## 2 Biosynthesis and Key Regulatory Networks of Volatile Terpenoids
### Regulatory Determinants of Volatile terpenoid Biosynthesis  
#### 2.2.1 Key Enzymes  
HMGR, DXS, and DXR serve as key regulatory checkpoints in the upstream metabolism of VTs, governing their biosynthetic cascades through the MVA and MEP pathways. HMGR is widely recognized as the key rate-limiting enzyme of the MVA pathway, controlling terpenoid flux in the cytosol [32]. HMGR orthologs have been cloned and functionally characterized in diverse plant systems, including _Arabidopsis thaliana_[33], _Salvia miltiorrhiza_[34], _Ginkgo biloba_[35], _Populus trichocarpa_[36], and _Glycine max_[37]. In _Vitis vinifera_, three HMGR paralogs (_VvHMGR1/2/3_) have been identified, among which _VvHMGR3_ has been functionally validated to enhance aroma volatile accumulation [38]. Transcriptomic analyses of roses revealed a strong positive correlation between terpene accumulation and the expression levels of _HMGR02_ and _HMGR10_[39]. Similarly, targeted suppression of _HMGR_ in _Jasminum sambac_ significantly reduced \(\alpha\)-farnesene emission, confirming its regulatory role in floral volatile biosynthesis [40]. DXR functions as a critical regulatory node in volatile terpenoid biosynthesis, catalyzing the isomerization and NADPH-dependent reduction of 1-deoxy-D-xylulose-5-phosphate (_DOXP_) to generate MEP [41]. This reaction represents the primary rate-limiting step in the DOXP/MEP metabolic pathway [42]. The transcript levels of _DXR_ exhibit a strong positive correlation with downstream terpenoid production. In _Arabidopsis thaliana_, genetic upregulation of _DXR_ led to a 2.3-fold increase in isoprenoid accumulation [43]. Functional characterization of _LCDXR_ from _Litsea cubeha_ demonstrated its constitutive expression across roots, stems, leaves, and fruits, with transgenic overexpression significantly enhancing monoterpene biosynthesis. Notably, heterologous expression of _LCDXR_ in _Nicotiana benthumiana_ resulted in a 5.9-fold increase in key monoterpene, including limonene, \(\alpha\)-pinene, 1,8-cineole, linalool, and terpineol [44], highlighting its conserved regulatory role across plant lineages. DXS functions as the first committed and rate-limiting enzyme in the MEP pathway. Its functional orthologs have been identified across diverse plant species, including _Arabidopsis thaliana_[45], _Mentha \(\times\) piperta_ L. [46], and tomato [47]. Overexpression of _DXS_ in _Arabidopsis_ resulted in a significant increase in terpene accumulation [48]. Transient overexpression of _AcDXS_ in kiwifruit (_Actinidia chinensis_) markedly upregulated the expression of the monoterpene synthase gene _AcTPS1_[49]. In grapevine (_Vitis vinifera_ L), _VvDXS_ transcript levels were positively correlated with linalool concentrations and total terpenoid content [50]. In _Ginkgo biloba_, a _DXS_ homolog exhibited constitutive expression across vegetative tissues, with _methyl jasmonate (MeJA)-inducible regulation and a positive correlation with ginkgolide biosynthesis [51]. Collectively, these findings establish _DXS_ as a central regulatory node governing terpenoid metabolic flux. Glycosyltransferases (GTs) comprise a crucial enzyme family that catalyzes the conjugation of free terpenoids into glycosylated, non-volatile derivatives, thereby enabling their intracellular storage [52; 53]. In tea (_Camelia sinensis_), terpenoid aroma compounds predominantly exist as glycosidic precursors, with low-temperature exposure inducing a concurrent accumulation of volatile neerolidol and its glycosylated form. Through an integrated analysis of cold-induced gene expression and enzymatic activity screening, UDP-glycostransferase _UGT91Q2_ was identified as the specific enzyme catalyzing meridold glycosylation. _UGT91Q2_ exhibits strict substrate specificity toward nerolidol, facilitating its conversion from a free monoterpene to a glycosidally bound form [54].