Uncovering tea-specific secondary metabolism using transcriptomic and metabolomic analyses in grafts of Camellia sinensis and C. oleifera

Camellia sinensis (L.) Kuntze and Camellia oleifera C. Abel (Theaceae) are closely related perennial woody shrubs, but the accumulation of metabolites and gene expression patterns are quite different between these two species. In order to understand the mechanisms behind the accumulation and biosynthesis of tea-specific secondary metabolites and the key genes that regulate their target pathways, 1-year-old clone cuttings of C. sinensis and C. oleifera were grafted in both directions, and self-grafted C. sinensis were used as controls. The transcriptomes and metabolomes of leaves and roots from the grafts were analyzed. We found that 1375 unigenes were up-regulated in the leaves of the CS-CO grafts (C. sinensis scion, C. oleifera stock), while 2437 unigenes were down-regulated. OPLS-DA models established for 7230 and 3223 mass spectra peaks were obtained in the positive and negative modes by LC-MS detection. Association analysis of the secondary metabolism pathways was performed, and the relative gene expressions of 14 genes from the transcriptome screening were verified by qRT-PCR. Among the differential metabolites screened and identified, we found that the relative levels of theanine and caffeine decreased significantly, and that many of the genes in these metabolic pathways were also down-regulated. In contrast, the levels of flavonoids apparently increased, and the expression of related genes in the flavonoid biosynthetic pathway were mostly up-regulated.