## 1 Introduction  
The cytochrome P450 (CYP) monooxygenenses, known as multifunctional oxidases, are encoded by a complex gene superfamily. This was the first group of enzymes classified as a \"super family\" and named after CYP because of the absorption peak at 450 nm produced by the combination of P450 and reductive carbon monoxide [1, 2]. A large number of _CYP_ genes have been reported in many plants including wheat (_Tritcium aestivum_), castor (_Ricinus communis_), and avocado (_Persen americana_) fruit [3, 4, 5, 6, 7]. Based on the similarities of the amino acid sequences encoded by the genes and the criterion of phylogeny and homology, the _CYP_ genes are divided into 11 clans: seven single-family clans (CYP51/74/97/710/711/727/746) and four multiple-family clans (CYP71/72/85/86). Each clan is named according to the lowest family number in the group [2, 8, 9]. Each CYP protein has about 500 amino acids, and the tertiary construction of the protein is generally configured by 13 alpha helices and four beta sheets. At present, determining if a gene belongs to the CYP450 gene family mainly depends on whether there is a conserved heme-binding domain (FxGxRxCxG) in its structure. If this exists, the gene can be grouped into the CYP450 gene family [10]. Over 20,000 CYP gene family members have been characterized but the functions of most remain to be revealed [8]. The CYP proteins with a broad range of catalytic activity are distributed in all branches in the evolution of life and participate in various secondary metabolic processes of plants [2]. Plant terpenoids have an impressive structural diversity. Among them, linalool, colimene, _cis_-linalool oxide, and _trans_-linalool oxide make major contributions of volatiles to the fragrance of sweet osmanthus (_Osmanthus fragrans_) [11; 12]. Complex combinations of terpen are the common constituents of the characteristic aromas of different cultivars [13]. Various volatile terpenoids are formed directly by catalysis using terpenoid synthases (TPSs). Previous studies provided evidence that cytochrome P450 enzymes involved in monoterrepenoid metabolism are usually co-expressed with TPSs [14; 15]. Gene co-expression analysis was used to predict which _P450_ genes were involved in monoterrepenoid metabolism [16]. The expression of _CYP76C1_ from Arabidopsis (_Arabidopsis thaliana_) is tightly coregulated with that of _TPS10_ and _TPS14_. These genes work together to promote and modulate the emission of linalool and the production of soluble carboxylic linalool in opening flowers [17]. _AtTPS10_ and _AtTPS14_ were also co-expressed in the same plant tissue with _AtCYP71B31_ and _AtCYP76C3_, respectively, and this tight co-expression pattern reveals linalool-coupled production and oxidation in Arabidopsis flowers [16]. Remarkably, _AtCYP71B31_ and _AtCYP76C3_, which belong to distinct subfamilies, are important regulators in oxidative metabolism of both (R)- and (S)-enantiomers of linalool in Arabidopsis [16]. In addition, other members of the CYP76C subfamily from Arabidopsis such as _CYP76C_ and _CYP76C4_ are also involved in linalool metabolism, synthesizing the main product 8-hydroxylinalool with 9-hydroxylinalool formed as a minor product [18]. Linalool and linalool derivatives not only comprise floral fragrance but also perform defense against floral antagonists [19; 20]. Many _CYP_ genes also participate in stress responses, including to biological and abiotic stress. In rice, _CYP94C2b_ promotes jasmonic acid metabolism and endows plant salinity tolerance [21]. In Arabidopsis, _AtCYP709B1_, _AtCYP09B2_, and _AtCYP709B3_ play roles in salt and abscisic acid stress [22]; and _AtCYP79B2_, _AtCYP79B3_, _AtCYP71A12_, _AtCYP71A13_, and _AtCYP1B15_ are involved in synthesis of camalexin-a phytoalexin with antibacterial, antifungal, and antiproliferative activities [23; 24; 25; 26]. Many studies have focused on the _CYP_ genes that play critical roles in plant stress responses and the synthesis of floral volatile organic compounds (VOCs) [16; 21; 27].