Polyunsaturated fatty acids (PUFAs) are essential membrane components in higher eukaryotes and are the precursors of many lipid-derived signaling molecules. Here, pathways for PUFA synthesis are described that do not require desaturation and elongation of saturated fatty acids. These pathways are catalyzed by polyketide synthases (PKSs) that are distinct from previously recognized PKSs in both structure and mechanism. Generation of cis double bonds probably involves position-specific isomerases; such enzymes might be useful in the production of new families of antibiotics. It is likely that PUFA synthesis in cold marine ecosystems is accomplished in part by these PKS enzymes.

Population growth, arable land and fresh water limits, and climate change have profound implications for the ability of agriculture to meet this century’s demands for food, feed, fiber, and fuel while reducing the environmental impact of their production. Success depends on the acceptance and use of contemporary molecular techniques, as well as the increasing development of farming systems that use saline water and integrate nutrient flows.

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Molecular gene transfer techniques have been used to engineer the fatty acid composition of Brassica rpa and Brassica napus (canola) oil. Stearoyl-acyl carrier protein (stearoyl-ACP) desaturase (EC 1.14.99.6) catalyzes the first desaturation step in seed oil biosynthesis, converting stearoyl-ACP to oleoyl-ACP. Seed-specific antisense gene constructs ofB. rapa stearoyl-ACP desaturase were used to reduce the protein concentration and enzyme activity of stearoyl-ACP desaturase in developing rapeseed embryos during storage lipid biosynthesis. The resulting transgenic plants showed dramatically increased stearate levels in the seeds. A continuous distribution of stearate levels from 2% to 40% was observed in seeds of a transgenic B. napus plant, illustrating the potential to engineer specialized seed oil compositions.Canola and other temperate vegetable oils are composed predominantly of unsaturated 18-carbon fatty acids: the monounsaturated oleic (18:1) and polyunsaturated linoleic (18:2) and linolenic (18:3) acids. In addition to these fatty acids, most oils also contain small but significant amounts of the saturated palmitic (16:0) and stearic (18:0) acids (1). The plastid-localized enzyme stearoyl-acyl carrier protein (stearoyl-ACP) desaturase (EC 1.14.99.6) catalyzes the initial desaturation reaction in fatty acid biosynthesis (Fig. 1A) and thus plays a key role in determining the ratio oftotal saturated to unsaturated fatty acids in plants (2,4,5).Specialized fatty acid compositions desired for edible and industrial purposes have been produced in oilseed crops through traditional breeding and selection alone or in combination with mutagenesis programs (6-9). Although the molecular basis for the changes is largely unknown, examples such as the removal of erucic acid from rapeseed oil to create canola (10), reduction of linolenic acid content in flax seed (11), and increases in stearate content of up to six times the wild-type level in safflower (up to 12% stearate) (12) and soybean (up to 30%o stearate) oil (13,14) demonstrate the plasticity of fatty acid composition in seed oil. It should also be possible to modify seed oil composition by the use of genetic engineering techniques (15-17). Antisense RNA technology has proven to be an effective means of reducing the level of specific enzymes in plants (18-21). Because fatty acid biosynthesis is an essential metabolic pathway in all tissues ofthe plant, modification of seed oil biosynthesis may require tissue-specific control of antisense RNA expression. Reduction of stearoyl-ACP desaturase in seeds should alter the ratio of saturated to unsaturated fatty acids and lead to the production of a novel storage oil without compromising the integrity of membrane lipids in leaf and other plant tissues.We report the isolation of a Brassica rapa (syn. Brassica campestris, turnip rape) stearoyl-ACP desaturase cDNAt and expression of antisense stearoyl-ACP desaturase constructs in seeds of B. rapa and Brassica napus. The activity and amount of stearoyl-ACP desaturase...

BackgroundWheat (Triticum spp.) is an important source of food worldwide and the focus of considerable efforts to identify new combinations of genetic diversity for crop improvement. In particular, wheat starch composition is a major target for changes that could benefit human health. Starches with increased levels of amylose are of interest because of the correlation between higher amylose content and elevated levels of resistant starch, which has been shown to have beneficial effects on health for combating obesity and diabetes. TILLING (Targeting Induced Local Lesions in Genomes) is a means to identify novel genetic variation without the need for direct selection of phenotypes.ResultsUsing TILLING to identify novel genetic variation in each of the A and B genomes in tetraploid durum wheat and the A, B and D genomes in hexaploid bread wheat, we have identified mutations in the form of single nucleotide polymorphisms (SNPs) in starch branching enzyme IIa genes (SBEIIa). Combining these new alleles of SBEIIa through breeding resulted in the development of high amylose durum and bread wheat varieties containing 47-55% amylose and having elevated resistant starch levels compared to wild-type wheat. High amylose lines also had reduced expression of SBEIIa RNA, changes in starch granule morphology and altered starch granule protein profiles as evaluated by mass spectrometry.ConclusionsWe report the use of TILLING to develop new traits in crops with complex genomes without the use of transgenic modifications. Combined mutations in SBEIIa in durum and bread wheat varieties resulted in lines with significantly increased amylose and resistant starch contents.