**1. Introduction**

After water, tea has been the most widely consumed beverage worldwide for several thousand years because of its unique aroma and taste. Tea plants (*Camellia sinensis* L.) contain abundant specialized secondary metabolites such as polyphenolic compounds, found in the largest proportions in tea plants alkaloids, terpenoids, and amino acids [1]. Tea polyphenols account for 30% of the dry weight of tea leaves. They can be roughly divided into the following five categories: flavanols, flavonols, flavones, proanthocyanidins, and phenolic acids [2,3].

Hydroxycinnamoyl transferase (HCT) catalyzes the transfer of hydroxycinnamoyl moiety to receptor substrates such as shikimic acid, quinic acid, anthocyanins, flavanol glycoside, polyamine, and long-chain fatty acids. Plants under environmental stresses can induce the related gene expression involved in the phenylpropanoid metabolic pathway to generate various secondary metabolites that resist or adapt to environmental stresses [4]. The HCT involved in the phenylpropanoid pathway catalyzes shikimic acid and quinic acid to participate in the upstream pathway of lignin biosynthesis. Lignins in plant cell walls provide a physical defense that protects polysaccharides in cell walls from degradation by microorganisms [5]. Low temperature, high salinity, drought, mechanical injury,

abscisic acids (ABAs), salicylic acid (SA), and hydrogen peroxide can induce *HcHCT* expression in *Hibiscus cannabinus*. HcHCT increases abiotic stress tolerance in plants [6]. In *Cucumis sativus*, *HCT* expression was reduced with pectinase treatment. In addition, directing the phenylpropanoid pathway to generate H-lignin caused p-coumaraldehyde accumulation [7].

HCT not only participates in secondary metabolite acylation but also regulates hypersensitive responses (HRs) in plants. HCT1806 or HCT4918 in *Zea mays* interacts with Rp1-D21 translated from resistance genes, thereby inhibiting HR generation. When pathogens attack plants, effectors secreted by the pathogens can change the protein structure of HCT1806 or HCT4918, which influences how they interact with Rp1-D21 and causes plants to generate HR. This prevents the spread of pathogens in local cell necrosis [8]. HCT is specific to a wide range of substrates such as gentisate, 3-hydroxybenzoate, hydroxyanthranilate, and protocatechuate, and competes with shikimic acid or quinate acid for the binding site on the enzyme, which in turn produces other acylation products [9–11].

In this study, to clarify the molecular characteristics of HCT in tea plants, we analyzed highly conserved domains in the amino acid sequences of HCT in *Arabidopsis thaliana*, *Nicotiana tabacum*, *H. cannabinus*, *Theobroma cacao*, and *Fragaria vesca*, designed degenerate primers for use in polymerase chain reaction (PCR), and cloned the genetic sequence of *CsHCT* from a Chin-Shin Oolong tea plant. Quantitative real-time PCR (qRT-PCR) was used to analyze *CsHCT* expression levels in the tissues of tea plants and seedlings. The results demonstrated a high level of *CsHCT* expression in the stem tissues of tea plants and seedlings. The amount of *CsHCT* transcribed in tea plants at various altitudes and in different seasons was also measured, and the results indicated that CsHCT expression levels were relatively high at high altitudes and at low temperatures. Moreover, an abiotic stress experiment revealed that low-temperature, drought, and high-salinity stresses induced *CsHCT* transcription. In addition, *CsHCT* expression increased with ABA treatment. Thus, this study concluded that CsHCT may be involved response to abiotic stress and ABA signaling pathways in tea plants.
