Elsevier

Phytochemistry

Volume 72, Issue 6, April 2011, Pages 435-457
Phytochemistry

Review
Molecular activities, biosynthesis and evolution of triterpenoid saponins

https://doi.org/10.1016/j.phytochem.2011.01.015Get rights and content

Abstract

Saponins are bioactive compounds generally considered to be produced by plants to counteract pathogens and herbivores. Besides their role in plant defense, saponins are of growing interest for drug research as they are active constituents of several folk medicines and provide valuable pharmacological properties. Accordingly, much effort has been put into unraveling the modes of action of saponins, as well as in exploration of their potential for industrial processes and pharmacology. However, the exploitation of saponins for bioengineering crop plants with improved resistances against pests as well as circumvention of laborious and uneconomical extraction procedures for industrial production from plants is hampered by the lack of knowledge and availability of genes in saponin biosynthesis. Although the ability to produce saponins is rather widespread among plants, a complete synthetic pathway has not been elucidated in any single species. Current conceptions consider saponins to be derived from intermediates of the phytosterol pathway, and predominantly enzymes belonging to the multigene families of oxidosqualene cyclases (OSCs), cytochromes P450 (P450s) and family 1 UDP-glycosyltransferases (UGTs) are thought to be involved in their biosynthesis. Formation of unique structural features involves additional biosynthetical enzymes of diverse phylogenetic background. As an example of this, a serine carboxypeptidase-like acyltransferase (SCPL) was recently found to be involved in synthesis of triterpenoid saponins in oats. However, the total number of identified genes in saponin biosynthesis remains low as the complexity and diversity of these multigene families impede gene discovery based on sequence analysis and phylogeny.

This review summarizes current knowledge of triterpenoid saponin biosynthesis in plants, molecular activities, evolutionary aspects and perspectives for further gene discovery.

Graphical abstract

Biosynthesis of triterpenoid saponins branches off phytosterol anabolism by alternative cyclization of 2,3-oxidosqualene. Mainly P450s and UGTs are involved in further biosynthetic steps.

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Research highlights

Saponins are bioactive compounds participating in plant defense. ► Saponins derive from phytosterol biosynthesis. ► OSCs mediate oxidosqualene cyclization. ► P450s catalyze sapogenin modification. ► UGTs are involved in conferring biological activity of saponins.

Introduction

The term ‘saponin’ defines a group of natural compounds that consist of an isoprenoidal-derived aglycone, designated genin or sapogenin, covalently linked to one or more sugar moieties. The name is deduced from the Latin word sapo (Engl.: soap) reflecting their wide spread ability to form stable soap-like foams in aqueous solutions. In fact, many plant extracts (e.g. from soapwort – Saponaria officinalis, soapbark – Quillaja saponaria, etc., Fig. 1) previously used for their soap-resembling properties often gained this ability due to saponins (Hostettmann and Marston, 1995). This characteristic trait is caused by the amphiphilic nature of saponins due to linkage of the lipophilic sapogenin to hydrophilic saccharide side chains.

Most known saponins are plant-derived secondary metabolites, though several saponins are also found in marine animals such as sea cucumbers (Holothuroidea) (e.g. Van Dyck et al., 2010) and starfish (Asteroidea) (e.g. Liu et al., 2008). The ability to synthesize saponins is rather widespread among plants belonging to the division of Magnoliophyta, covering both dicotyledons and monocotyledons. However, the majority of saponin-producing species has been found within dicotyledons (Vincken et al., 2007).

The biological role of saponins is not completely understood. In plants, they are generally considered to be part of defense systems due to anti-microbial, fungicidal, allelopathic, insecticidal and molluscicidal, etc. activities (reviewed in Francis et al., 2002, Sparg et al., 2004).

Saponin-producing plants generally accumulate saponins as part of their normal development. However, saponin accumulation is also known to be influenced by several environmental factors such as nutrient and water availability, light irradiation or combined effects (reviewed in Szakiel et al., in press). In addition, saponin distribution has been found to vary greatly in individual plant organs or tissues during ontogenesis or to show seasonal fluctuations. Variations in saponin distribution and levels have been suggested to represent varying needs for protection and to target specific herbivores and pests, respectively. For example, Ndamba et al. (1994) propose that maximal saponin accumulation in early stages of Phytolacca dodecandra (soapberry) berry development is to prevent fruit loss and to assure seed maturation. Lin et al. (2009) discuss that the highest saponin accumulation among all organs of Dioscorea pseudojaponica Yamamoto (yam) is in tubers to provide protection for this reproductive organ. Lastly, specific accumulation of saponins in the root epidermis of Avenae spp. (oats) has been demonstrated to counteract soil-borne fungi (Papadopoulou et al., 1999). Saponin levels are often increased in response to treatment with elicitors such as yeast extract or jasmonate derivatives (reviewed in Yendo et al., 2010). As particularly jasmonates are well known for triggering plant defense responses to herbivory (reviewed in Howe and Jander, 2008) this further corroborates the assumption of saponins being involved in plant defense.

