Production of novel oils in plants

https://doi.org/10.1016/S0958-1669(99)80031-7Get rights and content

Abstract

We have now isolated the great majority of genes encoding enzymes of storage oil biosynthesis in plants. In the past two years, particular progress has been made with acyltransferases, ketoacyl-acyl carrier protein synthetases and with desaturases and their relatives. In some cases, these enzymes have been re-engineered to create novel products. Nevertheless, the single or multiple insertion of such transgenes into oil crops has not always led to the desired phenotype. We are only now beginning to appreciate some of the complexities of storage and membrane lipid formation, such as acyl group remodelling and the turnover of unusual fatty acids. This understanding will be vital for future attempts at the rational engineering of transgenic oil crops. In parallel with this, the domestication of plants already synthesising useful fatty acids should be considered as a real alternative to the transgenic approach to producing novel oil crops.

Introduction

The manipulation of seed oil content via transgene insertion was one of the early successful applications of modern biotechnology in agriculture. Indeed, the first transgenic crop with a modified seed composition to be approved for unrestrictive commercial cultivation in the USA was a lauric oil rapeseed grown in 1995. There are two major reasons for this. Firstly, rapeseed, Brassica napus, is a species that is relatively amenable to transformation and regeneration, whereas many other major crops have proved more recalcitrant. Secondly, the metabolic pathways involved in storage oil biosynthesis appeared at first to be well defined and potentially straightforward to manipulate via single gene insertions.

Nevertheless, much of the early optimism for producing designer oilseeds has, over recent years, been tempered by setbacks in obtaining high yields of specific novel fatty acids in transgenic oilseed crops. During the past two years, there has been an increasing recognition of the complexity of the metabolic pathways involved in seed oil biosynthesis and several new enzymes have been discovered that contribute to these processes in quite unexpected ways. We have seen the isolation of some potentially key genes that contribute both to the quantity and quality of seed storage oils. There has also been an increasing appreciation of the importance of fatty acids, not only as storage or structural components, but also acting as, or giving rise to, important signalling molecules that regulate many aspects of plant development 1, 2. This illustrates the importance of ensuring that novel fatty acids in transgenic oil crops are correctly targeted to the storage oil and are hence unable to adversely affect membrane or signalling functions. The purpose of this article is to review some of the recent progress in understanding the mechanism and regulation of storage oil formation in plants, and how this may impact on its biotechnological manipulation.

Section snippets

Industrial and edible oils

Before reviewing some of the recent technical developments, it may be useful to consider what we are trying to achieve in modifying seed oils and why we are doing it. At present, over 80% of the 75 million tonnes of globally traded seed oils are used for edible purposes, most notably in the production of cooking oils, margarines and processed foods [3]. Global production of plant oils for industrial use (i.e. oleochemicals) is only about 15 million tonnes per year with a value of about $400–800

Non-oil products

In addition to producing seed oils with novel fatty acid compositions, there are numerous other actual or potential applications of transgenic oil crops. For example, as recently reviewed [11], following the insertion of a relatively small number of genes from certain bacteria, such as Alcaligenes spp, carbon can be diverted from oil synthesis towards the accumulation of polyhydroxyalkanoates. These polyesters are biodegradable thermoplastics. Their use is currently limited by their high price

Engineering fatty acid desaturases

An overview of the major pathways involved in storage lipid metabolism is shown in Figure 1. Over the past few years there has been considerable progress in isolating genes encoding the vast majority of these enzymes. Some of the most significant developments have taken place in characterising desaturases and related diiron-oxo proteins in plants, as recently reviewed in detail [13].

It now appears that plants contain two major families of diiron-oxo enzymes. Firstly, there are the soluble

Other key enzymes of fatty acid modification

One of the earliest successes in producing transgenic plants with modified storage oil was the addition of a California Bay thioesterase gene to rapeseed, resulting in the accumulation of ∼40% lauric acid in its seed triacylglycerol (TAG). The accumulation of higher levels (50–60%) of this C12 fatty acid required the additional transfer of a coconut sn-2 acyltransferase gene [3]. The important contribution of thioestereases to oil quality has also been shown by the accumulation of ∼20% stearic

Fatty acid segregation and recycling

An important challenge facing biotechnologists is to develop transgenic oil crops, such as rapeseed, with high levels of useful fatty acids, many of which are not normally produced by such species [3]. To date, most transgenic lines have been reported to accumulate relatively low (typically 1–40%) levels of the new fatty acids, such as ricinoleic 18, 19, stearic [24], or γ-linolenic [34]. One explanation for this is that rapeseed appears to be less efficient at segregating exotic fatty acids

Conclusions

Although nearly all of the genes encoding enzymes of storage lipid biosynthesis have now been cloned, there have been many surprising results when these genes are expressed in transgenic plants. This highlights our relative ignorance of the interactions between the components of this and other metabolic pathways in vivo. We also know very little about the mechanisms regulating the partitioning of carbon to storage products in sink tissues such as oilseeds. A very promising recent approach is to

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

References (46)

  • J. Fritsche et al.

    Analysis, occurrence, and physiological properties of trans fatty acids (TFA) with particular emphasis on conjugated linoleic acid isomers (CLA) — a review

    Fett/Lipid

    (1998)
  • I.S. Newton

    Long-chain polyunsaturated fatty acids — the new frontier in nutrition

    Lipid Technol

    (1998)
  • J.B. Ohlrogge

    Design of new plant products: engineering of fatty acid metabolism

    Plant Physiol

    (1994)
  • K.D. Richter et al.

    Fat infiltration in liver of rats induced by different dietary plant oils: high oleic-, medium oleic- and high petroselinic-acid-oils

    Z Ernahrungswiss

    (1996)
  • Anon: Chemical, Week, December, 24/31...
  • D.J. Murphy et al.

    Fatty acid desaturases: structure mechanism and regulation

  • D.M. Kurtz

    Structural similarity and functional diversity in diiron-oxo proteins

    J Bioinorganic Chem

    (1997)
  • Y. Lindqvist et al.

    Crystal structure of a delta-9 stearoyl-acyl carrier protein desaturase from castor seed and its relationship to other diiron proteins

    EMBO J

    (1996)
  • E.B. Cahoon et al.

    Redesign of soluble fatty acid desaturases from plants for altered substrate specificity and double bond position

    Proc Natl Acad Sci USA

    (1997)
  • E.B. Cahoon et al.

    A determinant of substrate specificity predicted from the acyl-acyl carrier protein desaturase of developing cat’s claw seed

    Plant Physiol

    (1998)
  • P. Broun et al.

    Accumulation of ricinoleic, lesquerolic, and densipolic acids in seeds of transgenic Arabidopsis plants that express a fatty acyl hydroxylase cDNA from castor bean

    Plant Physiol

    (1997)
  • P. Broun et al.

    A bifunctional oleate 12-hydroxylase: desaturase from Lesquerella fendleri

    Plant J

    (1998)
  • M. Lee et al.

    Identification of non-heme diiron proteins that catalyze triple bond and expoxy group formation

    Science

    (1998)
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