Roles of specialized metabolites in biological function and environmental adaptability of tea plant (Camellia sinensis) as a metabolite studying model

Tea is the second most popular beverage globally after water and contains abundant specialized metabolites. These metabolites give tea unique quality and are beneficial to human health. Some secondary metabolites are produced to help plants, including tea plants (Camellia sinensis), adapt to variable environment and grow normally. Therefore, whether abundant specialized metabolites have biological functions and play roles in the environmental adaptability of tea plants is of interest. Research progress regarding the biological functions of specialized metabolites (including catechins, l-theanine, caffeine, and volatile compounds) in tea plants is summarized. Furthermore, the main and characteristic scientific questions regarding tea plant growth in contrast to other economic crops are proposed, including (i) how tea plants adapt to acid soils, (ii) why tea plants have fewer diseases, and (iii) why tea plants and tea green leafhoppers have a symbiotic relationship. Accordingly, the potential adaptive mechanism is summarized, which is related to the function of specialized metabolites in tea plants. This is the most in-depth investigation of biological functions of volatile compounds in tea plants. Direct in vivo evidence in tea plants shows that volatile compounds help defend against insects through plant-to-plant signaling. Furthermore, abundant specialized metabolites are speculated to contribute to the environmental adaptability of tea plants. However, further in vivo evidence and exploration of relevant mechanisms are required for all aspects discussed. This review provides an important reference for basic biological research on the tea plant as a specialized metabolite studying model.