Due to their chemical properties and abilities as foaming agents, saponins or saponin-containing plant extracts are exploited by industry as additives to foods and cosmetics. They have the potential to be used for further industrial applications as, for instance, as preservatives, flavor modifiers and agents for removal of cholesterol from dairy products (reviewed in Güçlü-Üstündağ and Mazza, 2007, San Martín and Briones, 1999). Saponin-rich byproducts from tea oil production have also been explored as botanical vermicide for selective managing of earthworm casts on golf courses and sport fields (Potter et al., 2010). Saponins are also known to be major constituents of many traditional folk medicines (e.g. extracts of liquorice – Glycyrrhiza sp. or ginseng – Panax sp.). Although health beneficial effects of these herbal extracts have not unequivocally been confirmed in clinical studies (Glycyrrhiza sp.: reviewed in Asl and Hosseinzadeh, 2008; Panax ginseng: reviewed in Xiang et al., 2008), several of the saponins exhibit pharmacological activities and thus, attract attention as target for drug discovery. For example, several sapogenins (oleanolic acid: Sultana and Ata, 2008) and saponins are considered to possess activities such as anti-inflammatory (e.g. Sun et al., 2010b, Tapondjou et al., 2008), anti-cancerogenic (e.g. Musende et al., 2009; further reviewed in Man et al., 2010), anti-bacterial (e.g. De Leo et al., 2006; further reviewed in Saleem et al., 2010), anti-fungal (e.g. Coleman et al., 2010, Zhang et al., 2005) and anti-viral (e.g. Cinatl et al., 2003, Rattanathongkom et al., 2009, Zhao et al., 2008) effects. Saponins are also of interest as valuable adjuvants and the first saponin-based vaccines are introduced commercially (reviewed in Sun et al., 2009).

Present availability of saponins depends on their extractability from plants. Extraction procedures may be effective for saponins that occur in high concentrations in easily cultivable plants, and for applications that do not demand absolute purity. However, a steady supply of sufficient amounts of specific saponins from plants that accumulate mixtures of structurally related compounds is not feasible (Adams et al., 2010). Synthetic chemistry mainly attempts to address these issues by chemically linking desired side chains to extracted sapogenins (reviewed in Yu and Sun, 2009). However, the availability of the corresponding sapogenins, which may demand sophisticated modifications to derive from more common precursors, still constitutes bottlenecks. Biotechnological production of either complete saponins, or of saponin pathway intermediates that are not readily accessible, may circumvent the limitation of natural saponin availability. Bioengineering of crop plants with improved resistance towards specific pests as well as saponin reduction in other plants for increased food and feed quality (Dixon and Sumner, 2003) are additional motivations for elucidation of saponin biosynthesis.

Substantial efforts have been made to unravel saponin biosynthesis in plants. Although a general consensus on the basic organization of pathways and enzyme classes involved in saponin anabolism has been reached, discovery of individual genes in saponin biosynthesis has so far occurred rather sporadically. However, considerable progress has been accomplished since triterpenoid saponin biosynthesis was last reviewed (Haralampidis et al., 2002).

The aim of this review is to summarize this progress and provide an updated overview on the current knowledge of the enzymes involved in triterpenoid saponin biosynthesis. In addition, general aspects of molecular activities of saponins, with particular focus on their mode of action towards membranes, are outlined. Finally, the phylogeny of known enzymes in saponin pathways is reviewed and conclusions about evolution of saponin biosynthesis and consequences for further gene discovery are discussed.

Section snippets

Structural aspects and nomenclature

The commonly used nomenclature for saponins distinguishes between triterpenoid (also: triterpene) and steroidal saponins (Fig. 2). This differentiation is based on the structure and biochemical background of their aglycones. Both sapogenin types are thought to derive from 2,3-oxidosqualene, a central metabolite in sterol biosynthesis. In phytosterol anabolism, 2,3-oxidosqualene is mainly cyclized into cycloartenol. Triterpenoid sapogenins branch off the phytosterol pathway by alternative

Molecular activities on membranes

Plant-derived saponins are mainly considered to be part of plant defense systems against pathogens and herbivores. Numerous reports emphasize the fungicidal (Lee et al., 2001, Morrissey and Osbourn, 1999, Saha et al., 2010, Sung and Lee, 2008a), anti-microbial (Avato et al., 2006, Sung and Lee, 2008b), allelopathic (Waller et al., 1993), insecticidal (Sandermann and Funke, 1970, Shinoda et al., 2002, Kuzina et al., 2009, Nielsen et al., 2010a) and molluscicidal Aladesanmi, 2007, Gopalsamy et

