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Applied and Environmental Microbiology, April 2005, p. 1803-1810, Vol. 71, No. 4
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.4.1803-1810.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Azospirillum brasilense Produces the Auxin-Like Phenylacetic Acid by Using the Key Enzyme for Indole-3-Acetic Acid Biosynthesis

E. Somers,1 D. Ptacek,1 P. Gysegom,1 M. Srinivasan,2 and J. Vanderleyden1*

Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium,1 Yara Technology Centre, Hydro Research Park, Porsgrunn, Norway2

Received 23 August 2004/ Accepted 28 October 2004

An antimicrobial compound was isolated from Azospirillum brasilense culture extracts by high-performance liquid chromatography and further identified by gas chromatography-mass spectrometry as the auxin-like molecule, phenylacetic acid (PAA). PAA synthesis was found to be mediated by the indole-3-pyruvate decarboxylase, previously identified as a key enzyme in indole-3-acetic acid (IAA) production in A. brasilense. In minimal growth medium, PAA biosynthesis by A. brasilense was only observed in the presence of phenylalanine (or precursors thereof). This observation suggests deamination of phenylalanine, decarboxylation of phenylpyruvate, and subsequent oxidation of phenylacetaldehyde as the most likely pathway for PAA synthesis. Expression analysis revealed that transcription of the ipdC gene is upregulated by PAA, as was previously described for IAA and synthetic auxins, indicating a positive feedback regulation. The synthesis of PAA by A. brasilense is discussed in relation to previously reported biocontrol properties of A. brasilense.

* Corresponding author. Mailing address: Departement Toegepaste Plantwetenschappen, Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, Faculteit Landbouwkundige en Toegepaste Biologische Wetenschappen, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium. Phone: 32 16 321631. Fax: 32 16 321966. E-mail: Jozef.Vanderleyden{at}agr.kuleuven.ac.be.

Applied and Environmental Microbiology, April 2005, p. 1803-1810, Vol. 71, No. 4
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.4.1803-1810.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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