Saponins are secondary metabolites and provide protection against invading pathogens. They act by permeabilising plasma membrane (Mugford and Osbourn 2013). The mechanisms is based on their ability to form complexes with sterol present in the membrane of the microorganisms causing loss of membrane integrity. Plant may avoid the toxic effects of their own saponin by compartmentalizing them in cell vacuole or in other organelles whose membrane are protected because of low or different sterol composition (Osbourn 1996). Sterol structure give rise to significant differences in membrane properties and membrane mechanical properties (Hodzic et al., 2008). Plant cells contain phytosterol such as campesterol, sitosterol and stigmasterol, while animal cells contain cholesterol in their membrane (Kazan and Gardiner 2017). Sitosterol and stigmasterol are predominant in plant membrane (Hodzic et al., 2008). Cholesterol is present in teliospore of the corn smut fungus (Ustilago maydis) (Weete and Laseter 1974). The ergot fungus Claviceps purpurea has sterol ergosterol and the powdery mildew fungi too contain ergosterol in their membranes (Kazan and Gardiner 2017). Successful pathogens manipulate the plant cell membrane to retrieve more nutrient from cell. Wang et al.(2012) present that sterol content play an important role in plant innate immunity against bacterial infections by regulating nutrient efflux into the apoplast.
Saponins are surface active glycosides, which possess amphiphilic character originating from lipophilic aglycone (sapogenin) and hydrophilic sugar moieties. The hydrophobic part may be a sterol or triterpene. Saponin can possess one to three straight or branched sugar moieties. Monodesmosidic saponin means one positon of aglycone is glycosylated. The amphiphilic character of saponins allows them to aggregate in aqueous solution and interact with membrane components (Korchowiec et al., 2015). Subsequently saponin penetrate membrane where they complex with sterol, forming saponin/sterol complexes (Roddick and Drysdale 1984; Steel and Drysdale 1988; Armah et al., 1999). Upon incorporation of saponin molecule into the membrane, form complex with membrane sterol leading to pore formation (Iriti and Faoro 2009).The concomitant membrane permeabilization results in cell death (Colson et al., 2020).
The presence of appropriate glycosidases in cell membranes, are capable of converting saponins into their aglycones is a prerequisite for the membranolytic action of saponins. Removing only one sugar residue results in a complete loss of the pore-forming ability of avenacin A-1 (Armah et al., 1999). Furthermore removal of a single sugar from the tetrasaccharide chain of tomato steroidal glycoalkaloid alpha-tomatine results in a substantial reduction in antimicrobial activity, whereas complete loss of sugars leads to enhanced antifungal activity (Simon et al., 2006). The glycoalkaloid are able to interact with sterol containing membranes thereby causing membrane disruption specific for the type of glyalkaloid and sterol. The mode of action of the glycoalkaloid is proposed to consist following steps (Keuken et al., 1992):
- Insertion of the aglycone in the bilayer
- Complex formation of the glycoalkaloid with the sterols present
- Rearrangement of the membrane caused by the formation of a network of sterol-glycoalkaloid complexes resulting in a transient disruption of the bilayer during which leakage occur
The hydrogen-bonding between the sugar moieties of glycoalkaloids are important during matrix formation, the main factors for this process to occur are the structure and composition of these moieties. The lipid sugar groups exert hydrogen-bondings with sugar moieties of the membrane associated glycoalkaloids thereby prolonging the presence of the glycoalkaloids in the membrane, leading to formation of a membrane disruptive matrix (Keuken et al., 1995).
The important properties for sterol to interact with glycoalkaloids turned out to be a planar ring structure and a 3β-OH group (Keuken et al., 1995). The toxic effect of α-tomatine is attributed to mode of action that it forms a complex with fungal membrane sterols with free 3 β-hydroxyl group resulting in pore formation and loss of membrane integrity followed by leakage of cell components (Ito et al., 2007). α-Tomatine is toxic to a broad range of fungi because it binds to 3β-hydroxy sterol in fungal membrane (Sandrock and VanEtten 1998). α-Tomatine a pre-formed antifungal compound from tomato tissue disrupts liposome membranes containing a 3β-hydroxy sterol. The liposome membranes containing sterols lacking a 3β-hydroxy sterol were resistant to α-tomatine (Steel and Drysdale 1988).
The lysis of Penicillium notatum protoplasts by the potato glycoalkaloidsα-solanine and α-chaconine was studied (Roddick et al., 1988). The latter was more membrane disruptive compound. Aescin a saponin has a strong antifungal activity (Trda et al., 2019). The antifungal effect of aescin could be reversed by ergosterol, thus suggesting that aescin interferes with fungal sterol. Aescin activated plant defense through induction of salicylic acid (SA) pathway and oxidative burst. This defense response led to protection against both fungi and bacterial pathogens (Trda et al., 2019). The saponin-deficient mutants of diploid oat species Avena strigose are susceptible to a variety of fungal pathogen and the evidence suggests that compromised disease resistance is a direct consequence of saponin deficiency (Papadopoulou et al., 1999).
Targeting fungal sterol is a host defense strategy to counter pathogen attack. The antimicrobial properties of saponin indicate that it can be used as a biopesticide.
See Part A(i) for further information ………
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 