γ-Aminobutyric acid regulates mitochondrial energy metabolism and organic acids metabolism in apples during postharvest ripening

“Golden Delicious” apples were treated with 1.0 mmol L−1 γ-aminobutyric acid (GABA) to evaluate the changes of mitochondrial energy metabolism and organic acids metabolism during ripening. Results showed that GABA dipping enhanced malic, citric, succinic, and tartaric acids contents in apple fruit. GABA treatment enhanced phosphoenolpyruvate carboxylase (PEPC), NADP-dependent isocitrate dehydrogenase (NADP-IDH), cytoplasm aconitase (Cyt-ACO), citrate synthase (CS), and cytosolic NAD-dependent malate dehydrogenase (NAD-MDH) activities, but decreased cytosolic NAD phosphate-dependent malic enzyme (NADP-ME) activity. GABA also enhanced the expressions of MdPEPC, MdNADP-IDH, MdCyt-ACO, MdNAD-MDH and MdCS, and decreased MdNADP-ME expression in apples. Moreover, higher level of energy charge, adenosine triphosphate (ATP) content, H+-ATPase, cytochrome C oxidase (CCO), succinic dehydrogenase (SDH), and Ca2+-ATPase activities were recorded in GABA-treated fruit. Additionally, GABA up-regulated MdSDH, MdCCO, MdH+-ATPase, and MdCa2+-ATPase expressions of apples. In conclusion, GABA could regulate the key gene expression and enzyme activity in mitochondrial energy metabolism and organic acids metabolism during ripening to maintain freshness of apple fruit.