In Vitro Cellular and Developmental Biology 42, 89-99 (2006) MOLECULAR BREEDING STRATEGIES FOR THE MODIFICATION OF LIPID COMPOSITION DENIS J. MURPHY Biotechnology Unit, School of Applied Sciences, University of Glamorgan, CF37 1DL, Wales, United Kingdom email: dmurphy2@glam.ac.uk Summary Lipids are key components of all living cells. Acyl lipids and sterols provide the matrix of the biological membranes that both define the boundaries of cells and organelles, and act as sites for the trafficking of molecules within and into/out of cells. Lipids are also important metabolic intermediates and the most efficient form of energy storage that is available to a cell. It is the latter, energy-storing function that is of most relevance to this review. Storage lipids are accumulated in abundance in many of our most important crops, including maize, soybean, rapeseed and oil palm, giving rise to a commercial sector valued at over $50 billion/year. Because the storage lipids of the major global oil crops have a relatively restricted composition, there is great interest in using all available breeding technologies, whether traditional and modern, to enhance the variation in lipid quality in existing crops and/or to domesticate new crops that already accumulate useful novel lipids. Over the past few decades, there has been a great deal of effort to manipulate fatty acid composition in order to produce novel lipids, especially for industrial applications. However, these attempts, many based on genetic engineering have met with only limited commercial success to date. More recently, there has been a resurgence of interest in the modification of both acyl and non-acyl lipids to enhance the nutritional quality of plant oils. In this review, we will examine the background to plant lipid modification and some of the latest developments, with a particular focus on edible oils. Key words: lipids, oil crops, breeding, transgenic, fatty acids, edible oils Introduction For more than four decades, researchers around the world have been interested in the modification of lipid composition in many of our major crop plants. One of the most effective attempts to breed a new crop with an altered lipid composition was the development of the high-oleic canola varieties of oilseed rape by Downey et al in the 1960s (Downey & Craig, 1964). Such attempts have continued until the present time, with the announcement in 2005 of the genetic engineering of the oilseed rape relative, Brassica juncea, to produce very long chain polyunsaturates, such as arachidonic acid (Wu et al, 2005). The major reason for these efforts to modify lipid composition is the economic importance of plant oils in global markets. The annual traded volume of plant oils is about 110 million tonnes (MT), worth an estimated $50 billion. The major use of plant oils is for food products, although about 20% of output is employed as oleochemicals for a variety of industrial uses. Globally, plant lipids are the second most important source of edible calories in the human diet (after carbohydrates). Plant lipids are also sources of several essential vitamins and nutrients. For example, plant lipids are the ultimate source of the so- called ‘essential fatty acids’ that are an obligatory component of the diet of all mammals - ever since that time many millions of years ago when our distant animal ancestors lost the ability to introduce double bonds beyond the ∆ 9 position in long chain fatty acids. Since the dawn of agriculture, certain plant species have been cultivated specifically for their lipid composition. The earliest olive plantations have been dated to more than nine millennia before the present day and maize may have been domesticated in Mesoamerica as early as ten millennia ago (Murphy, 2006a). In addition to their acyl lipid ingredients, plants are also important dietary sources of a host of other lipophilic compounds, including vitamins A and E and a range of phytosterols. Most of our dietary plant lipid is derived from oil crops and is in the form of either ‘visible’ (e.g. oils, margarines, chocolate) or ‘invisible’ (cakes, confectionary, processed foods) fats. In the past, the lipid compositional requirements for these products have been provided by commodity plant oils that are blended and/or chemically modified (e.g. by hydrogenation) for a particular edible application. More recently, there has been a move towards a greater segmentation of the commodity plant oils market, with far more stress paid to the initial composition of the plant oil itself. Hence, the increasing demand for plant oils that are enriched in nutritionally beneficial monounsaturates, very long-chain ω-3 fatty acids, carotenoids, phytosterols, and tocotrienols. With an increased willingness by consumers to pay a premium for such nutritionally enhanced oils, it is becoming more economic for growers and processors to segregate such value-added products. This in turn is driving plant breeders to select new varieties of oil crop designed for consumers who are becoming ever more aware of lipid-related nutritional issues, such as the presence of trans fatty acids and saturates in foodstuffs of all kinds. In this article, I will describe how plants are being manipulated through various forms of breeding in order to supply this wide range of dietary lipids. Diversity of plant lipids About two-thirds of the 110 million tonnes of commercially produced plant oil is from soybean, palm and canola (genetically improved rapeseed). The major fatty acids from the world oil supply are palmitic, linoleic and oleic acids. In addition to these major edible fatty acids, many unusual fatty acids can accumulate in seed oils of other plant species, as is shown in Table 1. Sometimes these unusual fatty acids might comprise in excess of 90% of the seed oil (Hildebrand et al, 2005). As shown in Table 1, unusual fatty acid modifications include variations in carbon chain length and degree of unsaturation. Most naturally- 1