CATIONIC AMINO ACID TRANSPORTER 1 modulates amino acid distribution between stem and leaf in new shoots: A case study of theanine distribution in tea plants (<i>Camellia sinensis</i>)

Theanine, a tea plant (Camellia sinensis)-specific non-proteinogenic amino acid, is one of the most important components conferring the taste quality and health benefits of tea. It is primarily synthesized in roots of tea plants and transported to new shoots, where it is mainly distributed to the young stem; however, tea is predominantly produced from young leaves. To promote more theanine allocation to young leaves, the molecular mechanism underlying theanine distribution between stems and leaves requires elucidation. In this study, we found the ratios of stem-to-leaf theanine content in the new shoots of 11 tea plant cultivars ranged from 3.8 to 8.8. Analyses on transcriptome and gene expression demonstrated that the expression of CATIONIC AMINO ACID TRANSPORTER1 (CsCAT1), an amino acid transporter-encoding gene, was highly correlated with the ratios of theanine content in the stem and leaf (r = 0.97, P < 0.0001). Further analyses indicated that CsCAT1 localizes in the plasma membrane and has theanine transport activity. Moreover, CsCAT1 was predominantly expressed in the vascular ray cells in the stem. Finally, we found that repression of CsCAT1 increased theanine content in young leaves and the ratio of leaf-to-stem theanine content. These results indicate that CsCAT1 modulates theanine distribution between stem and leaf and provides a target for increasing theanine content in young leaves of tea plants.