Construct a gene-to-metabolite network to screen the key genes of triterpene saponin biosynthetic pathway in Panax notoginseng

Triterpene saponins are main active constituents of Panax notoginseng. Metabolites profiling of 12 triterpene saponins was analyzed by high-performance liquid chromatography-mass spectrometry in leaf, petiole, and root extracts of P.notoginseng. Most of the 20(S)-protopanaxatriol (PPT) type saponins, except ginsenoside Re, were mainly distributed in roots, while 20(S)-protopanaxadiol (PPD) type saponins were detected among various tissues. The total content of PPD-type saponins decreased in the order of leaf, petiole, and root. The expression patterns of four key genes (PnFPS, PnSQS, PnDS, and PnSE) in the triterpene saponin biosynthetic pathway were measured by real-time quantitative PCR (RT-qPCR). All the four investigated genes showed high expression levels in leaf. A gene-to-metabolite network was constructed through canonical correlation analysis. The results indicated that the expression levels of PnFPS, PnSQS, PnDS, and PnSE had high correlation with PPD-type saponins ginsenoside Rb-2, Rb-3, and Rc, while PnSQS was also highly correlated with Rb-1. Combining metabolic profiling, RT-qPCR, and gene-to-metabolite network, we inferred that the leaf of P.notoginseng was the main biosynthesis site of PPD-type saponins Rb-2, Rb-3, and Rc. The contribution to the biosynthesis of ginsenosides Rb-2, Rb-3, and Rc was in the order of PnSE > PnDS > PnSQS > PnFPS. PnSE and PnDS should be the preferred targets to regulate the production of PPD-type saponins Rb-2, Rb-3, and Rc in P.notoginseng by plant metabolic engineering.