Proposed model for theanine biosynthesis, transport and regulation at single cell resolution.
(A) Multicellular compartmentation of theanine biosynthesis. The high expression of CsTSI in pericycle cells suggests theanine is mainly synthesized in pericycle cells. CsAlaDC is highly expressed in cambium and protoxylem cells, suggesting that ethylamine (EA) is mainly synthesized in these cells. Thus, EA may move from cambium and protoxylem cells into pericycle cells, where it is a substrate for theanine biosynthesis. (B) In tea plant roots, CsAAP1 likely mediates in theanine retrieval from apoplast for its transport into cortical/endodermal cells. CsCAT2 imports theanine into vacuoles, especially in pericycle cells, which are the main cells for theanine biosynthesis. Theanine is exported, by an unidentified theanine exporter, from pericycle cells into the apoplastic pathway for entry into the xylem. (C) CsLBD37 co-regulates theanine biosynthesis and lateral root development. Increasing levels of nitrogen (N) promote theanine biosynthesis and accumulation. High accumulation of theanine and N induces CsLBD37 expression. CsLBD37 inhibits CsAlaDC expression to reduce ethylamine synthesis, which finetunes theanine biosynthesis in feedback loop. Besides, CsLBD37 is expressed in pericycle cells and also inhibits lateral root development. It is reported that apoplastic theanine can negatively regulate lateral root development, under high nitrogen levels (Chen et al., 2022). Therefore, CsLBD37 may also be involved in theanine-regulated lateral root development.