Seven lines of haploid Nicotiana tabacum tissue culture selected for resistance to normally toxic levels of the glycine analog glycine hydroxamate, a competitive inhibitor of the glycine decarboxylase reaction, were investigated. The presence of glycine hydroxamate greatly increased the intracellular concentration of both glycine and alanine in wild type and resistant cell lines, suggesting that the inhibitor blocks both glycine- and alanine-utilizing reactions. All the resistant cell lines, whether grown in the presence or absence of glycine hydroxamate, had high intracellular concentrations of the 12 free amino acids which were analyzed, including glycine and serine. (These lines averaged 3.6 times the total amino acid content of wild-type cells in the absence of the inhibitor). The resistant cell lines were indistinguishable from wild-type cell lines in their metabolism of radioactively labeled glycine hydroxamate and glycine. Comparison of the metabolism of radioactively labeled alanine, glycolate, and glyoxylate in wild-type and α resistant line also revealed no distinctive differences. Glycine decarboxylase activities were unaltered in the resistant cell lines. The cellular toxicity of glycine hydroxamate is considered in relation to (1) the competitive inhibition by glycine hydroxamate of the glycine- and alanine-utilizing enzymes and (2) the resultant imbalances caused by high intracellular concentrations of these amino acids. The significance of elevation of total free amino acid concentration in effecting resistance to the inhibitor is discussed.Plants were regenerated from 5 of these lines and callus cultures of explants were tested for glycine hydroxamate resistance. Plants from seedlings of two lines which retained the resistant characteristic in explanted callus did not have high amino acid levels in leaves.
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