From acetyl-CoA to 2,3-oxidosqualene – common biosynthetic origin with phytosterols

Current ideas of saponin biosynthesis in plants, consider them to be derived from metabolites of phytosterol anabolism. This assumption is supported by the reported concurrent upregulation of sterols and saponins in ginseng plants (P. ginseng) over-expressing squalene synthase (Lee et al., 2004), an enzyme catalyzing a step prior to the proposed branchpoint of the two pathways. Increased phytosterol levels in oat (Avena strigosa) sad1 (saponin-deficient) mutants (Qin et al., 2010) and increase

Conclusions

Enzymes belonging to the multigene families of oxidosqualene cyclases, cytochromes P450 and UDP-glycosyltransferases are key players in biosynthesis of plant triterpenoid saponins. The number of identified enzymes involved in saponin pathways derived from these gene families has increased considerably over the last decade, which now allows a first assessment of the evolution of saponin biosynthesis. Thus, despite the fact that members of these three multigene families presumably perform

Acknowledgments

We would like to thank Thomas Günther-Pomorski for his advice regarding the models of saponin activity towards membranes und his valuable comments on the manuscript. We would further like to acknowledge Emma O’Callahan and Fred Rook for their inestimable support in mastering uncertainties of the English language. Lastly, we acknowledge James Gaither, for the image of Quillaria saponaria (posted at www.flickr.com) and Ulf Liedén (www.floracyberia.com) for the image of Saponaria officinalis, both

Jörg M. Augustin did his Diploma studies at Martin Luther University, Halle-Wittenberg (Germany) finalizing with obtaining the Diploma Biochemist degree in 2008. For his Diploma thesis at the Leibniz Institute of Plant Biochemistry, Halle (Germany) he heterologously expressed a cycanobacterial O-methyltransferase in Arabidopsis thaliana thereby aiming to influence lignin biosynthesis. In 2009 he started his Ph.D. studies at the Department of Plant Biology and Biotechnology, Faculty of Life

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    Jörg M. Augustin did his Diploma studies at Martin Luther University, Halle-Wittenberg (Germany) finalizing with obtaining the Diploma Biochemist degree in 2008. For his Diploma thesis at the Leibniz Institute of Plant Biochemistry, Halle (Germany) he heterologously expressed a cycanobacterial O-methyltransferase in Arabidopsis thaliana thereby aiming to influence lignin biosynthesis. In 2009 he started his Ph.D. studies at the Department of Plant Biology and Biotechnology, Faculty of Life Sciences, University of Copenhagen (Denmark). His Ph.D. project is focused on identifying and characterizing genes involved in biosynthesis of triterpenoid saponins in Barbarea vulgaris.

    Vera Kuzina obtained her M.Sc. degree as Engineer Specialized in Biotechnology from Moscow State University of Food Production (Russia) in 1999. Her Ph.D. studies were carried out at University of Sevilla (Spain) and Wageningen University (Holland) in parallel and focused on regulation of terpenoid biosynthesis in filamentous fungi Phycomyces and Blakeslea. She is currently employed as a Post-doctoral researcher at the Department of Plant Biology and Biotechnology, University of Copenhagen (Denmark), where she works on elucidation of molecular mechanisms of natural plant resistance to insects with focus on saponins.

    Sven Bode Andersen obtained his M.Sc. degree in horticulture from the Royal Veterinary and Agricultural University, Copenhagen (Denmark) 1980 and his Ph.D. in plant breeding from the same institution in 1983. His research is focused around in vitro production and use of chromosome doubled haploid plants and genetic mapping of complex characters in breeding and science, particularly for cereals and horticultural species. He is currently employed as professor of plant breeding at the University of Copenhagen, Faculty of Life Sciences where he, in addition to research, provides teaching in plant breeding and plant genomics.

    Søren Bak obtained his M.Sc. in biochemistry at University of Copenhagen in 1993 and his Ph.D. in Plant Molecular Biology at the Department of Plant Biology, Royal Veterinary and Agricultural University. From 1998 to 2000 he was a visiting scientist at the Department of Plant Sciences, University of Arizona. He has since 2005 been employed as Professor within Systems Biology and Bioinformatics in Plants at the Department of Plant Biology and Biotechnology, University of Copenhagen, where he is heading the Molecular Evolution Group. The long term goal of the Molecular Evolution Research Group is to elucidate evolution of plant multigene families and how they have influenced plant genome organization, recruitment of ability to produce new bio-active natural products, natural variation, and interactions with insects and microbes. Since 1994 he has published more than 50 peer reviewed papers mainly on the molecular level of how plants respond to and adapt to environmental changes.

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