Terpene Synthase-b and Terpene Synthase-e/f Genes Produce Monoterpenes for Phalaenopsis bellina 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. Phalaenopsis bellina is a scented orchid with monoterpenes as major VOCs, comprising linalool, geraniol, and their derivatives. Comparative transcriptomics analysis identified four terpene synthase-b (TPS-b) genes and two TPS-e/f genes with differential gene expression between scented and scentless Phalaenopsis species. Here, we confirmed their differential expression between scented and scentless Phalaenopsis orchids and excluded one TPS-b candidate. We analyzed the temporal and spatial expression and functionally characterized these TPSs. Both TPS-b and TPS-e/f 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 PbTPS5 and PbTPS10. The TPS-b genes are expressed exclusively in reproductive organs, whereas the TPS-e/f genes are expressed in both reproductive and vegetative organs. In planta functional characterization of both PbTPS5 and PbTPS10 in tobacco and scentless Phalaenopsis plants did not produce terpenoids. Further ectopic expression in scented Phalaenopsis cultivar P. I-Hsin Venus showed that linalool was the main product, with PbTPS10 displaying 3-fold higher activity than PbTPS5. On in vitro enzyme assay with purified recombinant TPS-b proteins ectopically expressed in Escherichia coli, geraniol was the product catalyzed by PbTPS5 and PbTPS9. PbTPS3 was a linalool/(beta)-cis-ocimene synthase and PbTPS4 a linalool synthase. In conclusion, both TPS-b and TPS-e/f enzymes orchestrated floral monoterpene biosynthesis in P. bellina.

Automated summary

Orchids are highly diverse plants that interact with pollinators via volatile organic compounds. Scented orchids like Phalaenopsis bellina have specific genes related to floral scent biosynthesis, which show increased expression around blooming time.