The γ-Aminobutyric Acid (GABA) Synthesis Gene Regulates the Resistance to Water Core-Induced Hypoxia Stress for Pear Fruits

Watercore is a physiological disorder which often occurs in <i>Rosaceae</i> fruits, and it causes hypoxia stress, promoting fruit decay. γ-aminobutyric acid (GABA) was reported as being involved in different abiotic stresses, and glutamate decarboxylase (GAD) is the key enzyme of GABA synthesis in plants. Our previous transcriptome analysis found that <i>PpGAD2</i> was significantly induced in watercore fruit; however, the mechanism through which <i>PpGAD2</i> regulates watercore-induced hypoxia stress resistance in pears is unclear. The present study found that the fruit pulp ethanol, malondialdehyde (MDA) and H₂O₂ content was significantly inhibited by exogenous GABA. The transcript abundance of <i>PpGAD2</i> was significantly higher than that of other <i>PpGADs</i> in watercore fruit or healthy fruit. Tissue expression showed that the content of <i>PpGAD2</i> in mature fruit was higher than in young fruit. Moreover, subcellular localization showed that <i>PpGAD2</i> was located in the cytoplasm. Transient overexpression assays suggested that <i>PpGAD2</i> had a role in GABA synthesis. Several CML (calmodulin–like) genes were also significantly increased in watercore fruit. Moreover, <i>PpWRKY53</i> was significantly induced in watercore fruit, and the GUS activity assay showed that <i>PpWRKY53</i> can significantly increase the activity of the <i>PpGAD2</i> promoter. Taken together, these results demonstrate that <i>PpGAD2</i> played an important role in GABA synthesis to increase plants’ resistance to hypoxia stress, and its activity may be affected by <i>PpWRKY53</i> and several watercore-induced CML genes.