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 [N-15]anthranilate and [H-2(5)]tryPtophan, and the relative N-15 and H-2(5) enrichments of tryptophan and IAA were determined via GC-SIM-MS. The N-15 enrichment of tryptophan, 13% +/- 4%, was less than the N-15 enrichment of the IAA pool, 39% +/- 4%; therefore, IAA biosynthesis occurs via a tryptophan-independent pathway. The amount of H-2(5) incorporated by the plant into IAA from tryptophan (9% +/- 4%) was low and only slightly above the level of spontaneous, nonenzymatic conversion of [H-2(5)]tryptophan to [H-2(5)]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.
