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item Cahoon, Edgar

Submitted to: Biennial Conference on Molecular and Cellular Biology of the Soybean
Publication Type: Abstract Only
Publication Acceptance Date: 8/4/2006
Publication Date: 8/4/2006
Citation: Cahoon, E.B. 2006. Metabolic redesign of vitamin e biosynthesis in soybean for enhanced antioxidant content. Biennial Conference on Molecular and Cellular Biology of the Soybean.

Interpretive Summary:

Technical Abstract: The oxidative stability of soybean oil is determined by its fatty acid composition and antioxidant content. Oxidative stability is a critical factor for the use of vegetable oils in food processing and industrial lubricant applications. The primary antioxidants in soybean oil are tocopherols. Tocopherols, together with tocotrienols, comprise the vitamin E family of antioxidants in plants. While tocopherols are found in nearly all plant organs, the occurrence of tocotrienols is limited primarily to monocot seeds. Both forms of vitamin E are potent antioxidants that scavenge free radicals and protect unsaturated fatty acids of vegetable oils from oxidative breakdown. The committed step in tocopherol biosynthesis is the condensation of phytol diphosphate and homogentisate, which is catalyzed by homogentisate phytyltransferase (HPT). We have recently identified an enzyme designated “homogentisate geranylgeranyl transferase” (HGGT) from monocot seeds that shares approximately 60% identity with known HPTs. In contrast to HPT, recombinant HGGT from barley was approximately five-fold more active with geranylgeranyl diphosphate than with phytol diphosphate in condensation reactions with homogentisate. In addition, expression of barley HGGT under control of the CamV35S promoter in Arabidopsis was sufficient to confer tocotrienol biosynthetic ability and to increase the vitamin E antioxidant content of leaves by >10-fold. To test the efficacy of HGGT in soybean, a cDNA for the barley enzyme was expressed under control of the strong seed-specific promoter for the alpha’-subunit of beta-conglycinin gene. Seeds from the resulting transgenic events displayed a three- to five-fold increase in the total content of vitamin E antioxidants. These increases were due largely to the production of delta- and gamma-tocotrienols, which are normally found in only trace amounts in soybean seeds. Seeds from these lines will be used to assess the oxidative stability of the oil and to test its performance in food processing and lubricant applications.