Abstract
BackgroundPhalaenopsis bellina and its closely related species, P. violacea, emit linalool, geraniol and their derivatives as the predominant monoterpenes at the full-bloom stages. Geranyl diphosphate synthase (PbGDPS) is the key enzyme that converts precursors for monoterpene biosynthesis. Besides the monoterpenes being synthesized in concert with floral development stages, we noticed that the scent emission of P. bellina and P. violacea was detected mainly in the daytime.ResultsThe monoterpenes of P. violacea flowers displayed a diurnal emission pattern, which was regulated by an internal oscillator in the treatment of constant light. In contrast, constant dark diminished the scent emission levels, indicating that light also affects monoterpene emission in P. violacea. Further treating P. violacea with various light wavelengths showed that the monoterpene emission was greatest in white light condition. Other Phalaenopsis hybrids, including P. I-Hsin Venus ‘KHM2212’ and P. Meidarland Bellina Age ‘LM128’, responded differently to various light wavelengths but most of them still showed the highest scent emission under the whole spectra of light. A great number of light-responsive, HY5-interacting, and circadian-responsive elements was enriched on the promoters of both structural genes and transcription factors for monoterpene biosynthesis. Furthermore, several putative genes encoding components involved in light and circadian signaling pathways were also identified in the transcriptome of P. bellina flowers at consecutive stages (from the anthesis day to day 7 post anthesis).ConclusionsTaken together, both circadian clock and light factors had positive effects on orchid floral scent emission, and the regulation resided on the control of both structural genes and transcription factors for monoterpene biosynthesis.
Highlights
Phalaenopsis bellina and its closely related species, P. violacea, emit linalool, geraniol and their deriva‐ tives as the predominant monoterpenes at the full-bloom stages
Effects of light on P. violacea scent emission To determine how orchid floral scent emission varied throughout the day, P. violacea plants were first transferred to a 16 h/8 h L/D condition on Dd and grown under this photoperiod for 5 days
To investigate whether the floral monoterpene emission is regulated by an internal oscillator, plants were further treated with constant light or constant dark conditions over 4 days (D + 5 to D + 8)
Summary
Phalaenopsis bellina and its closely related species, P. violacea, emit linalool, geraniol and their deriva‐ tives as the predominant monoterpenes at the full-bloom stages. Besides the monoterpenes being synthesized in concert with floral development stages, we noticed that the scent emission of P. bellina and P. violacea was detected mainly in the daytime. The floral scent is often developmentally and rhythmically regulated to be associated with the activity of corresponding pollinators (Muhlemann et al 2014). As insect pollinators exhibit rhythmic activities (Saunders 1997), the flowers emit scent on specific timing in a day (Hoballah et al 2005; Fenske and Imaizumi 2016). This diurnal/nocturnal emission is proposed to be regulated by a circadian clock. The light receptors are responsible for inputting the light signal into the circadian clock (McClung 2006)
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