Production of Selected Fatty Acids | Genetics

Engineering crops for selected fatty acid production is one of the important aim for plant biotechnology. An attempt has been made to increase one particular fatty acid stearate in transgenic Brassica plant. Synthesis of high proportion of stearate (18: 0) can reduce the process of hydrogenation.

Normally hydrogenation generates high amount of trans fatty acids, which are associated with risk of coronary heart disease. Therefore plants generate cis fatty acid will benefit from an oil crop capable of accumulating high levels of stearate. Gen­erally most cultivated crop do not naturally produce high amount of stearate.

Through genetic engineering techniques, transgenic expression of modified acyl-specific acyl acyl carrier pro­tein (ACP) thioesterase (TE) produced transgenic plants capable of accumulating high level of specific fatty acids. In this approach, the seed specific expression of acyl-ACP TE allows the enzyme to block fatty acid synthesis pathway resulting accumulating high levels of specific fatty acid.

Similarly, Garcinia mangostana seeds contain oil that is rich in 18: 0 and this plant has an unusual acyl chain specificity. Recently, it has been reported that expressing Garcinia TE (Gar Fat A), in developing rape seeds (Brassica) causes significant accumulation of stearic acid, it is because Garcinia TE has higher activity with 18: 0-AC compared with Brassica TE.

In addition, Garcinia TE uses 18: 1-ACP three-fold more efficient than it was 18: 0-ACP. In the engineering of acyl-ACP thioesterase, cDNA of TE have been sequenced from more than 30 plant species and it has been demonstrated that there are two types of TE (Fat A and Fat B). Mutation in these TE increased the activity more than ten-fold.

Specific fatty acid production in plants can also be enhanced by applying novel antisense RNA technology. Deborah et al., (1992) have modified Brassica seed oil by increasing stearate level using antisense approach for steoroyl ACP desaturase. Steoroyl-acyl-carrier protein (stearoyl-ACP) desaturase catalyse first desaturation step in seed oil biosynthesis of converting stearoyl ACP to oleoyl-ACP.

Stearic acid level can be increased by reduc­ing convertion process from steoroyl-ACP to oleoyl-ACP using seed specific lupin promoter, antisense gene for B. rapa steroroyl-ACP desaturase were used to decrease this enzyme level. The outcome of antisense method in this plant resulted in decrease in desaturase enzyme and drastically increase stearic acid level from 2%-40% in the seed.

Although these techniques signifi­cantly alter the ratio of degree of saturated to unsaturated oil, however, they did not affect the availability of unsaturated fatty acid for membrane and triacyl glycerol biosynthesis. In addition, considerable efforts were made to increase oleic acid for food purpose by using antisense suppression of the ∆¹² desaturases in the seeds of oil seed rape. The resulting transgenic plants however, accumulated oleic acid level upto 88%.