A Functional Genomics Investigation of Allelochemical Biosynthesis in Sorghum Bicolor Root Hairs

Authors: Baerson, Scott; Dayan, Franck; Rimando, Agnes; NANAYAKKARA, N.P. DHAMMIKA - UNIVERSITY OF MISSISSIPPI; LIU, CHANG-JUN - BROOKHAVEN NATIONAL LAB; SCHRODER, JOACHIM - UNIV. FREIBURG-GERMANY; FISHBEIN, MARK - PORTLAND STATE UNIV.; Pan, Zhiqiang - Peter; Kagan, Isabelle; PRATT, LEE - UNIVERSITY OF GEORGIA; CORDONNIER-PRATT, MARIE-MICHELE - UNIVERSITY OF GEORGIA; Duke, Stephen

Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2007
Publication Date: 2/8/2008
Citation: Baerson, S.R., Dayan, F.E., Rimando, A.M., Nanayakkara, N., Liu, C., Schroder, J., Fishbein, M., Pan, Z., Kagan, I., Pratt, L.H., Cordonnier-Pratt, M., Duke, S.O. 2007. A Functional Genomics Investigation of Allelochemical Biosynthesis in Sorghum Bicolor Root Hairs. Journal of Biological Chemistry. 283(6):3231-3247.

Interpretive Summary: The sorghum plant produces compounds in the cells that cover the root system (root hairs) that inhibit the growth of other plant species. Sorgoleone is one of these compounds. Chemical analysis of the secretions produced by these cells identified a compound probably corresponding to one of the pathway intermediates produced by a type of enzyme called an O-methyltransferase, thus suggesting the type of compound normally used by this enzyme inside the sorghum plant. A database of gene sequences expressed in root hair cells was assembled to help identify the O-methyltransferase gene involved in manufacturing sorgoleone. We have studied the chemical reactions performed by one of the O-methyltransferase genes we found in the database, and it appears to have the properties one would expect for an O-methyltransferase enzyme involved in sorgoleone production. In the manuscript we propose that this enzyme, which we call SbOMT3, could be involved in the production of sorgoleone in sorghum root hair cells.

Technical Abstract: 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 analogues. 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 which 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 SAM-dependent O-methyltransferases and dihydroxylated by cytochrome P450 monooxygenases, although the precise sequence of reactions have not previously been determined. Analyses performed by GC-MS with sorghum root extracts identified a 3-methoxy derivative of the pentadecatrienyl 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. Additionally, an EST data set comprised of 5,469 sequences selected at random from a 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 RT-PCR and recombinant enzyme studies with putative O-methyltransferase sequences obtained from the EST data set have led to the identification of a novel root hair-specific O-methyltransferase which preferentially utilizes alk(en)ylresorcinolinic substrates (designated SbOMT3), proposed to be involved in the biosynthesis of sorgoleone.