## 1 Introduction  
_Gleditsia sinensis_ Lam. (Fabaceae) is a widely used traditional herbal medicine in China for treating various diseases and is widely distributed worldwide [1]. In China, there are six _Gleditsia_ species, including _G. sinensis_, _Gleditsia australis_ Wild., and two varieties: _Gleditsia japonica_ var. _delavayi_ (Franch.). [PERSON] and _Gleditsia japonica_ var. _Velutina_ [PERSON]. [2]. _G. sinensis_ is used for environmental protection landscaping and is an economically valuable tree species. Additionally, the pods are one of the primary medicinal materials in _G. sinensis_[3] and possess significant medicinal properties, including antimicrobial [4], anticancer [5], anti-inflammatory [6], and anticoagulant activities [7]. As a medicinal plant, the value of _G. sinensis_ pods is becoming increasingly important, with _G. sinensis_ being planted in rural areas more commonly for its pods in many countries [8]. Moreover, the main bioactive component is gleditsioside, which belongs to the triterpene saponins. Triterpenoid saponins are triterpene compounds containing one or more glycosyl groups connected to triterpene sapogenins [9]. The precursors of these active compounds are terpenoids, including monoterpenes, sesquiterpenes, diterpenes, triterpenes, and tetraterpenes. Terpenoids are the largest class of secondary metabolites produced by plants and play significant roles in plant growth and development [10]. The biosynthesis of triterpene saponins consists of three stages: terpene precursor biosynthesis, triterpene backbone biosynthesis, and the generation of various triterpene saponins. The first stage involves generating two types of terpene precursors: isopentenyl pyrophosphate (IPP) and its isomer, dimethylallyl diphosphate (DMAPP) [11]. These precursors are produced through two isoprenoid biosynthetic pathways: the mevalonate (MVA) pathway and the 2-C-methyl-d-erythritol 4-phosphate/1-deoxy-d-xylulose 5-phosphate (MEP/DOXP) pathway [12,13]. Interestingly, most plants, including _G. sinensis_, employ the classic cytosolic MVA pathway and chloroplastic MEP/DOXP pathway to produce IPP and DMAPP. In contrast, certain algae may possess a novel plasidic MEP/DOXP pathway, deviating from the conventional routes [14,15]. Furthermore, five-carbon units form via the cytosolic MVA pathway to serve primarily as precursors for the biosynthesis of sesquiterpenoids, phytosterols, brassinosteroids, triterpenoids, and other terpenoids in the cytosol and endoplasmic reticulum, as well as for the biosynthesis of ubiquinones in mitochondria. IPP and DMAPP derived from the plastidial MEP/DOXP pathway are preferentially used in the formation of hemi-, mono-, di-, sester-, and tetraterpenoids (such as carotenoids and their breakdown products), phytohormones (e.g., cytokins and gibberellins), chlorophyll, tocopherols, and plastoquinones [16]. Over the past few years, next-generation sequencing has been extensively used to explore novel genes underlying the biosynthesis of saponins and specialized terpenoids. Additionally, researchers have revealed the presence of noncanonical routes and gene clusters involved in forming precursors and downstream terpenoids in a few plant species, which adds to the complexities of terpenoid biosynthesis. The identification of genes encoding enzymes (such as cytochrome P450 monooxygenases (P450s) and UDP-glycostransferases) that catalyze distinct steps related to the biosynthetic pathway of saponins has been carried out in _Panax ginseng_ [PERSON]. [PERSON] and _Bacopa montieri_ ([PERSON]) [PERSON]. [17,18]. Comparative transcriptome analysis of root and leaf tissues has been reported to identify transcripts related to saponin biosynthesis in a few plants, such as _Hedera helix_ [PERSON], _Panax notoginseng_, and _Asparagus racemosus_ [PERSON]. [19, 20, 21, 22]. Elucidation of regulatory factors and gene clusters involved in the biosynthesis of specialized terpenoids in different plant species provides insights into enhancing the biosynthesis of specialized terpenoids.