Molecular Mechanism of Exogenous GABA in Regulating Salt Tolerance in Tomato (Solanum lycopersicum L.)

To explore the mechanism by which γ-aminobutyric acid (GABA) regulates the response of different salt-sensitive tomato seedlings under salt stress conditions, we used the previously selected salt-sensitive tomato 'M82' and the salt-tolerant introgression line 'IL-7-5-5'. The following three treatments were set up: (1) a normal nutrient solution concentration as the control, (2) a nutrient solution with 200 mmol·L^-1 NaCl, and (3) a nutrient solution with 200 mmol·L^-1 NaCl and 35 mmol·L^-1 GABA. The concentration of the reactive oxygen species metabolism-related compounds and antioxidant enzyme activity in the leaves of tomato seedlings subjected to the different treatments were measured, and transcriptome and metabolome analyses were conducted. After adding GABA, the SOD, POD, and APX activity in the leaves of the 'M82' seedlings significantly increased, while the GR activity significantly decreased. In the 'IL-7-5-5' seedlings, the CAT, APX, and GR activity significantly increased. The combined results from the transcriptome and metabolome analysis in leaves indicated that in 'M82' seedlings, 52 metabolic pathways were enriched, which included plant signal transduction pathways, phenylpropanoid biosynthesis pathways, and amino sugar and nucleotide sugar metabolism pathways. In the salt-tolerant 'IL-7-5-5' seedling leaves, 59 metabolic pathways were enriched, which included plant signal transduction pathways, amino acid biosynthesis pathways, and carbon metabolism pathways. A further analysis revealed that both varieties had a higher number of differentially enriched genes and differential metabolites belonging to the plant hormone signal transduction and amino acid biosynthesis pathways, indicating that GABA enhances the salt tolerance of tomato seedlings by regulating these two mechanisms.