## 4 Discussion
Our results show that the genes related to terpene synthesis of resistant _P. thunbergii_ and susceptible _P. thunbergii_ inoculated with PWNs were significantly upregulated (except GGPPS in susceptible _P. thunbergii_), but the gene expression levels of resistant _P. thunbergii_ were higher. Monoterpenes, sesquiterpenes and diterpenes are formed by the condensation of dimethylallyl diphosphate (DMAPP) with isopentenyl diphosphates (IPP) catalyzed by specific prenyltransferases [40]. HMDH1 is an upstream gene regulating DMAPP and IPP, while GGPPS is an upstream gene regulating diterpenoid synthesis (Figure 4). The results indicated that the differences in terpene synthesis pathways between _P. thunbergii_ with different levels of resistance may be concentrated in monoterpene and sesquiterpene pathways. This may be a potential explanation for the variations in terpenoid synthesis-related gene expression in _P. thunbergii_ of different resistances. It was found that the terpenoids in _P. thunbergii_ with different levels of resistance were mostly monoterpenoids and sesquiterpenoids, which was consistent with our prediction of terpenoid gene expression. Among the specific terpenoids of resistant _P. thunbergii_, although myrtrenal, linalool, \(\alpha\)-thujene, caryophyllene oxide and sabinene have bactericidal, insecticidal and anti-inflammatory effects, there is no report regarding their lethality against PWNs [40, 41, 42]. In addition, the analysis of common and specific terpenoids indicates that resistant _P. thunbergii_ can contain more terpenoids (resistant _P. thunbergii_: 39; susceptible _P. thunbergii_: 28). According to our analysis of the differences in the contents of terpenes shared by the differentially resistant _P. thunbergii_, we found that \(\alpha\)-pinene, \(\beta\)-pinene and caryophyllene were all present at high relative percentages in _P. thunbergii_ with different levels of resistance, with the contents in resistant _P. thunbergii_ always higher than those in susceptible _P. thunbergii_, suggesting that shared terpenes may have an inhibitory effect on PWNs and that differences in their contents may be one of the reasons for the resistance in pine trees. There may be localized increases in terpene concentrations around the infection site which may explain PWN activity and reproduction that is not captured in needle proportions. The contents of \(\alpha\)-pinene and \(\beta\)-pinene in highly resistant _P. massoniana_ were higher than those in the low-resistance _P. massoniana_[21], but [PERSON] et al. demonstrated that the \(\alpha\)- and \(\beta\)-pinene contents for highly resistant _P. massoniana_ were lower [41]. In the present study, the differences in the \(\alpha\)-pinene content were consistent with the former, while the differences in the \(\beta\)-pinene content were consistent with the latter. There were significantly more induced terpenoid in resistant _P. thunbergii_ (6) than susceptible _P. thunbergii_ (1). Among the terpenes specific to resistant _P. thunbergii_, pseudolimonene, \(\alpha\)-thujene and sabinene were present in large percentages, with sabinene being an induced terpene. It is speculated that terpenoid uniquely to resistant _P. thunbergii_ may inhibit the invasion, growth and population size of PWNs. [PERSON] found that the major enzymatic products of \(\alpha\)-pinene synthase (_PmtPS4_, primarily producing \(\alpha\)-pinene) and longfolene synthase (_PmtPS2_, produced \(\alpha\)-pinene) had inhibitory effects on PWNs in vitro [42], and \(\alpha\)-pinene may have the ability to kill PWNs. [PERSON] et al. [21] detected \(\alpha\)-pinene only in moderately and highly resistant _P. massoniana_. [PERSON] et al. [42] found that \(\alpha\)-pinene has antibacterial and anti-inflammatory effects. Finally, [PERSON] found that _B. xylophilus_ genes involved in transport, detoxification, and receptor activities were differentially expressed by \(\alpha\)-pinene [39].