Comparative Glutamine Metabolism in Tobacco Callus and Leaf Tissues1

Abstract

Tobacco (Nicotiana tabacum L.) callus tissue was found to contain at least a thirty‐fold higher concentration of free glutamine than leaves from greenhouse‐grown plants of the same genotype. The most important factor contributing to the observed difference in glutamine pool size was exogenous ammonium concentration. Also, the specific activities in cell‐free extracts of the ammonium assimilating enzymes, glutamine synthetase and glutamate dehydrogenase assayed at nonlimiting substrate concentrations were maximal in both tissues at exogenous ammonium concentrations of approximately 0.1 to 0.3 mM for glutamine synthetase and 10.0 to 30.0 mM for glutamate dehydrogenase. Labeling callus tissue with 14C‐glutamate allowed the calculation of approximate rates of in vivo transfer of carbon skeletons from glutamate to glutamine which were consistent with the effects of in vivo ammonium concentration on enzyme activities observed in cell‐free extracts.The results indicate that under conditions of ammonium limitation in either tissue, the glutamine synthetase/glutamate synthase pathway is important to ammonium assimilation. The data suggest that in vivo activities of glutamlne synthetase, glutamate synthase and glutamate dehydrogenase in both leaf and callus tissues are regulated by ammonium concentration.