Poster: Secondary Metabolism

Abs # 274: Biosynthesis of Sorgoleone: Complementary Retrobiosynthetic NMR and Root Hair Specific EST Analyses

Presenter:	Dayan, Franck E., fdayan@ars.usda.gov
Authors	Dayan, Franck E. (A)   Kagan, Isabelle A (A)   Rimand, A M (A)   Baerson, S  (A)
Affiliations:	(A): USDA-ARS Natural Products Utilization Research Unit

Sorgoleone is the main allelopathic component of the oily root exudate of sorghum (Sorghum bicolor). The putative biosynthetic pathway of this secondary metabolite was determined by combined chemical and molecular approaches. The labeling pattern obtained using various ¹³C-labeled precursors indicated that both the lipid tail and the quinone head of sorgoleone were derived from acetate units, but that the two moieties were synthesized in different subcellular compartments. The tail, derived from an unusual 16:3 fatty acid precursor synthesized in the plastids, is exported into the cytosol and converted into 5-pentadecatriene resorcinol by a polyketide synthase. This resorcinol intermediate, which was identified in root hair extracts, is subsequently methylated by a SAM-dependent O-methyltransferase and dihydroxylated by a P450 monooxygenase to yield the reduced form of sorgoleone. Earlier ultrastructural studies of S. bicolor roots demonstrated that the biosynthesis of sorgoleone is localized in root hair cells. Consequently, expressed sequence tag (EST) analysis was performed on approximately 5,000 random sequences from a cDNA library generated from purified root hair cells Highly expressed candidate sequences representing all of the putative target enzyme classes suggested by the retrobiosynthetic study, including fatty acid desaturases, polyketide synthases, O-methyltransferases, and P450 monooxygenases, were identified within the root hair EST data set. Functional analysis of some of these genes has thus far led to the characterization of a resorcinol specific O-methyltransferase, exhibiting a root hair-specific expression pattern, which seems likely to participate in the methylation of a key intermediate in sorgoleone biosynthesis.