Short-term nitrogen-induced modulation of phosphoenolpyruvate carboxylase in tobacco and maize leaves

Untransformed maize and tobacco plants and tobacco plants constitutively ex pressing nitrate reductase were grown with sufficient NO3- to support maxim al growth, Four days prior to treatment the tobacco plants were deprived of nitrogen, Excised maize leaves and tobacco leaf discs were fed with either 40 mM KNO3 or 40 mM KCl (control) in the light. Phosphoenolpyruvate (PEP) carboxylase (Case) activity was measured at 0.3 mM and 3 mM PEP. The light- induced increase in PEPCase V-max was greater in maize than tobacco, Furthe rmore light decreased malate sensitivity in maize (which was N-replete) but not in N-deficient tobacco, NO, treatment increased PEPOase V-max values i n both species and decreased the sensitivity to inhibition by malate, but e ffects of NO3- were much more pronounced in tobacco than maize. PEPCase kin ase activity was, however, greater in maize leaves fed NO3- than in the Cl- -treated controls, suggesting that it is responsive to leaf nitrogen suppl y. A correlation between foliar glutamine content and PEPCase activity was observed, It is concluded that PEPCase is sensitive to N metabolites which favour increased flow through the anapleurotic pathway in both C-3 and C-4 plants.