Tocotrienols are the primary form of vitamin E in seeds of most monocot plants, including cereals such as rice and wheat. As potent antioxidants, tocotrienols contribute to the nutritive value of cereal grains in human and livestock diets. cDNAs encoding homogentisic acid geranylgeranyl transferase (HGGT), which catalyzes the committed step of tocotrienol biosynthesis, were isolated from barley, wheat and rice seeds. Transgenic expression of the barley HGGT in Arabidopsis thaliana leaves resulted in accumulation of tocotrienols, which were absent from leaves of nontransformed plants, and a 10- to 15-fold increase in total vitamin E antioxidants (tocotrienols plus tocopherols). Overexpression of the barley HGGT in corn seeds resulted in an increase in tocotrienol and tocopherol content of as much as six-fold. These results provide insight into the genetic basis for tocotrienol biosynthesis in plants and demonstrate the ability to enhance the antioxidant content of crops by introduction of an enzyme that redirects metabolic flux.

Vegetable oils that contain fatty acids with conjugated double bonds, such as tung oil, are valuable drying agents in paints, varnishes, and inks. Although several reaction mechanisms have been proposed, little is known of the biosynthetic origin of conjugated double bonds in plant fatty acids. An expressed sequence tag (EST) approach was undertaken to characterize the enzymatic basis for the formation of the conjugated double bonds of ␣-eleostearic (18:3⌬ 9cis,11trans,13trans ) and ␣-parinaric (18:4⌬ 9cis,11trans,13trans,15cis ) acids. Approximately 3,000 ESTs were generated from cDNA libraries prepared from developing seeds of Momordica charantia and Impatiens balsamina, tissues that accumulate large amounts of ␣-eleostearic and ␣-parinaric acids, respectively. From ESTs of both species, a class of cDNAs encoding a diverged form of the ⌬ 12 -oleic acid desaturase was identified. Expression of full-length cDNAs for the Momordica (MomoFadX) and Impatiens (ImpFadX) enzymes in somatic soybean embryos resulted in the accumulation of ␣-eleostearic and ␣-parinaric acids, neither of which is present in untransformed soybean embryos. ␣-Eleostearic and ␣-parinaric acids together accounted for as much as 17% (wt͞wt) of the total fatty acids of embryos expressing MomoFadX. These results demonstrate the ability to produce fatty acid components of high-value drying oils in transgenic plants. These findings also demonstrate a previously uncharacterized activity for ⌬ 12 -oleic acid desaturase-type enzymes that we have termed ''conjugase.'' H undreds of unusual fatty acid structures are known to occur in the seed oils of various plant species (1). The biosynthetic pathways of many of these fatty acids are unknown or have not been well characterized. One such class consists of fatty acids with double bonds that are conjugated. This structural configuration is in contrast to that of linoleic (18:2⌬ 9cis,12cis ) and ␣-linolenic (18:3⌬ 9cis,12cis,15cis ) acids, the typical polyunsaturated fatty acids of plant seed oils, which contain double bonds that are separated by methylene (ϪCH 2 Ϫ) groups (Fig. 1). Among the fatty acids with conjugated double bonds that occur in the plant kingdom are ␣-eleostearic acid (18:3⌬ 9cis,11cis,13trans) and ␣-parinaric acid (18:4⌬ 9cis,11trans,13trans,15cis ) ( Fig. 1). ␣-Eleostearic acid accounts for Ͼ65% (wt͞wt) of the total fatty acids of tung oil, a high-value drying oil obtained from seeds of Aleurites fordii (2). Other sources of this fatty acid include the seed oil of Momordica charantia, which contains approximately 65% (wt͞wt) ␣-eleostearic acid (2). In addition, ␣-parinaric acid composes 30 to 65% (wt͞wt) of the seed oils of plants such as Parinarium laurinum and Impatiens species (1, 3, 4).The presence of conjugated double bonds in fatty acids markedly increases their rate of oxidation relative to polyunsaturated fatty acids with methylene-interrupted double bonds (5). This property makes seed oils, such as tung oil, that are enriched in fatty acids with conjugated double bonds...