Molecular Cloning of a TCHQD Class Glutathione S-Transferase and GST Function in Response to GABA Induction of Melon Seedlings under Root Hypoxic Stress

Glutathione-S-transferase (GST), a versatile enzyme that occurs widely in plants, plays a key role in plant resistance to environmental stresses. Previous results have demonstrated that GST proteins are involved in alleviating root hypoxic injury caused by gamma-aminobutyric acid (GABA); however, the induction mechanism of the GST gene in the melon under root hypoxic stress and its functional mechanisms remain unclear. In this study, based on gene cloning and bioinformatics analysis, GST gene expression and activity and glutathione (GSH) content were assessed under root hypoxic and normoxic conditions with or without GABA. The results showed that the CmGST locus includes an 804 bp gene sequence that encodes 267 amino acids. The sequence was highly similar to those of other plant TCHQD GSTs, and the highest value (94%) corresponded to Cucumis sativus. Real-time PCR results showed that the CmGST gene was induced by root hypoxic stress and GABA, and this induction was accompanied by increased GST activity and GSH content. Root hypoxic stress significantly upregulated CmGST expression in melon roots (0.5-6 d), stems, and leaves (0.5-4 d), and GST activity and GSH content were also significantly increased. Exogenous GABA treatment upregulated CmGST gene expression, GST activity, and GSH content, particularly under root hypoxic conditions. As a result, CmGST expression in GABA-treated roots and leaves at 0.5-4 d and stems at 0.5-6 d was significantly higher than that under root hypoxic stress alone. This study provides evidence that the TCHQD CmGST may play a vital role in how GABA increases melon hypoxia tolerance by upregulating gene expression and improving metabolism.