Sterols in plants are almost completely derived from acetyl-coenzyme A (acetyl-CoA). Acetyl-CoA is dimerized to acetoacetyl-CoA that then forms 3- hydroxy-3-methylglutaryl-CoA (HMG-CoA) by a condensation reaction. HMG-CoA is reduced to mevalonic acid by 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), a rate-limiting step in human isoprenoid biosynthesis. HMGR over-expression in transgenic tobacco plants resulted in increased levels of total sterols, indicating that HMGR is a limiting enzyme also in plant sterol biosynthesis (Schaller et al., 1995). The next step in sterol synthesis is to form squalene from mevalonic acid, and this is performend by a series of reactions where the final step is catalysed by the squalene synthase enzyme (SQS).
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The first fully cyclized sterol intermediates, cycloartenol and lanosterol, are made from squalene in a series of reactions. Cycloartenol then serves as common precursor for all end-product sterols in three parallel pathways leading towards cholesterol, campesterol, sitosterol (Benvineste, 2002). The role of lanosterol in plants is unclear. An increase in sterol sidechain length occurs by two alkylation reactions. Cycloartenol is methylated by the enzyme sterol methyltransferase type-1 (SMT1) at C-24. S-adenosyl methionine acts as the methyl donor in this process, thus introducing one nonacetyl-CoA carbon atom in the plant sterol structure. Transgenic potato plants overexpressing a SMT1 cDNA displayed an increase in total sterols, especially in isofucosterol and sitosterol as compared to …show more content…
Conversely, the smt1 mutant in Arabidopsis accumulates cholesterol and has less C-24 alkylated sterol contents (e.g. less campesterol, sitosterol and stigmasterol) (Diener et al., 2000). This indicates a pivotal role for SMT1 in the channeling of sterol synthesis between alkylated and non-alkylated sidechain sterols. A second methylation can further increase the sterol side-chain length. This step is catalyzed by sterol methyltransferase type-2 (SMT2), and the preferred substrate is 24-methylene lophenol. This leads towards increased synthesis of C10 side-chain sterols such as sitosterol, but at the expense of C8 and C9 sterols. The altered sterol profile in smt2 mutant Arabidopsis lines showed higher levels of campesterol at the expense of sitosterol, which led to a reduced stature and growth but which could not be rescued by exogenous BR treatment (Schaffer et al., 2001). This work indicates that appropriate ratios of all sterols within the membrane are necessary for normal growth and development of plants, and that alterations in their sterol profile may cause 32 aberrant growth and development. Another important step in plant sterol
The first fully cyclized sterol intermediates, cycloartenol and lanosterol, are made from squalene in a series of reactions. Cycloartenol then serves as common precursor for all end-product sterols in three parallel pathways leading towards cholesterol, campesterol, sitosterol (Benvineste, 2002). The role of lanosterol in plants is unclear. An increase in sterol sidechain length occurs by two alkylation reactions. Cycloartenol is methylated by the enzyme sterol methyltransferase type-1 (SMT1) at C-24. S-adenosyl methionine acts as the methyl donor in this process, thus introducing one nonacetyl-CoA carbon atom in the plant sterol structure. Transgenic potato plants overexpressing a SMT1 cDNA displayed an increase in total sterols, especially in isofucosterol and sitosterol as compared to …show more content…
Conversely, the smt1 mutant in Arabidopsis accumulates cholesterol and has less C-24 alkylated sterol contents (e.g. less campesterol, sitosterol and stigmasterol) (Diener et al., 2000). This indicates a pivotal role for SMT1 in the channeling of sterol synthesis between alkylated and non-alkylated sidechain sterols. A second methylation can further increase the sterol side-chain length. This step is catalyzed by sterol methyltransferase type-2 (SMT2), and the preferred substrate is 24-methylene lophenol. This leads towards increased synthesis of C10 side-chain sterols such as sitosterol, but at the expense of C8 and C9 sterols. The altered sterol profile in smt2 mutant Arabidopsis lines showed higher levels of campesterol at the expense of sitosterol, which led to a reduced stature and growth but which could not be rescued by exogenous BR treatment (Schaffer et al., 2001). This work indicates that appropriate ratios of all sterols within the membrane are necessary for normal growth and development of plants, and that alterations in their sterol profile may cause 32 aberrant growth and development. Another important step in plant sterol