Abstract
Artemisia annua, an annual herb used in traditional Chinese medicine, produces a wealth of monoterpenes and sesquiterpenes, including the well-known sesquiterpene lactone artemisinin, an active ingredient in the treatment for malaria. Here we report three new monoterpene synthases of A. annua. From a glandular trichome cDNA library, monoterpene synthases of AaTPS2, AaTPS5, and AaTPS6, were isolated and characterized. The recombinant proteins of AaTPS5 and AaTPS6 produced multiple products with camphene and 1,8-cineole as major products, respectively, and AaTPS2 produced a single product, β-myrcene. Although both Mg2+ and Mn2+ were able to support their catalytic activities, altered product spectrum was observed in the presence of Mn2+ for AaTPS2 and AaTPS5. Analysis of extracts of aerial tissues and root of A. annua with gas chromatography–mass spectrometry detected more than 20 monoterpenes, of which the three enzymes constituted more than 1/3 of the total. Mechanical wounding induced the expression of all three monoterpene synthase genes, and transcript levels of AaTPS5 and AaTPS6 were also elevated after treatments with phytohormones of methyl jasmonate, salicylic acid, and gibberellin, suggesting a role of these monoterpene synthases in plant–environment interactions. The three new monoterpene synthases reported here further our understanding of molecular basis of monoterpene biosynthesis and regulation in plant.
Highlights
Plants produce a plethora of organic compounds, among which terpenoids constitute the largest group with highly diversified structures and functionality
To analyze monoterpenes and sesquiterpenes in different parts of A. annua, fresh tissues of root, young and mature leaf, stem, and inflorescence were extracted with N-pentane and subjected to GC–MS
Artemisia triene was most abundant in stem; and β-myrcene was the major component in inflorescence but a minor one in leaf, whereas trans-carveol was detected only in leaf
Summary
Plants produce a plethora of organic compounds, among which terpenoids constitute the largest group with highly diversified structures and functionality. Extensive chemical analyses of plant extracts have demonstrated the presence of several classes of secondary metabolites, including terpenoids and flavonoids (Woerdenbag et al, 1990; Juteau et al, 2002; Lopes-Lutz et al, 2008) Among those with pharmacological activities, the sesquiterpene lactone artemisinin is widely used in the treatment of malaria, especially in the form of combination therapies (van Agtmael et al, 1999; Graham et al, 2010). 1/3 of plant monoterpene synthases characterized so far convert GPP into acyclic products (Degenhardt et al, 2009) These reactions proceed by ionization with the assistance of a divalent metal ion (usually Mg2+ or Mn2+) to the extended geranyl cation, followed by proton loss to form olefinic products including (E)-β-ocimene and β-myrcene or addition of water to form terpene alcohol such as geraniol or linalool. We cloned and functionally characterized three monoterpene synthases of A. annua: AaTPS2, AaTPS5, and AaTPS6, which produce β-myrcene, camphene, and 1,8-cineole as their major products, respectively
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