We all routinely consume plant secondary metabolites—such as caffeine—without necessarily considering the complex biochemistry that is behind their production. Secondary metabolites have been used as medicines or as components of industrial processes for millennia. The methods used to manipulate their production are discussed by Yazaki in Chapter 43. Some secondary metabolites are limited in supply because the natural source is either hard to cultivate or difficult to synthesise chemically. Therefore, an effort has been made to produce these compounds in vitro or increase their production through the metabolic engineering of plants.Yazaki first discusses how in vitro culture can be exploited to produce secondary metabolites. Thereafter, the strategies for metabolic engineering—that is, the use of genetic engineering methods to manipulate metabolism—are discussed in detail. It is evident from the review that understanding the biochemistry of their biosynthesis is the first step towards engineering the production of metabolites. Metabolic engineering differs from most molecular farming techniques for therapeutic protein production, because the aim is to manipulate complex plant metabolic pathways and to express proteins that are unlikely to have an effect on the plant cell itself. The successes in metabolic engineering reported by Yazaki are impressive.
The most telling and inspiring result so far has been the creation of ‘Golden Rice’, a transgenic rice line that has been metabolically engineered to produce high levels of the vitamin-A precursor ββ-carotene. This result exemplifies the potential of metabolic engineering to turn plants into living pharmacies.
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