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

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.