EVIDENCE FOR NEGATIVE FEEDBACK IN THE CONTROL OF ETHANOLAMINE BIOSYNTHESIS IN EXCISED TOMATO ROOTS

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Abstract
The isolation of a clone of excised tomato roots (clone R3), and the modifications of White's medium necessary for successful culture of this clone, are described. The medium includes pyridoxin. Either ethanolamine, dimethylethanolamine, glycine, L-serine, L-valine, DL-norleucine, or L-isoleucine (and possibly L-tryptophane) replaced pyridoxin to a greater or lesser extent. L-Alanine, L-lysine, and L-threonine did not replace pyridoxin. The replacement of pyridoxin by ethanolamine is not dependent upon the carry-over of pyridoxin in the inoculum and clone R3 has been subcultured for over a year in a medium containing ethanolamine but no added pyridoxin. Explanations of this fact are given.Nutritional experiments are described in which ethanolamine and amino acids, which will also partly replace pyridoxin, were supplied together in the absence of pyridoxin. The results support the hypothesis that exogenous ethanolamine leads to the loss of a vitamin B6dependent enzyme necessary for the biosynthesis of ethanolamine. The results are of incidental interest when considering the use of complex additives (e.g. yeast extract) in culture media, and to the explanation of toxicity of natural metabolites.