Progress in plant metabolic engineering

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Abstract

Over the past few years, there has been a growing realization that metabolic pathways must be studied in the context of the whole cell rather than at the single pathway level, and that even the simplest modifications can send ripples throughout the entire system. Attention has therefore shifted away from reductionist, single-gene engineering strategies and towards more complex approaches involving the simultaneous overexpression and/or suppression of multiple genes. The use of regulatory factors to control the abundance or activity of several enzymes is also becoming more widespread. In combination with emerging methods to model metabolic pathways, this should facilitate the enhanced production of natural products and the synthesis of novel materials in a predictable and useful manner.

Introduction

Metabolic engineering in plants involves the modification of endogenous pathways to increase flux towards particular desirable molecules. In some cases the aim is to enhance the production of a natural product, whereas in others it is to synthesize a novel compound or macromolecule. Over the past few years, significant advances have been made in metabolic engineering through the application of genomics and proteomics technologies to elucidate and characterize metabolic pathways in a holistic manner, rather than on a step-by-step basis [1]. This has been supported by direct studies at the level of the metabolome [2]. Further work has been carried out on the modeling of metabolic pathways on a genomic scale [3]. Such studies have shown that metabolic pathways are controlled at multiple levels and any form of perturbation can have wide-ranging effects at the whole-system level.

As a result, it has been realized that the manipulation of single genes is of only limited value in metabolic engineering, and attention has shifted towards more complex and sophisticated strategies in which several steps in a given pathway are modified simultaneously to achieve optimal flux. In this review, we look at recent examples of single and multiple gene metabolic engineering in primary and secondary metabolism that demonstrate how this new knowledge of metabolic systems is being applied in plants.

Section snippets

Strategies for metabolic engineering in plants

There are three basic goals of metabolic engineering in plants [4]: the production of more of a specific desired compound, the production of less of a specific unwanted compound, and the production of a novel compound (i.e. a molecule that is produced in nature, but not usually in the host plant, or a completely novel compound). Strategies for achieving these goals include the engineering of single steps in a pathway to increase or decrease metabolic flux to target compounds, to block

Carbohydrate metabolism

Carbohydrate metabolism in plants centers on the reversible conversion of sugars, the direct products of photosynthesis, into storage and structural carbohydrates such as starch and cellulose. Starch is a storage carbohydrate that accumulates transiently in leaves and stably in seeds, tubers and roots. It is a staple source of dietary carbohydrate for animals and also has a large number of industrial uses. Metabolic engineering has been used in an attempt to increase starch yields and to modify

Alkaloids

The alkaloids are perhaps the largest group of secondary products synthesized by plants and the pathways, enzymes and regulatory genes involved in their synthesis have been extensively reviewed [31]. Several distinct types of alkaloids can be distinguished, including the isoquinoline, indole and tropane classes. Park et al. [32] have recently demonstrated antisense suppression of the berberine bridge enzyme in poppy cells, reducing the amount of benzophenanthridine alkaloids but increasing the

Engineering novel metabolic pathways

Although metabolic engineering is often used to modify or extend existing pathways in the host plant, there are some examples where multigene engineering has been used to introduce completely novel pathways and thus to produce completely alien products. In one such experiment two multifunctional cytochrome P450 enzymes and a uridine diphosphate glucose (UDPG)-glucosyltransferase were transferred from sorghum (Sorghum bicolor) into A. thaliana, resulting in the production of the cyanogenic

Conclusions

Multipoint metabolic engineering is now beginning to supersede single-point engineering as the best way to manipulate metabolic flux in transgenic plants. As discussed above, several points in a given metabolic pathway can be controlled simultaneously either by overexpressing and/or suppressing several enzymes or through the use of transcriptional regulators to control several endogenous genes. Applied genomics, proteomics and metabolomics are continuing to expand our knowledge of metabolic

Update

Since this article was written, Lucker et al. [61] have reported the transformation of tobacco plants with three different monoterpene synthases from lemon. When the three transgenes were stacked in one transgenic line, the tobacco plants produced more terpenoids and displayed a different terpenoid profile to wild-type plants. Incidentally, this is also the first report of transgenic plants expressing multiple foreign enzymes competing for the same substrate.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • of special interest

  • ••

    of outstanding interest

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