## 3 Results
### Identification and Characterization of Candidate Terpenoid Biosynthesis Genes  
Fifty-six expressed unigenes encoding the terpenoid backbone biosynthesis pathway (ko00900) were identified in the KEGG pathway database based on previous transcriptomic  
Figure 1: Lactone concentrations and terpenoid biosynthesis pathway in ginkgo leaves: (**a**) Concentrations of lactones in dry leaves. All data were obtained from three biological replicates. The means were compared using [PERSON]’s multiple range test at \(p<0.05\) (*) or \(p<0.01\) (**). The error bars represent the standard deviations of the means; (**b**) MVA and MEP pathways for terpenoid biosynthesis in ginkgo determined by RNA-Seq. The enzymes are shown in green, with the exception of HMGS (red color, differentially expressed mRNA). The substrates and intermediates are shown in black, and the end products are shown in purple. The pathways that occur inside mitochondria and plastids are boxed. The arrows between the cytosol and plastid compartments represent the metabolic flow between these compartments. AACT, acetoacetyl-CoA thiolase; HMGS, 3-hydroxy-3-methylglutaryl CoA synthase; HMGR, 3-hydroxy-3-methylglutaryl CoA reductase; MK, mevalonate kinase; PMK, 5-phosphomevalonate kinase; MVD, 5-diphosphomevalonate decarboxylase; DXS, 1-deoxy-D-xylulose 5-phosphate synthase; DXR, 1-deoxy-D-xylulose 5-phosphate reductioisomerase; MCT, 2-C-methyl-D-erythmidt 4-phosphate cytidyltransferase; CMK, 4-(cytidine 5′-diphospho)-2-C-methyl-D-erythhitol kinase; HDS, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate synthase; HDR, 1-hydroxy-2-methyl-2-butenyl 4-diphosphate reductase; IDI, isopentenyl diphosphate isomerase; FPPS, farnesyl diphosphate synthase; GGPS, geranyldigmosphate synthase; MVA, mevalonic acid; MEP, 2-C-methyl-D-erythhitol 4-phosphate. The numbers in brackets of (**b**) correspond to the unigenes identified; DXS:CL24512 Conting1 unigene, CL4188 Conting1 unigene, CL16753 Conting1 unigene, CL24920 Conting1 unigene, CL2862 Conting1 unigene; DXR:CL27310 Conting1 unigene; MCT:19825 Conting1 unigene; CMK:CL29717 Conting1 unigene, CL26759 Conting1 unigene; MCS:CL20432 Conting1 unigene; HDS:CL5137 Conting1 unigene, CL25398 Conting1 unigene; HDR: CL5363 Conting1 unigene, CL26942 Conting1 unigene, CL31369 Conting1 unigene, CL2922 Conting1 unigene, CL23683 Conting1 unigene; IDI: CL19928 Conting1 unigene; AACT:CL9799 Conting1 unigene; HMGS:CL27234 Conting1 unigene, CL909 Conting1 unigene; HMGR:CL26720 Conting1 unigene, CL14958 Conting1 unigene, CL17244 Conting1 unigene; CL32074 Conting1 unigene; MK: CL25957 Conting1 unigene; PMK: CL31828 Conting1 unigene; MVD:CL11902 Conting1 unigene; FPPS: CL27679 Conting1 unigene; CL26373 Conting1 unigene, CL14415 Conting1 unigene; CL20545 Conting1 unigene; CL27568 Conting1 unigene; CL26432 Conting1 unigene; CL23650 Conting1 unigene; CL1404 Conting1 unigene; GGPS: CL20454 Conting1 unigene; CL27568 Conting1 unigene; CL26432 Conting1 unigene; CL26432 Conting1 unigene; CL26432 Conting1 unigene; CL26350 Conting1 unigene; CL4104 Conting1 unigene; comp71155_c0_seq1_2 unigene. (**c**) _GbHMGS1_ and _GbHMGS2_ gene expression patterns by the fragments per kilobase of transcript per million fragments calculation method. data [3]. In addition, 27 unigenes encoding terpenoid synthase, including six involved in monoterpenoid biosynthesis (ko00902), 15 involved in diterpenoid biosynthesis (ko00904), and six involved in sesquiterpenoid and triterpen biosynthesis (ko00909), were identified and characterized (Figure 1b). In the terpenoid backbone biosynthesis pathway (ko00900), only two _HMGS_ genes exhibited differential mRNA expression (Figure 1b). We selected the _HMGS_ genes for preliminary investigation and found that the _GbHMGS1_ gene was more highly expressed than the _GbHMGS2_ gene by the fragments per kilobase of transcript per million fragments calculation method (Figure 1c), and thus, we selected the _GbHMGS1_ gene for an in-depth study of its function.