Elucidation of the Biosynthetic Pathway of the Allelochemical Sorgoleone Using Retrobiosynthetic NMR Analysis

Abstract

NMR analyses of the labeling pattern obtained using various 13 C-labeled precursors indicated that both the lipid tail and the quinone head of sorgoleone, the main allelopathic component of the oily root exudate of Sorghum bicolor, were derived from acetate units, but that the two moieties were synthesized in different subcellular compartments. The 16:3 fatty acid precursor of the tail is synthesized by the combined action of fatty-acid synthase and desaturases most likely in the plastids. It is then exported out of the plastids and converted to 5-pentadecatriene resorcinol by a polyketide synthase. This resorcinol intermediate was identified in root hair extracts. The lipid resorcinol intermediate is then methylated by a S-adenosylmethionine-dependent O-methyltransferase and subsequently dihydroxylated by a P450 monooxygenase to yield the reduced form of sorgoleone.Sorgoleone (2-hydroxy-5-methoxy-3-[(Z,Z)-8Ј,11Ј,14Ј-pentadecatriene]-p-benzoquinone) (1) is the main allelopathic component of the oily root exudate of sorghum (Sorghum bicolor (L.) Moench) ( Fig. 1) (1-3). The term sorgoleone is also used to describe a group of lipophilic p-benzoquinones structurally related to 1 that are also produced by sorghum roots having a hydroxy and a methoxy substitution at positions 2 and 5, respectively, and either a 15-or 17-carbon aliphatic tail with various degrees of unsaturation at position 3 (3).The primary mechanism of phytotoxic action of 1 is associated with inhibition of photosynthesis in higher plant systems by competing for the binding site of plastoquinone on photosystem II (4 -6). This lipophilic p-benzoquinone is also known to hinder electron transfer reactions involved in mitochondrial respiration and to inhibit the enzyme p-hydroxyphenylpyruvate dioxygenase (7,8).The herbicidal and allelopathic properties of 1 make isolation of the genes responsible for its biosynthesis desirable, as manipulation of those genes in sorghum or their introduction in other plant species could provide a better understanding of the role of 1 in plant-plant interaction and natural weed control. To identify the genes and characterize the enzymes they encode, it is necessary to determine the steps involved in the biosynthesis of 1. Little has been reported on this subject.Sorgoleone is found exclusively as a hydrophobic droplet exuding from the tips of root hairs (2), and a recent investigation of sorghum roots suggested that the production of 1 is compartmentalized in the highly physiologically active root hairs (9). The biosynthesis of secondary metabolites occurring in root hairs has not been studied in detail, although other specialized cells at the interface between a plant surface and its environment (i.e. trichomes) are known to produce unique secondary metabolites (10 -12).It has been postulated that the biosynthesis of 1 and related resorcinolic lipids is the result of the convergence of two pathways, namely the fatty acid biosynthetic pathway for the synthesis of the aliphatic tail and the activity of polyketide synthase-...

A Stilbene Synthase Gene (SbSTS1) Is Involved in Host and Nonhost Defense Responses in Sorghum

“…Alternatively, SbSTS1 may utilize substrates other than phenylpropanoid-CoA esters leading to the formation of a more complex secondary metabolite in sorghum. Dayan et al (2003) demonstrated recently that an STS-type reaction is involved in the biosynthesis of the sorghum root exudate sorgoleone. Sorgoleone and its derivatives are benzoquinone-containing aliphatic tails of 15 or 17 carbons with various degrees of unsaturation (Netzly et al, 1988).…”

Probing allelochemical biosynthesis in sorghum root hairs

“…Subsequently, both the predicted 5-n-pentadecatrienyl resorcinol as well as a 3-methyl ether derivative of this compound were identified in sorghum root extracts, indicating that dihydroxylation of the resorcinol ring is preceded by O-methylation at the 3-hydroxyl position. 14,16 The resulting chemically unstable hydroquinone rapidly oxidizes to the bioactive sorgoleone benzoquinone once released into the rhizosphere, where it may persist in soil for extended periods. 7,8,17 De novo synthesis of sorgoleone from available palmitoleoyl-CoA, would therefore likely require, at a minimum, the participation of a Δ 12 and Δ 15 fatty acid desaturase (DES), ARS, a 3-O-methyltransferase (OMT), and a cytochrome P450 (Fig.…”

Temporal expression of the sor1 gene and inhibitory effects of Sorghum bicolor L. Moench on three weed species

“…Although the cellular localization and the steps of the biosynthetic pathway of sorgoleone have been determined (Dayan et al 2003;Pan et al 2007;Baerson et al 2008aBaerson et al , 2008b, there is a lack of studies investigating its differential expression among genotypes and its bioactivity in vivo. Therefore, the evaluation of accessions that show variability for sorgoleone synthesis could contribute to the selection of the top lines in breeding programs for food production, as well as for weed control, in farming systems.…”

“…Molecular studies have described the metabolic pathway of sorgoleone, and a number of genes involved in the event cascade have been identified and characterized (Buchanan et al 2000;Dayan et al 2003;Pan et al 2007;Baerson et al 2008aBaerson et al , 2008b. According to Dayan et al (2003), the gene sor1, which encodes a membrane desaturase, is the main precursor of sorgoleone synthesis.…”

“…According to Dayan et al (2003), the gene sor1, which encodes a membrane desaturase, is the main precursor of sorgoleone synthesis.…”