Abstract
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.
Highlights
The tryptamine pathway is one of five proposed pathways for the biosynthesis of indole-3-acetic acid (IAA), the primary auxin in plants
N-hydroxytryptamine was analyzed by liquid chromatography (LC)-electrospray ionisation (ESI) mass spectrometry (MS) (Fig. 2A), along with commercially obtained serotonin (5-hydroxytryptamine; Fig. 2B) and tryptamine (Supplemental Fig. S1; both compounds were obtained from Sigma-Aldrich)
To determine whether tryptamine acts as an IAA precursor in other parts of the pea plant, we investigated IAA synthesis in young developing seeds, about 14 d postanthesis (DPA), when the seeds weighed between 40 and 130 mg and still contained liquid endosperm
Summary
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. It has been reported that AtYUC1 (Arabidopsis thaliana; Zhao et al, 2001) and its putative homologs in other species (Exposito-Rodrıguez et al, 2007; LeClere et al, 2010) catalyze the formation of N-hydroxytryptamine from tryptamine Both Zhao et al (2001) and LeClere et al (2010) based this conclusion on mass spectral identification of the product of an in vitro assay, using recombinant YUC (from Arabidopsis and maize [Zea mays], respectively) as the enzyme and tryptamine as the substrate. We reexamine the tryptamine pathway by determining the mass spectral properties of our previously synthesized N-hydroxytryptamine (Quittenden et al, 2009) for comparison with
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call