Abstract
Osmanthus fragrans is an ornamental and economically important plant known for its magnificent aroma, and the most important aroma-active compounds in flowers are monoterpenes, mainly β-ocimene, linalool and linalool derivatives. To understand the molecular mechanism of monoterpene production, we analyzed the emission and accumulation patterns of these compounds and the transcript levels of the genes involved in their biosynthesis in two O. fragrans cultivars during flowering stages. The results showed that both emission and accumulation of monoterpenes varied with flower development and glycosylation had an important impact on floral linalool emission during this process. Gene expression demonstrated that the transcript levels of terpene synthase (TPS) genes probably played a key role in monoterpene production, compared to the genes in the MEP pathway. Phylogenetic analysis showed that OfTPS1 and OfTPS2 belonged to a TPS-g subfamily, and OfTPS3 and OfTPS4 clustered into a TPS-b subfamily. Their transient and stable expression in tobacco leaves suggested that OfTPS1 and OfTPS2 exclusively produced β-linalool, and trans-β-ocimene was the sole product from OfTPS3, while OfTPS4, a predictive sesquiterpene synthase, produced α-farnesene. These results indicate that OfTPS1, OfTPS2, and OfTPS3 could account for the major floral monoterpenes, linalool and trans-β-ocimene, produced in O. fragrans flowers.
Highlights
Sweet osmanthus (Osmanthus fragrans Lour.), belonging to the Oleaceae family, is a wellknown ornamental and economically important, aromatic woody plant, with the flower having a long history in China
Solid-phase microextraction-gas chromatograph (GC)-MS analysis identified a total of 33 volatile compounds in two cultivars of ‘Gecheng’ and ‘Liuye’ at the initial flowering stage (S2), assigned to monoterpene, norisoprenoid, aromatic and fatty acid-related compounds (Supplemental Table S1)
The volatile products of the OfTPS transformed plants were detected by solid-phase microextraction (SPME)-GC-MS, and the results were the same as those from transient expression in plants (Figure 7B). These results suggest that both OfTPS1 and OfTPS2 are involved in synthesis of the same terpene product, β-linalool, and OfTPS3 and OfTPS4 are related to trans-β-ocimene and α-farnesene synthesis in planta, respectively
Summary
Sweet osmanthus (Osmanthus fragrans Lour.), belonging to the Oleaceae family, is a wellknown ornamental and economically important, aromatic woody plant, with the flower having a long history in China. Because of its extremely powerful and unique aroma, flowers and the essential oils of O. fragrans are in high demand for the production of expensive perfumes, flavorings and cosmetics (Wang et al, 2009; Cai et al, 2014). The fresh flowers are very rich in floral volatiles, including terpenoids, aromatic compounds, C6 compounds and esters. The terpenoids, including β-ionone, β-ocimene, β-linalool, and linalool derivatives, have been detected as dominant components of fresh flower volatiles and essential oils (Wang et al, 2009; Sun et al, 2012; Xin et al, 2013), and are important in the aroma formation of O. fragrans (Cai et al, 2014). The molecular mechanism for the formation of monoterpenes such as β-ocimene and linalool in O. fragrans is not clear
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