The diurnal emission of floral scent in oncidium hybrid orchid is controlled by CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) through the direct regulation on terpene synthase

Abstract

Background: To adapt the periodic fluctuation of environmental factors, plants are subtle to monitor the natural variation for the growth and development. It is well known that the daily activities and physiological functions in coordination with the natural variation are regulated by circadian clock genes. The circadian emission of floral scents is one of the rhythmic physiological activity controlled by circadian clock genes. Here, we study the circadian emission pattern and molecular mechanism of floral terpenoids compounds (ocimene/linalool) in Oncidium Sharry Baby (Onc. SB) orchid, which were diurnally emitted under light condition. Results: GC-Mass analysis revealed that Onc. SB periodically emitted ocimene and linalool during 6:00AM to 2:00PM daily. Terpene synthase, one of the key gene in the terpenoid biosynthetic pathway is expressed in coordination with scent emission. Investigation on the promoter structure of terpene synthase revealed a circadian binding site (CBS), 5’-AGATTTTT-3’ for the binding motif of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) transcription factor. EMSA data performed confirms the binding affinity between CCA1 protein and CBS of cis-acting DNA element. It suggests that the diurnal circadian emission of floral scent is controlled by CCA1.Conclusions: The work validates that the mechanism of circadian emission of floral scents in Onc. SB is controlled by the oscillator gene, CCA1(CIRCADIAN CLOCK ASSOCIATED 1) under light condition. CCA1 transcription factor regulates terpene synthase (TPS) by direct binding on CBS motif, 5’-AGATTTTT-3’ of promoter region. The circadian expression of TPS is the key gene to affect the biosynthesis and emission of floral scent in Onc. Sharry Baby.