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).