4.1 CsTPS1 is a geraniol synthase in tea
Geraniol has a sweet, floral aroma similar to that of roses, and it contributes to the characteristic floral aroma and flavor of many fruits. Tea plants are important evergreen crops that are grown in temperate and subtropical regions. In response to herbivore and pathogen invasion, tea plants release volatiles, such as 3-hexenol, geraniol, β-ocimene, β-caryophyllene, and α-farnesene (Zhou et al., 2019). Tea green leafhopper, a major pest of tea plants, can significantly induce the emission of geraniol from tea leaves (Zhou et al., 2019). Other studies have shown that the higher content of geraniol in tea plants might be responsible for their stronger resistance to the pathogen causing tea leaf blight (Zhang et al., 2006). In addition, geraniol is considered one of the most abundant terpenes in tea, and it contributes greatly to its aroma (Yang et al., 2013). Geraniol is an important defense-inducing substance in tea plants; however, the biosynthesis of geraniol in tea leaves has not yet been clarified.
Although geraniol synthase genes have been reported in Vitis vinifera , Glycine max , Coffea arabica , and other plants (Martin et al., 2010b; Liu et al., 2014), geraniol synthase genes have not yet been identified in tea plants. Only a few TPS genes have been identified in tea trees to date (Zhou et al., 2020), such asCsNES , nerolidol synthase (Zhou et al., 2017), CsLIS/NES , linalool/nerolidol synthase (Liu et al., 2018), CsAFS , α-farnesene synthase (Wang et al., 2019b), CsOCS , and β-ocimene synthase (Xu et al., 2018). CsTPS1 was first identified by analysis of gene expression levels and geraniol accumulation in tea plants, and both in vitro and in vivo analysis showed that it functions as a geraniol synthase in tea plants (Figure 2C and 4B).
Plant TPS s are divided into seven families (TPS-a to TPS-g) (Nieuwenhuizen et al., 2013). Although phylogenetic analyses of terpenes can provide insights into the function of TPSs, TPS s on the same branch might have different functions. In our study, CsGES(CsTPS1/1-AS ) and CsOCS were in the same branch (Figure 6F); their homologous sequence alignments were similar, but their functions were quite different. Phylogenetic analysis showed thatCsTPS1 clustered with CsOCS2 , which belongs to theTPS-b gene family (Figure 6F). The TPS-b subfamily is the second largest in C. sinensis , and it includes approximately 37.5% of all TPS genes in tea (Zhou et al., 2020).CsOCS specifically catalyzes the synthesis of β-ocimene from GPP (Xu et al., 2018), and CsGES (CsTPS1/1-AS ) catalyzes the conversion of GPP to both geraniol and β-ocimene, and mainly catalyzed the synthesis geraniol. Therefore, the latter gene thus encodes the main enzyme that catalyzes the synthesis of geraniol (Figure 2C and 2D).