Abstract
The monoterpenes linalool and its oxides are the key aroma-active compounds in Osmanthus fragrans Lour. flowers. The glycosides of these monoterpenes accumulate throughout flowering, leading to considerable storage of potential aroma constituents that account for the majority of non-volatile aroma compounds. However, the UDP-glycosyltransferase (UGT) responsible for the glycosylation of linalool and its oxides has not been clarified. Four candidate OfUGTs (UGT85A82, UGT85A83, UGT85AF3, and UGT85A84) with high homology to the known terpenoid UGTs were screened by transcriptome sequencing. Over-expression of the candidate OfUGTs in tobacco showed that UGT85A84 glycosylated linalool oxides in planta. Since the transcript levels of UGT85A84 were positively correlated with glycoside accumulation, the recombinant UGT85A84 protein was subjected to reactions with aglycones and sugar donors. Two formate adducts were exclusively detected in UDP-Glc with linalool and linalool oxide reactions by liquid chromatography-mass spectrometry (LC-MS), indicating that UDP-Glc was the specific sugar donor. The kinetic parameters demonstrated that UGT85A84 glycosylated both linalool and lianlool oxides in vitro. Further analysis demonstrated that the transcription levels of MEP pathway genes might play an important role in mediating terpenoid glycosylation. Our findings unraveled the mechanism underlying the glycosylation of essential aroma compounds in flowers. This study will facilitate the application of potential aroma contributors in future industries.
Highlights
Osmanthus fragrans Lour., one of the most popular ornamental plants in China, is appreciated for its unique and pleasant aroma (Zheng et al, 2017)
The results showed that considerable glycosides of linalool and its oxides were accumulated in the leaves with substrate injection (CK), while no glycoside was detected from wild-type tobacco leaves, indicating that the injection effectively promoted the absorption and conversion of the substrates by the leaves
Volatile aroma compounds have been extensively studied for decades because they act as pollinator attractant for better reproduction and enhance the aroma value of flowers and fruits (Keisuke et al, 2013)
Summary
Osmanthus fragrans Lour., one of the most popular ornamental plants in China, is appreciated for its unique and pleasant aroma (Zheng et al, 2017). Previous studies have shown that terpenoids, such as linalool, ionone, ocimene, and their derivatives are the key characteristic aroma-active compounds in both natural fresh flowers and in artificial industrial products (Wang et al, 2009; Cai et al, 2014). Linalool and its oxides account for a large proportion of aroma-active compounds in all O. fragrans cultivars. They impart floral and citrus notes to the flowers and corresponding products. They are identified as the principle desirable aroma compounds (Cai et al, 2014)
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