The MdWRKY50-MdHD14 Module Regulates Fruit Senescence by Affecting γ-Aminobutyric Acid Synthesis

Fruit senescence negatively affects apple quality and postharvest storage. γ-Aminobutyric acid (GABA) accumulates rapidly following stress. Here, we found that the HD-ZIP transcription factor (TF) MdHD14 induces GABA accumulation by upregulating MdGAD2 and MdGAD4. This subsequently increases antioxidant enzyme activity in apple fruits and delays fruit senescence. The overexpression of MdGAD2/4 enhances GABA levels and decreases reactive oxygen species levels. Conversely, MdGAD2/4 silencing revealed that MdGAD2/4 mediates GABA synthesis and delays the senescence of apple fruit. We also found that MdHD14 can activate the transcription of MdGAD2 and MdGAD4. Both the overexpression of MdHD14 and its editing indicate that MdHD14 alters MdGAD2 and MdGAD4 expression to regulate GABA synthesis. The expression of MdHD14 was regulated by the TF MdWRKY50. Antioxidant enzyme activity, transcript levels of MdGAD2/4, and the GABA content were increased in plants overexpressing MdWRKY50. The molecular mechanism by which MdWRKY50, MdHD14, and MdGAD2/4 collectively regulate GABA homeostasis and fruit senescence in apple was clarified. These findings clarify the molecular mechanisms of senescence resistance in apple.