Application of γ-aminobutyric acid induces disease resistance in apples through regulation of polyamine metabolism, GABA shunt and reactive oxygen species metabolism

Apples were treated with γ-aminobutyric acid (GABA) to investigate its effect on blue mold, reactive oxygen species (ROS) metabolism and GABA shunt. The results showed that GABA significantly inhibit blue mold of apples. Further studies demonstrated that GABA remarkably increased H2O2, ascorbic acid, and glutathione contents; superoxide dismutase, NADPH oxidase, catalase, glutathione reductase, ascorbate peroxidase, dehydroascorbate reductase, and monodehydroascorbate reductase activities in apple fruit. GABA also activated glutamate decarboxylase, glutamate dehydrogenase and glutamate synthase, while inhibited GABA transaminase and succinate hemialdehyde dehydrogenase in apples. GABA treatment increased endogenous GABA, pyruvic acid and glutamate contents in the fruit. In addition, exogenous GABA dipping up-regulated MdMT, MdMS, MdSAMS, MdSAMDC, MdODC, MdADC and MdSPDS, and down-regulated MdPAO and MdDAO in apples. These findings indicate that GABA enhances disease resistance in apples through regulation of GABA shunt, ROS and polyamine metabolism.