## 3 Discussion
Four experimentally validated GES enzymes from other plant families (highlighted with underlining in Figure 2) were included in our phylogenetic analysis [19; 21; 42; 23]. The resulting tree revealed their distribution across distinct clades of the TPS family (TPS-b and TPS-g), and despite their substantial sequence divergence, all retained the essential NSE/DTE and DDxxD conserved motifs (Figure S3). This result suggests the occurrence of convergent evolution, where distinct TPS subfamilies independently evolved the same function and evolutionary divergence from different plant families [43]. These bioinformatics-derived characteristics not only suggested UrGES to be a typical member of the TPS family, but also provided testable hypotheses for its enzymatic function in plastitial monoterpene biosynthesis. Geraniol was identified as the major product of the recombinant UrGES protein enzymatic assays in vitro, in agreement with previous studies. For example, CoTPS4 in C. _odorata_ belongs to TPS-g and exclusively produces geraniol from GPP [20]. To provide in vivo evidence of UrGES function, the gene was heterologously expressed in tobacco. Figure 3c shows that _UrGES_-overexpressing transgenic transgenic plants produced relatively high levels of geraniol compared to the controls. These results suggest that UrGES functions specifically as a geraniol synthase. Similarly, transient _CoGES_ (_Canphora officinarum_) expression in _N. benthamiana_ resulted in increased geraniol accumulation (relative content), whereas the empty vector control only produced basal levels of geraniol [23]. Functional characterization revealed that CsTPS10 from tea plant (_C. sinensis_) failed to produce detectable metabolites in a prokaryotic expression system, whereas transient expression in _N. benthamiana_ leaves yielded measurable geraniol [35]. The ectopic expression of _LiTPS2_ from _Lilium_ 'Siberia' in tobacco significantly increased monoterpene levels in both flowers and leaves, with linalool content in transgenic flowers exceeding that of wild-type plants 2-3-fold [44]. Studies have validated geraniol synthase's function in tea plants, showing that silencing _CsTPS1-AS_ reduces geraniol accumulation, increases pathogen susceptibility, and downregulates defense-related and salicylic acid pathway genes, whereas silencing _CsTPS1_ has no significant effect, indicating its role in geraniol formation and defense through alternative splicing [19]. Given these findings, the functional characterization of UrGES in its native host is essential, particularly to validate its proposed role as the key initiating enzyme in the iridoid pathway. MeJA is involved in responses to various stresses, as well as regulating secondary metabolite accumulation, including terpenes and alkaloids [45; 46]. Numerous genes and transcriptional regulators associated with the TIA pathway in _Catharanthus. roseus_ have been shown to respond to MeJA induction [46]. The MeJA-responsive transcription factor ORCA3 enhances biosynthesis of TIA by upregulating metabolic genes in _C. roseus_, highlighting a connection between the responses of jasmonic-acid-mediated stress and plant secondary metabolite production [47]. Moreover, MeJA treatment induced significant upregulation of the _CrGES_ transcription level in _C. roseus_ cells [15]. In MeJA-treated _Rauvolifa verticillata_ hairy roots, the _RvTDC_ expression was slightly upregulated, which was positively correlated with ajmalicine accumulation [48]. Similarly, MeJA-induced TIA accumulation and the upregulation of _UrGES_ gene expression were observed in _U. nynchophylla_ leaves (Figure 5), suggesting that the biosynthesis of TIA in _U. nynchophylla_ is regulated by MeJA signaling and that _UrGES_ is an elicitor-responsive gene activated by MeJA induction. Furthermore, strong positive correlation between _UrGES_ expression and MeJA-induced TIA accumulation implies that UrGES might play a key role in TIA biosynthesis. However, the precise molecular mechanisms through which MeJA signaling regulates TIA biosynthesis in _U.