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Amino Acid Interactions in the Regulation of Nitrate Reductase Induction in Cotton Root Tips

Western Cotton Research Laboratory, Agricultural Research Service, United States Department of Agriculture, Phoenix, Arizona 85040

Glycine, asparagine, and glutamine inhibited the induction by nitrate of nitrate reductase activity in root tips of cotton (Gossypium hirsutum L.). This inhibition was partially or entirely prevented when the inhibitor was applied in combination with any of several other amino acids. Studies of ¹⁴C-labeled amino acid uptake showed that, in most cases, the apparent antagonism resulted simply from competition for uptake. However, certain antagonists did not curtail uptake. The most effective of these were leucine (against all three inhibitors), and isoleucine and valine (against asparagine or glutamine, but not glycine). These results show that interactions among amino acids in the regulation of nitrate reductase induction result from at least two mechanisms, one acting on uptake of inhibitory amino acids, and the other involving true antagonism.