Diurnal regulation of the floral scent emission by light and circadian rhythm for pollinator attraction in phalaenopsis orchids

Floral scent plays a vital role in pollinator attraction. Full-bloom flowers of Phalaenopsis bellina and its closely related species, P. violacea, emit linalool, geraniol, and their derivatives as the predominant monoterpenes. Geranyl diphosphate synthase (PbGDPS) is the critical enzyme producing the precursor, geraniol diphosphate, for monoterpene biosynthesis. Besides the monoterpenes synthesized in concert with floral development stages, we notice that the scent emission of P. bellina is detected mainly in the daytime. The monoterpenes of P. violacea flowers display a diurnal emission pattern, which is regulated by an internal oscillator in the treatment of constant light. In contrast, constant dark diminishes the scent emission levels, indicating that light is also essential for monoterpenes emission in P. violacea. Further treating P. violacea with various light wavelengths shows that the monoterpenes emission is highest in white light conditions. Other Phalaenopsis hybrids, included P. Meidarland Bellina Age "LM128" and P. I-Hsin Venus, respond differently to various light wavelengths, but most of them still show the highest scent emission under the whole spectra of light. Transcriptomic analysis of P. bellina flowers at consecutive stages shows that several genes involved in light and circadian signaling pathway display concurrent expression patterns as those of PbGDPS. These include PHYTOCHROME (PHY), CRYPTOCHROME (CRY), CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1), CIRCADIAN CLOCKASSOCIATED 1 (CCA1), and GIGANTEA (GI). Furthermore, a significant higher number of light-responsive and HY5-interacting elements are present on the promoters of five transcription factors regulating monoterpene biosynthesis than that on the promoters of structural genes for monoterpene biosynthesis. Taken together, both circadian clock and light factors have positive effects on orchid floral scent emission, and the regulation resides on the control of TFs for monoterpene biosynthesis.