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

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Watercore is a physiological disorder which often occurs in Rosaceae 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 PpGAD2 was significantly induced in watercore fruit; however, the mechanism through which PpGAD2 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 PpGAD2 was significantly higher than that of other PpGADs in watercore fruit or healthy fruit. Tissue expression showed that the content of PpGAD2 in mature fruit was higher than in young fruit. Moreover, subcellular localization showed that PpGAD2 was located in the cytoplasm. Transient overexpression assays suggested that PpGAD2 had a role in GABA synthesis. Several CML (calmodulin–like) genes were also significantly increased in watercore fruit. Moreover, PpWRKY53 was significantly induced in watercore fruit, and the GUS activity assay showed that PpWRKY53 can significantly increase the activity of the PpGAD2 promoter. Taken together, these results demonstrate that PpGAD2 played an important role in GABA synthesis to increase plants’ resistance to hypoxia stress, and its activity may be affected by PpWRKY53 and several watercore-induced CML genes.