Integrated Transcriptomic and Biochemical Profiling Reveals the Regulatory Mechanism of Light Intensity-Induced Anthocyanin Biosynthesis in a Purple-Leaf Tea Cultivar

Anthocyanins are water-soluble flavonoid pigments that contribute to leaf coloration, tea quality, and have potential health-promoting properties. Purple-leaf tea plants have attracted significant interest due to their high anthocyanin levels; however, cultivar-specific evidence linking light intensity with pigment accumulation, enzyme activity, and transcriptional regulation in purple-leaf tea remains limited. In this study, the purple-leaf tea cultivar ‘Ziyan’ was exposed to three light intensity treatments: low (LL, 100 μmol·m−2·s−1), medium (ML, 200 μmol·m−2·s−1), and high (HL, 400 μmol·m−2·s−1). Increasing light intensity resulted in deeper purple coloration and significantly higher anthocyanin accumulation. The contents of delphinidin, cyanidin, pelargonidin, and total anthocyanins under HL increased by 124.2%, 63.4%, 57.2%, and 105.9%, respectively, compared to the control. Activities of major biosynthetic enzymes (CHS, CHI, F3H, F3′H, F3′5′H, DFR, ANS) were enhanced under ML and HL, whereas ANR activity decreased. Transcriptome sequencing identified 1349 differentially expressed genes (DEGs) (404 up-regulated, 945 down-regulated) in the LL vs. HL comparison. Notably, TFBS motif enrichment analysis revealed that up-regulated DEGs were dominated by a single cohesive SPL/SBP regulatory module, while down-regulated DEGs partitioned into independent NAC, HSF, and EIL modules, suggesting a multi-layered transcriptional regulatory network governing light-responsive anthocyanin biosynthesis. Several transcription factors, including MYB44, MYB75, bHLH162, and WRKY40, were also identified as candidate regulators. These results indicate that increasing light intensity is associated with enhanced anthocyanin accumulation, accompanied by coordinated changes in phenotype, flavonoid metabolism, enzyme activities, and gene expression, providing evidence for a possible regulatory mechanism of light-responsive pigmentation in purple-leaf tea and a basis for cultivation optimization and molecular breeding.