WRINKLED1 and ACYL-COA:DIACYLGLYCEROL ACYLTRANSFERASE1 regulate tocochromanol metabolism in Arabidopsis.

Abstract

Photosynthetic organisms such as plants, algae and some cyanobacteria synthesize tocochromanols, a group of compounds that encompasses tocopherols and tocotrienols and that exhibits vitamin E activity in animals. While most vitamin E biosynthetic genes have been identified in plant genomes, regulatory genes controlling tocopherol accumulation are currently unknown. We isolated by forward genetics Arabidopsis enhanced vitamin E (eve) mutants that overaccumulate the classic tocopherols and plastochromanol-8, and a tocochromanol unknown in this species. We mapped eve1 and eve4, and identified the unknown Arabidopsis tocochromanol by using a combination of analytical tools. In addition, we determined its biosynthetic pathway with a series of tocochromanol biosynthetic mutants and transgenic lines. eve1 and eve4 are two seed lipid mutants affecting the WRINKLED1 (WRI1) and ACYL-COA:DIACYLGLYCEROL ACYLTRANSFERASE1 (DGAT1) genes, respectively. The unknown tocochromanol is 11'-12' γ-tocomonoenol, whose biosynthesis is VITAMIN E 1 (VTE1) - and VTE2-dependent and is initiated by the condensation of homogentisate (HGA) and tetrahydrogeranylgeranyl pyrophosphate. This study identifies the first two regulatory genes, WRI1 and DGAT1, that control the synthesis of all tocochromanol forms in seeds, and shows the existence of a metabolic trade-off between lipid and tocochromanol metabolisms. Moreover, it shows that Arabidopsis possesses a tocomonoenol biosynthetic pathway that competes with tocopherol synthesis.