Biosynthesis and Metabolism of Indol-3yl-acetic acid: II. IN VIVO EXPERIMENTS WITH 14C-LABELLED PRECURSORS OF IAA IN TOMATO AND BARLEY SHOOTS

Our previous investigation of the naturally occurring indolecompounds in barley and tomato shoots suggested that the biosynthesisof indol-3yl-acetic acid (IAA) from tryptophan might proceedvia either the indol-3yl-pyruvic acid or tryptamine pathwaysin both species. The results further indicated that the indol-3yl-lacticacid pathway for IAA formation might also be operative in tomato.In the present study, tryptophan-3-¹⁴C and tryptamine-2-¹⁴Cwere fed to excised shoots of both barley and tomato, and indol-3yl-lacticacid-3-¹⁴C was also fed to shoots of tomato. All three compoundswere found to give rise to radioactive IAA with little dilutionin specific activity. Feeding tryptophan-3-¹⁴C also resultedin the labelling of indol-3yl-pyruvic acid, indol-3yl-acetaldehyde,and tryptamine, which were isolated and chemically identifiedfrom both species, and radioactive indol-3yl-lactic acid andtryptophol were also produced in tomato. Indol-3yl-acetaldehydewas found to be labelled in both species after administrationof tryptamine-2-¹⁴C, while the principal metabolite of indol-3yl-lacticacid-3-¹⁴C was radioactive tryptophan. These findings, alongwith the results from a quantitative study of the radioactivemetabolites, indicate that both the indol-3yl-pyruvic acid andtryptamine pathways can operate in both species, while the formationof IAA from indol-3yl-lactic acid in tomato probably occursindirectly, via tryptophan. These conclusions were supportedby the demonstration of the enzymes, L-tryptophan transaminase,L-trypto-phan decarboxylase, and indol-3yl-acetaldehyde dehydrogenasein cell-free extracts of both tissues, and of indol-3yl-pyruvicacid decarboxylase in the tomato extract. No indol-3yl-lacticacid decarboxylase activity was observed in the extracts fromeither tissue.