Abstract
Osmanthus fragrans Lour. is one of the top 10 traditional ornamental flowers in China famous for its unique fragrance. Preliminary study proved that the terpenoids including ionone, linalool, and ocimene and their derivatives are the dominant aroma-active compounds that contribute greatly to the scent bouquet. Pollination observation implies the emission of aromatic terpenoids may follow a circadian rhythm. In this study, we investigated the variation of volatile terpenoids and its potential regulators. The results showed that both volatile and non-volatile terpenoids presented circadian oscillation with high emission or accumulation during the day and low emission or accumulation during the night. The volatile terpenoids always increased to reach their maximum values at 12:00 h, while free and glycosylated compounds continued increasing throughout the day. The depletion of non-volatile pool might provide the substrates for volatile emission at 0:00–6:00, suggesting the sequestration of non-volatile compounds acted like a buffer regulating emission of terpenoids. Further detection of MEP pathway genes demonstrated that their expressions increased significantly in parallel with the evident increase of both volatile and non-volatile terpenoids during the day, indicating that the gene expressions were also closely associated with terpenoid formation. Thus, the expression of MEP pathway genes and internal sequestration both played crucial roles in modulating circadian rhythm of terpenoid emission in O. fragrans.
Highlights
Most plants release diverse blends of low molecular weight and high vapor pressure organic compounds from flowers, leaves and fruits into the atmosphere
The results showed that 14 MEP pathway genes including DXS1, DXS2, DXR, CMK1, CMK2, MCT1, MCT2, MECPS, HDS, IDS1, IDS2, IDS3, IDI, GPPS basically confined to uniform circadian oscillation
Volatile terpenoids tended to increase quickly at 6:00–12:00 h, coinciding with the significant increase of MEP pathway genes, TPSs, CCD1, and CCD4 expressions. This result demonstrated that the diurnal fluctuations of the terpenoid emission were probably a result of the rhythmic gene expressions
Summary
Most plants release diverse blends of low molecular weight and high vapor pressure organic compounds from flowers, leaves and fruits into the atmosphere. These volatile compounds represent approximately 1% of the whole secondary metabolites (Dudareva et al, 2004). By emitting aromatic compounds from flowers, plants attract pollinators and repel antagonists to serve plant fitness (Yu et al, 2014; Florian, 2015). The specific circadian rhythm of volatile terpenoids emission and corresponding regulator along with potential biological function remain unclear
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