Strategies for Vitamin E Metabolic Engineering in Plants

Metabolic engineering in plants is generally defined as the redirection of one or more enzymatic reactions to produce new compounds in an organism, improvement of existing compounds, or mediation of the degradation of compounds. Although progress in discovering biosynthetic pathway genes and in our ability to manipulate gene expression in transgenic plants has been most impressive during the past two decades, attempts to use these tools to engineer plant metabolism has met with more limited success. Though there are notable exceptions, most attempts at metabolic engineering have focused on modifying the expression of single genes affecting pathways.

There are three basic goals of metabolic engineering in plants: high production of a specific desired compound, low production of a specific unwanted compound, and the production of a novel compound, for example, a molecule that is produced in nature, but not usually in the host plant, or a completely novel compound. This includes two complementary strategies. (1) Introducing the genes that encode enzymes can increase the flux through the tocopherol biosynthetic pathway to produce elevated levels of total tocols. (2) Introducing the genes that encode enzymes can affect the composition of tocols and make a-tocopherol the predominant form of vitamin E, so as to elevate the activity of vitamin E.

There are two basic goals of vitamin E metabolic engineering in plants: (1) increasing the flux through the vitamin E biosynthetic pathway to enhance the levels of total tocochromanols and (2) altering the tocochromanol composition in favor of a-tocopherol. The approaches to achieving these goals include overexpression of single-gene, multiple-gene combinations or a transcription factor to establish single-gene or multigene control in the biosynthetic pathway for vitamin E, or use of RNAi/antisense knockout of a gene controlling the flux of some other metabolic pathway in order to increase the content or change the composition of vitamin E (Fig. 18.3).

Though progress in the discovery of genes of the biosynthetic pathway and our ability to manipulate gene expression in transgenic plants have been most impressive during the past two decades, attempts to use these tools to engineer plant metabolism has met with more limited success. Though there are notable exceptions, most attempts at metabolic engineering have focused on modifying the expression of single genes affecting pathways.

Vitamin E metabolic engineering in plants

Goals

Increasing vitamin E content

j

Modulating vitamin E composition

Approaches

Approaches

Over-expression of single gene

Over-expression of multiple genes combination

Over-expression of transcription fact or

RNAi/Antise nse knockout

Fig. 18.3 Strategies of vitamin E metabolic engineering in plants

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