Integrated metabolomic and transcriptomic strategies to understand the effects of dark stress on tea callus flavonoid biosynthesis

Flavonoid biosynthesis is a crucial secondary metabolism process for tea plants. Its metabolism is affected by multiple environmental factors, especially light. Shade, also known as dark stress (DS), is generally used during cultivation to improve tea quality by influencing the flavonoid accumulation. To explore the molecular mechanisms of flavonoid biosynthesis under DS, metabolomics and transcriptomics (METR) analyses were performed in tea callus via culturing the plants in vitro using 12 h light/12 h dark cycles (A) or completely dark (B) conditions for 30 days. In total, 161 differential metabolic products (DMPs) and 3592 differential expression genes (DEGs) were identified. The major flavonoids including epicatechin gallate, catechin gallate, gallocatechin-catechin, cyanidin 3-O-glucoside and the total of catechin, anthocyanin and proanthocyanidin contents were all remarkably down-regulated in tea callus under DS. Meanwhile, 9 genes including CsPAL, Cs4CL, CsCHS, CsFLS, CsDFR, CsANS, CsLAR, CsANR, and CsUFGT determined to be responsible for the flavonoid biosynthesis. In addition, 2 transcription factors (TFs) including CsMYBT1 and CsMYBT2 verified to play key role in regulation the flavonoid biosynthesis. These results helped us further understand the underlying molecular mechanism of flavonoid metabolism in tea plants.