Transcriptional regulation of volatile terpenoid biosynthesis in phalaenopsis orchids

A synchronized floral development and monoterpenes biosynthesis as floral scents are assumed to attract pollinators in Phalaenopsis bellina, but their coordination regulation is still unclear. To unveil the developmental regulation of monoterpenes biosynthesis, we analyze the volatiles and transcriptomes of the scent and scentless orchids at various floral developmental stages. We identify differentially expressed structural genes and isolate the geranyl diphosphate gene PbGDPS, whose ectopic transient expression leads to monoterpene elevation in scentless orchids. The GDPS promoters from scent (designated PbGDPSp) and scentless (designated PaGDPSpA and PaGDPSpB) orchids are next isolated. The promoter activity of PbGDPSp and PaGDPSpA is much higher in the scent than in the scentless orchids. The nucleotide sequence of PaGDPSpB differs from the other two, and its transactivation is barely detectable in planta. We then isolate five transcription factors (TFs) with sequential elevation expression throughout the floral development in P. bellina, and transient assays indicate that they transactivate promoters of structural genes. Further ectopic transient expression of the five TFs in scentless orchids demonstrates that their roles in promoting monoterpene production. Our study reveals an elaborate control of monoterpenes biosynthesis involving sequential and coordinated expression of TFs and structural genes throughout the orchid floral development.