Involvement of Camellia sinensis AAP3.1 in nitrogen allocation is validated in tea plants and Arabidopsis

Abstract

Amino acids are the primary assimilated form of nitrogen (N) in plants, and their transport and distribution between source and sink organs determine the yield and quality of tea plants (Camellia sinensis L.). Amino acid permeases (AAPs) play critical roles in amino acid uptake, transport, and distribution, yet the specific functions of most CsAAPs remain largely unelucidated. Here, we aimed to identify the functional role of CsAAP3.1, in amino acid transport and N allocation dynamics. Based on bioinformatics analysis and the expression pattern profiling, we identified CsAAP3.1 as a candidate gene. Its expression patterns across tissues and N conditions were characterized and its function using heterologous overexpression in Arabidopsis and virus-induced gene silencing (VIGS) in tea plants. CsAAP3.1 showed distinct transcript levels in leaves, veins, and roots. Under low nitrogen (LN, 0.25 mmol · L-1 N) conditions, its expression was repressed in roots but induced in shoots. Functional assays revealed that CsAAP3.1 is a broad-specificity amino acid transporter, capable of transporting theanine (Thea) and twelve other amino acids. Compared to wild-type (WT), CsAAP3.1 overexpressing Arabidopsis lines exhibited lower amino acid levels and enhanced N accumulation under low-N conditions, and higher amino acid levels with reduced N allocation in young leaves under normal nitrogen (NN, 5 mmol · L-1 N) conditions at maturity. Moreover, in tea plants, VIGS-mediated silencing of CsAAP3.1 reduced free amino acids and N in young leaves, with 15N tracing showing inhibited amino acid transport from mature to young leaves and downregulated key nitrogen assimilation genes in young leaves, indicating CsAAP3.1 mediated source-to-sink amino acid transport and regulates nitrogen accumulation in young leaves. Taken together, these findings suggest that CsAAP3.1 optimizes N allocation and partitioning to acclimate to low-N, which may have applications for tea quality improvement by upregulating amino acid level.