This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Triterpenoids Biosynthesis|
Some taxa known to possess this pathway include : Arabidopsis thaliana col
Expected Taxonomic Range: Brassicaceae
Triterpene compounds are plant secondary metabolites that are often involved in plant defense and may also have beneficial effects when ingested by humans. They arise from numerous different cyclizations of 2,3-oxidosqualene catalyzed by various oxidosqualene cyclases (OSCs) [Phillips06, Liby07, Papadopoulou99]. Attempts to determine the functional specificity of members of the OSC family of enzymes in Arabidopsis led to the discovery that THAS produces thalianol when expressed in yeast [Nelson04]. Later studies with plants confirmed that this metabolite is present at low levels in Arabidopsis roots, and that THAS is required for its synthesis [Field08]. Characterization of plants with mutations in the two adjacent genes coding for enzymes from Brassicaceae-specific families of P450 proteins demonstrate that THAH and THAD further modify thalianol. Like the avenacin gene cluster found in oats, this set of three genes forms another operon-like group of metabolically-related enzymes [Field08].
The functional importance of this pathway is still under investigation. The three thalianol-related metabolites can be identified in Arabidopsis roots and an accumulation of thalianol and thalian-diol can increase root growth [Field08]. But, despite the expected role for triterpenes in plant defense, roots of thas, thad, and thah mutants do not appear more susceptible to a few strains of bacterial and plant pathogens that have been tested [Field08]. In addition, over-expression of THAS and THAD, leading to the ectopic production of thalianol and thalian-diol in aerial tissues can cause stunted plant growth [Field08]. Further work should help clarify the biological role of thalianol and its derivatives in Arabidopsis thaliana and related species.
Nelson04: Nelson DR, Schuler MA, Paquette SM, Werck-Reichhart D, Bak S (2004). "Comparative genomics of rice and Arabidopsis. Analysis of 727 cytochrome P450 genes and pseudogenes from a monocot and a dicot." Plant Physiol 135(2);756-72. PMID: 15208422
Papadopoulou99: Papadopoulou K, Melton RE, Leggett M, Daniels MJ, Osbourn AE (1999). "Compromised disease resistance in saponin-deficient plants." Proc Natl Acad Sci U S A 96(22);12923-8. PMID: 10536024
Corey93: Corey EJ, Matsuda SP, Bartel B (1993). "Isolation of an Arabidopsis thaliana gene encoding cycloartenol synthase by functional expression in a yeast mutant lacking lanosterol synthase by the use of a chromatographic screen." Proc Natl Acad Sci U S A 1993;90(24);11628-32. PMID: 7505443
Dunkley06: Dunkley TP, Hester S, Shadforth IP, Runions J, Weimar T, Hanton SL, Griffin JL, Bessant C, Brandizzi F, Hawes C, Watson RB, Dupree P, Lilley KS (2006). "Mapping the Arabidopsis organelle proteome." Proc Natl Acad Sci U S A 103(17);6518-23. PMID: 16618929
Hamberger06: Hamberger B, Bohlmann J (2006). "Cytochrome P450 mono-oxygenases in conifer genomes: discovery of members of the terpenoid oxygenase superfamily in spruce and pine." Biochem Soc Trans 34(Pt 6);1209-14. PMID: 17073787
Kolesnikova07: Kolesnikova MD, Obermeyer AC, Wilson WK, Lynch DA, Xiong Q, Matsuda SP (2007). "Stereochemistry of water addition in triterpene synthesis: the structure of arabidiol." Org Lett 9(11);2183-6. PMID: 17474751
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