H

CDP-choline 18:1

fad 2

Linoleoyi desaturase

18:3

Oleoyl desaturase

Linoleoyi desaturase

18:3

Figure 1 A simplified pathway of triglyceride biosynthesis. Fatty acid biosynthesis from C2 to C16:0, C18:1 occurs in the plastid. Critical steps affecting oil quality are the stearoyl desaturase and the thioesterases. The remaining steps in TAG biosynthesis are located in the smooth endoplasmic reticulum. The oleoyl and linoleoyi desaturases are the most critical extraplastidic steps affecting oil quality.

(18:2, 18:3), combined with a reduction in total saturates (16:0, 18:0). This would yield more a chemically stable oil (high 18:1) with reduced total saturated fat content. Another important target has been to increase the total 18:0 and 18:1 content of the plant oil so that it has an acceptable solid fat functionality for use in margarine and other confectionery applications but without the perceived deleterious health effects of partially hydrogenated oils rich in trans-fatty acids.

It has been shown that the fat B class of acyl:ACP thioesterases control the release of 16:0 into the cytoplasm, making it available for TAG biosynthesis. Thus, inactivation of fat B should give a low 16:0 phenotype. Conversely, overexpression of fat B should give a high 16:0 phenotype. The insertion of the first double bond into fatty acyl chains is a plastidic reaction catalyzed by a soluble desaturase enzyme, stearoyl-ACP desaturase (AAD1). Inactivation of the aad 1 gene should result in a high 18:0-coenzyme A (CoA) pool in the cytoplasm and a resultant high 18:0 content of seed TAG. Oleoyl-CoA is also incorporated into membrane phosphatidylcholine (PC), where it is desaturated to linoleoyl-PtdCho by a membrane-bound 5-12 desaturase encoded by a fad 2 gene (13). Inactivation of the fad 2 gene should give a high 18:1 phenotype.

High-oleic mutants of corn, peanut canola, and sunflower have been described, with an 18:1 ranging from 60 to 90%. The high-oleic sunflower is particularly noteworthy because it is a dominant mutation in a structural fad 2 gene (14). Interestingly, no good high-oleic mutant of soybean has yet been found by chemical mutagenesis despite intensive screening by both private and public breeders. In contrast to most high-oleic mutants, which are primarily recessive mutations, transgenic soy lines in which the seed-specific expression of fad 2 has been inactivated result in a consistently high oleic (>80%) content of the seed oil. This phenotype is not affected by environment and has no yield penalty (15).

Low-saturate mutants of soy also exist, primarily due to altered fat B activity (16), but again they are recessive and are the result of mutations in several genes. Transgenic soy lines in which the fat B thioesterase activities are suppressed have a 50% reduction in total 16:0 and are dominant (15).

Again, both high-16:0 (inactive kas 2) and high-stearate (reduced aad 1 activity) mutants of soy exist. The high-16:0 mutants (>40% 16:0 in seed oil) are relatively stable across different environments, although the mutants are recessive (17). Transgenic soybean plants overexpressing/ar B also produce >40% 16:0 in the seed oil but as a dominant trait (15).

The high (20-30% 18:0) stearate mutant of soy, by contrast, has severe phenotypic consequences associated with the trait including poor germina-bility and loss of vigor (17). These characteristics were overcome in both transgenic soy and canola in which one or more endogenous seed-specific aad 1 genes were inactivated in the transgenic seeds. The resultant oil in both cases was enriched in stearate (35-40% 18:0) and the vegetative tissue of the transgenic plants was normal. In both cases, however, the seed germination was impaired, possibly because of the presence of 18:0 in the membrane phospholipids as well as the TAG (15,18). Sunflower, by contrast, has a high-stearate mutant (25% 18:0) that appears to be normal in all other respects (19). Further, transgenic sunflowers that contain 35% 18:0 in the TAG (seed-specific suppression of aad 1) germinated normally, although the seed oil content was reduced (S. Coughlan, unpublished observations).

There is also a report of a specialized thioesterase involved in the production of stearate in the seeds of mangosteen (Garcinia mangostana) (20). Stearate is the predominant fatty acid (45-50% of total fatty acids) in this seed. When the mangosteen thioesterase gene (fat A) was expressed in canola seeds, stearate levels of 20% were observed (20).

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