*3.4. Effects of Abiotic Stresses and Hormone Signaling on CsHCT Transcript Levels in Oolong Tea Seedlings*

To investigate whether *CsHCT* expression is induced by abiotic stress, we exposed 1-year-old tea seedlings to stresses such as high temperature (35 ◦C), low temperature (5 ◦C), high salinity (300 mM NaCl), and drought; subsequently extracted the total RNA from one-tip-two-leaf tissues of oolong tea seedlings; and then performed qRT-PCR analysis. The results demonstrated that compared with the control group, *CsHCT* expression was higher at low-temperature stress and lower under high-temperature stress, thus verifying the association between *CsHCT* expression and temperature. Furthermore, *CsHCT* transcription levels also increased in response to the high-salinity and drought treatments (Figure 6A).

**Figure 6.** *CsHCT* expression levels under various abiotic stress and phytohormone treatment;

qRT-PCR analysis of *CsHCT* transcription levels in tea seedlings after (**A**) abiotic stress and (**B**) phytohormone treatment. Total RNA was isolated from YLs of tea seedlings after cold treatment (5 ◦C) for 12 h, heat treatment (35 ◦C) for 12 h, drought (no water) for 3 days, and salt treatment (300 mM NaCl) for 5 days. The phytohormone treatment included treatment with 100 µM ABA, SA, MeJA, and ACC for 6 h. *CsHCT* transcription levels were calculated and normalized to that of *18S rRNA*. Values represent means ± SD from four biologically independent experiments. \* *p* < 0.05, versus value in control check (CK) treatment (Student's *t* test).

When a plant is under biotic or abiotic stresses, stress-related hormone signaling initiates the plant's defense mechanisms and increases its stress tolerance. To assess whether CsHCT expression is induced by various stress hormone signals, we treated the 1-year-old tea seedlings with ABA, SA, MeJA, and the ethylene precursor ACC for 6 h and subsequently collected the one-tip-two-leaf tissues for analysis of *CsHCT* expression through qRT-PCR. The results indicated that *CsHCT* expression was induced under ABA and MeJA treatment and that the expression level was the highest in the ABA group. By contrast, the *CsHCT* expression level in the SA group was significantly lower than that in the control (Figure 6B). Accordingly, this study determined that CsHCT expression can be induced by abiotic stresses such as low temperature, high salinity, and drought, and inferred that CsHCT may be involved in the ABA signaling pathways.
