and Genes Produce Monoterpenes for Floral Scent.

Orchids are the most species-rich plants and highly interactive with pollinators via visual or olfactory cues. Biosynthesis and emission of volatile organic compounds (VOCs) to the atmosphere facilitate the olfactory cues and ensure successful pollination. is a scented orchid with monoterpenes as major VOCs, comprising linalool, geraniol, and their derivatives. Comparative transcriptomics analysis identified four () genes and two genes with differential gene expression between scented and scentless species. Here, we confirmed their differential expression between scented and scentless orchids and excluded one candidate. We analyzed the temporal and spatial expression and functionally characterized these . Both and genes showed an increased expression on blooming day or 3 days post-anthesis (D + 3) before the optimal emission of floral scent on D + 5, with especially high expression of and . The genes are expressed exclusively in reproductive organs, whereas the genes are expressed in both reproductive and vegetative organs. functional characterization of both and in tobacco and scentless plants did not produce terpenoids. Further ectopic expression in scented cultivar I-Hsin Venus showed that linalool was the main product, with displaying 3-fold higher activity than On enzyme assay with purified recombinant TPS-b proteins ectopically expressed in , geraniol was the product catalyzed by and . was a linalool/(β)-cis-ocimene synthase and a linalool synthase. In conclusion, both and enzymes orchestrated floral monoterpene biosynthesis in .