Arabidopsis thaliana auxotrophs reveal a tryptophan-independent biosynthetic pathway for indole-3-acetic acid

Jennifer Normanly, Jerry D. Cohen, Gerald R. Fink

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We used tryptophan auxotrophs of the dicot Arabidopsis thaliana (wall cress) to determine whether tryptophan has the capacity to serve as a precursor to the auxin, indole-3-acetic acid (IAA). Quantitative gas chromatography-selected ion monitoring-mass spectrometry (GC-SIM-MS) revealed that the trp2-1 mutant, which is defective in the conversion of indole to tryptophan, accumulated amide- and ester-linked IAA at levels 38-fold and 19-fold, respectively, above those of the wild type. Tryptophan and free IAA were isolated from the trp2-1 mutant grown in the presence of [15N]anthranilate and [2H5]tryptophan, and the relative 15N and 2H5 enrichments of tryptophan and IAA were determined via GC-SIM-MS. The 15N enrichment of tryptophan, 13% ± 4%, was less than the 15N enrichment of the IAA pool, 39% ± 4%; therefore, IAA biosynthesis occurs via a tryptophan-independent pathway. The amount of 2H5 incorporated by the plant into IAA from tryptophan (9% ± 4%) was low and only slightly above the level of spontaneous, nonenzymatic conversion of [2H5]tryptophan to [2H5]IAA. These results show that the dicot Arabidopsis is similar to the monocot Zea mays in that the major route of IAA biosynthesis does not occur through tryptophan.

Original languageEnglish (US)
Pages (from-to)10355-10359
Number of pages5
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number21
StatePublished - Nov 1 1993


  • Auxin
  • Gas chromatography-selected ion monitoring-mass spectrometry


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