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.
Tivendale
Nathan D
ND
School of Plant Science, University of Tasmania, Hobart, Tasmania 7001, Australia.
Davies
Noel W
NW
Molesworth
Peter P
PP
Davidson
Sandra E
SE
Smith
Jason A
JA
Lowe
Edwin K
EK
Reid
James B
JB
Ross
John J
JJ
D017360:Arabidopsis / Q000378:metabolism; D002853:Chromatography, Liquid; D007210:Indoleacetic Acids / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D012701:Serotonin / Q000502:physiology*; D021241:Spectrometry, Mass, Electrospray Ionization; D053719:Tandem Mass Spectrometry; D003313:Zea mays / Q000378:metabolism
D007210:Indoleacetic Acids; D012701:Serotonin