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© 1972 Oxford University Press

RESEARCH-ARTICLE

Biosynthesis and Metabolism of Indol-3yl-acetic acid

II. IN VIVO EXPERIMENTS WITH 14C-LABELLED PRECURSORS OF IAA IN TOMATO AND BARLEY SHOOTS

R. A. GIBSON 1, ELNORA A. SCHNEIDER and F. WIGHTMAN

Department of Biology, Carleton University Ottawa, Canada

Our previous investigation of the naturally occurring indole compounds in barley and tomato shoots suggested that the biosynthesis of indol-3yl-acetic acid (IAA) from tryptophan might proceed via either the indol-3yl-pyruvic acid or tryptamine pathways in both species. The results further indicated that the indol-3yl-lactic acid pathway for IAA formation might also be operative in tomato. In the present study, tryptophan-3-14C and tryptamine-2-14C were fed to excised shoots of both barley and tomato, and indol-3yl-lactic acid-3-14C was also fed to shoots of tomato. All three compounds were found to give rise to radioactive IAA with little dilution in specific activity. Feeding tryptophan-3-14C also resulted in the labelling of indol-3yl-pyruvic acid, indol-3yl-acetaldehyde, and tryptamine, which were isolated and chemically identified from both species, and radioactive indol-3yl-lactic acid and tryptophol were also produced in tomato. Indol-3yl-acetaldehyde was found to be labelled in both species after administration of tryptamine-2-14C, while the principal metabolite of indol-3yl-lactic acid-3-14C was radioactive tryptophan. These findings, along with the results from a quantitative study of the radioactive metabolites, indicate that both the indol-3yl-pyruvic acid and tryptamine pathways can operate in both species, while the formation of IAA from indol-3yl-lactic acid in tomato probably occurs indirectly, via tryptophan. These conclusions were supported by the demonstration of the enzymes, L-tryptophan transaminase, L-trypto-phan decarboxylase, and indol-3yl-acetaldehyde dehydrogenase in cell-free extracts of both tissues, and of indol-3yl-pyruvic acid decarboxylase in the tomato extract. No indol-3yl-lactic acid decarboxylase activity was observed in the extracts from either tissue.


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