Metabolic Alterations in Organic Acids and γ-Aminobutyric Acid in Developing Tomato (Solanum lycopersicum L.) Fruits

Salt stress improves the quality of tomato fruits. To clarify the mechanism(s) underlying this phenomenon, we investigated metabolic alterations in tomato fruits exposed to 160 mM salt, focusing on metabolism of organic acids related to the tricarboxylic acid (TCA) cycle and gamma-aminobutyric acid (GABA). Quantitative analyses revealed that most amino acids increased in response to salt stress throughout fruit development, and the effect of the stress was greater in the pericarp than in the columella, whereas organic acids did not show a remarkable tendency to salt stress. The transcript levels of 20 genes encoding enzymes of the TCA cycle and peripheral pathways were also analyzed in salt-stressed fruit. Genes responsive to salt stress could be categorized into two types, which were expressed during early development or ripening stages. During fruit development, phosphoenolpyruvate carboxylase 2 and phosphoenolpyruvate carboxykinase displayed contrasting expression patterns between early development and ripening, suggesting a switch of carbohydrate metabolism after the turning stage. Our results revealed a new metabolic pathway for GABA during the development of tomato fruits. At the start of ripening, GABA is first converted to malate via succinate semialdehyde, and it passes into a shunt through pyruvate. Then, it flows back to the TCA cycle and is stored as citrate, which contributes as a substrate for respiration during fruit maturation.