5.3.2. Heterologous expression
As noted above, C. acuminate is a camptothecin-producing plant in which geraniol is the biosynthetic precursor of secologanin and camptothecin. A heterologous host strategy may be useful to gain a more detailed understanding of camptothecin biosynthesis to enable improved production to improve its production by employing genetic engineering approaches. In this regard, genes from C. roseus encoding strictosidine synthase and geraniol 10-hydroxylase (G10H ), were transformed into Ophiorrhiza pumila hairy roots. Co-overexpression of both genes enhances camptothecin production by 56% in comparison to non-transgenic lines and expression of the single gene(s) in the heterologous host (Cui et al., 2015). Microbial organisms are multipurpose heterologous hosts for pharmaceutically important natural products through metabolic engineering of their biosynthetic pathways (Morrone et al., 2010). Expression of the geraniol synthase gene from C. acuminate for geraniol production has successfully been achieved. Heterologous expression of geraniol synthase gene with optimization condition in recombinant E. coliproduces 48.5 mg/l geraniol. This yield was higher than using transgenic tobacco and yeasts (Chen et al., 2016). Likewise, co-overexpression of strictosidine synthases (STR ) and geraniol 10-hydroxylase (G10H ) genes from C. roseusinduced a 56% increase in camptothecin (CPT) production (Cui et al., 2015) (Table 4 ).