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J. Biol. Chem., Vol. 283, Issue 6, 3231-3247, February 8, 2008

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A Functional Genomics Investigation of Allelochemical Biosynthesis in Sorghum bicolor Root Hairs^*

Scott R. Baerson¹, Franck E. Dayan, Agnes M. Rimando, N. P. Dhammika Nanayakkara, Chang-Jun Liu^¶, Joachim Schröder^||, Mark Fishbein^**, Zhiqiang Pan, Isabelle A. Kagan², Lee H. Pratt, Marie-Michèle Cordonnier-Pratt, and Stephen O. Duke

From the United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University, Mississippi 38677, the National Center for Natural Products Research, School of Pharmacy, University of Mississippi, University, Mississippi 38677, the ^¶Biology Department, Brookhaven National Laboratory, Upton, New York 11973, the ^||Universität Freiburg, Institut für Biologie II, Schänzlestrasse 1, D-79104 Freiburg, Germany, the ^**Department of Biology, Portland State University, Portland, Oregon 97207, and the Department of Plant Biology, University of Georgia, Athens, Georgia 30602

Sorghum is considered to be one of the more allelopathic crop species, producing phytotoxins such as the potent benzoquinone sorgoleone (2-hydroxy-5-methoxy-3-[(Z,Z)-8',11',14'-pentadecatriene]-p-benzoquinone) and its analogs. Sorgoleone likely accounts for much of the allelopathy of Sorghum spp., typically representing the predominant constituent of Sorghum bicolor root exudates. Previous and ongoing studies suggest that the biosynthetic pathway for this plant growth inhibitor occurs in root hair cells, involving a polyketide synthase activity that utilizes an atypical 16:3 fatty acyl-CoA starter unit, resulting in the formation of a pentadecatrienyl resorcinol intermediate. Subsequent modifications of this resorcinolic intermediate are likely to be mediated by S-adenosylmethionine-dependent O-methyltransferases and dihydroxylation by cytochrome P450 monooxygenases, although the precise sequence of reactions has not been determined previously. Analyses performed by gas chromatography-mass spectrometry with sorghum root extracts identified a 3-methyl ether derivative of the likely pentadecatrienyl resorcinol intermediate, indicating that dihydroxylation of the resorcinol ring is preceded by O-methylation at the 3'-position by a novel 5-n-alk(en)ylresorcinol-utilizing O-methyltransferase activity. An expressed sequence tag data set consisting of 5,468 sequences selected at random from an S. bicolor root hair-specific cDNA library was generated to identify candidate sequences potentially encoding enzymes involved in the sorgoleone biosynthetic pathway. Quantitative real time reverse transcription-PCR and recombinant enzyme studies with putative O-methyltransferase sequences obtained from the expressed sequence tag data set have led to the identification of a novel O-methyltransferase highly and predominantly expressed in root hairs (designated SbOMT3), which preferentially utilizes alk(en)ylresorcinols among a panel of benzene-derivative substrates tested. SbOMT3 is therefore proposed to be involved in the biosynthesis of the allelochemical sorgoleone.

Received for publication, August 8, 2007 , and in revised form, October 9, 2007.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) EF189706, EF189707, EF189708, and EH406574 to EH412041.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1 and Tables 1 and 2.

² Present address: U. S. Department of Agriculture, Agricultural Research Service, FAPRU, N220 Agricultural Sciences North, Lexington, KY 40546.

¹ To whom correspondence should be addressed: U. S. Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, P. O. Box 8048, University, MS 38677. Tel.: 662-915-7965; Fax: 662-915-1035; E-mail: sbaerson{at}ars.usda.gov.

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