Synthesis of 'cineole cassette' monoterpenes in Nicotiana section Alatae: gene isolation, expression, functional characterization and phylogenetic analysis.

The scent bouquets of flowers of Nicotiana species, particularly those of section Alatae, are rich in monoterpenes, including 1,8-cineole, limonene, β-myrcene, α- and β-pinene, sabinene, and α-terpineol. New terpene synthase genes were isolated from flowers of Nicotiana bonariensis, N. forgetiana, N. longiflora, and N. mutabilis. The recombinant enzymes synthesize simultaneously the characteristic 'cineole cassette' monoterpenes with 1,8-cineole as the dominant volatile product. Interestingly, amino acid sequence comparison and phylogenetic tree construction clustered the newly isolated cineole synthases (CIN) of section Alatae together with the catalytically similar CIN of N. suaveolens of section Suaveolentes, thus suggesting a common ancestor. These CIN genes of N. bonariensis, N. forgetiana, N. longiflora, and N. mutabilis are distinct from the terpineol synthases (TERs) of the taxonomically related N. alata and N. langsdorfii (both Alatae), thus indicating gene diversification of monoterpene synthases in section Alatae. Furthermore, the presence of CINs in species of the American section Alatae supports the hypothesis that one parent of the Australian section Suaveolentes was a member of the present section Alatae. Amino acid sequences of the Nicotiana CINs and TERs were compared to identify relevant amino acids of the cyclization reaction from α-terpineol to 1,8-cineole.