Integrative Multi-Omics Elucidates the Molecular Mechanisms Underlying Sweet-Mellow Taste Formation During Withering of Niangniang Tea (Large-Leaf Yellow Tea)

Withering is a critical but poorly understood step in yellow tea processing. This study integrated metabolomics (ultra-performance liquid chromatography–tandem mass spectrometry, UPLC-MS/MS), transcriptomics (RNA sequencing, RNA-seq), and sensory evaluation to investigate dynamic changes in key metabolites and gene expression during withering (0–10 h) of Niangniang tea (NNT). Transcriptomic analysis was performed on leaves sampled directly during withering, while metabolomic profiling and sensory evaluation were conducted on finished teas processed from the corresponding withered leaves. A total of 1064 metabolites were identified. Sensory evaluation showed that 8 h of withering yielded the highest sweet-mellow taste scores. Theanine, L-aspartic acid, and L-arginine peaked at 8 h, while galloylated catechins decreased. Key regulatory genes (CsTA, CsSCPL, CsPDX2.1) were identified, with expression patterns correlating with sweet-related amino acid accumulation and catechin conversion. Integrative analysis delineated a molecular roadmap in which downregulation of CsPDX2.1 after 6 h was associated with reduced theanine hydrolysis, and upregulation of CsTA correlated with galloylated catechin conversion, collectively contributing to sweet-mellow taste. These findings provide a scientific basis for optimizing withering duration in large-leaf yellow tea production.