Quantitative assessment of crosstalk between the two isoprenoid biosynthesis pathways in plants by NMR spectroscopy

Plants have been shown to use the mevalonate pathway for the biosynthesis of sterols and triterpenes in the cytoplasm and the recently discovered deoxyxylulose phosphate pathway for the biosynthesis of a variety of hemiterpenes, monoterpenes, diterpenes, as well as for the biosynthesis of carotenoids and the phytol side chain of chlorophyll in plastids. Despite the compartmental separation, at least one terpene precursor can be exchanged between the two pathways. In order to assess quantitatively the crosstalk between the two isoprenoid pathways, [2-13C1]mevalonolactone or [U-13C6]glucose were supplied to cell cultures of Catharanthus roseus grown under illumination or in darkness. Sitosterol, lutein and phytol were isolated and analysed by NMR spectroscopy. The incorporations of exogenous [2-13C1]mevalonolactone were 48% and 7% into the DMAPP and IPP precursors of sitosterol and lutein, respectively. With [U-13C6]glucose as precursor, at least 95% of sitosterol precursors were obtained via the mevalonate pathway, whereas phytol appeared to be biosynthesised via the deoxyxylulose phosphate pathway (approximately 60%) as well via the mevalonate pathway (approximately 40%). The apparent ratios for the contribution of the two pathways depend on the nature of the precursor supplied as well as the nature of the target compound. Thus, crosstalk between the two terpenoid pathways cannot be explained in detail by a simple two compartment model and requires an additional in depth study of complex regulatory mechanisms.