Sites of biosynthesis and storage of Taxol in Taxus media (Rehder) plants: Mechanism of accumulation

Abstract

Taxol is a cytotoxic agent against various types of cancers. The cytotoxic activities of Taxol can be extended to its synthesizing plant. Here, Taxol is shown to have special synthesis, storage and transport mechanisms that avoid the toxic effects on its source plant. The sites of Taxol biosynthesis, transport and storage were revealed by quantification of plant Taxol, its intermediate baccatin III, the polyphenol side chain precursor , gene expression analysis of the major Taxol biosynthetic genes and in situ immuno-labeling. Although the biosynthesis of Taxol was limited by the expression of its biosynthetic genes and the presence of baccatin III, its presence did not correlate to baccatin III accumulation, nor to the expression of biosynthetic genes. However, Taxol presence positively correlated to polyphenol accumulation (late stage in Taxol assembly) and the resin-like hydrophobic bodies (HB, storage organelles). These results indicate that the presence of Taxol requires two complementary steps, biosynthesis followed by storage. Each step is limited by the availability of different precursors, which differ in their localization within the plant. Thus, the sites of biosynthesis, transport and storage of Taxol are different. Taxus media (Rehder) plant wood showed high concentrations of baccatin III and the expression of biosynthetic genes. However, the concentrations of Taxol, polyphenol and HB were very high in the plant outer layers including phloem and dead bark (rhytidome). Furthermore, in situ immuno-labeling showed that taxadiene synthase (the rate-limiting enzyme in Taxol biosynthesis) was mainly found in the wood, while Taxol primarily localized to the outer tissues. Conclusively, wood can be considered as the site of Taxol biosynthesis. Our data also propose that Taxol then accumulates into HB in order to permit its transport within the living plant tissues without causing toxic effects. This is followed by Taxol storage in the outer tissues including phloem and dead bark.