## 3 Discussion
For example, (_E_)-\(\beta\)-ocimene synthase in _Antitrinium majus_ belongs to the TPS-g subfamily [39], (_E_)-\(\beta\)-farnesene synthase in _Mentha_\(\times\)_piperta_ belongs to the TPS-a subfamily [40], \(\alpha\)-farnesene synthase in _Pinus taeda_ and (_E_,_E_)-\(\alpha\)-farnesene synthase in _Pica abies_ belong to the TPS-d1 subfamily [38; 41], and _MdAFS_ coding (_E_,_E_)-\(\alpha\)-farnesene synthase in _Malus domestica_ belongs to the TPS-b subfamily [16]. To identify the \(\alpha\)-farnesene synthase in tea plants, we compared these homologous sequences of monoterpenes and sesquiterpenes synthases among the TPS family. There is no successful genetic transformation system in tea plants, such that the evidence of metabolic pathways is difficult to obtain _in vivo_. Therefore, we used _N. benthamiana_ as model plants to provide internal evidence. In our study, the results of _E. coli_ expression _in vitro_ indicated that the \(\alpha\)-farnesene synthase, CsAFS protein, can produce \(\alpha\)-farnesene or \(\beta\)-ocimene from different substrates (Figures 2 and 3). For the different locations for monoterpene and sesquiterpene formation, we used subcellular localization to further confirm the functions potentially performed by this enzyme in tea plants (Figures 3 and 4). These findings showed that \(\alpha\)-farnesene synthase was an enzyme found in the cytoplasm, and that it was more likely to convert the substrate FPP into \(\alpha\)-farnesene in tea plants. In the present study, we still could not exclude the possible occurrences of other _CsAFS_ genes in tea because the expression of _CsAFS_ was induced around 8 h after JA treatment, however, the accumulation of \(\alpha\)-farnesene was detected around 4 h (Figure 5C,D). Future study on function validation of CsAFS (proposed in this study) using RNAi system in tea plants may find out if CsAFS has _in vivo_ function in tea plants. SA has an important effect on plant defense system activation after attack by pathogens [42] or herbivores [43; 44] and the plant stress mechanism when confronting a poor environment [45; 46]. Tobacco mosaic virus (TMV) has been reported to not only enhance the SA content in damaged leaves but also increase the endogenous SA levels [47]. These findings suggested that SA had a significant effect on plant defense signal transduction. [PERSON] et al. indicated that SA was not responsible for the signal transduction to induce acquired resistance but was necessary in this translocation process [48]. Furthermore, exposure to \(\beta\)-ocimene has been indicated to arouse defense mechanisms in plantsthrough SA, JA, and ET-mediated signaling pathways [49]. In plants suffering various stresses, it has been reported that ET is usually exposed and induced by 1-aminocyclopropane-1-carboxylate synthase (ACS) by improving the content of ET precursors [50; 51]. Therefore, _ACS_ and _EIN_, which belong to important genes involved in ET formation [52], were analyzed to determine the changes in ET. In this study, the SA content showed an increasing trend under \(\alpha\)-farnesene treatment (Figure 6), while JA level and genes expression related to ET formation showed no significant changes, possibly due to antagonistic/synergistic cross-talk between phytohormone pathways [33]. Recently, a predominant isochorismate-derived biosynthesis of SA has been discovered in _Arabidopsis_ plant. The key involved enzymes are isochorismate synthase 1 (ICS1), avrPphB susceptible 3 (PBS3), and enhanced disease susceptibility 5 (EDS5) [53]. It would be interesting to identify functions of these key involved enzymes in tea leaves and further investigate influence of \(\alpha\)-farnesene treatment on them. \(\beta\)-Glucanases are considered important proteins for the response to fungal infection in pea endocarp. Sustained expression of the \(\beta\)-glucanase gene resulted in the general defense ability being enhanced by the enzyme [54].