppr essed the NO(3)(-)dependent activation of phosphoenolpyruvate carboxyl ase. Tungstate restored the rate of sucrose synthesis to the level of the control (absence of nitrate), while methionine sulfoximine enhance d it. Whatever way the glutamine level was increased in tissues (feedi ng the leaves with glutamine or with azaserine which inhibits glutamat e synthase), enhancement of the light-dependent activation of phosphoe nolpyruvate carboxylase and reduction of sucrose synthesis were observ ed 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 e ffector for controlling the N-dependent activation of phosphoenolpyruv ate carboxylase, and that the glutamine/glutamate ratio might regulate phosphoenolpyruvate carboxylase activation and the rate of sucrose sy nthesis in wheat leaves. Glutamine and glutamate were shown to be posi tive and negative effectors respectively of in vitro PEPcase-protein k inase. In addition glutamine and glutamate were shown to control PEPca se by induction and repression of RNA synthesis respectively. Thus, tw o aspects of PEPcase modulation by these metabolites must be considere d, (1) control of the PEPcase gene expression and (2) regulation of th e PEPcase-protein kinase.