Peter P. Molesworth

Auxin Biosynthesis in Pea: Characterization of the Tryptamine Pathway

One pathway leading to the bioactive auxin, indole-3-acetic acid (IAA), is known as the tryptamine pathway, which is suggested to proceed in the sequence: tryptophan (Trp), tryptamine, N-hydroxytryptamine, indole-3-acetaldoxime, indole-3-acetaldehyde (IAAld), IAA. Recently, this pathway has been characterized by the YUCCA genes in Arabidopsis (Arabidopsis thaliana) and their homologs in other species. YUCCA is thought to be responsible for the conversion of tryptamine to N-hydroxytryptamine. Here we complement the genetic findings with a compound-based approach in pea (Pisum sativum), detecting potential precursors by gas chromatography/tandem-mass spectrometry. In addition, we have synthesized deuterated forms of many of the intermediates involved, and have used them to quantify the endogenous compounds, and to investigate their metabolic fates. Trp, tryptamine, IAAld, indole-3-ethanol, and IAA were detected as endogenous constituents, whereas indole-3-acetaldoxime and one of its products, indole-3-acetonitrile, were not detected. Metabolism experiments indicated that the tryptamine pathway to IAA in pea roots proceeds in the sequence: Trp, tryptamine, IAAld, IAA, with indole-3-ethanol as a side-branch product of IAAld. N-hydroxytryptamine was not detected, but we cannot exclude that it is an intermediate between tryptamine and IAAld, nor can we rule out the possibility of a Trp-independent pathway operating in pea roots.

Reassessing the Role of N-Hydroxytryptamine in Auxin Biosynthesis

The tryptamine pathway is one of five proposed pathways for the biosynthesis of indole-3-acetic acid (IAA), the primary auxin in plants. The enzymes AtYUC1 (Arabidopsis thaliana), FZY (Solanum lycopersicum), and ZmYUC (Zea mays) are reported to catalyze the conversion of tryptamine to N-hydroxytryptamine, putatively a rate-limiting step of the tryptamine pathway for IAA biosynthesis. This conclusion was based on in vitro assays followed by mass spectrometry or HPLC analyses. However, there are major inconsistencies between the mass spectra reported for the reaction products. Here, we present mass spectral data for authentic N-hydroxytryptamine, 5-hydroxytryptamine (serotonin), and tryptamine to demonstrate that at least some of the published mass spectral data for the YUC in vitro product are not consistent with N-hydroxytryptamine. We also show that tryptamine is not metabolized to IAA in pea (Pisum sativum) seeds, even though a PsYUC-like gene is strongly expressed in these organs. Combining these findings, we propose that at present there is insufficient evidence to consider N-hydroxytryptamine an intermediate for IAA biosynthesis.

Hydrogen/deuterium exchange on aromatic rings during atmospheric pressure chemical ionization mass spectrometry.

It has been demonstrated that substituted indoles fully labelled with deuterium on the aromatic ring can undergo substantial exchange back to partial and even fully protonated forms during atmospheric pressure chemical ionisation (APCI) liquid chromatography/mass spectrometry (LC/MS). The degree of this exchange was strongly dependent on the absolute quantity of analyte, the APCI desolvation temperature, the nature of the mobile phase, the mobile phase flow rate and the instrument used. Hydrogen/deuterium (H/D) exchange on several other aromatic ring systems during APCI LC/MS was either undetectable (nitrobenzene, aniline) or extremely small (acetanilide) compared to the effect observed for substituted indoles. This observation has major implications for quantitative assays using deuterium-labelled internal standards and for the detection of deuterium-labelled products from isotopically labelled feeding experiments where there is a risk of back exchange to the protonated form during the analysis.

Synthesis and phytotoxicity of structural analogues of thaxtomin natural products

Structural analogues of the phytotoxic thaxtomin natural products have been synthesized by building upon a piperazinedione core and from l-phenylalanine. The compounds were evaluated for their phytotoxic activity against Arabidopsis thaliana seedlings and some of the key features for activity have been identified.

Chemoselective reduction of 2-acyl-N-sulfonylpyrroles: synthesis of 3-pyrrolines and 2-alkylpyrroles.

The partial reduction of pyrroles is not a common practice even though it offers a potential route to pyrroline building blocks, commonly used for synthesis. We have investigated the reduction of 2-acyl-N-sulfonylpyrroles and by varying the hydride source and solvent, achieved a chemoselective reduction, leading to 3-pyrrolines and alkyl pyrroles in high yield.

Reassessing the role of YUCCAs in auxin biosynthesis

It is remarkable that although auxin was the first growth-promoting plant hormone to be discovered, and although more researchers work on this hormone than on any other, we cannot be definitive about the pathways of auxin synthesis in plants. In 2001, there appeared to be a dramatic development in this field, with the announcement of a new gene,i and a new intermediate, purportedly from the tryptamine pathway for converting tryptophan to the main endogenous auxin, indole-3-acetic acid (IAA). Recently, however, we presented evidence challenging the original and subsequent identifications of the intermediate concerned.2

Use of the anti-oxidant Butylated Hydroxytoluene in situ for the synthesis of readily oxidized compounds: application to the synthesis of the moth pheromone (Z,Z,Z)-3,6,9-Nonadecatriene

The triene (Z,Z,Z)-3,6,9-nonadecatriene was synthesized in three steps from methyl linolenate. The key to the synthesis was the use of the anti-oxidant butylated hydroxytoluene in situ to provide protection of the unstable triene from autoxidation during reaction workup. This simple modification resulted in an increase in the yield from 20 to 85% over three steps.