Mass spectrometry imaging of gamma-aminobutyric acid and glutamic acid decarboxylase reactions at various stages of banana ripening.

Banana is the fourth most consumed crop worldwide, and its high economic value and health benefits have made it very popular. Bananas are climacteric fruits that ripen after harvesting. It has been reported that the endogenous substances in bananas change significantly during the ripening process. This study focused on levels of gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD), an enzyme that catalyzes the synthesis of GABA, which reportedly fluctuates during the ripening stage. Previous studies have shown that GAD expression is associated with banana ripening; however, changes in its distribution during ripening have not been verified. This study aimed to clarify the relationship between GABA and GAD during ripening of ethylene-treated bananas. Visualization of the localization of endogenous GABA and GAD was performed using mass spectrometry imaging. To visualize GAD reaction, a glutamate-d (labeled substrate) was supplied to the sample, and a GABA-d (labeled product) was regarded as the localization of the enzymatic reaction. Liquid chromatography-mass spectrometry was also used to confirm the amount of GABA and activity of the GAD. This will allow us to clarify the direct relationship between GABA and GAD and to understand the role of the GAD reaction in phytohormones.