## 2 Results
### Expression Analysis of Genes Related to Terenoid Synthesis and Metabolism in C. mollissima  
Terpenoid compounds mainly involve four KEGG processes, including the terpenoid backbone synthesis pathway (ko00900), monoterpenoid biosynthesis (ko00902), diterpenoid biosynthesis (ko00904), and sesquiterpenoid and triterpenoid biosynthesis (ko00909). A total of 31 differentially expressed genes related to terpenoid biosynthesis were screened (Figure 4). The terpenoid backbone synthesis pathway is the initial step for the synthesis of various terpenoid compounds. The terpenoid backbone synthesis pathway consisted of the MVA (Mevaluation) pathway and MEP (2-C-methyl-D-erythriitol 4-phosphate) pathway [11; 12]. In the MVA pathway, Acetyl-CoA biosynthesis IPP (Isopentenyl-PP ) though the enzymes of AACT (Acetyl-CoA acetyltransferase 1), HMGS (Hydroxymethylglutaryl-CoA synthase), HMGR (3-hydroxy-3-methylglutaryl-coenzyme A reductase), MK (Mevaluation kinase), PMK (Phosphomevalonate kinase), and PPMD (Diphosphomevalonate decarboxylase) [11]. In this study, _HMGCS (EVM0020603)_, _HMGCR (EVM0028745)_, and _PMK (EVM0002367)_ were up-regulated during the FFS period. In the MEP pathway, Pyruvate and GA-3-P biosynthesize Dimethylallyl-PP (DPP) though the enzymes of DXS (1-deoxy-D-xylulose 5-phosphate synthetase), DXR (1-deoxy-D-xylulose 5-phosphate reductoisomerase), MCT (2-C-Methyl- D-erythriitol 4-phosphate), CMK (4-(Cvidine 5'-diphospho)-2-C-methyl-D-erythriitol), MDS (2-Phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythriitol), HDS (2-C-Methyl- D-erythriitol 2,4-cyclodiphosphate), and HDR (4-Hydroxy-3-methylbut-2-enyl-diphosphate). Only _DXS (EVM0009849)_ and _HDS (EVM0009307)_ were differentially expressed between the IFS and FFS stages, and _DXS_ was down-regulated at the FFS stage, while _HDS_ was up-regulated at the FFS stage. IPP and DPP convert under the action of IDI (isopentenyl-diphosphate Delta-isomerase). In this study, _IDI (EVM0027391)_ expression was up-regulated during the FFS stage. Subsequently, IPP was transformed into FPP (Farnesyl-PP). FPP synthesizes sesquiterpenoids and triterpenoids or converts to trans-farnesol under the action of PCME (prenylcysteine alpha-carboxyl methylesterase). Trans-farnesol can also be transformed into FPP under the action of FLDH (NAD(P)-binding Rossmann-fold superfamily protein). The expression of _PCME (EVM0028365)_ and _FLDH (EVM0004964)_ were also up-regulated at the FFS stage. In the MEP pathway, DPP firstly transformed into GPP (Geranyl diphosphate), and then synthesizes diterpenoid [11]. The first key enzyme gene _Probable terpene  
Figure 3: Volcano plot of differentially expressed genes between the IFS and FFS. _syntthase \(12\) PTS12 (EVM0020228)_ involved in the GPP synthesis of monoterpenoids was up-regulated at the FFS stage. In the monoterpenoids synthesis pathway, there were seven (+)-Neomenthol biosynthesis gene _(+)-neomenthol dehydrogenase SDRs_, all of which are up-regulated during the FFS stage. There are six differentially expressed genes in the direct-penoid biosynthesis pathway, among which the key genes _Ent-kaur-16-ene synthase GA2 (EVM0014037)_ and _Ent-kaurenoic acid oxidase KAO (EVM0030158)_ for GA12 synthesis are up-regulated during the FFS stage, while the expression levels of _Ent-kaurene oxidase GA3 (EVM0024918, EVM0027610)_, _Gibbrellin 2-beta-dioxygenase GA2 ox (EVM0022124)_, and _Gibbrellin 20 oxidase GA20 ox (EVM0006681)_ are down-regulated. Six differentially expressed genes were upregulated in the sesquiterpenoid and triterpenoid biosynthesis pathway, including _Probable alpha alpha-trehalose-phosphate synthase [UDP-forming] 2 TPS2 (EVM0023005)_, _Squielene epoxidase 1 SQEI (EVM0021771)_, _(E,E)-alpha-faresene synthase AFS1 (EVM0011286)_, _(-)-germacrene D synthase GERD2 (EVM0029474)_, _Taraxerol synthase TARS (EVM0008352)_, and _Beta-amyrin synthase BAS (EVM0005414)_. _TPS2_ and _GERD2 (EVM0029474)_ are responsible for synthesizing germacrene-type sesquiterpenoids, while _AFS1_ is a key gene for acyclic sesquiterpenoids.