## 3 Discussion
In our study, _OfCYP227_, which has high sequence identity to _AtCYP73A5_, showed consistently high expression in all organs and was extremely highly expressed in flowers. Seven genes similar to _OfCYP227_, which were specifically expressed in blooming flowers, were selected to verify their expression patterns (Figure 5A). The results indicated that these _OfCYP_s had high expression levels in flowers and may play a critical role in the synthesis and emission of fragrance by _O. fragrans_ flowers. The expression of genes involved in stress responses usually responds differently to different stresses [42]. The analysis of RNA-seq data under abiotic stress showed that some _OfCYP_s were upregulated, but the highest expression level was either at 12 h (_OfCYP142_), 24 h (_OfCYP196_), 72 h (_OfCYP248_), or 120 h (_OfCYP262_) in response to cold stress. _OfCYP142_ was also upregulated after salt treatment for 6 and 72 h (Figure 6)-thus this gene was induced by both cold and salt stress. In addition, _OsCYP94C2D_ was demonstrated to endow rice with salinity tolerance [21]. _OfCYP142_ has very high homology with _OsCYP94C2b_, further confirming that _OfCYP142_ was induced by salt stress. Previous studies have shown that CYP proteins that participate in monoterenoid metabolism are usually co-expressed with terpene synthases [16; 43], and gene co-expression analysis has been used to predict which _P450_ genes are involved in monoterenoid metabolism [16]. _OfTPS2_ was reported to play an important role in the synthesis of linalool in _O. fragrans_[44]. To gain insight into the functions of _P450_ genes in linalool synthesis and metabolism, the relationships between the expression patterns of _OfCYP_s and _OfTPS2_ were investigated (Figure 5C). _OfCYP142_ which had an expression pattern with the highest positive correlation with _OfTPS2_, was selected as a candidate gene to further explore its function. Consistent with the above predictions, _OfCYP142_ overexpression in _O. fragrans_ petals promoted linalool accumulation (Figure 7C), implying that _OfCYP142_ may play a critical role in stimulating the synthesis of linalool. Linalool is formed by a dephosphorylation reaction and a hydroxylation reaction of geranyl pyrophosphate (GPP) catalyzed by TPS [45]. Many _CYP_ genes have been reported to be involved in the process of linalool hydroxylation conversion to linalool oxides [16; 17]. We speculate that _OfCYP142_ may be involved in the hydroxylation reaction of GPP to promote the synthesis of linalool. A previous study demonstrated that expression of several MEP monoterpene substrate pathway genes preceding terpene formation was positively correlated with corresponding downstream volatile terpen [46]. Transcriptome profiling of the transgenic flowers indicated that many key regulation genes in the MEP pathway showed an upregulated expression trend (Figure 8B). The co-ordinated regulation relationships are found in most of MEP pathway genes [47]. We speculate the overexpression of _OfCYP142_ could trigger some co-ordinated regulation mechanisms, and upregulate the expression levels of cor responding MEP pathway genes which can promote the floral VOCs accumulation. The results of tobacco transformation further verified the above conclusion, with _OfCYP142_ overexpression significantly elevating some floral VOCs including linalool, leaf alcohol, and (-)-caryophyllene oxide in tobacco flowers (Figure 9E-H). Leaf alcohol is present in a wide range of foods, beverages, and fine fragrances, due to its characteristic intense smell of fresh cut grass or leaves [48; 49]; and (-)-caryophyllene oxide is a major representative sesquiterpenoid, and an important resource for organic and medicinal chemistry [50; 51]. Many _CYP_ genes have been reported to participate in sesquiterpene biosynthesis [52]. These results imply that _OfCYP142_ is not only involved in linalool biosynthesis but can also promote accumulation of the other floral VOCs.