Gamma-aminobutyric acid (GABA) alleviates salt damage in tomato by modulating Na+ uptake, the GAD gene, amino acid synthesis and reactive oxygen species metabolism

Abstract

Background Gamma-amino butyric acid (GABA), a four-carbon nonprotein ogenic amino acid, is involved in plant abiotic stress resistance. Previous studies have reported that GABA acts as a signal substance or metabolic product by regulating cytoplasmic pH, polyamine biosynthesis and degradation, NO3− reduction and assimilation, and antioxidant responses in a variety of crops under various environmental stresses. The main purpose of our study was to explore the regulatory mechanism by which exogenous GABA enhances salt tolerance in tomato (Solanum lycopersicum L.) and its effects on the functions of key enzymes. Results Exogenous application of 5 mM GABA significantly reduced the salt damage index and increased the plant height, chlorophyll content and dry and fresh weights of tomato plants treated with 175 mM NaCl. GABA significantly reduced Na+ accumulation in leaves and roots by preventing Na+ influx in roots and transportation to leaves. Cloning of the sequences of four SlGAD genes revealed that SlGAD genes played an important role in enhancing the resistance of tomato plants to NaCl stress with GABA application. Among the SlGAD genes, SlGAD1 was the most sensitive and contributed the most to the increase in GAD activity under salt stress even if the SlGAD2 transcriptional expression was the prominent under normal conditions. GABA increased the GAD activity and amino acid contents in tomato leaves compared with the levels under salt stress alone, especially the levels of GABA and proline. In addition, GABA treatment significantly alleviated the active oxygen-related injury of seedlings under salt stress by increasing the activities of antioxidant enzymes and decreasing the contents of active oxygen species (O2∙ and H2O2) and malondialdehyde (MDA). Conclusion Our data revealed a positive effect of GABA on the resistance of tomato seedlings to salt stress, which was closely associated with GABA's effects on Na+ flux and transportation, the expression and activity of SlGADs, amino acid contents and the metabolism of reactive oxygen species. Exogenous GABA influences NaCl-treated tomato plants by reducing Na+ influx into root and inducing osmotic regulation and antioxidant reactions by increasing SlGAD1 expression and GAD activity, the contents of endogenous GABA and proline and antioxidant enzyme activity.