An increase in the oil content of the main oil-producing plants has obvious advan­tages, especially with soybean. Soybean is grown as a source of high protein animal feed but supplies large amounts of oil in the USA, although it only contains low levels of oil (18-22%). The limiting step in fatty acid synthesis would appear to be the acetyl-CoA carboxylase production of malonyl-CoA. However, genetic manipulation increasing the malonyl-CoA pool failed to increase oil yields significantly, which sug­gests that there are additional controls in fatty acid synthesis.

The quality of the oil produced can be altered by making single gene changes. The oleic acid content of soybean was increased to 86% by suppression of the enzyme oleoyl desaturase (Thelen and Ohlrogge, 2002). Rapeseed has been engineered to accumulate 58% short-chain lauric acid (12:0) in its oil by expression of a thioesterase from the California bay plant. However, not all gene transfers have been successful and breakdown of the target product can occur in some cases.

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One of the possibilities of increasing oil supplies is to introduce new plants, espe­cially those that can be grown on marginal land not suited to food crops. One group of plants is the oilseed shrubs which include castor bean (Ricinus communis), Ponga — mia pinnata, Calophyllum inophyllum and Jatropha curcas. Oil from these plants has been tested for its suitability as biodiesel (Forson et al., 2004; Chapter 7, Table 7.17). Castor oil is best suited as a lubricant, and for many other industrial uses. J. curcas has attracted interest as the oil is suitable as a biofuel and the plant can grow in the desert without addition of water and fertilizer (Openshaw, 2000). However, as a commercial crop J. curcas has a number of problems. It has to be harvested by hand and contains toxic alkaloids, phorbol esters and curcin which make the meal unsuitable as animal feed. It is perhaps why J. curcas is known as black vomit nut, purge nut, physic nut and the extracted oil, hell oil. The other plants also have toxic compounds in their seeds. Castor beans contain ricin, a neurotoxin, along with allergens and P. pinnata and C. inophyllum both have bitter and poisonous compounds in their seeds. To make these plants commercial crops they will need modification which could involve genetic manipulation. The characteristics which need to be introduced are as follows:

• Dwarf stalks for easy harvesting.

• Suppression of branching to allow for mechanical harvesting.

• Introduction of anti-shattering gene to stop fruit drying and scattering seeds.

• The elimination of the toxic compounds.

The genetic manipulation of microalgae has been demonstrated in a few algal species (Rosenberg et al., 2008). An increase in lipid content in the diatom Cyclotella cryp — tica was attempted by overexpression of ACCase but as found with higher plants it did not have a significant effect on lipid yields. More research is required to determine the control mechanisms of lipid synthesis.