The expression of the tobacco genes encoding plastidic glutamine synthetase or ferredoxin-dependent glutamate synthase does not depend on the rate of nitrate reduction, and is unaffected by suppression of photorespiration

The expression of the tobacco genes encoding the ammonium-assimilating enzymes plastidic glutamine synthetase (GS-2) or ferredoxin-dependent glutamate synthase (Fd-GOGAT) was examined in relation to the rates of two major ammonium-producing processes in leaves, i.e. nitrate reduction and photorespiration. Impairment of nitrate reduction induced by the application of tungstate, a molybdate analogue inhibiting nitrate reductase (NR), resulted in an increase in the NR and the nitrite reductase (NiR) mRNA pools. It is shown that this effect could in part be reversed by the application of ammonium. These results support negative regulation by nitrate-derived N-metabolites of both the NR and NiR mRNA abundance. Both the GS-2 and the Fd-GOGAT transcript level and the corresponding enzymatic activities remained largely unaffected by impairment of nitrate reduction, suggesting that both GS-2 and Fd-GOGAT gene expression are not under the control by end products of nitrogen assimilation. Growth of tobacco at a CO 2 partial pressure sufficiently high to suppress photorespiration (300 Pa CO 2 ) had no effect on either the GS-2 or Fd-GOGAT transcript levels. It is concluded that the rate of photo-respiratory ammonium production does not exert a metabolic control on GS-2 or Fd-GOGAT gene expression.