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J. Biol. Chem., Vol. 282, Issue 7, 4326-4335, February 16, 2007

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Functional Characterization of Desaturases Involved in the Formation of the Terminal Double Bond of an Unusual 16:3^{9, 12, 15} Fatty Acid Isolated from Sorghum bicolor Root Hairs^*

Zhiqiang Pan¹², Agnes M. Rimando¹, Scott R. Baerson, Mark Fishbein, and Stephen O. Duke

From the United States Department of Agriculture, Agricultural Research Service, Natural Products Utilization Research Unit, University of Mississippi, University, Mississippi 38677 and the Department of Biology, Portland State University, Portland, Oregon 97207

Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is likely responsible for much of the allelopathic properties of sorghum root exudates against broadleaf and grass weeds. Previous studies suggest that the biosynthetic pathway of this compound initiates with the synthesis of an unusual 16:3 fatty acid possessing a terminal double bond. The corresponding fatty acyl-CoA serves as a starter unit for polyketide synthases, resulting in the formation of 5-pentadecatrienyl resorcinol. This resorcinolic intermediate is then methylated by an S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated, yielding the reduced (hydroquinone) form of sorgoleone. To characterize the corresponding enzymes responsible for the biosynthesis of the 16:3 fatty acyl-CoA precursor, we identified and cloned three putative fatty acid desaturases, designated SbDES1, SbDES2, and SbDES3, from an expressed sequence tag (EST) data base prepared from isolated root hairs. Quantitative real-time RT-PCR analyses revealed that these three genes were preferentially expressed in sorghum root hairs where the 16:2 and 16:3 fatty acids were exclusively localized. Heterologous expression of the cDNAs in Saccharomyces cerevisiae revealed that recombinant SbDES2 converted palmitoleic acid (16:1⁹) to hexadecadienoic acid (16:2^9,12), and that recombinant SbDES3 was capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3^9,12,15). Unlike other desaturases reported to date, the double bond introduced by SbDES3 occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain. Collectively, the present results strongly suggest that these fatty acid desaturases represent key enzymes involved in the biosynthesis of the allelochemical sorgoleone.

Received for publication, July 3, 2006 , and in revised form, December 18, 2006.

^* 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 nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) EF206346 [GenBank] , EF206347 [GenBank] , and EF206348 [GenBank] .

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed: USDA-ARS Natural Products Utilization Research Unit, P. O. Box 8048, University, MS 38677. Tel.: 662-915-1548; Fax: 662-915-1035; E-mail: zpan{at}msa-oxford.ars.usda.gov.

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				F. E. Dayan, J. Howell, and J. D. Weidenhamer Dynamic root exudation of sorgoleone and its in planta mechanism of action J. Exp. Bot., May 1, 2009; 60(7): 2107 - 2117. [Abstract] [Full Text] [PDF]

				S. R. Baerson, F. E. Dayan, A. M. Rimando, N. P. D. Nanayakkara, C.-J. Liu, J. Schroder, M. Fishbein, Z. Pan, I. A. Kagan, L. H. Pratt, et al. A Functional Genomics Investigation of Allelochemical Biosynthesis in Sorghum bicolor Root Hairs J. Biol. Chem., February 8, 2008; 283(6): 3231 - 3247. [Abstract] [Full Text] [PDF]