Abstract
The development of nitrate reductase (NR) activity was studied in tobacco mesophyll protoplast-derived cells. When cells were grown on nitrate as the sole nitrogen source, NR activity developed during the growth phase. When cells were grown on ammonium succinate as the sole nitrogen source, NR activity developed only at the beginning of the stationary phase. Under these latter conditions of development of NR activity, 4 mM ammonium was still remaining in the culture medium and large amounts of glutamine were accumulated in cells. The regulation of NR by exogenously supplied amino acids was studied by measuring enzyme activity, and by comparing the growth inhibition of cells supplied with amino acids in the presence of nitrate or ammonium. In contrast with previous reports, no close relationship between amino acid toxicity and inhibition of NR activity was found. The implications of these results regarding the regulation of nitrate reductase activity are discussed.
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