Enhancement of seed phytosterol levels by expression of an N-terminal truncated Hevea brasiliensis (rubber tree) 3-hydroxy-3-methylglutaryl-CoA reductase.

Dietary intake of phytosterols (plant sterols) has been shown to be effective in reducing blood cholesterol levels, thereby reducing the risk of cardiovascular disease. Phytosterols are most commonly sourced from vegetable oils, where they are present as minor components. We report here the generation of transgenic tobacco seeds substantially enhanced in phytosterol content by the expression of a modified form of one of the key sterol biosynthetic enzymes, 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR).

Co-expression of N-terminal truncated 3-hydroxy-3-methylglutaryl CoA reductase and C24-sterol methyltransferase type 1 in transgenic tobacco enhances carbon flux towards end-product sterols.

The enzymes 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) and C24-sterol methyltransferase type 1 (SMT1) have been proposed to be key steps regulating carbon flux through the sterol biosynthesis pathway. To further examine this hypothesis, we co-expressed the catalytic domain of Hevea brasiliensis HMGR (tHMGR) and Nicotiana tabacum SMT1 in tobacco, under control of both constitutive and seed-specific promoters, resulting in increased accumulation of total sterol in seed tissue by 2.5- and 2.

Sterol C-24 methyltransferase type 1 controls the flux of carbon into sterol biosynthesis in tobacco seed.

The first committed step in the conversion of cycloartenol into Delta(5) C24-alkyl sterols in plants is catalyzed by an S-adenosyl-methionine-dependent sterol-C24-methyltransferase type 1 (SMT1). We report the consequences of overexpressing SMT1 in tobacco (Nicotiana tabacum), under control of either the constitutive carnation etched ring virus promoter or the seed-specific Brassica napus acyl-carrier protein promoter, on sterol biosynthesis in seed tissue. Overexpression of SMT1 with either promoter increased the amount of total sterols in seed tissue by up to 44%.