Molecular basis of flavonoid accumulation in tea leaves grafted with Camellia sinensis var. assamica cv. "Yinghong9" as rootstock based on multi-omics analysis

Grafting plays a significant role in tea plant cultivation and influences the growth and quality of tea plants. However, the specific molecular mechanisms through which tea rootstocks affect the metabolic regulation in grafted leaves have not been thoroughly investigated. Our results showed that compared with self-root grafting (EC1-EC1), the total amount of flavonoids and catechins in the leaves of "Echa 1" (scion) and "Yinghong9" (rootstock) increased, while the total free amino acids decreased, particularly bitter amino acids. To further understand the molecular mechanisms of metabolic regulation in tea plant grafting, "EC1-EC1" and "EC1-YH9" were used to analyze the metabolome and transcriptome. Metabolomic analysis identified 1091 metabolites, among which 173 were differentially accumulated metabolites, with 87 upregulated and 86 downregulated metabolites. Notably, the most abundant metabolite class was the flavonoids in the grafted leaves of YH9 as rootstock. This increase was primarily attributed to the upregulated expression of key genes involved in the flavonoid biosynthesis pathways, including chalcone isomerase, dihydroflavonol 4-reductase, and UDPglycosyltransferase 75C1. In addition, the rootstock reduced the abundance of amino acid compounds and the expression levels of related metabolic pathway genes, which also affected the quality of grafted leaves. Overall, these results provide valuable genetic resources for studying the molecular insights of tea rootstocks on the metabolic regulatory network of grafted leaves.