## 3 Biosynthesis of Lily Fragrance Components
### Biosynthesis Pathways of Terpenoids  
Terpenoid volatiles are the most abundant components of lily fragrance, mainly including monoterreponoids and sesquiterpenoids [7; 29]. These are formed via two pathways: the C5 precursor of monoterreponoids is synthesized through the methylerythnitol phosphate (MEP) pathway in plastids, derived from pyruvate and glyceraldehyde-3-phosphate (G3P). One molecule of dimethylallyl diphosphate (DMAPP) and one molecule of IPP condense head-to-tail to form geranyl diphosphate (GPP), which is then catalyzed by monoterreponid synthase to produce monoterreponoids [7], such as (_E_)-\(\beta\)-ocimene and myrcene. The cytosolic mevalonate (MVA) pathway, derived from acetyl-CoA, synthesizes isopentenyl diphosphate (IPP), and two molecules of IPP and one molecule of DMAPP condense to form farresyl diphosphate (FPP), which is then catalyzed by sesquiterpenoid synthase to produce sesquiterpenoids [30].  
High activation levels of the genes encoding _DXS_, _DXR_, _HDS_, _HDR_, _IDI_, and _GPS_/_GPPS_ in the upstream MEP pathway promote the synthesis of monoterreponoid volatiles [20]. Transferring _LiDXS_ and _LiDXR_ resulted in increased levels of monoterreponids and sesquiterpenoids such as cryophyllene and linalool in tobacco [31]. _LiMCT_ from _Lilium_ 'Sorbonne' plays a crucial role in regulating the synthesis of monoterreponids and other isoprenoid precursors in transgenic _Arabidopsis_[32]. Additionally, [PERSON] et al. identified the genes related to terpenoid biosynthesis, such as _LiDXS2_, _LiLIS_, and _LiMYS_, in _Lilium_ 'Sorbonne', and confirmed their involvement in monoterreponoid biosynthesis through heterologous transformation in _Arabidopsis_[33]. In the transcriptome of _Lilium_ 'Siberia', the _MCT_, _CMK_, and _MDS_ genes encoding enzymes in the MEP pathway were identified, along with six genes encoding enzymes in the MVA pathway, including two _AACT_, one _HMGS_, two _HMGR_, and one _MK_. The expression of these key genes varies with floral scent changes but is more complex during the circadian rhythm [21]. In studies on the gene function of terpenoid synthases, the expression levels of _TPS_ genes in seven ly varieties were strongly positively correlated with the content of monoterreponoids such as linalool and \(\beta\)-ocimene [11]. In the transcriptome of _Lilium_ 'Siberia', _19 TPS_ genes were identified, including linalool synthase (_LIS_), (_E_)-\(\beta\)-ocimene synthase, myrcene synthase, and sesquiterpenoid (3S,6E)-nerolidol synthase (_NES_) genes. Most of these _TPS_ gene expression changes are related to terpenoid biosynthesis during flower development and the circadian rhythm [21]. In enzyme function studies, most researchers have chosen the fragrant 'Siberia' lily as a model plant to study the molecular mechanisms of lily fragrance biosynthesis. Five _TPS_ genes have been successfully cloned, including _LoTPS1_, which is localized in plastids and catalyzes the formation of (_Z_)-\(\beta\)-ocimene and linalool from GPP, and _LoTPS3_, which is localized in mitochondria and catalyzes the formation of linalool from GPP and _cis_-nerolidol from FPP [19]. _LoTPS2_, localized in the cytoplasm, catalyzes the formation of (_E_,_E_)-\(\alpha\)-farnesene from FPP, while _LoTPS4_, a multi-product enzyme localized in plastids, catalyzes the formation of D-limonene and \(\beta\)-ocimene from GPP and _trans_-bergamotene from FPP [18]. _LoTPS5_, localized in plastids, catalyzes the formation of squalene from FPP [34]. Other teams have also successfully isolated _LiTPS2_ from 'Siberia', a terpene synthase localized in the chloroplasts that catalyzes the formation of linalool and _trans_-nerolidol from GPP and FPP, respectively. Overexpression 'Siberia' _LiTPS2_ in tobacco significantly increased the release of linalool, myrcene, and (_E_)-\(\beta\)-ocimene [35]. Additionally, 'Sorbonne' (LsoTPS-1, LsoTPS-2, and LsoTPS-3) and 'Red Life' (LlrTPS-1 and LrlTPS-3) were able to convert GPP into (_E_)-\(\beta\)-ocimene, \(\alpha\)-pinene, and limonene.