## 1 Introduction  
Belonging to the Rubiaceae family, _Unaria rhynchophylla_ (Miq.) Miq. ex Havil. holds an important status in traditional Chinese medical practices and is primarily employed in the treatment of hypertension, convulsions, epilepsy, eclampsia, and encephalopathy [1]. The principal active constituents in _U. rhynchophylla_ are terpenoid indole alkaloids (TIAs), including rhynchophyllline, isorhynchophylline, corynoxeine, and isocorynoxeine [2], which have demonstrated therapeutic potential in managing Alzheimer's disease, hypertension, and liver injury [3, 4, 5]. However, the current supply of these alkaloids is insufficient to meet medicinal demands due to their low natural abundance and the high cost of traditional cultivation methods. As such, synthetic biology and metabolic engineering strategies are promising alternatives, necessitating the mining of functional genes participating in TIA pathway. Advances in high-throughput sequencing technologies have enabled genome and transcriptome sequencing of various medicinal plants, including alkaloid-rich species such as _Camptotheca acuminata_[6], _Ophiorhiza pumila_[7], and _Dendrobium officinal_[8], thereby facilitating the identification of candidate biosynthetic genes and advancing our understanding of alkaloid secondary metabolite pathways. In _U. rhynchophylla_, several enzymes participating in TIA biosynthesis, along with key regulatory transcription factors, have been discovered. Through combined metabolomic and transcriptomic analyses, three key enzymes--strictoisidine-\(\beta\)-D-glucosidase (UrSGD1), strictosidine synthase (UrSTR2), and L-tryptophan decarboxylase (UrTDC)--have been identified and functionally validated in vitro [9]. Additionally, the overexpression of _UrSTR1_ and _UrSTR5_ in transgenic hairy roots resulted in increased TIA accumulation, whereas RNA interference (RNAi)-mediated silencing of these genes led to a decline in TIA levels [10]. Under low-light conditions, the phytochrome-interacting factor UrPIF3 was shown to enhance the expression of _UrSGD_ and _UrSTR_, whereas its silencing reduced isorhychophylline content and significantly suppressed these genes [11]. Furthermore, the basic helix-loop-helix transcription factor UrbHLH1 has been observed to interact with promoters of 10-hydroxygeraniol dehydrogenase (_Ur10 HG0_) and geraniol 10-hydroxylase (_UrG10H_), implicating a regulatory role in TIA biosynthesis [12]. Geranyl pyrophosphate (GPP) serves as an immediate precursor of monoterpenes and, catalyzed by terpene synthase (TPS), forms various monoterpene compounds [13]. GPP is produced by the condensation reaction of dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP), with these precursor molecules being synthesized via two distinct metabolic routes: the plastid-localized methylerythnitol phosphate (MEP) pathway and the cytosolic mevalonate (MVA) pathway [14]. Accordingly, the classic TPS-based pathway in which geraniol synthase (GES) catalyzes GPP to produce geraniol in plastids [15] and a specific mechanism for the catalytic reaction of Nudix hydrolase in the cytoplasm [16]. The iridoid biosynthetic cascade commences with GPP conversion to geraniol; subsequently, secologanin is generated through a series of enzymatic reactions and condenses with indole pathway-derived tryptamine to form strictosidine, which is the direct precursor of TIAs such as vinblastine [15], camptothecin [17], and rhynchophylline [18]. As a TPS family member, GES is able to catalyze the conversion of GPP to geraniol [19]. _GES_ genes were characterized across diverse plants, and encoded proteins were capable of catalyzing the geraniol formation in vitro [20, 21, 22, 23]. In addition, a cytoplasmic-located Nudix hydrolase, RhNUDX1, was discovered in rose that catalyzes the conversion of GPP to geranyl monophosphate, which then further generates geraniol in the presence of a petal-derived phosphatase [24].