Tangy scent in Toona sinensis (Meliaceae) leaflets: isolation, functional characterization, and regulation of TsTPS1 and TsTPS2, two key terpene synthase genes in the biosynthesis of the scent compound

Toona sinensis (Chinese Mahogany; Meliaceae), a subtropical deciduous tree, has a tangy scent resembling a mix of shallots and garlic. T. sinensis has long been known for its medicinal efficacy for treating enteritis, dysentery, itch and some cancers. However, its volatile components and their biosynthesis remain unexamined. In this study, we identified the spectrum of volatile compounds, isolated and functionally characterized two terpene synthase genes, Tstps1 and Tstps2, responsible for terpenoid synthesis in T. sinensis leaflets. TsTPS1 and TsTPS2 afford multiple products upon incubation with geranyl and farnesyl diphosphate respectively and mainly regulate the biosynthesis of (+) limonene and β- elemene in vitro, respectively. Headspace analyses show that 98% of leaflet volatiles were sesquiterpenoids and the developing leaflets released a greater diversity and quantity of volatiles than the mature leaflets did, and that β-elemene was the dominant component in both of them. These data suggested that tangy scent of T. sinensis consists of a combination of terpenoids and that Tstps2 was the major gene involved in the terpenoid biosynthesis in T. sinensis. In situ hybridization revealed that glandular cells of the leaf rachises accumulated abundant Tstps1 mRNA transcripts. Our GFP-based assay further unprecedentedly demonstrated that the transit-peptide of TsTPS1 targets specifically to the mitochondria.