Comprehensive co-expression analysis provides novel insights into temporal variation of flavonoids in fresh leaves of the tea plant (Camellia sinensis).

Flavonoids are the major class of characteristic secondary compounds in Camellia sinensis that affect quality of tea. However, the temporal variation and the underlying regulatory mechanism of flavonoid biosynthesis during different growth months require a further investigation. Here, we combined analyses of the metabolomics and transcriptomics to tea leaves freshly collected during five different months for a comprehensive understanding of flavonoid metabolism regulation in tea plants. Through loading plot analysis, significant changes in the contents of metabolites during growing months were discovered, and further co-expression and association analysis indicated that one flavone glycoside (naringenin-7-O-glucoside) and two flavonol glycosides (quercetin-3-O-galactoside and kaemferol-3-O-(6″-O-p-courmaroyl)-glucoside) were evaluated as growth markers, which may explain the high bitterness and astringency of August teas; additionally, the high levels of two flavan-3-ols (gallocatechin and catechin gallate) may contribute to the flavor formation of April tea. Meanwhile, multiple flavonoid-related structural genes, MYB and bHLH transcription factors exhibit specific expression patterns to modulate the biosynthesis of these key flavonoids. A co-expression regulatory sub-network was constructed based on profiles of differentially expressed genes; one CsbHLH and six transcription factors (three CsbHLHs and three CsMYBs) exhibited negative and positive roles in the regulation of flavonoid biosynthetic genes, respectively. Taken together, our results provide new insights into the regulation of principle flavonoids for unique flavor of tea regulated by many flavonoid-related structural genes and transcription factors during different growth months.