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The resinous portions of Aquilaria plants, called agarwood, have been used as medicines and incenses. Agarwood contains a great variety of sesquiterpenes, and a study using cultured cells of Aquilaria showed the production of sesquiterpenes (α-guaiene, α-humulene, and δ-guaiene) to be induced by treatment with methyl jasmonate (MJ). In this study, the accumulation and production of sesquiterpenes were quantified. The amounts accumulated and produced reached a maximum at 12 h, and the most abundant product was α-humulene at 6 h and δ-guaiene after 12 h. However, a headspace analysis of the cells revealed that α-humulene is likely to be volatilized; so overall, the most abundant sesquiterpene in the cells was δ-guaiene. A cDNA library from RNA isolated from MJ-treated cells was screened using PCR methodologies to isolate five clones with very similar amino acid sequences. These clones were expressed in Escherichia coli, and enzymatic reactions using farnesyl pyrophosphate revealed that three of the clones yielded the same compounds as extracted from MJ-treated cells, the major product being δ-guaiene. These genes and their encoded enzymes are the first sesquiterpene synthases yielding guaiane-type sesquiterpenes as their major products to be reported. Expression of a fourth terpene synthase gene in bacteria resulted in the accumulation of the protein in insoluble forms. Site-directed mutagenesis of the inactive clone and three-dimensional homology modeling suggested that the structure of the N-terminal domain was important in facilitating proper folding of the protein to form a catalytically active structure.
The genera Aquilaria and Gyrinops of the Thymelaeaceae are large evergreen trees found mainly in Southeast Asia. The resinous portions of their branches and trunks, known as agarwood, have been used in natural medicine as a digestive, sedative, and antiemetic, and also as incense. However, the source of agarwood is facing serious depletion because of uncontrolled collection in forests and the rapid loss of tropical rain forests. Consequently, Aquilaria and Gyrinops have been listed in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora since 2005 (Ito and Honda, 2008), and the international import and export of their products are under strict control.
The main fragrant compounds of agarwood are sesquiterpenes and phenylethyl chromone derivatives, and a great variety of sesquiterpenes are contained in high-quality...