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Review
, 49 (6), 439-62

Metabolic and Functional Diversity of Saponins, Biosynthetic Intermediates and Semi-Synthetic Derivatives

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Review

Metabolic and Functional Diversity of Saponins, Biosynthetic Intermediates and Semi-Synthetic Derivatives

Tessa Moses et al. Crit Rev Biochem Mol Biol.

Abstract

Saponins are widely distributed plant natural products with vast structural and functional diversity. They are typically composed of a hydrophobic aglycone, which is extensively decorated with functional groups prior to the addition of hydrophilic sugar moieties, to result in surface-active amphipathic compounds. The saponins are broadly classified as triterpenoids, steroids or steroidal glycoalkaloids, based on the aglycone structure from which they are derived. The saponins and their biosynthetic intermediates display a variety of biological activities of interest to the pharmaceutical, cosmetic and food sectors. Although their relevance in industrial applications has long been recognized, their role in plants is underexplored. Recent research on modulating native pathway flux in saponin biosynthesis has demonstrated the roles of saponins and their biosynthetic intermediates in plant growth and development. Here, we review the literature on the effects of these molecules on plant physiology, which collectively implicate them in plant primary processes. The industrial uses and potential of saponins are discussed with respect to structure and activity, highlighting the undoubted value of these molecules as therapeutics.

Keywords: Glycoalkaloid; plant development; plant growth; steroid; structure–activity relationships; triterpenoid.

Figures

Figure 1.
Figure 1.
Overview of structural diversity in saponin aglycones. The saponin biosynthesis pathway begins with acetyl-CoA, which via the mevalonate pathway is converted to the linear 30-carbon precursor squalene (yellow). Squalene is oxidized to 2,3-oxidosqualene, which is cyclized to the cycloartane aglycone, the dedicated precursor for sterol biosynthesis in plants (green). Cholesterol is the dedicated precursor for the synthesis of steroidal saponins (violet) and steroidal glycoalkaloids (blue). The triterpenoid saponins (orange) are synthesized from 2,3-oxidosqualene that is cyclized to specialized triterpene aglycones. Enzyme and pathway names are italicized. Dashed arrows imply multiple steps in the pathway.
Figure 2.
Figure 2.
Organ- and tissue-specific expression of oxidosqualene cyclase genes in Arabidopsis thaliana and Avena strigosa. (A) Heat map depicting the expression profiles for 12 of the 13 A. thaliana OSC genes in various plant organs. Transcripts for CAMS1 (At1g78970) were not detected and hence not included. Expression data were retrieved from Genevestigator V3. BARS1, baruol synthase; CAS1, cycloartenol synthase; LSS1, lanosterol synthase; LUP1, lupeol synthase; LUP2, β-amyrin synthase; LUP4, β-amyrin synthase; LUP5, tirucalla-7,21-dien-3β-ol synthase; MRN1, marneral synthase; PEN1, arabidiol synthase; PEN3, tirucalla-7,24-dien-3-ol synthase; PEN6, bauerenol synthase; THAS1, thalianol synthase. From Thimmappa et al. (2014). (B) Distribution of β-amyrin synthase (Sad1) and cycloartenol synthase (CS1) gene transcripts in the root tip of A. strigosa. In situ mRNA hybridization of young root tips shows Sad1 transcripts in the epidermis, weakly in parts of the subepidermis and some columella cells. CS1 expression is restricted to the cortex in the elongation zone above the root tip. From Wegel et al. (2009).
Figure 3.
Figure 3.
Plant growth and development effects upon external application of saponins and their intermediates. (A) Effect of application of chromosaponin I (CSI, isolated from Pisum sativum) on Arabidopsis thaliana root growth. Root length, root cell number and length of mature cells increase in both Landsberg erecta (Ler) and Columbia (Col) wild-type (WT) ecotypes upon CSI application. However, the root diameter decreases upon CSI treatment in both ecotypes. From Rahman et al. (2000). (B) Effect of application of 0.018% betulin or its glucosides on Medicago sativa radicle and hypocotyl growth. No effects were observed on hypocotyl growth, but a clear inhibitory effect was seen on radicle growth with increasing number of glucosides attached to betulin. From Ohara & Ohira (2003). (C) Inhibitory effect of application of α-tomatine on stem elongation and chlorophyll accumulation in Sesbania exaltata, Senna obtusifolia, Vigna radiata, Triticum aestivum and Sorghum vulgare. From Hoagland (2009). The structures of the applied compounds are given. Gal, galactose, Glc, glucose, GlcA, glucuronic acid; Rha, rhamnose; Xyl, xylose.
Figure 4.
Figure 4.
Plant growth and development effects resulting from altered in planta saponin biosynthesis. Various developmental effects resulting from the over or reduced accumulation of saponins and/or their intermediates is depicted in Solanum lycopersicum, Medicago truncatula, Arabidopsis thaliana, Lotus japonicus and Avena strigosa. Gene names in upper and lower cases imply gene overexpression and knock out, respectively.
Figure 5.
Figure 5.
Phenotypic similarity of Medicago trunctula and Avena strigosa mutants accumulating incompletely glycosylated saponins. (A) Chemical structures of a representative monoglycosylated saponin, 3-O-Glc-Medicagenic acid accumulating in Mkb1 silenced M. truncatula roots (left) and avenacin A-1 lacking the β-1,4-linked d-glucose (marked with a red line) accumulating in sad3 and sad4 oat mutants (right). (B) Confocal microscopy analysis (left) of control (CTR, top) and Mkb1 silenced (bottom) M. trunctula hairy roots, compared to bright field (left) and UV illumination (right) microscopy of wild type (WT), sad3 and sad4 oat mutants (right) show similar root cap and root epidermal layer defects in both plants. From Mylona et al. (2008) and Pollier et al. (2013).
Figure 6.
Figure 6.
Overview of chemical structures of triterpenoids and saponins with industrial potential.
Figure 7.
Figure 7.
Overview of biological activities of triterpenoids and saponins in animals. The structures of representative compounds for each activity are given, together with the various molecular targets and affected pathways. Gene, pathway or metabolite activation and inhibition are indicated by arrow heads and blunt ends, respectively. Akt, serine/threonine-specific protein kinase; Bcl2, B-cell lymphoma 2; bFGF, basic fibroblast growth factor; cIAP, cellular inhibitor of apoptosis; COX-2, cyclooxygenase-2; CTL, cytotoxic T-lymphocyte; CXCR4/CXCL12, C-X-C chemokine receptor type 4 complex with C-X-C motif chemokine 12; DR, death receptor; EGFR, epidermal growth factor receptor; ICAM-1, intercellular adhesion molecule 1; IFN-γ, interferon γ; IgG, immunoglobulin G; IKK, IκB kinase; IL-2, interleukin 2; i-NOS, inducible nitric oxide synthase; MMP9, matrix metalloproteinase 9; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; Nrf2-ARE, nuclear factor (erythroid-derived 2)-like 2, antioxidant responsive element; PDGF, platelet-derived growth factor; PPARγ, peroxisome proliferator-activated receptor-γ; STAT3, signal transducer and activator of transcription 3; Th1, T helper 1; VEGF, vascular endothelial growth factor.
Figure 8.
Figure 8.
Overview of chemical structures. (A) Carbon and ring numbering of oleanane, ursane, lupane and dammarane aglycones. (B) Examples of triterpenoids and saponins discussed in structure-activity relationship studies. α-l-Ara, α-l-arabinose; α-l-Rha, α-l-rhamnose; Glc, glucose; Xyl, xylose.

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