Patch-Clamp- and N Isotope Tracer-Based Techniques Reveal the Transport Characteristics of SlGAT1 and the Metabolic Flow of Exogenous GABA Under Salt Stress.

γ-Aminobutyric acid (GABA) plays an important role in plant growth, development, and stress resistance. Exogenous GABA works by increasing the content of endogenous GABA; therefore, GABA transport from the apoplast to the cytosol is critical. In this study, two GABA transporter genes, SlGAT1 and SlGAT2 were identified in tomato. It was verified that the transporter efficiency of SlGAT1 is greater than that of SlGAT2 via a two-electrode voltage clamp (TEVC) and quantitative real-time PCR (qRT-PCR). In combination with its subcellular and tissue expression, SlGAT1 is localised on the plasma membrane and is widely distributed in plants in response to salt stress. In addition, N isotope tracing confirmed that exogenous GABA can be absorbed by plant roots and transported to stems and leaves. After transmembrane transport to the cytosol, GABA is converted into glutamate, which is then catabolized to arginine and three branched-chain amino acids (BCAAs), valine, leucine, and isoleucine through four metabolic pathways. Among them, the metabolic processes of glutamate-aspartate-threonine-glycine-serine-BCAAs are crucial. Our findings provide the first evidence that SlGAT1 is a high affinity GABA transporter located on the tomato plasma membrane, that regulates plant cell metabolic pathways, promotes plant growth and enhances plant salt tolerance through GABA transport.