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United States Department of Agriculture

Agricultural Research Service

Title: Functional Characterization of Desaturases Involved in the Formation of the Terminal Double Bond of a Unique 16:3 Fatty Acid Isolated from Sorghum Bicolor Root Hairs

Authors
item Pan, Zhiqiang
item Rimando, Agnes
item Baerson, Scott
item Fishbein, Mark - PORTLAND STATE UNIVERSITY
item Duke, Stephen

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 2006
Publication Date: February 16, 2007
Citation: Pan, Z., Rimando, A.M., Baerson, S.R., Fishbein, M., Duke, S.O. 2007. Functional characterization of desaturases involved in the formation of the terminal double bond of a unique 16:3 fatty acid isolated from sorghum bicolor root hairs. Journal of Biological Chemistry. V. 282(7):4326-4335.

Interpretive Summary: Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is responsible for much of the allelopathic properties of sorghum root exudates against broadleaf and grass weeds. Biosynthetic pathway of sorgoleone begins with the synthesis of a 16:0 fatty acid desaturated to 16:3, having a terminal double bond. This fatty acyl-CoA serves as a starter unit for polyketide synthases, followed by a number of steps involved in a series of reactions by a methyltransferase and P450 monooxygenases to yield the reduced form of sorgoleone. Three putative fatty acid desaturases, designated SbDes1, SbDes2 and SbDes3, were cloned for the study of the terminal desaturation of the fatty acid in the pathway. These genes were identified from an expressed sequence tag (EST) database containing approximately 5,500 sequences randomly selected from a sorghum cDNA library prepared from isolated root hairs. Heterologous expression of the cDNAs in S. cerevisiae resulted in the conversion of palmitoleic acid to hexadecadienoic acid by the recombinant SbDes2, and the conversion of hexadecadienoic acid into hexadecatrienoic acid by recombinant SbDes3. The double bond introduced by SbDes3 occurred between carbons 15 and 16 resulting in a terminal double bond aliphatic chain.

Technical Abstract: Sorgoleone, produced in root hair cells of sorghum (Sorghum bicolor), is 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 begins with the synthesis of a 16:0 fatty acid desaturated to 16:3, having a terminal double bond. This fatty acyl-CoA serves as a starter unit for polyketide synthases, resulting in the formation of 5-pentadecatriene resorcinol. This resorcinol intermediate is then methylated by a SAM-dependent O-methyltransferase and dihydroxylated by a P450 monooxygenase to yield the reduced (hydroquinone) form of sorgoleone. To characterize the corresponding enzymes responsible for the terminal desaturation 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) database containing approximately 5,500 sequences randomly selected from a sorghum cDNA library 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 S. cerevisiae revealed that recombinant SbDes2 converted palmitoleic acid (16:1- delta9) to hexadecadienoic acid (16:2-delta9, 12), and that recombinant SbDes3 was capable of converting hexadecadienoic acid into hexadecatrienoic acid (16:3-delta9,12,15). 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.

Last Modified: 7/25/2014