Chamomile sesquiterpene biosynthesis

Adam, K.R and Zapp, J. (1998) Biosynthesis of the isoprene units of chamomile sesquiterpenes. Phytochemistry, 48, 953-9. 

This does not mean that all sesquiterpene biosynthesis is independent of the 1-DXP pathway, however. In one plant system, quite the opposite has been found to be true. Bis-abolol oxides (85 and 86), matricin (81) and chamazulene (82) from chamomile flowers have been shown to contain two isoprene units derived from the 1-DXP pathway and one derived at least partially from the mevalonate pathway (Adam et al. 1999). A proposed explanation for this labelling pattern is shown in Figure 3.30. Here, sesquiterpene biosynthesis is thought to begin, unusually, in the plastids with the formation of IPP and GPP via... 

Figure 3.30 Compartmentation of sesquiterpene biosynthesis in chamomile as proposed by Adam etal. (1 999). Blocks represent isoprenoid precursor units (IPP or DMAPP).

The full details of sesquiterpene biosynthesis in chamomile are not yet known. Isolation of the prenyltransferase(s) responsible for the final steps in the biosynthesis and identification of the site of biosynthesis are likely to shed further light on the conundrum of biosynthetic compartmentation in flowering plants. These studies may also offer a challenge to the accepted wisdom that sesquiterpene biosynthesis in other flowering plants is always simply a matter of the processing of mevalonate-derived FPP in the cytosol. 

Cross-talk is also evident in the biosynthesis of certain sesquiterpenes and diterpenes. In chamomile flowers (see Section 3.5.1) it is thought that export of isoprenoid intermediates formed by the 1-DXP pathway takes place into a cellular space which has access to IPP derived from both 1-DXP and mevalonate pathways (Adam etal. 1999). 

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