Glutathione S-transferases (GSTs) constitute a large family of enzymes with a wide range of cellular functions. Recently, plant GSTs have gained a great deal of attention due to their involvement in the detoxification of electrophilic xenobiotics and peroxides under adverse environmental conditions, such as salt, cold, UV-B and drought stress. A previous study reported that a GST gene (CsGSTU8) in tea plant was distinctly induced in response to drought, suggesting this gene plays a critical role in the drought stress response. In this study, by using quantitative real-time PCR (qRT-PCR) and β-glucuronidase (GUS) reporter lines, we further demonstrated that CsGSTU8 was upregulated in response to drought stress and exogenous abscisic acid (ABA) treatments. Overexpression of CsGSTU8 in Arabidopsis resulted in enhanced drought tolerance as indicated by the improved scavenging of excess amounts of reactive oxygen species (ROS) under drought conditions. Furthermore, we found that CsWRKY48 acts as a transcriptional activator and that its expression is induced in response to drought stress and ABA treatment. Electrophoretic mobility shift assays (EMSAs), dual-luciferase (LUC) assays and transient expression assays in tea plant leaves revealed that CsWRKY48 directly binds to the W-box elements in the promoter of CsGSTU8 and activates its expression. Taken together, our results provide additional knowledge of drought stress responses in tea plant.

Drought stress severely limits growth and causes losses in the yield of tea plants. Exogenous application of 24-epibrassinolide (EBR) positively regulates drought responses in various plants. However, whether EBR could contribute to drought resistance in tea plants and the underlying mechanisms has not been investigated. Here, we found that EBR application is beneficial for the drought tolerance of tea plants. The transcriptome results revealed that EBR could contribute to tea plant drought resistance by promoting galactinol and abscisic acid (ABA) biosynthesis gene expression. The content of galactinol was elevated by EBR and EBR-responsive CsDof1.1 positively regulated the expression of the galactinol synthase genes CsGolS2-1 and CsGolS2-2 to contribute to the accumulation of galactinol by directly binding to their promoters. Moreover, exogenous EBR was found to elevate the expression of genes related to ABA signal transduction and stomatal closure regulation, which resulted in the promotion of stomatal closure. In addition, EBR-responsive CsMYC2-2 is involved in ABA accumulation by binding to the promoters CsNCED1 and CsNCED2 to activate their expression. In summary, findings in this study provide knowledge into the transcriptional regulatory mechanism of EBR-induced drought resistance in tea plants.

No abstract

Salt and alkali stress are considered major abiotic stresses in agriculture. Tea plant (Camellia sinensis), an acidophilic economic crop, is seriously affected by salt and alkali stress, which severely restricts its widespread planting. However, the mechanisms underlying the response of tea plants to salt and alkali stress remain unclear. To understand the physiological and molecular responses of tea plants to salt and alkali stress, tea plants were treated with NaCl and NaHCO3 to study the effects of salt and alkali stresses, respectively. Tea plants exhibited different symptoms under the NaCl and NaHCO3 treatments. The leaves of tea plants suffered varying degrees of damage, and concentrations of the quality components epigallocatechin gallate, caffeine, and theanine in the leaves decreased significantly under the NaCl treatment. However, after NaHCO3 treatment, epigallocatechin and theanine levels were significantly reduced. Photosynthesis and antioxidant enzymes changed significantly to varying degrees under both the NaCl and NaHCO3 treatments. The stomata closed under both NaCl and NaHCO3 treatments. After the transcriptome analysis of tea samples treated for one, three, and seven days with NaCl or NaHCO3, a large number of differentially expressed genes related to photosynthesis and the antioxidant system were identified. Analysis of the promoters of key differentially expressed genes revealed many light-responsive, hormone-responsive, and stress-responsive elements, and many corresponding upstream transcription factors were also differentially expressed. These results provide a basis for understanding the physiological and molecular responses of tea plants to salt and alkali stress.