Semiliquidambar cathayensis Hung T. Chang, is a plant species belonging to the family Altingiaceae and the genus Semiliquidambar , is endemic to China and is scarce and scattered among the mountainous regions of South Jiangxi, North Guangxi, South Guizhou, Guangdong (including Hainan Island), South Hunan, and other areas. With a long history of medicinal use, S. cathayensis Chang roots, branches, leaves, bark, and nectar are used to dispel wind, remove dampness, relax muscles, and promote blood circulation. In China, the plant is also used to treat rheumatoid arthritis, lumbar muscle strain, postpartum wind paralysis, hemiplegia, bruises, and sprains [2, 3]. As it is relatively difficult to harvest its branches, roots, and nectar, the leaves of this plant are typically boiled into medicinal solutions for medicated baths, rubs, or for oral ingestion .
Modern pharmacological studies have demonstrated that S. cathayensis Chang crude extracts exert analgesic, anti-inflammatory, and anti-hepatitis B effects and pharmacological activities, such as promoting blood circulation and removing blood stasis . Li et al.  isolated 35 alkaloids, 12 flavonoids, nine fatty acid types, seven terpenoids, seven cyclic peptide types, five phenylpropanoids, and ten other ingredients from S. cathayensis Chang. Meanwhile, using silica gel and Sephadex LH-20 column chromatography, Lu et al.  separated ten compounds: 3-acetoxy oleanolic acid methyl ester, β-sitosterol, 3-acetoxyalkene phenolic acid, 2α,3β-dihydroxy-20(29)-ene-lupin-28-acid, (24R)-5α-stigmaster-3,6-dione, betulinic acid, stearic acid, 3-keto-oleanolic acid, arjunolic acid, and carotin. Zhou et al.  reported that the ethyl acetate extract exerts effective anti-inflammatory effects upon croton oil stimulation of mouse ear and identified nine active monomer compounds—oleanolic acid, 3-carbonyl oleanolic acid, 2 α,3β-dihydroxy oleanolic acid, 2 α,3β,2 3-trihydroxy oleanolic acid, tannic acid-3,3′-di methyl ether, tannin-3,3',4-trimethyl ether, tannin-3,3'-dimethyl ether-4-O-β-D-xyloside, β-sitosterol, and stearic acid—via silica gel column chromatography, mass spectrometry, and nuclear magnetic separation. The main active ingredients of S. cathayensis Chang are secondary metabolites, with terpenoids being important components. However, because genome or transcriptome technology has not yet been applied for identifying the genes encoding active secondary metabolic components, the molecular biosynthesis mechanisms in S. cathayensis Chang remain unclear. Studies have been conducted on terpenoid biosynthesis in other traditional Chinese medicinal plants, including Catharanthus roseus , Andrographis paniculata , Aconitum napellus , Panax ginseng , and Salvia miltiorrhiza . Recent research studies have focused on the identification, characterization, expression, and regulation of genes encoding enzymes that play important roles in terpenoid biosynthesis.
High-throughput transcriptome sequencing has played an increasingly important role in the elucidation of medicinal plant functional genes , metabolic pathways, molecular regulation mechanisms, and molecular markers . Transcriptome analysis has allowed the exploration of tissue-specific gene expression patterns, clarifying secondary metabolite pathways and networks in medicinal plants . Transcriptome sequencing applied to traditional Chinese medicinal plants, such as Eucommia ulmoides , Magnolia officinalis , S. miltiorrhiza (cinnabar) root , Aquilaria sinensis (baimuxiang) , Glycyrrhiza uralensis (ural) licorice , and Blumea balsamife (ainaxiang) , has generated abundant genetic data for mining the genes involved in the key secondary metabolite synthesis pathways. In this study, we used Illumina sequencing technology to conduct comparative transcriptomic analysis on tissues from the leaves, stem epidermis, stem xylem, root epidermis, and root xylem of S. cathayensis Chang. Through the differential expression patterns of S. cathayensis Chang genes, the terpenoids metabolic mechanism could be clarified at the molecular level. Understanding the metabolic mechanisms will provide the foundation for further study on terpenoid biosynthesis, metabolic pathways, regulatory mechanisms, and resource utilization.