Article

Biosynthesis of Phenolic Glycosides from Phenylpropanoid and Benzenoid Precursors in Populus

School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931, USA.
Journal of Chemical Ecology (Impact Factor: 2.75). 02/2010; 36(3):286-97. DOI: 10.1007/s10886-010-9757-7
Source: PubMed

ABSTRACT

Salicylate-containing phenolic glycosides (PGs) are abundant and often play a dominant role in plant-herbivore interactions of Populus and Salix species (family Salicaceae), but the biosynthetic pathway to PGs remains unclear. Cinnamic acid (CA) is thought to be a precursor of the salicyl moiety of PGs. However, the origin of the 6-hydroxy-2-cyclohexen-on-oyl (HCH) moiety found in certain PGs, such as salicortin, is not known. HCH is of interest because it confers toxicity and antifeedant properties against herbivores. We incubated Populus nigra leaf tissue with stable isotope-labeled CA, benzoates, and salicylates, and measured isotopic incorporation levels into both salicin, the simplest PG, and salicortin. Labeling of salicortin from [13C6]-CA provided the first evidence that HCH, like the salicyl moiety, is a phenylpropanoid derivative. Benzoic acid and benzaldehyde also labeled both salicyl and HCH, while benzyl alcohol labeled only the salicyl moiety in salicortin. Co-administration of unlabeled benzoates with [13C6]-CA confirmed their contribution to the biosynthesis of the salicyl but not the HCH moiety of salicortin. These data suggest that benzoate interconversions may modulate partitioning of phenylpropanoids to salicyl and HCH moieties, and hence toxicity of PGs. Surprisingly, labeled salicyl alcohol and salicylaldehyde were readily converted to salicin, but did not result in labeled salicortin. Co-administration of unlabeled salicylates with labeled CA suggested that salicyl alcohol and salicylaldehyde may have inhibited salicortin biosynthesis. A revised metabolic grid model of PG biosynthesis in Populus is proposed, providing a guide for functional genomic analysis of the PG biosynthetic pathway.

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    • "This molecule is a departure from the common salicinoid molecules, e.g. salicin and salicortin, typically found in the Salicaceae, and which have been shown to be biosynthesised from phenylalanine via cinnamic acid (Babst et al. 2010). Acutifoliside Downloaded by[does not possess the salicyl alcohol unit in its structure and, therefore, represents a different class of molecule, presumably arising from different biosynthetic origins.Bartsch et al. (2010)identified 2,3-dihydroxybenzoic acid xyloside in Arabidopsis and suggested that this molecule had arisen from isochorismate and that its accumulation depended on eDS1 during ageing or resistance responses to pathogens. "
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    ABSTRACT: Ultra high-performance liquid chromatography-mass spectrometry (UHPLC-MS) profiling of a polar solvent extract of juvenile stem tissue of Salix acutifolia Willd. identified a range of phenolic metabolites. Salicortin, 1, a well-known salicinoid, was the major compound present and the study identified young stem tissue of this species as a potential source of this compound for future studies. Three further known metabolites (salicin 2, catechin 3 and tremuloidin 4) were also present. The UHPLC-MS analysis also revealed the presence of a further, less polar, unknown compound, which was isolated via HPLC peak collection. The structure was elucidated by high-resolution mass spectroscopic analysis, 1- and 2-D NMR analysis and chemical derivatisation and was shown to be a novel benzoic acid glycoside 5, which we have named as acutifoliside.
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    • "This reaction could theoretically be a source of salicyl benzoate (33), which might then be converted to salicyl HCH (36) (salicyl hydroxy-2-cyclohexen-one) and salicortin (2) directly. However, this scheme contradicts the model proposed by Babst et al. (2010), and the authors' experimental data showed a lack of conversion of salicyl alcohol (8) to salicortin (2), though the label does readily become incorporated into salicin (1). This would suggest that there is no direct conversion from salicyl alcohol (8) to salicortin (2), as might be proposed if PtSABT is within the core pathway. "
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    ABSTRACT: Salicinoids are phenolic glycosides (PGs) characteristic of the Salicaceae and are known defenses against insect herbivory. Common examples are salicin, salicortin, tremuloidin, and tremulacin, which accumulate to high concentrations in the leaves and bark of willows and poplars. Although their biosynthetic pathway is not known, recent work has suggested that benzyl benzoate may be a potential biosynthetic intermediate. Two candidate genes, named PtACT47 and PtACT49, encoding BAHD-type acyl transferases were identified and are predicted to produce such benzylated secondary metabolites. Herein described are the cDNA cloning, heterologous expression and in vitro functional characterization of these two BAHD acyltransferases. Recombinant PtACT47 exhibited low substrate selectivity and could utilize acetyl-CoA, benzoyl-CoA, and cinnamoyl-CoA as acyl donors with a variety of alcohols as acyl acceptors. This enzyme showed the greatest Km/Kcat ratio (45.8 nM-1 s-1) and lowest Km values (45.1 uM) with benzoyl-CoA and salicyl alcohol, and was named benzoyl-CoA: salicyl alcohol O-benzoyltransferase (PtSABT). Recombinant PtACT49 utilized a narrower range of substrates, including benzoyl-CoA and acetyl-CoA and a limited number of alcohols. Its highest Km/Kcat (31.8 nM-1 s-1) and lowest Km (55.3 uM) were observed for benzoyl-CoA and benzyl alcohol, and it was named benzoyl-CoA: benzyl alcohol O-benzoyltransferase (PtBEBT). Both enzymes were also capable of synthesizing plant volatile alcohol esters, such as hexenyl benzoate, at trace levels. Although the activities demonstrated are consistent with roles in salicinoid biosynthesis, direct tests of this hypothesis using transgenic poplar must still be performed.
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    • "Radiolabelled salicylaldehyde 23 was readily glucosyled to yield b-D-helacin 30 when fed to S. purpurea which, subsequently underwent reduction at the carbonyl group to give b-D-salicin 1 [7] [16]. In addition, using radiolabelled b-D-helacin 30 undergoes similar reduction to give b-D-salicin 1 [27]. "
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