Exogenous Activation of the Ethylene Signaling Pathway Enhances the Freezing Tolerance of Young Tea Shoots by Regulating the Plant's Antioxidant System

Tea plants (Camellia sinensis (L.) O. Kuntze) frequently suffer severe damage as a result of freezing temperatures in early spring, which severely affect tea quality and tea production in China. Emerging evidence has demonstrated that the ethylene signaling pathway plays an important role in tea plants' freezing responses. However, how ethylene modulates the response to freezing in sprouting tea shoots is not clear. This study verified that the measurement of relative electrolyte leakage in young shoots after 1 h at 5 degrees C is a rapid way to evaluate their freezing tolerance in the laboratory. Further exploration of the mechanism involved in increasing tea-shoot freezing tolerance by monitoring changes in the transcription of ethylene-related genes and cold signaling-related genes, and the physiological and biochemical changes after the application of ethephon (2-chloroethylphosphonic acid, an ethylene release reagent), revealed that exogenous ethephon significantly increased the freezing tolerance of tea shoots within 3 days of treatment, while concomitantly altering the expression of the ethylene signaling pathway-related genes (i.e., CsETR1, CsETR2, and CsEBF1). Moreover, antioxidant enzyme activities, including superoxide dismutase, catalase, and peroxidase, were uniformly upregulated, which might constitute a major physiological change induced by ethylene signaling and may be responsible for the observed increase in freezing resistance. Nevertheless, soluble sugars and starch, trehalose metabolism, and cold signaling-related genes did not appear relevant to the freezing tolerance increase following ethephon application. This study demonstrated that the freezing tolerance of sprouting tea shoots can be rapidly increased by the exogenous activation of the ethylene signaling pathway and upregulation of the plant's antioxidant system.