Multi-omics analysis reveals the role of <b><i>γ</i></b>-aminobutyric acid in maintaining the postharvest quality of baby mustard (<i>Brassica</i> <i>juncea</i> var. <i>gemmifera</i>)

γ-Aminobutyric acid (GABA) is a nonprotein amino acid involved in plant stress responses and metabolic regulation. In this study, we investigated the effects of exogenous GABA on postharvest senescence in baby mustard stored at 20 °C. GABA treatment preserved external quality and delayed the loss of chlorophylls, ascorbic acid, and phenolics. Quasi-targeted metabolomics identified 195 differentially accumulated metabolites (DAMs) in the comparison between untreated and GABA-treated samples, including amino acids (e.g., threonine, phenylalanine), soluble sugars (e.g., trehalose), and phenylpropanoids (e.g., sinapine, astragalin). Transcriptomic profiling of the same comparison revealed 5,912 differentially expressed genes (DEGs). Many of these DEGs were enriched in phenylpropanoid biosynthesis and antioxidant pathways, with GABA upregulating antioxidant enzyme genes and contributing to redox stability. Moreover, GABA treatment suppressed ethylene signaling by downregulating ETR, CTR1, and 40 out of 42 ethylene-responsive (ERF) transcription factors, thereby repressing ethylene-responsive transcription. These integrated responses delayed oxidative damage and maintained nutritional stability. This study provides the first omics-based evidence of GABA-mediated alleviation of senescence in baby mustard, highlighting its potential as a postharvest preservation strategy for Brassica vegetables.