How do MEJA, GABA, and melatonin shape flavonoid profiles differently? Insights from integrated transcriptomics and metabolomics in mulberry leaves

Flavonoid content is the core quality indicator of medicinal and edible mulberry ( Morus alba L.) leaves. This study investigated the regulatory effects of 100 μmol/L MEJA, 200 mg/L GABA and 50 μmol/L MT on flavonoid accumulation, physiological traits, in vitro antioxidant and hypoglycemic activities of mulberry leaves, and elucidated the underlying molecular mechanisms via integrated transcriptomics and metabolomics. All three hormones significantly elevated total flavonoid (TFC), total phenol (TPC) and four characteristic flavonoid contents, with maximum accumulation at 20 d. MEJA showed the optimal effect, increasing TFC and TPC by 89.7% and 191.2% at 20 d, with the strongest DPPH scavenging (97.49% at 0.2 mg/mL) and α-amylase inhibitory activity (62.08% at 1.0 mg/mL). MT exhibited superior α-glucosidase inhibition, while GABA significantly promoted 1-deoxynojirimycin (DNJ) accumulation. Multi-omics analysis revealed that all three hormones upregulated key flavonoid biosynthetic genes ( PAL , 4CL , CHS , CHI ) with distinct regulatory patterns. This study systematically clarified the differential regulatory mechanisms of three exogenous hormones on mulberry leaf flavonoid synthesis, providing a theoretical basis for the targeted directional improvement of mulberry leaf functional quality.