The balance between lignin and flavonoid metabolism has a central role in the changes of quality in young shoots of the tea plant (Camellia sinensis)

Tenderness is a core index for evaluating the quality of young tea plant shoots and ultimately influences the quality and yield of tea products in terms of bioactive components, processing adaptability, tenderness-keeping ability, and other factors. In this study, we describe the underlying regulatory mechanism of tenderness changes in young shoots of tea plants, combined with biochemical composition determination and transcriptome analysis. The high accumulation of lignin, cellulose, hemicellulose, and pectin is a direct factor leading to a decrease in the tenderness of young shoots, which is dependent on the upregulated expression of genes encoding enzymes related to their biosynthesis. The accumulation of lignin during tenderness decrease in young shoots was accompanied by a decrease in the major flavonoid catechins and anthocyanins, and the upregulated expression of lignin biosynthesis-related genes was also accompanied by the downregulated expression of flavonoid biosynthesis-related genes, implying that the competition between the lignin and flavonoid biosynthesis pathways influences tenderness and bioactive component changes in young tea plant shoots. In addition, as core factors, hydroxycinnamoyl-CoA: shikimate/quinate hydroxycinnamoyl transferase (HCT) and chalcone synthase (CHS) synergize with downstream enzymes to maintain the balance between lignin and flavonoid metabolism. MYB transcription factors in the G10 and G11 groups regulated the expression of genes related to lignin and flavonoid biosynthesis pathways, respectively, and other transcription factors may also play roles in regulating this balance. This study elucidates the regulatory mechanisms of quality changes in young tea plant shoots from the perspective of tenderness.