title: Arabidopsis thaliana auxotrophs reveal a tryptophan-independent biosynthetic pathway for indole-3-acetic acid.
abstract: 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.
authors.affiliation: Whitehead Institute for Biomedical Research/Massachusetts Institute of Technology, Cambridge 02142.
authors.forename: J
authors.initials: J
authors.lastname: Normanly
authors.forename: J D
authors.initials: JD
authors.lastname: Cohen
authors.forename: G R
authors.initials: GR
authors.lastname: Fink
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D003903:Deuterium; D008401:Gas Chromatography-Mass Spectrometry; D005838:Genotype; D007210:Indoleacetic Acids / Q000302:isolation & purification / Q000378:metabolism*; D008027:Light; D008954:Models, Biological; D009587:Nitrogen Isotopes; D014364:Tryptophan / Q000378:metabolism*; D062367:ortho-Aminobenzoates / Q000378:metabolism
chemical_list: D007210:Indoleacetic Acids; D009587:Nitrogen Isotopes; D062367:ortho-Aminobenzoates; C031385:anthranilic acid; C030737:indoleacetic acid; D014364:Tryptophan; D003903:Deuterium