Characterisation of an (S)-linalool synthase from kiwifruit (Actinidia arguta) that catalyses the first committed step in the production of floral lilac compounds

Kiwifruit (Actinidia spp. Lindl.) flowers and fruit contain many compounds of interest to the flavour and fragrance industries. In particular, Actinidia arguta (Sieb. et Zucc.) Planch. ex Miq. flowers produce β-linalool and important derivatives thereof, including linalool oxides, lilac aldehydes, alcohols and alcohol epoxides. Dynamic headspace sampling of whole A. arguta flowers showed that the peak emission rate of linalool, lilac alcohols and lilac aldehydes occurred around 0800 hours. After solvent extraction, linalool levels remained constant throughout the day and night, but lilac alcohol levels peaked at noon. In whole flowers, linalool was found predominantly in pistils and petals, and the lilac compounds were found mainly in petals. Two highly homologous (96.6% nucleotide identity) terpene synthase cDNA sequences, AaLS1 and ApLS1, were isolated from A. arguta and Actinidia polygama (Sieb. et Zucc.) Maxim flower EST libraries respectively. Real-time PCR analysis revealed that AaLS1 was expressed constitutively throughout the day and night, and primarily in petal tissue. Functional analysis in Escherichia coli showed that AaLS1 and ApLS1 each encoded a linalool synthase which was confirmed by transient expression in planta. Enantioselective gas chromatography revealed that both terpene synthases produced only (S)-(+)-linalool. AaLS1, therefore, is likely to be the key enzyme producing the (S)-linalool precursor of the lilac alcohols and aldehydes in A. arguta flowers.