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Published on March 18, 2009; 10.1104/pp.109.135848

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Received January 18, 2009
Accepted March 3, 2009

An Aldehyde Oxidase in Developing Seeds of Arabidopsis thaliana Converts Benzaldehyde to Benzoic Acid

Mwafaq Ibdah , Ying-Tung Chen , Curtis G. Wilkerson , and Eran Pichersky *

Department of Molecular, Cellular and Developmental Biology, University of Michigan, 830 North University Street, Ann Arbor, MI 48109-1048 (MI, Y-TC, EP); Department of Energy Plant Research Laboratory and Michigan Proteome Consortium, Michigan State University, East Lansing, MI 48824 (CGW)

* Corresponding author; email: lelx{at}umich.edu.

Arabidopsis siliques synthesize high levels of benzoic acid (BA) which is incorporated into several glucosinolate compounds. The origin of BA in the siliques has not yet been determined. Here we show that siliques have higher levels of benzaldehyde (BD) oxidizing activity relatives to leaves. The BD oxidizing activity was purified from siliques in several chromatographic steps, and a 145 kDa protein was identified as the enzyme most likely to possess this activity. The protein was trypsinized and the sequence of the resulting peptides was determined by Mass Spectrometry, identifying it as the product of gene At1g04580, also designated as AAO4. AAO4 had previously been shown to be highly and specifically expressed in developing seeds, and its protein to belong to a family of aldehyde oxidases. Here we show that the AAO4 protein is an aldehyde oxidase that can use several substrates but that, among the substrates tested, has the lowest Km value (23 µM) with BD. AAO4 is able to oxidize BD without NAD+, but its activity increases by 50% when this cofactor is added. The pH optimum of AAO4 is 7.0. Plants homozygous for a null allele in AAO4 showed a reduction of 30-45% in the total levels of BA in seeds, as well as a 7-9% and 32-38% decreases in the levels of 3-benzoyloxypropylglucosinolate and 4-benzoyloxybutylglucosinolate, respectively. Expressing AAO4 in Escherichia coli resulted in 3-fold increase of BD-oxidizing activity in crude bacterial extracts over endogenous levels. These findings indicate that in A. thaliana seeds oxidation of BD contributes in part to the synthesis of BA.






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