The production of docosahexaenoic acid (DHA) in transgenic plants: a sustainable source of omega-3 fish oils

Fish oils have been historically associated with health-beneficial properties and over the last few years a large number of scientific studies have demonstrated the benefits of a diet rich in these oils. In particular, some of the fatty acids found in fish oils seem to play a role in preventing heart attacks and other circulatory problems. These fatty acids are the omega-3 long chain polyunsaturated fatty acids (abbreviated to omega-3 LC-PUFAs), and they are now widely viewed as vital constituents of human diet. As well as being able to play a role in preventing diseases, fish oil omega-3 LC-PUFAs are also very important in human growth and development. For example, breast milk contains these fatty acids, and it is for this reason formula (replacement) milks are now enriched in these fats. The primary source of omega-3 LC-PUFAs is fish oils, but unfortunately global fish stocks are now in severe decline (mainly due to decades of over-fishing). This not only represents an ecological crisis, but may also, in the future, severely hamper the availability of fish oils to maintain a healthy diet. Moreover, there are growing concerns about the contamination of current wild fish stocks with pollutants such as heavy metals, plasticizers and dioxins. Therefore, there is an urgent need to find a new sustainable source of these very important fatty acids. One approach that we are undertaking is to try and make fish oils in plants. This requires genetic engineering of a suitable plant (ideally an oilseed), because there are no known examples of higher plants which synthesise omega-3 LC-PUFAs. To carry out this work, the genes which direct the synthesis of omega-3 LC-PUFAs need to be introduced in a plant. These genes come from the tiny microbes (such as algae) which live in the ocean and synthesise omega-3 LC-PUFAs, so the project involves moving these genes into plants, to allow the synthesis of these important fatty acids in a clean and sustainable manner.

The aim of this project is to produce the twenty two carbon omega-3 long chain polyunsaturated fatty acid (LC-PUFA) docosahexaenoic acid (DHA; 22:6, n-3) in transgenic plants, in particular in the triacylglycerols of seed storage lipid reserves. This proposal builds on the collective expertise and data of the applicants, who have demonstrated the feasibility of the synthesis of the twenty carbon omega-3 LC-PUFA eicosapentaenoic acid (EPA; 20:5, n-3) in transgenic plants. However the conversion of EPA to DHA (mediated by elongation and desaturation) appears from several independent studies to represent a major bottleneck in the production of DHA. The aim of this proposal is to systematically determine and overcome this bottleneck, resulting in the accumulation of DHA in transgenic plants. This in turn will represent a novel sustainable source of these omega-3 LC-PUFAs which are now known to be important for human health and nutrition. The key approaches to optimising the synthesis and accumulation of DHA can be summarised as: 1) Overcoming the substrate-dichotomy present in the biosynthetic pathway, in which desaturation requires fatty acid substrates linked to phospholipids whereas elongation requires substrate fatty acids as acyl-CoAs. 2) Increasing the conversion of omega-6 fatty acids to omega-3 forms, thereby increasing the flux towards EPA and DHA. 3) Optimising the accumulation of DHA in storage lipid, by using omega-3 LC-PUFA-specific triacylglycerol biosynthesis activities. 4) Increasing the flux (by enhancing substrate-specificity) through the microsomal elongase. Collectively, these approaches are predicted to deliver the accumulation of DHA in transgenic plants to the level of 15-25% of total fatty acids. This represents levels similar to that achieved by the primary producers (i.e. the aquatic microbes) of omega-3 LC-PUFAs, with the crucial difference being that DHA will accumulate predominantly in triacylglycerols of transgenic plants.