Exogenous GABA stimulates endogenous GABA and phenolic acid contents in tomato plants under salt stress

Gamma aminobutyric acid (GABA) is a non protein amino acid found in various organisms including plants. In recent years, the signal role of GABA in the stress response has of special interest. However, the effects of exogenous GABA on phenolic compounds which have special roles as antioxidants are not well known. In this study, the effects of exogenous GABA on endogenous GABA and phenolic contents were analyzed in tomato plants under salt stress. Tomato plants were grown in a growth chamber under controlled conditions and NaCl and GABA were applied in Hoagland solution. Qualitative and quantitative analysis of GABA and phenolic compounds (Benzoic, caffeic, chlorogenic, gallic, hydroxybenzoic, syringic, rosmarinic, p-coumaric, sinapic, t-cinnamic, t-ferulic acids, catechin, epicatechin, hesperidin and quercetin) were measured by HPLC. Differences were found in chlorogenic acid, coumaric acid and gallic acid among the phenolic substances. We found a significant increase in gallic acid and coumaric acid contents under 200 mM NaCl and GABA applications. Exogenous GABA treatment caused a slight increase in endogenous GABA content. The increase in GABA content under GABA+salt treatments were higher than that of single salt and GABA applications. According to our results we can suggest that exogenous GABA could enhance the stress response by enhancing some phenolic substances and GABA content under salt stress.