## 1 Introduction  
Sacred lotus belongs to Nelumbonaceae, which contains only two species, sacred lotus (_Nelumboucifera_ Gaertn.) and American lotus (_Nelumbo lutea_ Pers.) [1]. Sacred lotus is one of the economically best-known aquatic plants in East Asia, with a history of cultivation for nearly 2000 years in China, and is commonly used for medicine and ornamentation [2, 3]. A large number of metabolites, including alkaloids, steroids, flavonoids, triterpenoids, glycosides, and polyphenols, were widely detected in sacred lotus and are tightly related to their pharmacological activities, such as anti-ischaemia, antioxidant, anticancer, antiviral, as well as anti-obesity [4, 5, 6, 7, 8, 9].  
As the largest group of structurally diverse metabolites, terpenoids are versatile natural compounds that act as metabolism mediators, plant volatiles, and ecological communicators. triterpenoids are important plant secondary metabolites with pharmacological effects, such as anticancer, antiviral, and cholesterol lowering. According to the number of rings in the structure, triterpenoids can be classified into monocyclic triterpenes, bicyclic triterpenes, tricyclic triterpenes, tetracyclic triterpenes, and pentacyclic triterpenes, etc. Among them, pentacyclic triterpenoids have received more attention because of more types and functions [10, 11, 12, 13]. Biologically, triterpene saponins are considered to be defensive compoundsagainst external stresses [14; 15]. Trepenoids are sequentially biosynthesized from the universal C5 precursors as dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP) [16; 17; 18]. These C5 precursors are sequentially catalyzed by prenyltransferases to form prenyl diphosphates, including geranyl pyrophosphate (GPP), farnesyl pyrophosphate (FPP), and geranylgeranyl pyrophosphate (GGPP). These prenyl diphosphates are further used as the immediate precursors for the biosynthesis of various terpenoids, such as monoterpenes (C10), sesquiterpenes (C15), diterpenenes (C20), triterpenenes (C30), tetrterpenes (C40), as well as polyterpenes (more than C40) by the action of terpene synthase (TPS). Transcription factors (TFs) regulate the accumulation of terpenoids by activating or repressing the promoters of TPS genes to control their expression and five TFs have been identified as being involved in the regulation of terpene synthesis [19; 20].  
Despite the scientific and industrial interest in sacred lotus, terpenoid components have not been identified. Moreover, the genes involved in terpenoid biosynthesis were been completely studied in aquatic botany. In recent years, RNA sequencing (RNA-Seq) and metabolite analysis were developed as powerful tools for identifying genes involved in the biosynthesis of various secondary metabolites in higher plants, including _Salvia officinalis_ [PERSON]. [21], _Cinnamomum camplora_ [PERSON][PERSON]. [10], and _Salvia guaranitica_ [PERSON]. [22].  
As one of the best-known medicinal plants, few terpenoid compounds have been identified in plant parts of \(N\). _uncifera_, and the molecular genetic basis of terpenoid biosynthesis pathways is still unveiled. Considering the specific characteristics of sacred lotus, it was of interest to profile the terpenoid biosynthesis and key genes related with the terpenoid metabolic pathway. In this study, the contents and compositions of terpenoids in seeds, young leaves, mature leaves, and flowers with different colors were analyzed with LC-MS/MS. In addition, the key genes and transcription factors involved in terpenoid biosynthesis were screened by RNA-seq. We believe these results will shed light on the mechanism of terpenoid biosynthesis in sacred lotus.