Integrated Transcriptomics and Metabolomics Analysis of the Formation Mechanism of Low Bitterness and Astringency in “Huangjinya” Tea

The flavor variation among different tea plant cultivars significantly influences their economic value. ‘Huangjinya’ (HJY), a typical light-sensitive etiolated tea cultivar, is prized for its fresh and mellow taste with low bitterness and astringency, and commands a high market value, serving as excellent germplasm for breeding superior-flavor tea varieties. Previous studies have largely focused on the mechanisms underlying fresh and umami tastes, whereas the regulatory mechanisms underlying bitterness and astringency remain poorly understood. In this study, two tea plant (Camellia sinensis (L.) O. Kuntze) cultivars with distinct bitterness and astringency phenotypes, namely ‘Fuxuan 9’ (FX) and HJY, were used to investigate the formation mechanism of reduced bitterness and astringency through integrated metabolomic and transcriptomic analyses. Metabolomic profiling identified 601 differentially accumulated metabolites between the two cultivars, which were significantly enriched in the flavonoid biosynthesis pathway. Notably, although the total flavonoid content was higher in HJY, the accumulation of non-esterified catechins (C, EC, EGC), which are key contributors to bitterness, was significantly lower than in FX. By contrast, the levels of esterified catechins (EGCG, GCG) showed no marked differences between cultivars. This distinct accumulation pattern provides a metabolic basis potentially associated with the reduced bitterness and astringency in HJY. Transcriptomic analysis revealed that the catechin biosynthetic gene CsLAR was significantly down-regulated in HJY, consistent with the reduced accumulation of non-esterified catechins. Promoter cloning and functional validation confirmed that the CsLAR promoters in both cultivars were active, which differed by only one regulatory element. Under shading treatment, CsLAR expression in HJY was more sensitive to light and decreased more markedly, suggesting that the expression of CsLAR may be regulated by transcription factors. Furthermore, 385 differentially expressed transcription factors were identified, with predicted binding sites in the CsLAR promoter region, implying their potential involvement in modulating catechin accumulation through modulation of CsLAR expression. Together, these multi-omics findings reveal molecular features potentially contributing to the low bitterness and astringency of HJY, providing a theoretical basis and candidate regulators for breeding tea cultivars with improved flavor profiles.