Ethylamine, beyond the synthetic precursor of theanine: CsCBF4-CsAlaDC module promoted ethylamine synthesis to enhance osmotic tolerance in tea plants

The tea plant (Camellia sinensis) is a perennial green plant, and its tender leaves are rich in secondary metabolites, such as theanine. Ethylamine (EA), a small amine, is an important prerequisite for theanine synthesis. However, beyond its involvement in theanine synthesis, the other physiological functions of EA in tea plants remain unknown. In vitro experiments indicate that EA may function as scavengers of reactive oxygen species (ROS) to protect the plant against damage caused by osmotic stress. Additionally, a significant correlation between EA levels and osmotic tolerance has been observed in different tea varieties. From the results, alanine decarboxylase (CsAlaDC)-silenced tea leaves and overexpressed CsAlaDC Arabidopsis thaliana lines decreased and increased EA levels, respectively, and mediated ROS homeostasis, thus exhibiting a sensitive and tolerant phenotype. In addition, the transcription factor (TF) CsCBF4 was functionally identified, which can directly bind to the CsAlaDC promoter. CsCBF4-silenced tea leaves significantly reduced the expression levels of CsAlaDC and in turn EA content, resulting in excess ROS accumulation and an osmotic-sensitive phenotype. Taken together, these results established a new regulatory module consisting of CBF4-CsAlaDC responsible for EA accumulation and ROS homeostasis in response to osmotic stress.