## 3 Discussion  
Herbivore attack is a common phenomenon in the process of plant growth and development, and plants can emit HIPVs in response to invasion [20; 21; 22]. Elicitors and mechanical damage are the main factors that induce HIPVs formation. Elicitors include fatty acid-amino conjugates (FACs) from herbivores' oral secretion, hydrolase from herbivore saliva, and other compounds isolated from herbivores that react with plant cells, which can all induce specific production of HIPVs. Compound _N_-(17-hydroxylinolenoid)-l-glutamine, which has been isolated from the oral secretions of beet armyworm caterpillars, can induce corn to emit volatile compounds similar to those emitted by corn under beet armyworm caterpillar attack when used on damaged leaves of corn [2]. Another report has indicated that _Heliothis virescens_ and _Helioverpa zea_ can induce two different types of  
Figure 6: Changes in phytohormones content and expression levels of resistant related genes under \(\alpha\)-farnesene in different treatment time, 24 h and 36 h. (**A,B**) All of the tea samples were collected in September 2018; (**C,D**) All of the tea samples were collected in June 2019. CK, dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)) treatment collected in different treatment time. T, \(\alpha\)-farnesene dissolving in CH\({}_{2}\)Cl\({}_{2}\) treatment collected in different treatment time. Significant differences between control and treatment at the same treatment time are indicated (\({}^{*}p\leq 0.05\), and \({}^{**}p\leq 0.01\)). SA, salicylic acid; _BGL_, _\(\beta\)-1, 3-glucanase_; FW, fresh weight. HIPVs when attacking cotton, corn, and tobacco [23]. Owing to their specificity, these types of HIPV could be candidates for narrow-spectrum insect resistance. In addition to elicitors, mechanical damage is a common factor resulting from all herbivore attacks. For example, when lima beans are subjected to continuous wounding treatment, the emitted volatiles are similar to those from lima beans exposed to _Spodoptera litterorialis_ attack [24]. During mechanical damage, phytohormones play an important role in volatiles formation, with the JA-mediated pathway considered the most important signal transduction pathway leading to HIPV emissions [25]. When suffering mechanical damage, plants can rapidly produce JA by activating its biosynthetic genes, and the synthetic JA can upregulate the formation of HIPVs through the downstream signal transduction networks [26; 27; 28]. In tea plants, mechanical damage can change the HIPV emissions significantly. After damage by _Tetranychus kanzawai_, tea geometrid, and tea green leafhopper, tea plants can release different levels of \(\alpha\)-farnesene, (_E_)-3,8-dimethyl-1,4,7-nonatriene (DMNT) and (_E_)-\(\beta\)-ocimene [29; 30]. Therefore, these HIPVs, including \(\alpha\)-farnesene, resulting from wounding (Figure 1) might be broad-spectrum insect resistance candidates, which can be used for direct anti-insect compounds or attractants for insect enemies or plant defense-enhancing compounds. Terpenoids are the most abundant class of natural products, showing structural diversity, including monoterpenes, diterpenoid, sesquiterpenoid, and triterpenoid and are the main components of HIPVs [31]. Many reports have confirmed that biotic and abiotic stresses can dramatically enhance most volatiles by regulating related genes important to the network, including terpene formation in tea plants [32; 33; 34; 35; 36]. \(\alpha\)-Farnesene is a common volatile emitted by tea trees when suffering attack and has an important effect on insect resistance in many plant species. There are many researches reporting the genes on formation of farnesene or other terpen in different plant species, while the \(\alpha\)-farnesene synthesis genes with regards to tea plants have not been directly reported in recent studies. These genes coding terpene synthases belong to the TPS family, and are involved in both primary and secondary metabolism [37; 38].